Life cycle comparison of industrial-scale lithium-ion battery recycling and mining supply chains

Domain: earth_science
Published: 2025-01-24
Editorial outcome: major_revision

Abstract
Recycling lithium-ion batteries (LIBs) can supplement critical materials and improve the environmental sustainability of LIB supply chains. In this work, environmental impacts (greenhouse gas emissions, water consumption, energy consumption) of industrial-scale production of battery-grade cathode materials from end-of-life LIBs are compared to those of conventional mining supply chains. Converting mixed-stream LIBs into battery-grade materials reduces environmental impacts by at least 58%. Recycling batteries to mixed metal products instead of discrete salts further reduces environmental impacts. Electricity consumption is identified as the principal contributor to all LIB recycling environmental impacts, and different electricity sources can change greenhouse gas emissions up to five times. Supply chain steps that precede refinement (material extraction and transport) contribute marginally to the environmental impacts of circular LIB supply chains (<4%), but are more significant in conventional supply chains (30%). This analysis provides insights for advancing sustainable LIB supply chains, and informs optimization of industrial-scale environmental impacts for emerging battery recycling efforts.

Introduction
The rise of intermittent renewable energy generation and vehicle electrification has created exponential growth in lithium-ion battery (LIB) production beyond consumer electronics. By 2030, the electric vehicle (EV) sector is projected to dominate LIB growth, accounting for 82% of an estimated 2.4 TWh yr−1 of total global LIB production (Supplementary Fig. 1). However, the limited supply of critical materials (e.g., Li, Ni, Co, and Cu1) needed for prominent LIB chemistries has exacerbated environmental, economic, national security, and human rights concerns2,3. Critical LIB materials are projected to reach major global supply–demand balance deficits before 2030 (Supplementary Fig. 1) without additional investment to improve supply chains. Further, both mining of LIB materials and improper disposal of end-of-life LIBs can damage natural and human cause occupational hazards during handling, and result in monetary losses4.

Recycling critical materials in end-of-life LIBs can help alleviate growing environmental concerns and is essential for the long-term sustainability of electrified transportation. While recycled materials may not contribute substantially to global LIB demand for decades, the establishment of domestic circular supply chains is iterative, requiring multiple learning curves as the dominant supply of end-of-life LIB chemistries and form factors evolve and as supply grows. Factors central to the success of recycling include the cost of collecting products, the cost of recycling processes, and the economic value of recovered materials. Considering LIB prices between 2018 and 2021, Li, Ni, and Co comprise the highest embodied economic value (Fig. 1a)5–12, and Al and Cu account for a significant weight percentage of EV battery packs (~25%)13. While 99% of lead-acid batteries are recycled in the USA, LIBs exhibit 2–10 times higher economic values but are only recycled 2%–47% globally14. The environmental benefits of circularity also strongly motivate LIB recycling given the vast LIB production and emission-intensive mining refinement for key constituent metals. There is a critical need to evaluate the environmental opportunity across several application scales, from numerous small-scale consumer electronic LIBs (e.g., 10–100 Wh) to fewer large-scale transportation and stationary storage LIB packs (e.g., 10–100 kWh)15. In addition, the preferred chemistries by automakers have evolved to hedge potential critical mineral shortages and react to market shifts (e.g., increasing emphasis on lithium iron phosphate and sodium-ion batteries), such as the near tripling of lithium carbonate prices in early 2022. Existing LIB variation and supply chain complexity highlight the need for a methodical and comparative life cycle assessment (LCA) between circular (i.e., recycling end-of-life batteries) and conventional supply chains, which is needed for incumbent LIBs today and for prospective recycling strategies with various battery chemistries in the future.

Despite significant progress, current understanding of the environmental impacts of recycling LIBs is still incomplete. The most significant environmental differences between LIB production from circular and conventionally mined cathode materials have not been uniformly attributed to specific supply chain steps which we refer to as extraction, transport, and refinement steps (together referred to as “cradle-to-gate,” Fig. 1b). The gate-to-gate refinement processes utilized at established and emerging circular refinement facilities include mechanical separation (Me), pyrometallurgy (Py)16,17, and hydrometallurgy (Hy)18,19. Specifically, Me physically dismantles LIBs into constituent components, Py leverages elevated temperature to facilitate material transformations, and Hy separates materials in the aqueous phase via leaching, precipitation, and solvent extraction processes. Previous efforts have worked toward calculating environmental impacts (e.g., energy consumption, greenhouse gas emission, and water consumption) of LIB refinement pathways and all cradle-to-gate supply chain steps. However, gate-to-gate analyses of circular refinement processes reported environmental impacts differing by over 30%18–21 due to inconsistent methodologies. In addition, advancing decision-making capabilities to scale sustainable LIB supply chains requires LCA with more granular data at each step. Incorporating industrial-scale refinement operational data can uniquely inform rational design of refinement technologies. The future development of LIB manufacturing and drivers for a circular battery economy have been projected by academic and industrial researchers16,22, but industrial-level understanding of the environmental influences of different feedstocks and refinement products is still lacking.

In this study, we quantify the cradle-to-gate environmental impacts of battery-grade cathode material salts manufactured in conventional and circular supply chains across three major steps: material extraction, transport, and refinement (Fig. 1b), focusing on the refinement step. First, we quantify the refinement of mined concentrate from natural deposits into battery-grade materials in conventional supply chains and compare with production of these materials by Redwood Materials (a recycling company in Nevada, USA) in 2021. Two LIB feedstocks are explored: non-energized LIB production scrap from manufacturing facilities and energized end-of-life LIBs collected from consumers. This study contributes insights to inform circular battery manufacturing by addressing three critical gaps in the literature. First, industrial-scale operational data provided by Redwood Materials are analyzed from a more granular level than previous reports and compared to conventional LIB supply chain values based on Argonne National Laboratory’s Greenhouse Gases, Regulated Emissions, and Energy use in Technologies (GREET 2021) model23. This use of industrial-scale recycling quantitatively identifies the dominant role of input grid electricity in circular refinement at the balancing area level on environmental impacts metrics based on industrial-scale LIB recycling data. Second, the influences of the product formats in the circular refinement pathways on environmental impacts are examined by varying the industrial-scale refinement pathways. For both conventional and circular refinement, impacts of producing mixed Ni–Co compounds and discrete salts are analyzed, showing that lower-impact mixed products are worth further investigation. Third, the environmental impacts of upstream processes before gate-to-gate refinement are modeled. The upstream assessment includes the extraction of LIB material from conventional (i.e., mined ore) or circular (i.e., collected batteries) sources and the transport of extracted material to relevant refinement facilities for the production of battery-grade cathode materials as Li, Co, and Ni sulfate or carbonate salts. This upstream modeling shows that refinement is the major contributor to environmental impacts in the circular case, which highlights opportunities to further improve refinement-associated energy and emissions. This study provides LCA insights with primary industrial-scale circular refinement data that includes stepwise, cradle-to-gate comparison of conventional and circular LIB supply chains. With the methodologies and results reported in this study, researchers can prioritize major opportunities to improve process efficiencies, practitioners can benchmark their environmental impacts, and policymakers can incentivize better environmental practices in LIB supply chain management. Granular insights provided by this study based on industrial operation data can also help recyclers optimize the environmental impacts of their refinement processes, and spark more academic-industrial collaborations to further advance the field.

Results
Refinement pathways
In this study, analyses of environmental impacts were presented with a focus on the refinement step, followed by analysis of the upstream material extraction and transport steps. In LIB supply chains, the refinement step converts the collected feedstocks into battery-grade salts for further manufacturing (Fig. 2). In both conventional and circular supply chains, the refinement pathways vary significantly depending on multiple factors. Five refinement pathways were compared in this study (Fig. 2). Conventional refinement starts with mined ores/brines (Scenarios (1) and (2) in Fig. 2). Battery scrap generated from manufacturing and assembly is considered a primary recycling source today, and is projected to account for approximately half of the recycling source material in the next decade as battery production outpaces the generation of end-of-life energized batteries16,22. Therefore, circular refinement was analyzed starting with either end-of-life batteries ((3) and (4)) or battery scrap (5). Ni and Co in refinement products for subsequent manufacturing can be discrete salts ((1) and (3)) or mixed compounds ((2), (4), and (5)). Target products of the conventional and circular pathways were based on the GREET model and practical recycling operations, respectively. In the following sections, the overall refining environmental impacts were first analyzed, followed by influences of product formats on the refinement step and key contributors to the refining environmental impacts. Lastly, upstream environmental impacts were analyzed and compared to the refinement step. Unless specifically noted, all major analyses were based on 2021 data (data reference years are summarized in Supplementary Table 2).

Refining lithium-ion batteries lowered environmental impacts
Environmental impacts of refinement pathways in conventional and circular LIB supply chains were analyzed in Fig. 3. The upstream steps of material extraction and transport, which did not have the same granular primary data as refinement, were considered in later sections. Energy consumption, greenhouse gas emissions (CO2-equivalents, CO2-eq; additional criteria air pollutants are detailed in Supplementary Table 3), and water consumption were chosen as key metrics to analyze the environmental impacts of LIB supply chains in this study19,24,25. One kg of lithium–nickel–cobalt–aluminum–oxide cathode-equivalent material (NCA-eq) was employed as a functional unit throughout this study for supply chain comparison, accounting for the elemental requirements to produce stoichiometric LiNi0.80Co0.15Al0.05O2. NCA chemistry was selected because it accounted for the second-largest category of EV battery chemistries following NMC batteries in 202115,26, and is projected to utilize less Co compared to NMC14. Excluding the environmental impacts of material extraction and transport steps, the gate-to-gate production of 1 kg NCA-eq battery-grade material from state-of-the-art conventional mined natural materials consumed 193.9 MJ and 77.3 L H2O while emitting 14.5 kg CO2-eq (Fig. 3). Refinement of mined material concentrate into battery-grade Ni material dominated NCA environmental impacts, representing >57% of total values. Note that the results were based on GREET 2021 to match the period when circular refinement data were collected at Redwood; more recent environmental impacts of the conventional supply chain based on the 2023 model were 17.0%–18.3% higher than that of 2021, and are presented in Supplementary Data. The greenhouse gas emissions values were comparable with previous studies based on GREET datasets23,27 (comparison of environmental impacts with data in literature is detailed in Supplementary Fig. 2d).

The environmental impacts of two circular refinement pathways were presented in each graph in Fig. 3 for mixed-stream LIB feedstocks processed at Redwood Materials: non-energized production scrap from LIB production facilities (recycled scrap) and energized, end-of-life LIBs collected from consumers (recycled battery). Using a limiting-reagent approach of output products to produce 1 kg NCA-eq material, energy requirements for processing recycled scrap and recycled battery streams were 22.0 MJ and 44.4 MJ per kg NCA-eq materials, significantly lower than conventional refinement by 88.7% and 77.1%, respectively (Fig. 3a). Relatedly, 2.8 and 6.1 kg CO2-eq per kg NCA-eq materials were generated from scrap and battery streams, respectively, a substantial reduction in CO2-eq emissions by 80.9% and 57.7% (Fig. 3b). Water consumption was also lower by 87.7% for scrap and 72.2% for battery streams relative to the conventional scenario, resulting from the consumption of 9.5 and 21.5 L H2O per kg NCA-eq materials, respectively (Fig. 3c). Note that while the elemental stoichiometry was identical, the output battery-grade materials varied slightly between conventional (Li2CO3, NiSO4, CoSO4) and circular (Li2SO4, (Ni,Co)SO4) refinement in the “Methods” section. Metal sulfates are commonly produced and traded in the battery recycling market28–31. Converting the final lithium product to Li2CO3 did not substantially change the environmental impacts of the circular supply chains (Supplementary Note 3, Supplementary Fig. 2), and impacts of producing discrete or mixed products are examined in the following section.

To produce battery-grade cathode materials, Redwood Materials used a combination of reductive calcination (RC) pyrometallurgical, mechanical (Me), and hydrometallurgical (Hy) LIB refinement processes (pathways detailed in Supplementary Fig. 5). RC is an industrial-scale exothermic pyrometallurgical process that reduces the cathode metal oxide compounds under oxygen-free conditions for subsequent refinement. Unlike dominant industrial pyrometallurgical processes (e.g., direct roasting or smelting) that require high temperature >1400 °C17,32, the RC process optimizes the working conditions, favoring carbothermal reduction without using graphitic carbon, thus avoiding direct fossil fuel inputs, graphite combustion, and substantial Li loss. Because RC is not required for non-energized LIB production scrap materials, the two feedstock streams (recycled scrap and recycled batteries) were analyzed separately. Energy consumption and CO2-eq emissions of representative existing recycling pathways from the literature, including pyrometallurgy (Py*), hydrometallurgy (Hy*), and direct recycling (Direct*), are also presented in Fig. 3 for comparison. Note that a mechanical processing step is often included in the Hy* refinement. In general, the RC+Me+Hy pathway exhibited comparable energy consumption and CO2-eq emissions with Hy* and Direct* literature values19, and substantially lower environmental impacts than Py*. Similar to RC, emerging oxygen-free pyrometallurgical processes rely on carbothermic and thermite reduction for recycling cathode metal compounds at moderate temperatures (600–1000 °C)33–35. While most carbothermic and thermite reduction processes have been investigated at the lab scale33–35, our results showed that the RC step accounted for 5.5%–7.5% of the total environmental impacts of the circular refinement step (Fig. 3), demonstrating the environmental feasibility of industrial-scale carbothermal reduction-based pyrometallurgy. Note that RC pyrometallurgy can process energized batteries of varying states of charge, health, and formats with minimal modification, whereas traditional hydrometallurgy often requires discharging energized batteries in a salt bath or removing electrolyte for safe mechanical processing. While this analysis was focused on Redwood Materials refinement pathways, the methodology can be used to evaluate additional refinement pathways (e.g., analysis of a representative hydrometallurgy pathway for energized end-of-life batteries detailed in Supplementary Fig. 2d), or others that use different material feedstocks, refinement processes, and energy supplies.

Among the few studies that directly compare the environmental impacts of circular and conventional NCA refinement using industrial-scale operational data, 35% lower greenhouse gas emissions (Supplementary Fig. 2) were reported for Me+Hy circular refinement compared with the current study19,27. However, direct comparison can be inexact due to varying underlying assumptions and data sources. For example, Argonne National Laboratory’s GREET and EverBatt models leveraged a combination of technology descriptions from patent applications, literature data on process flow consumptions, industry site visits and surveys, expert advice solicitation, and stated assumptions to form complete pathways. Further, Ciez and Whitacre quantified environmental impacts using output products represented as “metal offsets” for pyrometallurgy or with metals in solution for hydrometallurgy19 (Supplementary Note 3), rather than cathode salts in this study. In addition, the previous studies included a portion of recycled metal materials in conventional supply chain analysis, whereas this work referenced only mined natural deposits in conventional supply chains to fully deconvolute the environmental impacts27. The different conclusions highlight divergent LCA approaches, processing conditions, and the utility of primary industrial data access in addition to modeling processes from literature sources21.

Formats of refinement products influenced environmental impacts
Product format is an important factor in understanding and properly comparing LIB refinement pathways (Fig. 2). Ni and Co are key elements for battery manufacturing, and can be traded in the format of mixed metal salts or discrete salt products between battery refiners and battery manufacturers36,37. To examine the influences of the refinement product formats, environmental impacts were compared for refinement to mixed salt (e.g., mixed NiSO4 and CoSO4) and refinement to discrete sulfate salts, NiSO4 and CoSO4 (Fig. 4). Both conventional and circular refinement pathways were analyzed.

The GREET model was employed to analyze different conventional mining pathways generating different product formats (detailed in the “Methods” section). In conventional mining, refining to mixed hydroxide precipitate, (Ni,Co)(OH)2 (Scenario (2) in Fig. 2), increased energy consumption and CO2-eq emissions by 77.% and 89.4%, respectively, over the discrete salts-based pathway (Fig. 4a, b, “Conv. Mined”). The discrete products NiSO4 and CoSO4 are separately produced from Ni-rich and Co-rich ores. In contrast, generating mixed hydroxide salt starts from the Ni laterite ore, which has a low concentration of Co (0.01-0.15%) relative to Ni (0.66-2.4%)38. This low Co concentration limits the NCA stoichiometry and increases the total energy cost to generate 1 kg NCA-equivalent materials. On the other hand, water consumption of refining mixed hydroxides was slightly lower (−6.6%) than that of producing discrete salts. In general, the discrete salts-based pathway is favorable for conventional refinement to reduce environmental impacts.

Circular pathways refining batteries to different products were analyzed using the industrial RC+Me+Hy data and the modeling of a representative battery recycling method combining mechanical and hydrometallurgy (Me+Hy, analogous to the Hy* in Fig. 3) refinement (Scenario (3) in Fig. 2). The RC+Me+Hy pathway refines recycled batteries to mixed metal sulfate, (Ni,Co)SO4, whereas the representative Me+Hy produces discrete NiSO4 and CoSO4. Refining into mixed metal sulfate exhibited lower energy consumption (−72.3%), CO2-eq emissions (−41.4%), and water consumption (−8.0%) than the Me+Hy pathway (Fig. 4), because it avoids additional treatment separating (Ni,Co)SO4 to discrete salts. Overall, our results indicated that refining batteries to mixed metal salts instead of discrete salts can substantially save environmental impacts while still satisfying the needs of circular LIB supply chains. Our findings also provide important insights to optimizing plant-scale battery refining operations. In the following sections, mixed salt-based pathways were analyzed for refinement.

Electricity consumption dominated circular refinement
To further understand the performance limiting factors in the refinement step, the relative environmental impacts of input consumables (e.g., energy, water, commodity chemicals) in the gate-to-gate refinement processes were disaggregated in Fig. 5 (additional criteria air pollutants in Supplementary Tables 10, 11 and Supplementary Figs. 3, 6). Note that the embodied environmental impacts of electricity consumption in Fig. 3 were based on the Nevada Power Company (NEVP) at the Redwood Materials location. Electricity consumption was found to be a principal factor dominating the environmental impacts. For both LIB feedstock pathways (Scenarios (4) and (5) in Fig. 2), electricity accounted for 70.3%–91.0% of the total energy consumption, 70.5%–83.4% of the total CO2-eq emissions, and 56.9%–66.1% of water consumption (Fig. 5a). For both feedstocks, Hy processes comprised the majority of environmental impacts, contributing more than 87.3% to energy consumption, 85.9% to CO2-eq emission, and 88.6% to water consumption. Notably, the additional RC step required for processing energized batteries only marginally contributed to CO2-eq emissions (7.5% of total). Unlike conventional pyrometallurgical processes that require external energy sources19,32, RC pyrometallurgy is primarily autothermic because it leverages process heat released from exothermic reactions of the LIB materials39,40. In addition to electricity consumption, chemical reagents used in circular refinement processes also contributed to embodied environmental impacts. Alkali reagents used to precipitate metals contributed between 7.6% and 19.9% of environmental impacts (largest relative contribution to water consumption). H2O2 was used to reduce high oxidation state metal compounds for hydrometallurgical leaching of scrap material, and accounted for 10.7%–20.1% of environmental impacts (largest relative contribution to energy consumption).

Because electricity dominated the environmental impacts of LIB recycling processes, we compared several electricity grid balancing areas that emit a range of CO2-eq emissions per MWh (averaged for 2021)41–43 in Fig. 5b (additional criteria air pollutants detailed in Supplementary Table 12). Substituting NEVP electricity with other balancing areas including Bonneville Power Administration Transmission (BPAT), California Independent System Operator (CISO), Western Area Power Administration of Colorado-Missouri (WACM), and a 100% Renewable Energy Tariff in Nevada (NV*), yielded a significant reduction in CO2-eq emissions of up to 93.3% (recycled scrap) and 87.4% (recycled battery) relative to conventional refinement (Fig. 5b). Conversely, employing low-carbon electricity grids can increase water consumption compared with NEVP-based operation, following the order of NV*>BPAT>WACM>CISO>NEVP (Fig. 5b). Note that NV*- and BPAT-based circular refinement processes exceeded the water consumption level of conventional refinement due to significant contributions from hydro- and geothermal power. Further investigation into the grid electricity sources of balancing areas revealed a tradeoff between CO2-eq emissions and water consumption based on electricity generation type (Fig. 5c); most electricity sources with relatively low CO2-eq emissions (e.g., those based on bio-, hydro-, or geothermal energy) exhibited high water consumption, and vice versa. This tradeoff also explained the different influences of electricity sources on environmental impacts of the Redwood Materials refinement step and other pathways (detailed in Supplementary Fig. 2d). However, the electricity sources for each balancing area will affect both CO2-eq emissions and water consumption. For example, because NEVP-based electricity includes a relatively large proportion (70%) from CO2-eq emissions-intensive natural gas with low water consumption, a switch to hydro-intensive (73%) BPAT electricity would decrease CO2-eq emissions while increasing water consumption.

Upstream environmental impacts were lower in circular supply chains
Before the refinement step, LIBs undergo the upstream steps of material extraction and transport to refinement facilities (Fig. 1b). Environmental impacts of these upstream steps were analyzed for two representative LIB chemistries and battery use cases: NCA in EV battery packs, and lithium cobalt oxide (LiCoO2 or LCO) in smartphones. California was chosen to assess circular extraction because it has the largest population and EV market share in the USA44,45. Smartphones were considered extracted when collected, aggregated, and transported from all California residents (analyzed per census block) to the nearest existing collection facility (CF)46. A shortest-path route for collection was determined by minimizing the distances from block group to CF for the whole state (Fig. 6a; the model is summarized in the “Methods” section and detailed in Supplementary Note 4)44. To quantify conventional material extraction environmental impacts from mining, global supply chain data were adapted from GREET23 (Supplementary Fig. 4, Supplementary Tables 14, 15). Smartphone extraction in the circular supply chain emitted only 0.0186 kg CO2-eq per kg LCO-eq, significantly lower than conventional mining (1.96 kg CO2-eq per kg LCO-eq) by 99.0%. Energy and water consumption were similarly lower in the circular supply chain (Supplementary Table 15).

After extraction, LIB material concentrates transported along domestic and international routes by truck, train rail, and maritime cargo ship to refinery locations (a portion of the network model is presented in Supplementary Fig. 4, and data summarized in Supplementary Tables 4–9, 15, and 16)23,47–57. The environmental impacts of transport were quantified by calculating the shortest distance along major transport routes among the participating countries weighted by the relative contributions of the countries to the market for each element (the case of cobalt is presented as an example in Fig. 6b). Details of the modeling method can be found in Supplementary Note 5. Conventional mine-to-refinery environmental impacts were calculated for 1 kg of embodied Li, Ni, Co, and Al metal (Supplementary Table 14). While transport emissions for Li, Ni, and Co ranged from 5.4–6.4 kg CO2-eq per kg embodied metal, Al was three times lower. For the circular case applied to California, smartphones and EV battery packs collected at CFs were transported to a hypothetical central LIB circular refinement facility at the population-weighted center (i.e., gravity point) of California (near Bakersfield)44. In conventional supply chains, transporting mined material concentrates accounted for 3.68 kg CO2-eq per kg NCA-eq and 4.32 kg CO2-eq per kg LCO-eq. By comparison, emissions for the transport of aggregated end-of-life NCA EV battery packs (i.e., not disassembled) and LCO smartphone batteries (not separated from phones) to a circular refinement facility were 0.073 kg CO2-eq per kg NCA-eq and 0.47 kg CO2-eq per kg LCO-eq, 98.2% and 89.1% lower than transport of mined concentrate, respectively. The reduction in CO2-eq emissions was attributed to differences in elemental concentrations of transported materials and aggregate transport distance (e.g., a weighted average of 224 km for circular NCA-eq materials, and 57,600 km for conventional NCA-eq materials).

Refinement-dominated circular and conventional supply chains
Combining material extraction, transport, and refinement steps yielded a cradle-to-gate comparison of the differentiated steps of conventional and circular LIB supply chains for producing battery-grade cathode materials (Fig. 6d–f). Here the environmental impacts of the LIB refinement step in California were analyzed for a hypothetical scenario employing the same circular multi-step refinement technologies as the RC+Me+Hy pathway in Nevada, but using California (CISO) electricity to produce battery-grade cathode materials. A circular supply chain in California for NCA EV and LCO smartphone batteries lowered energy and greenhouse gas emissions by at least 47.3% and water consumption by over 42.2%. In the case of recycling NCA EV batteries in California, the entire cradle-to-gate greenhouse gas emissions of the circular supply chain were lower than the transport emissions of mined concentrate in conventional supply chains (Fig. 6d–f and Supplementary Table 15). Circular production of LCO-grade materials led to higher environmental impacts than that of NCA-grade materials based on the mixed-stream feedstock composition analyzed in this study. Note that LCO has relatively lower packing densities of active materials compared with NCA (detailed in Supplementary Table 5, thus elevating the environmental impacts of transporting LCO). Overall, upstream steps (extraction and transport) contributed marginally to the total environmental impacts of both circular supply chains, accounting for ≤4.1% CO2-eq emission, ≤8.2% energy consumption, and ≤0.36% water consumption. Accordingly, the refinement process dominated the environmental impacts of the circular supply chain. In contrast, upstream steps in the conventional supply chain played a larger role (still smaller than refinement) in cradle-to-gate environmental impacts, contributing between 7.8% and 30.4% to the environmental metrics considered (Supplementary Table 15).

Methods
Goal and scope
The goal of this study was to compare stepwise cradle-to-gate environmental impacts (energy consumption, CO2-eq emission, and water consumption) for two supply chains: a conventional, linear supply chain fed by natural mined material for refinement into battery materials, and a circular supply chain fed by LIBs. Both supply chains produced battery-grade cathode materials. A comprehensive cradle-to-gate analysis of both supply chains considered steps of material extraction, transport, and refinement, and gate-to-gate analysis investigated the refinement step, the focus of this study. A gate-to-gate scope was broadly defined as the boundary surrounding processing facility operations. In this analysis, gate-to-gate refinement only considered direct processing (e.g., alteration, concentration, precipitation) of the feedstock material once it was extracted from its original state and transported to the refinement location (shown in Fig. 1b). For Redwood Materials, this scope included mechanical processing, reductive calcination, and hydrometallurgy (Supplementary Fig. 5). The system boundary did not include other operations outside of the direct refinement processes as discussed in study limitations below.

Two LIB feedstock streams were evaluated: (1) battery production scrap and (2) mixed, spent LIBs from consumers (Supplementary Fig. 5). Upstream of the gate-to-gate supply chain included both material extraction and transport steps. For conventional extraction, GREET was used to quantify the environmental impacts of mining. Transport between the supply chain steps and the circular extraction step was quantified using a logistics transportation model developed in this study, where limitations were summarized below.

Methodology
An attributional LCA was conducted to quantify and compare conventional and circular LIB supply chains for the production of battery-grade cathode materials. This analysis complied with the International Organization for Standardization (ISO) 14040 standards but omitted conversion to environmental impact indicators and external review67. Data for conventional material extraction (e.g., mining) and refining were adapted from the Argonne National Laboratory’s Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation (GREET®) 2021 model. GREET and the ecoinvent 3.3 database68 were employed for lifecycle inventory (LCI) data of chemical consumables for the conventional and circular supply chains.

To assess circular LIB refinement, primary operational data detailing energy, water, on-site emissions, and consumables usage were provided by Redwood Materials and normalized to mass flows of different elements of interest in input feedstocks and output products. A representative prevailing circular refinement, Method (2) in Fig. 2, was modeled with the software HSC Sim69, based on the technical procedures available in the literature58,70–72 and the practical feedstock amount received by Redwood.

Conventional refinement was modeled by aggregating the environmental impacts of the individual refining pathways for each LIB cathode element (Supplementary Table 3), normalizing by the mass of the individual element of interest within the output product (e.g., Li in Li2CO3) and then normalizing again by the mass of that element in the functional unit for this LCA (defined in the next section). For elements where more than one pathway of production exists in the GREET model (i.e., Ni and Li), the overall environmental impacts were calculated by averaging pathways weighted by their respective share of global production (45% Li production from brine and 55% from ore, and 60% Ni production from mixed hydroxide precipitate and 40% from Class 1 Ni). Both discrete and mixed output products were considered. Discrete salts from conventional refinement were Li2CO3, NiSO4, CoSO4, and Al2O3; alternatively, (Ni,Co)(OH)2 was considered as the mixed product. Lithium outputs produced by Redwood Materials were Li2SO4 (environmental impacts for converting to Li2CO3 are detailed in Supplementary Note 3), and other outputs existed as mixed metal sulfates of (Ni,Co)SO4 or as Al2O3 and Al(OH)3. With additional treatment further transforming the mixed metal sulfate into separate Ni and Co compounds, discrete salts as NiSO4 and CoSO4 were analyzed based on modeling of a prevailing Hy+Me refinement pathway.

In the cradle-to-gate analysis, material transportation between stages was not included because it was not consistently available in the GREET model. Electricity sources vary between elements, as well as between pathway stages. For example, crude production of Co(OH)2 used a distributed electricity source in the Democratic Republic of the Congo, and the refinement of these materials into CoSO4 and CoCl2 used a distributed electricity source in China. See Supplementary Data for the breakdown of the conventional refining data workflow.

Defining functional units
Functional units standardize comparisons of the resource consumption and emissions in LCAs. In this study, two functional units were considered to normalize environmental impacts between conventional and circular supply chains: the battery-grade material required to make 1 kg of stoichiometric lithium nickel cobalt aluminum oxide (LiNi0.80Co0.15Al0.05O2, NCA-eq) and lithium cobalt oxide (LiCoO2, LCO-eq) cathode material. Mass was selected as the primary normalizing factor because any energy-based functional unit (e.g., per kWh) could vary based on battery manufacturing and cycling characteristics. NCA chemistry was selected because future cathodes were projected to utilize less Co compared to NMC batteries in EVs15, and NCA comprised the second-largest category of EV battery chemistries in 2016, following NMC batteries. LCO was a representative chemistry used in handheld rechargeable devices (e.g., cellphones and laptops) which are currently available to recycle in larger quantities than EV LIBs. Environmental impacts of other LIB-relevant materials (Cu and Mn) in conventional supply chains can be found in Supplementary Table 14.

In both conventional and circular supply chains, the extraction, transport, and refinement steps were converted into environmental impacts for the production of battery-grade materials and normalized by NCA and LCO functional units. A limiting-reagent approach was used to quantify the environmental impacts of a functional unit in circular refinement pathways. According to current multi-step pathways using mixed-stream LIB feedstocks (either recycled scrap or recycled battery), the Li output was the limiting element for creating 1 kg of NCA-eq materials from recycled scrap, where other refined elemental products were produced in excess. Relatedly, Ni was the limiting output element from recycled batteries. For multi-step refinement processes, the recovery rate of Ni and Co was 95% and for Li was 92%. Additionally, a sensitivity analysis of environmental impacts from circular refinement was conducted based on facility location in different grid balancing areas and their associated electricity sources.

Lifecycle inventory and assessment
The LCI data for conventional mining pathways were normalized by each critical metal element: Li, Ni, Co, Al, Cu, and Mn (Supplementary Table 14). The LCI data for consumables in the Redwood process were adapted from the GREET 2021 model and ecoinvent 3.3 (Supplementary Table 10)23,68. The LCI for the Redwood processes also listed water consumption and criteria emissions for different electricity sources by grid balancing areas in the Western USA (Supplementary Table 12). Three categories of environmental impacts were detailed in this study: energy consumption, air pollutant emissions, and water consumption. Energy consumption included the input electricity for different applications and the energy required to produce required consumables. Criteria air pollutant emissions included the embodied emissions generated by the production of input electricity and the consumed reagents. CO2, CH4, CO, NOx, N2O, SOx, PM10, and PM2.5 were the air pollutants provided in the GREET model and considered here. Greenhouse gas emissions were reported as CO2 equivalents (CO2-eq) summing CO2, CH4, and N2O weighed by the corresponding 100-year global warming potential (GWP). Water consumption considered withdrawn water not returned to the original source, and both the input city water usage and the embodied water consumption in electricity generation and the manufacturing of consumable materials were included.

Estimating environmental impacts of material extraction
For conventionally mined ore and brine, energy consumption, CO2-eq emission, and water consumption values were separated for the material extraction processes found in the GREET model. For the circular extraction case, LCO-based smartphones were assumed to be collected and transported to existing private and municipal collection facilities (CFs) from each census-block group in CA, assuming every person owned a cell phone and purchased a new phone every 3 years. A shortest-route method was developed for modeling LCO collection at the closest municipal collection facility. Census blocks centered by their CFs were first determined for the state of California by k-means clustering method, and the pathways of transporting LCOs to CFs were identified through minimizing the possible travel distance, realized by Dijkstra’s algorithm73. Details of the methods are described in Supplementary Note 4.

Estimating environmental impacts of material transport
In the conventional supply chain, a network model of primary transport routes was established that connects mines to refinery locations for Li, Co, Ni, and Al on a country-level basis (Supplementary Tables 4–9, 15, and 16) because the amount of mined material transported from each mine to each refinery was not known. The distances of the shortest-path routes were calculated between mines and refineries by country, predicated on the closest available modes of transport (including road, rail, and maritime). Details of the modeling method are described in Supplementary Note 5. A major mine cluster or refinery location was selected to represent country-level transport values (Supplementary Tables 7 and 8) based on production volumes, and distances were quantified between international destinations. These distances were used to calculate the energy consumption, CO2-eq emissions, and water consumption associated with the transportation of critical materials as mined concentrate. Mined concentrate referred to ore or brine that was concentrated locally beyond natural concentration values to reduce weight for transport to a refinery. By considering the total elemental mass and elemental weight percentage of the mined concentrate transported along a route (Supplementary Tables 7 and 8), the environmental impacts on a per-element basis were calculated as a global weighted average (Supplementary Table 4) with additional process details in Supplementary Notes 4 and 5.

For the circular case applied to California, end-of-life EV NCA LIBs were aggregated at one CF per county closest to its centroid, where county-level data were the most granular data available. All smartphones were aggregated at their nearest CFs. Aggregated smartphone and EV batteries were assumed transported via truck to a single recycling facility located at the gravity point of California’s population based on census block-level data (detailed in Supplementary Note 4). The mass-distances traveled were converted to energy consumption, CO2-eq emission, and water consumption (Supplementary Tables 4 and 15).

Summary of study limitations
Limitations based on key assumptions of supply chain steps (extraction, transport, refinement) in each supply chain (conventional and circular) are briefly discussed in this section.

Mining data in conventional supply chains in GREET often only refer to one mining country per material, indicating that the global supply chain was difficult to capture. Transport required between mining unit processes (e.g., crushing, flotation, and concentration) prior to refinement was excluded from the current analysis due to limited information in GREET. In collection of end-of-life batteries in smartphones, inefficient transport to a CF (e.g., driving each smartphone individually or taking longer transport routes to a CF) was not considered. In addition, all end-of-life EV battery packs were assumed to be driven to each CF in their original vehicles, which was attributed to the “product use” stage instead of extraction in LCA; therefore, zero CO2-eq emissions were assumed for the extraction step of EV batteries. Batteries may be collected as hazardous wastes, but the potential influences of the safety costs were not considered.

An inter-country LIB material transportation assessment was performed as a weighted distribution between all major mining and refining countries. Results were sensitive to the weight percentage of critical material in transported concentrate presented in Supplementary Tables 7 and 8. Transport between a domestic mine and refinery was not considered, resulting in net zero use of resources in such cases. The resources required to separate an embedded battery from its device prior to a refinement facility were not considered in circular supply chains. Similarly, the effect of transporting only LIBs separated from the devices was not considered. Incorporating the domestic transport and battery separation operations can increase environmental impacts.

Refinement data in conventional supply chains were limited to the country scenarios reported in GREET, and transport between refinement unit processes was not included. Ancillary processes (e.g., transport between unit processes) beyond direct refinement unit processes and embodied resources of the capital equipment used for material refinement were not considered for the circular supply chain. The chemical formats of output products differ between the conventional and circular supply chains, but converting them to the same products would not substantially change the results due to the similarity between the cathode salts of the two supply chains (Supplementary Note 3). Battery-grade cathode metal sulfates were chosen as the refinement products for the major comparison, differing from the ready-to-use materials for manufacturing, but not substantially influencing the fairness of the comparison.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_2": "This study reveals that recycling LIBs into battery-grade materials significantly reduces environmental impacts. The research highlights that electricity consumption is the major factor in the environmental impacts of LIB recycling. It emphasizes the relatively minor contribution of material extraction and transport in circular supply chains, while these steps have a more notable impact in traditional supply chains. The inclusion of primary industrial-scale data from Redwood Materials is an interesting contribution, adding some real-world relevance to the findings. However, some arguments may not be fully supported, and some descriptions of important points are inadequate. The following are the detailed comments and critiques to be addressed:\n1. While the study mentions the potential to help recyclers optimize environmental impacts, it would be strengthened by providing more concrete examples or strategies for such optimization. Having Redwood Materials as a case study is interesting, but there are several others using more mature or advanced facilities in US or global markets.\n2. The study appears to be well-structured and clear in its objectives and methodologies. However, ensuring that the data sources, assumptions, and limitations are transparently communicated will be crucial for the study's credibility and usefulness. More importantly, the data used in Fig 1 is based on market value from 2018 to 2021, which is quite outdated. In the battery industry, the prices change in weekly or even daily basis, and fluctuation can be large. One typical exam is Li, its prices has changed by about 75% since 2022.\n3. The methodology seems robust, utilizing the GREET 2021 model for conventional supply chain analysis and incorporating data from actual industrial operations for the circular supply chain. However, the model is also quite outdated.\n4. The results indeed are different from some of the recent findings that transportation can be a significant cost factor, especially collecting spent lithium- ion batteries and transport them as Class 9 hazardous wastes is a huge cost burden. But the authors seem to have not included this factor in discussing recycling as those spent batteries will not automatically concentrate in the recycling facility.\n5. The analysis comparing pyrometallurgical and hydrometallurgical processing, and the identification of the environmental benefits of the RC pathway, is valuable. However, the study could benefit from a deeper exploration of the trade-offs between these methods, particularly in terms of scalability and economic viability.\n6. The study's focus on upstream impacts, including extraction and transport, is an important aspect that often gets overlooked. The suggestion for further concentrating materials near mine sites or adjusting locations of refineries to reduce environmental impacts is a practical recommendation.\n7. The mention of designing and manufacturing LIBs for recyclability is a crucial point, suggesting a shift towards a more sustainable, circular economy. However, this aspect could be explored in more detail, particularly in terms of how current manufacturing practices could be adapted.\n8. Give a clearer, more detailed explanation of methodologies, especially where the authors mention modeling and the “shortest route” approach. This provides context and helps readers understand how your conclusions were reached.\n9. Where discussing environmental impacts, also consider mentioning the economic aspects more explicitly. Balancing environmental benefits with economic feasibility is crucial for practical applications.\n10. The study's broader implications for strategic placement of LIB facilities and policy recommendations are valuable. However, more concrete suggestions or models for policy implementation would enhance the study's practical utility. It would also be beneficial if the study included specific recommendations or identified key areas where policy interventions could be most effective.\n11. Expand on the limitations of study and the implications for future research. This not only adds credibility to your work but also guides future studies in this area.\nMinor comments:\n1. Some phrases might be ambiguous or unclear. For example, line 478: \"building reinterests closer to sources\" is unclear and might need rephrasing based on the intended meaning.\n2. Maintain a consistent verb tense throughout the text. For example, switch from \"The analysis reveals\" to \"The analysis revealed\" if the rest of your text is in past tense.\n3. Thoroughly proofread the text to catch any typographical errors, misplaced commas, or incorrect word usage.\n4. Where you refer to figures (e.g., \"Fig. S3\", \"Fig. 5\"), ensure these are easily accessible to the reader and consider including brief descriptions in the text to aid understanding for those who might not have immediate access to the figures.\n5. Many references are in random formats.", "reviewer_3": "This paper presents a comparative life cycle analysis for primary and secondary supply chains for lithium ion batteries (LiBs). The paper is well written, the topic is of interest, the core methodology is sound, and outcomes relevant for advancing the conversation around designing circular supply chains for LiBs. The major concerns I have about this study have to do with the case for its uniqueness, the significance of some of the key outcomes and the appropriateness of the context for interpreting these results. I think it needs to be updated before considering it for publication. I provide these as comments below for consideration.\nPlease see attached document for my comments.\n\nREVIEW SUMMARY\nThis paper presents a comparative life cycle analysis for primary and secondary supply chains for lithium ion batteries (LiBs). The paper is well written, the topic is of interest, the core methodology is sound, and outcomes relevant for advancing the conversation around designing circular supply chains for LiBs. The major concerns I have about this study have to do with the case for its uniqueness, the significance of some of the key outcomes and the appropriateness of the context for interpreting these results. I think it needs to be updated before considering it for publication. I provide these as comments below for consideration.\n\nMAJOR COMMENTS\nLines 128 – 130, 438 – 440: The authors make the case that a distinguishing contribution from this study is the representation of (1) industrial scale circular refinement data, & (2) stepwise cradle-to-gate supply chain comparison, but it is not clear to me what this claim really amounts to.\n• For (1), I imagine this has to do with using data from Redwood’s process, so what is the value proposition then? Does using data from Redwood’s process provide more accurate (or trustworthy) recycling impacts compared to previous studies? Since the authors correctly pointed out that there are significant differences in reported environmental impacts for LiB recycling (Lines 91-94), how does this study address and resolve that problem, and not simply add another data point to literature on LiB recycling impacts?\n• For (2), Since several studies have already looked at both primary and secondary battery supply chains and compared the impacts of the various process steps (even if they did not categorize it as extraction and refining, those steps are implicit in the reference process description) and across different environmental metrics, I would like more clarity on how and why this work is different, and why that difference is important.\nLines 153 – 154, 286-287, …: Some of the major study findings, such as that “recycling exhibit lower environmental impacts than primary production” or that “electricity consumption dominates recycling environmental impacts” have already been established in previous studies. I think the authors need to make a clearer case of what their results add to our current understanding. For instance, are they providing an important update to the magnitude of the comparative differences, or greater clarity about relative impacts, or a more granular level of detail not available in previous studies but which are potentially important? For example, something I don’t remember in other studies is an analysis like that shown in Figure 5b, which seems like an outcome that could be elevated and used as a basis for the broader discussion that incorporates consideration for (grid) energy transitions.\n201 – 204, Figure 3, Figure 5,..: While it is interesting that the authors include Recycled scrap in the analysis, I would expect some additional context for how to interpret its relevance. For instance, what are the relative material flows of scrap vs recycled batteries in secondary supply chains (currently or projected for the future)? For example, If scrap-to-recycled battery stream ratio is 1:100, then the additional reduction in water consumption from 74% for battery streams to 88.4% for scrap does not really amount to much, and is not particularly important to designing strategies for water efficiency and resilience in circular supply chains.\n\nADDITIONAL COMMENTS\nFigure 1: What is the distinction between a conventional “material refinery facility” and a circular “recycling facility” in Figure 1b?\nFigure 2 – Process descriptions: It would help if the authors included (perhaps as supplemental material) the details of the underlying process descriptions for the different pathways illustrated, specifically to help with distinguishing between Redwood process and “Representative Circular (RC)” pathway. For instance, do they share common/similar process steps and are the assumptions about these overlapping steps identical? Do they use the same chemicals? What degree of chemicals/solvent recovery or reuse is assumed? Does the RC Hy* process (in Figure 3) also include mechanical processing - which one would expect, but which is not obvious from the figure?\nFigure 3 caption: Please correct the b and c captions of Figure 3 – it appears they are switched.\nLines 234 – 249: It is good to see that the authors highlighted these differences in predicted LCA impacts between studies, even if just for reference. However, how exactly should we interpret the results from the current work against the backdrop of those other results? Are there obvious limitations in other studies that is corrected for here? Can we have greater transparency in the underlying process and data for the current work?\nLines 258 – 260: The authors suggest that in conventional supply chains, producing mixed salts has higher energy and carbon footprint compared to producing discrete salts. This result is counter-intuitive, since I'd expect that producing individual salts would theoretically include an additional separation step, which adds energy, and consequently, CO2 emissions. Could the authors explain this non-intuitive trend, and how come it is different from that seen for the circular pathway? It would help if they also provide the underlying process description as well.\nFigure 6: I am not sure why Figure 6 is presented as a result, and not as supplemental information illustrating the approach used to model the transport logistics.\nLines 354 – 355, Figure 6: I think there is a very important element that is missing, which is scale.\n• To compare primary and secondary logistics fairly, it would be good to try to make the scale more comparable. For example, expanding the collection radius from just California to say, the entire U.S. significantly changes the associated impact footprints. While I expect this to still be smaller than conventional, the comparison at this scale will be more justifiable.\n• The converse argument is that for a much smaller scale of primary manufacturing, it is possible to identify a domestic supply chain that ends up with a much smaller footprint than the transnational supply chain considered in the model in Figure 6.\nFigure 7: Can the authors clarify the reason for the difference in transport energy and CO2 footprint between EV and smartphone batteries? Does the difference arise from relative battery compositions, or transport distances? In addition, are there any key differences in their respective recycling steps, and how do these translate to differences in overall lifecycle impacts?\nLines 485 – 487 – Can the authors say more/ be a bit more specific here? Anyone can mention this trade- off without analysis. I would expect that the current analysis can supply enough information to recommend which of the two options – disassembly before transport vs transport before disassembly - is potentially a more impactful approach?\nLines 489 – 490: A number of studies mention the potential of rail transport to reduce energy and environmental impacts of secondary battery supply chain logistics relative to trucking, but how practical is this, especially for this analysis which is restricted to California? I imagine there are major limitations that make this nearly impossible, such as inadequate local rail coverage, competition with other high value cargo, etc.\nImpact allocation method: Could the authors clarify what type of impact allocation method they used. For instance, was it based on the embodied economic value of the target products in the recycled or primary pathways?"}, "author_response": "Manuscript Title: Life cycle comparison of industrial-scale lithium-ion battery recycling and\nmining supply chains\nManuscript ID: NCOMMS-23-46463\nIn this document, we respond point-by-point to comments from the reviewers and the editor on\nour original manuscript submission. We thank the reviewers for their insightful feedback, and have\nincorporated several of their suggestions to further improve the manuscript before publication. The\nlegend for this document is as follows:\nReviewer comments are in italics.\nAuthor responses are in standard type and indented.\nDeletions and additions to the original manuscript are marked in red and blue texts, respectively.\nLine numbers are specified for either the original manuscript or the revised manuscript.\n\n REVIEWER 2\nReviewer: This study reveals that recycling LIBs into battery-grade materials significantly\nreduces environmental impacts. The research highlights that electricity consumption is the\nmajor factor in the environmental impacts of LIB recycling. It emphasizes the relatively minor\ncontribution of material extraction and transport in circular supply chains, while these steps\nhave a more notable impact in traditional supply chains. The inclusion of primary industrial-\nscale data from Redwood Materials is an interesting contribution, adding some real-world\nrelevance to the findings. However, some arguments may not be fully supported, and some\ndescriptions of important points are inadequate. The following are the detailed comments and\ncritiques to be addressed:\nOur response: We are grateful to the reviewer for the favorable evaluation. Below, we have\naddressed the constructive comments with a focus on improved descriptions and support for our\nmajor arguments based on practical data from an operational battery recycling facility.\nReviewer: 1. While the study mentions the potential to help recyclers optimize environmental\nimpacts, it would be strengthened by providing more concrete examples or strategies for such\noptimization. Having Redwood Materials as a case study is interesting, but there are several\nothers using more mature or advanced facilities in US or global markets.\nOur response: We thank the reviewer for the suggestions, and take this opportunity to highlight\nour key insights related to environmental impacts of the lithium-ion battery supply chains. Notably,\nour study does provide concrete suggestions for optimizing environmental impacts in battery\nrefinement. In addition, our analysis includes a case study, but has generalizable contributions to\nthe literature enabled by granular Redwood data and our rigorous analysis of gate-to-gate and\ncradle-to-gate environmental impacts. Our response below highlights two major areas: (1) insights\n1\nand strategies for optimizing environmental impacts and (2) importance and broader impacts of\nemploying data from Redwood Materials.\n1. Insights and strategies for optimizing environmental impacts.\nThe first novelty of our analysis is that it provides quantitative evaluations for industrial-scale\ncircular refinement of different feedstocks using different technologies within the same process\nflow. While both recycled scrap and batteries are projected to be important recycling sources in\nthe coming decades, our findings quantify the different environmental impacts of refining each\nfeedstock, and give concrete suggestions for advancing corresponding pyrometallurgical and\nhydrometallurgical technologies. We have revised the manuscript to strengthen these novel\ncontributions (lines 262–267, 524–533, 541–546):\n“Similar to RC, emerging oxygen-free pyrometallurgical processes rely on carbothermic and\nthermite reduction for recycling cathode metal compounds at moderate temperatures (600–\n1000℃)19-21. Our results show that the RC step accounts for 5.5%–7.4% of the total environmental\nimpacts in Redwood (Fig. 3), demonstrating the environmental feasibility of industrial-scale\ncarbothermal reduction-based pyrometallurgy.”\n“Our findings showed the promise of carbothermal reduction-based pyrometallurgy for\npretreatment of LIB recycling; however, we note that controlling process conditions to achieve\noptimal environmental impacts and product formats will elevate the costs. Existing\nhydrometallurgy exhibits advantages over pyrometallurgy in terms of energy efficiency and\nrefinement precision, but requires costly pretreatment and purification, and suffers from limited\nscalability and from generation of secondary waste liquids34,35. Because Our analysis identified\nchemical consumables such as H O are as important contributors to hydrometallurgy, indicating\n2 2\nthat environmental impacts of Hy processes could can be reduced through more sustainable (e.g.,\nelectrochemical) production methods for chemical inputs (e.g., electrochemical generation of\nH O )36.”\n2 2\n“Our findings provide important guidance for sources and products in future refinement. Battery\nscrap is currently the primary recycling source due to the proliferation of gigafactories, but will\ndecrease in the future with quality control advances in manufacturing8. While refining recycled\nbatteries required higher greenhouse gas emissions, energy input, and water demand than refining\nbattery scrap (Fig. 3), it is critical to improve the technologies for recycling energized batteries\nfor future challenges.”\nSecond, by contrasting the environmental impacts of producing mixed metal salts and discrete salts\n(Fig. 4), we demonstrated the environmental and economic benefits of refining batteries to mixed\nmetal salts, while most current recycling facilities are still producing discrete salts. We highlighted\nsuch optimization strategies in lines 318–321 and 547–549.\n“Overall, our results indicated that refining batteries to mixed metal salts instead of discrete salts\ncan substantially save environmental impacts while still satisfying the needs of circular LIB supply\nchains. Our findings also provide important insights to optimizing plant-scale battery refining\noperations.”\n“Our findings also demonstrated the benefits of mixed metal sulfates as refinement products over\nsingle salts in the circular supply chain, indicating that the further separations between Ni and Co\nsalts can be avoided.”\ntechnologies compared side-by-side at the same facility (Fig. 5), providing suggestions for\noptimizing the circular environmental impacts using suitable input electricity. These suggestions\nare informed by our unique focus on practical facility-level data regarding overall and element-\nspecific mass balances, along with key inputs such as energy, water, and chemical reagents. In\ncontrast, many existing studies refer to other reports for primary operational data and do not benefit\nfrom the iterative insights enabled by our academic-industrial partnership.\nAs an example of enacting insights from our life cycle assessment, we note that Redwood Materials\nhas already referred to our findings and optimized their refining products accordingly, including\n(1) choosing mixed (Ni,Co)SO as their major refining products to avoid unnecessary separations,\n4\n(2) substituting the original Nevada Power Company-based input electricity with that generated\nfrom sustainable sources to lower environmental impacts, and (3) using key chemical inputs (e.g.,\nH O ) produced from low environmental-impact methods identified by our study. Additionally,\n2 2\nour findings and suggestions have gained great interest from numerous battery production and\nbattery recycling companies in conferences and industrial meetings. For example, the tradeoff we\nidentified between water consumption and greenhouse gas emissions has been noted by several\ncompanies as a novel insight worth considering in the placement and design of pilot scale battery\nrecycling facilities (lines 556–558):\n“Therefore, considering water consumption and CO -eq emissions is necessary for selecting\n2\nrecycling facility locations, particularly in water-sensitive or emissions-sensitive scenarios.”\n2. Importance and broader impacts of employing data from Redwood Materials.\nOur analysis based on technologies and data from Redwood Materials is not only a specific case\nstudy; rather, utilizing the state-of-the-art and versatile technologies and practical data in Redwood\nenable us to investigate different factors in the circular LIB refinement from a more granular level\nthan commonly provided in similar literature reports, and provide important insights for the whole\narea of LIB recycling.\nFirst, Redwood Materials represents state-of-the-art circular refinement technologies that are\ncomparable to existing and emerging technologies. Reductive calcination (RC) is a\npyrometallurgical process relying on carbothermal reduction with optimized conditions.\nPyrometallurgy based on carbothermal reduction has been rapidly developed at lab scale. Our\nstudy, to the best of our knowledge, reports the first evaluation of the industrial-scale application\nof the emerging technology, providing important insights to all pyrometallurgical refinement\ntechnologies. The hydrometallurgical processes at Redwood that refine both energized batteries\nand scrap are analogous to the prevailing hydrometallurgy, and our analysis identifying\nenvironmental impact contributors to the Hy process similarly advances Hy processes beyond\nRedwood. The manuscript has been revised to better generalize the working conditions of\nRedwood technologies, and compare with existing refinement methods (lines 513–528):\n“Disaggregated analysis of LIB refinement pathways at Redwood Materials provides provided\nimportant insights into the performance and potential of different refinement processes.\nPyrometallurgical technologies are advantageous in scalability and operating safety9,10,33, While\npyrometallurgical processing is but are widely considered as more environmentally intensive than\nhydrometallurgy31,32 due to the high reaction temperature,. Oxygen-free carbothermic reduction\nand thermite reduction have been recently investigated at lab scale for energy-efficient LIB cathode\n3\npyrolysis19-21. Our analysis showed that Redwood Materials’ industrial-scale carbothermal\nreduction-based RC pathway pyrometallurgy (reductive calcination in Redwood Materials)\nexhibits exhibited much lower environmental impacts than prevailing direct roasting and smelting\npyrometallurgical and current Hy-containing hydrometallurgical pathways reported in practice\nand in literature (Fig. S3). The optimized conditions of RC processing minimizes the combustion\nof carbon-containing LIB materials, significantly reducing CO -eq emissions while simultaneously\n2\ngenerating products that are amenable for hydrometallurgical separation.. Our findings showed\nthe promise of carbothermal reduction-based pyrometallurgy for pretreatment of LIB recycling;\nhowever, we note that controlling the process conditions to achieve optimal environmental impacts\nand product formats will elevate the costs.”\nSecond, operational data from Redwood Materials provide more granular insights to verify and\ncorrect the existing model-based studies. This Redwood-based study helped identify the gaps\nbetween models and industrial operations, and we highlighted the needs for advancing the whole\nfield in the revised manuscript (lines 531–541):\n“Because Our analysis identified chemical consumables such as H O are as important\n2 2\ncontributors to hydrometallurgy, indicating that environmental impacts of Hy processes could can\nbe reduced through more sustainable (e.g., electrochemical) production methods (e.g.,\nelectrochemical generation of H O )36.”\n2 2\n“We note the inconsistencies among existing studies on assessing conventional LIB refinement due\nto different product compositions, which underscores the need to standardize the functional unit in\nreporting environmental analyses for future studies in the field. The deviations between our results\nof circular refinement and model-based literature data highlight the importance of reconciling\nmodels with industrial operating data.”\nThird, the versatile technologies at Redwood Materials enable treatment of different feedstocks\n(energized batteries and battery scrap) and generation of different product formats (mixed metal\nsulfate and discrete salts), offering opportunities to investigate influences of multiple factors on\nthe circular refinement. Using Redwood data, we analyzed the refinement of energized batteries\nand scrap, providing suggestions for developing generalizable strategies for different recycling\nsources (Fig. 3 and lines 542–547). Furthermore, while prevailing recycling produces discrete salts,\nwe analyzed different refinement scenarios and demonstrated the advantages of producing mixed\nsalts (Fig. 4 and lines 315–320).\n“Our findings provided important guidance for sources and products in future refinement. Battery\nscrap is currently the primary recycling source due to the proliferation of gigafactories, but will\ndecrease in the future with quality control advances in manufacturing8. While refining recycled\nbatteries requires higher greenhouse gas emissions, energy input, and water demand than refining\nbattery scrap (Fig. 3), it is critical to improve the technologies for recycling energized batteries\nfor future challenges.”\n“The RC pathway (RC+Me+Hy) exhibits exhibited lower energy consumption (−72.3%), CO -eq\n2\nemissions (−39.5%), and water consumption (−12%) relative to the Me+Hy pathway (Fig. 4),\nbecause it avoids additional treatment separating (Ni,Co)SO to discrete salts. Overall, our results\n4\nindicate indicated that refining batteries to mixed metal salts instead of discrete salts can\nsubstantially save environmental impacts while still satisfying the needs of circular LIB supply\nchains.”\nthe complete operating data are not public or cannot be readily accessed. Our analyses relying on\npractical data from Redwood provide exceptionally important and granular insights to several\nresearch communities (e.g., battery recycling, life cycle assessment, battery synthesis), and can\nspark more academic-industrial collaborations to further advance the field.\nThe manuscript has been revised to better highlight our contributions (lines 147–152):\n“With the methodologies and results reported in this study, researchers can prioritize major\nopportunities to improve process efficiencies, practitioners can benchmark their environmental\nimpacts, and policymakers can incentivize best environmental practices in LIB supply chain\nmanagement. Granular Iinsights provided by this study based on practical data can also help\nrecyclers optimize the environmental impacts of their refinement processes, and spark more\nacademic-industrial collaborations to further advance the field.”\nReviewer: 2. The study appears to be well-structured and clear in its objectives and\nmethodologies. However, ensuring that the data sources, assumptions, and limitations are\ntransparently communicated will be crucial for the study's credibility and usefulness. More\nimportantly, the data used in Fig 1 is based on market value from 2018 to 2021, which is quite\noutdated. In the battery industry, the prices change in weekly or even daily basis, and\nfluctuation can be large. One typical exam is Li, its prices have changed by about 75% since\n2022.\nOur response: We agree with the reviewer regarding the importance of transparent\ncommunication regarding data sources, assumptions, and limitations. To this end, we have\nincluded a robust Limitations section in the manuscript, and have cited all data sources\nappropriately for reproducible analysis. We were especially careful regarding reproducibility and\ntransparency because not all existing reports include data that can achieve these goals. Our\ncommitment to reproducible analysis shows the rigor and contributions to the field of the submitted\nstudy. Regarding Figure 1, the purpose is not to reflect the most updated pricing, but to highlight\nthe motivations; with this purpose in mind, our data sources limitations are very transparent. We\nare well aware of the rapidly changing market prices of common recycled commodities and\nlithium-ion batteries, but the purpose of presenting Fig. 1a is for highlighting the important drivers\nfor recycling LIBs. Of note, these prices reported are spot market prices, and are meant to show\ntrends across critical materials and battery chemistries. We chose the years 2018–2021 because\nthat 2021 is when our study started and reflects the timing of the original data from Redwood\nMaterials as well.\nThroughout the manuscript, except for the confidential information from our industrial\ncollaborators, we have provided all the data sources in Supplementary Information, clearly stated\nthe assumptions in the main manuscript, and summarized the study limitations in the Methods\nsection.\nReviewer: 3. The methodology seems robust, utilizing the GREET 2021 model for\nconventional supply chain analysis and incorporating data from actual industrial operations\nfor the circular supply chain. However, the model is also quite outdated.\nupdates in GREET 2023. Compared with the 2021 version, GREET 2023 provided additional\nrefining pathways for Ni and Al and updated the environmental impacts for several relevant\ncomponents. Employing data from GREET 2023 yields 17.0%–18.3% and 2.2%–7.4% higher\nenvironmental impacts than those in the submitted manuscript for producing discrete and mixed\nsalt, respectively (Figs. 3–4). No change has been identified for the life cycle inventory of\nconsumables, i.e., the environmental impacts of circular refinement based on Redwood data (Figs.\n3–5) are not influenced by updating GREET. Overall, applying GREET 2023 data will increase\nthe savings of environmental impacts for circular compared to conventional refinement, but will\nnot change the main conclusions of the study.\nFirst shaped in 2021, the study analyzed the practical data from Redwood based on their operations\nin 2021, and employed the GREET 2021 model to guarantee a fair comparison with the\nconventional supply chains. We keep the results based on GREET 2021 in the main manuscript\nwith further data corrections, and have provided an additional Supplementary Data File B\nappended to the revised manuscript with results based on GREET 2023 to compensate for the\nlimitations of the 2021-based analysis. This approach prioritizes similar time periods for the\nconventional data for GREET and the circular data from Redwood; it also conservatively estimates\n(i.e., underestimates) savings provided by circular compared to conventional refinement\napproaches (lines 191–197).\n“Excluding the environmental impacts of material extraction and transport steps, the gate-to-gate\nproduction of one kg NCA-eq battery-grade material from state-of-the-art conventional mined\nnatural materials consumes consumed 193.9 MJ and 77.3 L H O while emitting 14.5 kg CO -eq\n2 2\n(Fig. 3). Note that the results are based on GREET 2021 to match the period when circular\nrefinement data were collected at Redwood; more recent environmental impacts of the\nconventional supply chain based on the 2023 model are 17.0%–18.3% higher than that of 2021,\nand are presented in Supplementary Data File B.”\nReviewer: 4. The results indeed are different from some of the recent findings that\ntransportation can be a significant cost factor, especially collecting spent lithium- ion batteries\nand transport them as Class 9 hazardous wastes is a huge cost burden. But the authors seem\nto have not included this factor in discussing recycling as those spent batteries will not\nautomatically concentrate in the recycling facility.\nOur response: We are aware of the different understanding of transport costs in LIB recycling,\nbut note that the current study is focused on environmental impacts, with emphasis on the\nrefinement step. Examining cost considerations in the circular battery supply chain warrants\nadditional investigation and is beyond the scope of the current study.\nThe manuscript has been revised to reflect the importance of these considerations for future\nresearch (lines 600–603, and 606–610):\n“Business models for collection of all LIB types and sizes will likely vary from manufacturer-led\nto municipal or private collection programs., and can be influenced by the safety costs while\ncollecting used batteries as hazardous wastes.”\n“Additionally, designing and manufacturing LIBs for recycling in a circular economy can reduce\nresource usage identified in this study42. Recyclable battery packing can save energy and chemical\n6\ninputs used in refinement, and manufacturing battery cells and modules favorable for extraction\nand integration will lower the environmental impacts of the upstream steps.”\nReviewer: 5. The analysis comparing pyrometallurgical and hydrometallurgical processing,\nand the identification of the environmental benefits of the RC pathway, is valuable. However,\nthe study could benefit from a deeper exploration of the trade-offs between these methods,\nparticularly in terms of scalability and economic viability.\nOur response: We are grateful to the reviewer for the helpful suggestion. Lines 267–270 in the\noriginal manuscript compare tradeoffs between different methods:\n“Note that traditional pyrometallurgy and Redwood Material’s reductive calcination can process\nenergized batteries of varying states of charge, health, and formats with minimal modification,\nwhereas traditional hydrometallurgy may need to discharge energized batteries in salt bath or\ncryogenically remove electrolyte for safe mechanical processing.”\nWe also added lines 513–528 and 528–534 in the revised manuscript to extend the discussion as\nsuggested:\n“Disaggregated analysis of LIB refinement pathways at Redwood Materials provides provided\nimportant insights into the performance and potential of different refinement processes.\nPyrometallurgical technologies are advantageous in scalability and operating safety9,10,33, While\npyrometallurgical processing is but are widely considered as more environmentally intensive than\nhydrometallurgy31,32 due to the high reaction temperature,. Oxygen-free carbothermic reduction\nand thermite reduction have been recently investigated in lab scale for energy-efficient LIB cathode\npyrolysis33-35. Our analysis showed that Redwood Materials’ industrial-scale carbothermal\nreduction-based RC pathway pyrometallurgy (reductive calcination in Redwood Materials)\nexhibits exhibited much lower environmental impacts than prevailing direct roasting and smelting\npyrometallurgical and current Hy-containing hydrometallurgical pathways reported in practice\nand in literature (Fig. S3). The optimized conditions of RC processing minimizes the combustion\nof carbon-containing LIB materials, significantly reducing CO -eq emissions while simultaneously\n2\ngenerating products that are amenable for hydrometallurgical separation.. Our findings showed\nthe promise of carbothermal reduction-based pyrometallurgy for pretreatment of LIB recycling;\nhowever, we note that controlling the process conditions to achieve optimal environmental impacts\nand product formats will elevate the costs.”\n“Existing hydrometallurgy exhibits advantages over pyrometallurgy in terms of energy efficiency\nand refinement precision, but requires costly pretreatment and purification, and suffers from the\nlimited scalability and generation of secondary waste liquids34,35. Because Our analysis identified\nchemical consumables such as H O are as important contributors to hydrometallurgy, indicating\n2 2\nthat environmental impacts of Hy processes could can be reduced through more sustainable (e.g.,\nelectrochemical) production methods (e.g., electrochemical generation of H O )36.”\n2 2\nReviewer: 6. The study's focus on upstream impacts, including extraction and transport, is an\nimportant aspect that often gets overlooked. The suggestion for further concentrating materials\nnear mine sites or adjusting locations of refineries to reduce environmental impacts is a\npractical recommendation.\nOur response: We thank the reviewer for the favorable comments on our findings and their novel\ncontributions to the literature.\npoint, suggesting a shift towards a more sustainable, circular economy. However, this aspect\ncould be explored in more detail, particularly in terms of how current manufacturing practices\ncould be adapted.\nOur response: The manuscript has been revised to extend the discussion (lines 606–610):\n“Additionally, designing and manufacturing LIBs for recycling in a circular economy can reduce\nresource usage identified in this study30. Recyclable battery packing can save energy and chemicals\nused in refinement, and manufacturing battery cells and modules favorable for extraction and\nintegration will lower the environmental impacts of the upstream steps.”\nReviewer: 8. Give a clearer, more detailed explanation of methodologies, especially where\nthe authors mention modeling and the “shortest route” approach. This provides context and\nhelps readers understand how your conclusions were reached.\nOur response: We have provided detailed descriptions in the submitted manuscript of the\nmodeling of extraction and transport in Note 4 and Note 5 in Supplementary Information,\nrespectively. Representative descriptions are listed below:\n“California (CA) was chosen because it is the most populated US state (39.6 million, 2022) and\nhas the highest number of electric vehicles, representing 42% of the entire country in 202018.\nExisting Collection Facilities (CFs) are provided by the California Department of Resources\nRecycling and Recovery (CalRecycle)18. Two different LIB products are considered: end-of-life\nLCO-based smartphones and end-of-life NCA-based EV batteries. The assumed mass of\ntransported smartphones and EV battery packs along with the mass of active cathode material is\nsummarized in Table S10.”\n“In the first scenario, transport of smartphones from the census-block level to the nearest CF is\nused to estimate environmental impacts of extraction from consumers. Population in every census\nblock is obtained from the U.S. Census Bureau19. It is assumed that one smartphone is owned by\neach person in CA and is recycled every three years. To determine the closest CF, k-means\nclustering is used for the allocation of census blocks to CFs based on minimizing the distance\nbetween a census block’s centroid and a singular CF, for all census blocks. This process produces\nclusters of block groups whose LIB-embedded smartphones are allocated to the closest CF. The\ndistance from the census blocks to the CFs is determined by the shortest route using Dijkstra’s\nalgorithm. The number of clusters, “k,” used in this algorithm is fixed and equals the total number\nof CFs. The relative size of a CF in the inset of Fig. 6 indicates the number of collected smartphones\nas compared to nearby CF clusters, and the smartphone collection area is color-coded for\ndistinction for each CF. Multiplying the resulting distances with the average mass of a smartphone\nyields a mass-distance product (i.e., kg × km), which are converted to environmental impacts using\nvalues for trucking (Table S12). Note that the shortest-route value represents a lower bound of\nemissions.”\n“For a particular element, the shortest route utilizing all combinations between mines and\nrefineries is calculated with Dijkstra’s algorithm27.”\n“The normalized mass-based environmental impacts of LIB material concentrate transported, , are\ncalculated by Eq. S, where is mass-based environmental impacts of the material i with a certain\ntransport model (Table S9), d is the segmental transport distance in the shortest-route model, n\ni d\n8\nis the total segments along that route, and w is the element weight percent from the country of\ni\norigin.\nn d ( em d )\nem = i=1 Mat,i i (S1)\nMat,i w\ni\nEnvironmental impacts with weighted distributions, E , for each element i are calculated to\nMat,i\nconsider all routes for the three environmental impacts:\nE =n d ( em m ) (S2)\nMat,i i=1 Mat,i i\nwhere m denotes the global percentage of an element considered for mined production or\ni\nrefinement production in 2019 (values summarized in Tables S9–S10).”\nWe have added lines 407–413, 446–451, 722–727, 731–734 in the main manuscript and Methods\nto better summarize the methodology:\n“Smartphones are were considered extracted when collected, aggregated, and transported from\nall California residents (analyzed per census block) to the nearest existing collection facility (CF)24.\nThe analytical model for this circular extraction is depicted in Fig. 6a, where a A shortest-path\nroute for collection is was determined by minimizing the distances from block group to CF is\nmodeled for the whole state (the model is summarized in Methods and detailed in Note 4 in\nSupplementary Information) 30.\n“An algorithm is developed to quantify The environmental impacts of transport are were quantified\nby based on a weighted distribution of participating countries and calculating the shortest distance\nalong major transport routes among the participating countries weighted by the relative\ncontributions of the countries to the market (the case of cobalt is presented as an example in Fig.\n6b). Details of the modeling method are presented in Note 5 of Supplementary Information.”\n“A shortest-route algorithm method was used developed for modeling LCO collection at the closest\nmunicipal collection facility. Census blocks centered by their CFs were first determined for the\nstate of California by k-means clustering method, and the pathways of transporting LCOs to CFs\nwere identified through minimizing the possible travel distance, realized by the Dijkstra’s\nalgorithm50 (details of the methods are described in Note 4 in Supplementary Information).”\n“The distances of the shortest-path routes are were calculated between mines and refineries by\ncountry, predicated on the closest available modes of transport (including road, rail, and maritime).\nDetails of the modeling method are described in Note 5 of Supplementary Information.”\nReviewer: 9. Where discussing environmental impacts, also consider mentioning the economic\naspects more explicitly. Balancing environmental benefits with economic feasibility is crucial\nfor practical applications.\nOur response: We acknowledge the importance of balancing environmental and economic\nconsiderations, but emphasize that the current study is focused on environmental impacts,\nespecially of the refinement step; economic feasibility is beyond the scope of the current study and\nwarrants further investigation. Thus, we have restricted our claims to the environmental analysis\nconducted. We have mentioned this scoping point in the original main manuscript (lines 604–606,\nand 610–612):\n9\n“Notably, analogous economic and environmental impacts to local ecosystems of conventional\nmining are not considered in this analysis, and warrant future studies41.\n“Future efforts should also focus on optimizing refinement processes for subsequent steps of the\ncircular supply chain in LIB manufacturing, product performance, and economic cost.”\nWe have also added lines 519–528 and 528–534 to mention the economic aspects in the discussion\nof the present results (bolded text refers explicitly to costs):\n“Our analysis showed that Redwood Materials’ industrial-scale carbothermal reduction-based RC\npathway pyrometallurgy (reductive calcination in Redwood Materials) exhibits exhibited much\nlower environmental impacts than prevailing direct roasting and smelting pyrometallurgical and\ncurrent Hy-containing hydrometallurgical pathways reported in practice and in literature (Fig.\nS3). The optimized conditions of RC processing minimizes the combustion of carbon-containing\nLIB materials, significantly reducing CO -eq emissions while simultaneously generating products\n2\nthat are amenable for hydrometallurgical separation.. Our findings showed the promise of\ncarbothermal reduction-based pyrometallurgy for pretreatment of LIB recycling; however, we note that\ncontrolling the process conditions to achieve optimal environmental impacts and product\nformats will elevate the costs.”\n“Existing hydrometallurgy exhibits advantages over pyrometallurgy in terms of energy efficiency\nand refinement precision, but requires costly pretreatment and purification, and suffers from\nlimited scalability and from generation of secondary waste liquids34,35. Because Our analysis\nidentifies chemical consumables such as H O are as important contributors to hydrometallurgy,\n2 2\nindicating that environmental impacts of Hy processes could can be reduced through more\nsustainable (e.g., electrochemical) production methods for chemical inputs (e.g., electrochemical\ngeneration of H O )36.”\n2 2\nReviewer: 10. The study's broader implications for strategic placement of LIB facilities and\npolicy recommendations are valuable. However, more concrete suggestions or models for\npolicy implementation would enhance the study's practical utility. It would also be beneficial\nif the study included specific recommendations or identified key areas where policy\ninterventions could be most effective.\nOur response: We thank the reviewer for the favorable comments on the broader implications of\nour work, but politely note that the current study is focused on technically investigating the\nenvironmental impacts of battery recycling; proposing specific recommendations on policy\nintervention is beyond the scope of the current study. Instead, we anticipate that future policy-\nfocused studies and interventions will leverage our results to inform policy analysis. However, we\nhave highlighted the potential implications of our findings for advancing the field in terms of\noptimization and standardization, which are closely related to policy intervention (lines 556–558):\n“Therefore, considering water consumption and CO -eq emissions is necessary for selecting\n2\nrecycling facility locations, particularly in water-sensitive or emissions-sensitive scenarios.”\nWe also added lines 531–541 and 606–610 to the revised manuscript to strengthen the discussion:\n“Because Our analysis identified chemical consumables such as H O are as important\n2 2\ncontributors to hydrometallurgy, indicating that environmental impacts of Hy processes could can\nbe reduced through more sustainable (e.g., electrochemical) production methods (e.g.,\nelectrochemical generation of H O )38.…We note the inconsistencies among existing studies on\n2 2\n10\nassessing conventional LIB refinement due to different product compositions, which underscores\nthe need to standardize the functional unit in reporting environmental analyses for future studies\nin the field. The deviations between our results of circular refinement and model-based literature\ndata highlight the importance of reconciling models with industrial operating data.”\n“Additionally, designing and manufacturing LIBs for recycling in a circular economy can reduce\nresource usage identified in this study42. Recyclable battery packing can save energy and chemicals\nused in refinement, and manufacturing battery cells and modules favorable for extraction and\nintegration will lower the environmental impacts of the upstream steps.”\nReviewer: 11. Expand on the limitations of study and the implications for future research. This\nnot only adds credibility to your work but also guides future studies in this area.\nOur response: We are grateful to the reviewer for the suggestion. We note that a subsection\nSummary of Study Limitations in Methods in provided in Methods, and has been expanded in\nthe revised manuscript. Specifically, lines 531–541 and 600–603 have been added to the revised\nmanuscript to strengthen the implications:\n“Because Our analysis identified chemical consumables such as H O are as important\n2 2\ncontributors to hydrometallurgy, indicating that environmental impacts of Hy processes could can\nbe reduced through more sustainable (e.g., electrochemical) production methods (e.g.,\nelectrochemical generation of H O )38.…We note the inconsistencies among existing studies on\n2 2\nassessing conventional LIB refinement due to different product compositions, which underscores\nthe need to standardize the functional unit in reporting environmental analyses for future studies\nin the field. The deviations between our results of circular refinement and model-based literature\ndata highlight the importance of reconciling models with industrial operating data.”\n“Business models for collection of all LIB types and sizes will likely vary from manufacturer-led\nto municipal or private collection programs, and can be influenced by the safety costs while\ncollecting used batteries as hazardous wastes.”\nReviewer: Minor comments:\n1. Some phrases might be ambiguous or unclear. For example, line 478: \"building reinterests\ncloser to sources\" is unclear and might need rephrasing based on the intended meaning.\nOur response: We have revised the typographical errors throughout the manuscript, including in\nlines 479–481 and 566–568:\n“Circular production of LCO-grade materials leads to higher environmental impacts than that of\nNCA-grade materials based on the mixed-stream feedstock composition processed by Redwood\nMaterials.”\n“Further concentrating materials near mine sites or building reinterests refineries closer to\nsources can efficiently reduce environmental impacts of the conventional mined materials.”\nReviewer: 2. Maintain a consistent verb tense throughout the text. For example, switch from\n\"The analysis reveals\" to \"The analysis revealed\" if the rest of your text is in past tense.\nactions taken during our study to past tense. Some example changes are presented below (lines\n176–180, 292–297, 332–338, 401–407, and 467–472):\n“Refining lithium-ion batteries into battery-grade materials exhibits exhibited lower\nenvironmental impacts than production from mined natural materials. Environmental impacts of\nrefinement pathways in conventional and circular LIB supply chains were analyzed in Fig. 3, while\nthe The upstream steps of material extraction and transport are were considered in later sections.”\n“Formats of refinement products influence influenced environmental impacts. …To examine the\ninfluences of the refinement product formats, the environmental impacts of refinement to mixed salt\nare were compared to the refinement to discrete sulfate salts, NiSO and CoSO (Fig. 4). Both\n4 4\nconventional and circular refinement pathways are were analyzed.”\n“Electricity consumption dominates dominated the environmental impacts of lithium-ion battery\ncircular refinement. The relative environmental impacts of input consumables (e.g., energy, water,\ncommodity chemicals) in the gate-to-gate refinement processes are were disaggregated in Fig. 5\n(additional criteria air pollutants in Tables S2–S3, Figs. S4–S5). Note that the embodied\nenvironmental impacts of electricity consumption in Fig. 3 are were based on the Nevada Power\nCompany (NEVP) at the Redwood Materials location. Electricity consumption is was found to be\na principal factor dominating the environmental impacts.”\n“Environmental impacts of material extraction and transport are were significantly lower in\ncircular lithium-ion battery supply chains than in conventional supply chains. …Environmental\nimpacts of these upstream steps are were analyzed for two representative LIB chemistries and\nbattery use cases: NCA in EV battery packs, and lithium cobalt oxide (LiCoO or LCO) in\n2\nsmartphones. California is was chosen to assess circular extraction because…”\n“The refinement step dominates dominated environmental impacts of circular and conventional\nsupply chains. Combining material extraction, transport, and refinement steps yields yielded a\ncradle-to-gate comparison…”\nReviewer: 3. Thoroughly proofread the text to catch any typographical errors, misplaced\ncommas, or incorrect word usage.\nOur response: We thank the reviewer for the close read; we have thoroughly proofread the\ndocument and made several improvements for readability, including (lines 94–96):\n“The most significant environmental differences between LIB production from circular and\nconventionally mined cathode material lie early in supply chains, comprised of extraction,\ntransport, and refinement steps (together as “cradle-to-gate,” Fig. 1b).”\nAdditional changes were made (including the previous comments directly above), but are\nenumerated in detail here for brevity.\nReviewer: 4. Where you refer to figures (e.g., \"Fig. S3\", \"Fig. 5\"), ensure these are easily\naccessible to the reader and consider including brief descriptions in the text to aid\nunderstanding for those who might not have immediate access to the figures.\nOur response: We note that Nature Communications is an open access journal where the readers\ncan access the main manuscript and source data including the supplementary figures. We have\n12\nprovided adequate descriptions in the original manuscript while citing the figures, e.g. (lines 155–\n156, 228–229, 333–335):\n“In LIB supply chains, the refinement step converts the collected feedstocks into battery-grade salts\nfor further manufacturing (Fig. 2a).”\n“The environmental impacts of two circular refinement pathways are were presented in each graph\nin Fig. 3 for mixed-stream LIB feedstocks processed at Redwood Materials…”\n“The relative environmental impacts of input consumables (e.g., energy, water, commodity\nchemicals) in the gate-to-gate refinement processes are were disaggregated in Fig. 5 (additional\ncriteria air pollutants in Tables S2–S3, Figs. S4–S5).”\nAdditional figure descriptions (lines 197–199, 271–274, 393–395, and 443–446) were added to\nthe revised manuscript to improve clarity:\n“The values of energy consumption and greenhouse gas emissions are were comparable with\nprevious studies based on GREET datasets12,15 (comparison of environmental impacts with data in\nliterature is detailed in Fig. S3b).”\n“While this analysis is was focused on Redwood Materials refinement pathways, the methodology\ncan be used to evaluate additional refinement pathways (e.g., analysis of the representative\nhydrometallurgy was detailed in Fig. S3c), or others that use different material feedstocks,\nrefinement processes, and energy supplies.”\nThis tradeoff also explains explained the different influences of electricity source on environmental\nimpacts of the Redwood Materials refinement step and other pathways (detailed in Fig. S3d).\nAfter extraction, LIB material concentrates are transported along domestic and international\nroutes by truck, train rail, and maritime cargo ship to refinery locations (a portion of the network\nmodel is presented in Fig. S76, and data are summarized in Table S8–S15; complete references in\nSupplementary Information).\nReviewer: 5. Many references are in random formats.\nOur response: We thank the reviewer for raising the errors, and have adjusted the citations below.\nNote that the corrected reference numbers may change in the revised manuscript. Some original\nreferences had missing information, including Refs. 2–3, 22–24, 27, 29, 33, and 36–37. We have\nrevised the respective citations to:\n“2 National blueprint for lithium batteries 2021–2030. (Department of Energy, 2021).”\n“3 Global supply chains of EV batteries. (International Energy Agency, 2022).”\n“30 Electric Vehichle Registrations by State. (U.S. Department of Energy, 2022).”\n“31 United States Census Bureau Database. (United States Census Bureau, 2020).”\n“32 Where to recycle: Map of public recycling locations. (CalRecycle, 2022).”\n“39 Hazardous Materials: Transportation of Lithium Batteries. (Pipeline and Hazardous\nMaterials Safety Administration, 2014).”\n“41 Daily, G. C. & Ruckelshaus, M. 25 years of valuing ecosystems in decision-making. Nature\n606, 465-466 (2022).”\n“45 Korolev, I. et al. in 29th International Mineral Processing Congress, IMPC 2018.”\n“47 Battery recycling through extraction of metals from spent lithium-ion batteries. (Syensqo,\n2023).”\n13\n“48 Creating a circular and sustainable battery supply chain. (Li-Cycle, 2023).”\nOther original references had incorrect journal abbreviations, including Refs. 4, 6–8, 11, 16–17,\n25, 28, and 30. We have been revised to:\n“4 Winslow, K. M., Laux, S. J. & Townsend, T. G. A review on the growing concern and potential\nmanagement strategies of waste lithium-ion batteries. Resour. Conserv. Recycl. 129, 263-277\n(2018).”\n“6 Mayyas, A., Moawad, K., Chadly, A. & Alhseinat, E. Can circular economy and cathode\nchemistry evolution stabilize the supply chain of Li-ion batteries? Extr. Ind. Soc. 14, 101253\n(2023). ”\n“7 Jacoby, M. It’s time to get serious about recycling lithium-ion batteries. Chem. Eng. News 97,\n29-32 (2019). ”\n“11 Ciez, R. E. & Whitacre, J. F. Examining different recycling processes for lithium-ion batteries.\nNat. Sustain. 2, 148-156 (2019). ”\n“12 Crenna, E., Gauch, M., Widmer, R., Wäger, P. & Hischier, R. Towards more flexibility and\ntransparency in life cycle inventories for Lithium-ion batteries. Resour. Conserv. Recycl. 170,\n105619 (2021).\n“24 Makuza, B., Tian, Q., Guo, X., Chattopadhyay, K. & Yu, D. Pyrometallurgical options for\nrecycling spent lithium-ion batteries: A comprehensive review. J. Power Sources 491, 229622\n(2021). ”\n“26 Kim, S. et al. A comprehensive review on the pretreatment process in lithium-ion battery\nrecycling. J. Clean. Prod. 294, 126329 (2021). ”\n“37 Roithner, C., Cencic, O. & Rechberger, H. Product design and recyclability: How statistical\nentropy can form a bridge between these concepts-A case study of a smartphone. J. Clean. Prod.\n331, 129971 (2022). ”\n“40 Sun, M., Yang, X., Huisingh, D., Wang, R. & Wang, Y. Consumer behavior and perspectives\nconcerning spent household battery collection and recycling in China: a case study. J. Clean. Prod.\n107, 775-785 (2015). ”\n“42 Berger, K., Schöggl, J.-P. & Baumgartner, R. J. Digital battery passports to enable circular\nand sustainable value chains: Conceptualization and use cases. J. Clean. Prod. 353, 131492\n(2022). ”\n14\n REVIEWER 3\nReviewer: Review Summary: this paper presents a comparative life cycle analysis for primary\nand secondary supply chains for lithium ion batteries (LiBs). The paper is well written, the\ntopic is of interest, the core methodology is sound, and outcomes relevant for advancing the\nconversation around designing circular supply chains for LiBs. The major concerns I have\nabout this study have to do with the case for its uniqueness, the significance of some of the key\noutcomes and the appropriateness of the context for interpreting these results. I think it needs\nto be updated before considering it for publication. I provide these as comments below for\nconsideration.\nOur response: We thank the reviewer for the favorable reception of our study. We have carefully\nconsidered their comments on uniqueness, significance, and results interpretation and revised the\nmanuscript as detailed below.\nReviewer: Lines 128 – 130, 438 – 440: The authors make the case that a distinguishing\ncontribution from this study is the representation of (1) industrial scale circular refinement\ndata, & (2) stepwise cradle-to-gate supply chain comparison, but it is not clear to me what this\nclaim really amounts to.\nFor (1), I imagine this has to do with using data from Redwood’s process, so what is the value\nproposition then? Does using data from Redwood’s process provide more accurate (or\ntrustworthy) recycling impacts compared to previous studies? Since the authors correctly\npointed out that there are significant differences in reported environmental impacts for LiB\nrecycling (Lines 91-94), how does this study address and resolve that problem, and not simply\nadd another data point to literature on LiB recycling impacts?\nOur response: We thank the reviewer for raising the questions concerning the importance of\nanalyzing Redwood Materials processes. In general, our analyses based on Redwood data provide\ncomprehensive and granular insights for advancing various refining technologies, and\nRedwood processes allow us to systematically investigate impacts of different factors in\nrecycling LIBs. Below we summarize the important value propositions and potential broad\nimpacts of employing Redwood’s processes, and have revised the main manuscript to better\nhighlight these points. Note that this comment resembles the first comment from Reviewer 2; thus\nwe have adapted our response from that comment (Page 1 of this document).\nFirst, Redwood Materials represents state-of-the-art circular refinement technologies that are\ncomparable to existing and emerging technologies, and our findings provide insights for advancing\nvarious circular refining technologies. Reductive calcination (RC) is a pyrometallurgical process\nrelying on the carbothermal reduction with optimized conditions. Pyrometallurgy based on\ncarbothermal reduction has been rapidly developed at lab scale, while our study, to the best of our\nknowledge, reports the first evaluation of the industrial-scale application of the emerging\ntechnology, providing important insights to all pyrometallurgical refinement technologies. The\nhydrometallurgical processes in Redwood that refine both energized batteries and scrap are\nanalogous to the prevailing hydrometallurgy, and our analysis identifying environmental impact\ncontributors to the Hy process similarly advances Hy processes beyond Redwood. The manuscript\n15\nhas been revised to better generalize the working conditions of Redwood technologies (lines 262–\n267 and 513–528), and compare with existing refinement methods (lines 531–534):\n“Similar to RC, emerging oxygen-free pyrometallurgical processes rely on carbothermic and\nthermite reduction for recycling cathode metal compounds at moderate temperatures (600–\n1000℃)19-21. Our results show that the RC step accounts for 5.5%–7.4% of the total environmental\nimpacts in Redwood (Fig. 3), demonstrating the environmental feasibility of industrial-scale\ncarbothermal reduction-based pyrometallurgy.”\n“Disaggregated analysis of LIB refinement pathways at Redwood Materials provides provided\nimportant insights into the performance and potential of different refinement processes.\nPyrometallurgical technologies are advantageous in scalability and operating safety9,10,33, While\npyrometallurgical processing is but are widely considered as more environmentally intensive than\nhydrometallurgy31,32 due to the high reaction temperature,. Oxygen-free carbothermic reduction\nand thermite reduction have been recently investigated in lab scale for energy-efficient LIB cathode\npyrolysis19-21. Our analysis showed that Redwood Materials’ industrial-scale carbothermal\nreduction-based RC pathway pyrometallurgy (reductive calcination in Redwood Materials)\nexhibits much lower environmental impacts than prevailing direct roasting and smelting\npyrometallurgical and current Hy-containing hydrometallurgical pathways reported in practice\nand in literature (Fig. S3). The optimized conditions of RC processing minimizes the combustion\nof carbon-containing LIB materials, significantly reducing CO -eq emissions while simultaneously\n2\ngenerating products that are amenable for hydrometallurgical separation.. Our findings showed\nthe promise of carbothermal reduction-based pyrometallurgy for pretreatment of LIB recycling;\nhowever, we note that controlling the process conditions to achieve optimal environmental impacts\nand product formats will elevate the costs.”\n“Because Our analysis identified chemical consumables such as H O are as important\n2 2\ncontributors to hydrometallurgy, indicating that environmental impacts of Hy processes could can\nbe reduced through more sustainable (e.g., electrochemical) production methods (e.g.,\nelectrochemical generation of H O )36.”\n2 2\nSecond, operational data from Redwood Materials provide more granular insights to verify and\ncorrect the existing model-based studies. This Redwood-based study helped identify the gaps\nbetween models and industrial operations, and we highlighted the needs for advancing the whole\nfield in the revised manuscript (line 107–114 and 536–541):\n“Previous efforts have worked towards calculating There is a critical need for transparency and\ndetailed insights into the environmental impacts (e.g., energy consumption, greenhouse gas\nemission, and water consumption) of LIB refinement pathways and all cradle-to-gate supply chain\nsteps. Previous efforts have worked towards addressing this need11,12, and this study builds on the\ncomparative methodology of a recent step-by-step study to provide higher resolution and more\nactionable primary data, insights, and recommendations. however, gate-to-gate analyses of\ncircular refinement processes reported environmental impacts differing by >30%10,11,13 due to\ninconsistent methodologies.”\n“We note the inconsistencies among existing studies on assessing conventional LIB refinement due\nto different product compositions, which underscores the need to standardize the functional unit in\nreporting environmental analyses for future studies in the field. The deviations between our results\nof circular refinement and model-based literature data highlight the importance of reconciling\nmodels with industrial operating data.”\n(energized batteries and battery scrap) and generation of different product formats (mixed metal\nsulfate and discrete salts), offering opportunities to investigate influences of multiple factors on\nthe circular refinement. Using Redwood data, we analyzed the refinement of energized batteries\nand scrap, providing suggestions for developing generalizable strategies for different recycling\nsources (Fig. 3 and lines 542–547). Furthermore, while prevailing recycling produces discrete salts,\nwe analyzed different refinement scenarios and demonstrated the advantages of producing mixed\nsalts (Fig. 4 and lines 315–320).\n“Our findings provided important guidance for sources and products in future refinement. Battery\nscrap is currently the primary recycling source due to the proliferation of gigafactories, but will\ndecrease in the future with quality control advances in manufacturing11. While refining recycled\nbatteries requires higher greenhouse gas emissions, energy input, and water demand than refining\nbattery scrap (Fig. 3), it is critical to improve the technologies for recycling energized batteries\nfor future challenges.”\n“The RC pathway (RC+Me+Hy) exhibits exhibited lower energy consumption (−72.3%), CO -eq\n2\nemissions (−39.5%), and water consumption (−12%) relative to the Me+Hy pathway (Fig. 4),\nbecause it avoids additional treatment separating (Ni,Co)SO to discrete salts. Overall, our results\n4\nindicate indicated that refining batteries to mixed metal salts instead of discrete salts can\nsubstantially save environmental impacts while still satisfying the needs of circular LIB supply\nchains.”\nWe have revised the introduction to better sharpen the research gaps answered by our Redwood-\nbased analysis and importance of the findings based on Redwood data (lines 97–122 and 145–152):\n“While several previous studies have investigated cradle-to-gate environmental impacts, gate-to-\ngate analyses of circular refinement processes are inconsistent, reporting environmental impacts\nthat differ by >30%8-10, and are not completely based on industrial-scale LIB recycling operations.\nThe gate-to-gate refinement processes utilized at established and emerging circular refinement\nfacilities may include mechanical separation (Me), pyrometallurgy (Py)8,9, and hydrometallurgy\n(Hy)10,11. Specifically, Me physically dismantles LIBs into constituent components, Py leverages\nelevated temperature to facilitate thermally-driven material transformations, and Hy separates\nmaterials in the aqueous phase via leaching, precipitation, and solvent extraction processes.\nVariations in environmental impacts arise from the specific operational choices at refinement\nfacilities that utilize different processing pathways and from the methods to evaluate them. Previous\nefforts have worked towards calculating There is a critical need for transparency and detailed\ninsights into the environmental impacts (e.g., energy consumption, greenhouse gas emission, and\nwater consumption) of LIB refinement pathways and all cradle-to-gate supply chain steps. Previous\nefforts have worked towards addressing this need11,12, and this study builds on the comparative\nmethodology of a recent step-by-step study to provide higher resolution and more actionable\nprimary data, insights, and recommendations. however, gate-to-gate analyses of circular\nrefinement processes reported environmental impacts differing by >30%10,11,13 due to inconsistent\nmethodologies. In addition, Aadvancing decision-making capabilities to scale sustainable LIB\nsupply chains requires life cycle assessment with more granular data at each step, inclusion of and\nincorporating industrial-scale refinement operations with practical mixed-stream battery\nfeedstocks can uniquely inform rational design of refinement technologies,. The development of\nLIB manufacturing and needs for the recycling market have been projected by academic and\nindustrial researchers8,14, but industrial-level understanding of the environmental influences of\ndifferent feedstocks and refinement products is still lacking. documentation of operational\n17\nparameters, and qualification of results in terms of limitations and applicability to real-world\nscenarios.”\n“Findings of this study provide important guidance to advancing comprehensive refinement\ntechnologies. With the methodologies and results reported in this study, researchers can prioritize\nmajor opportunities to improve process efficiencies, practitioners can benchmark their\nenvironmental impacts, and policymakers can incentivize best environmental practices in LIB supply chain\nmanagement. Granular Iinsights provided by this study based on practical data can also help\nrecyclers optimize the environmental impacts of their refinement processes, and spark more\nacademic-industrial collaborations to further advance the field.”\nReviewer: For (2), Since several studies have already looked at both primary and secondary\nbattery supply chains and compared the impacts of the various process steps (even if they did\nnot categorize it as extraction and refining, those steps are implicit in the reference process\ndescription) and across different environmental metrics, I would like more clarity on how and\nwhy this work is different, and why that difference is important.\nOur response: Regarding the stepwise cradle-to-gate analysis, we found in our literature review\nthat different reports combine parts of the battery production chain in unique fashions, which leads\nto opaque comparisons (both comparing different steps and non-reproducible analyses). While\nthese steps may be implicit, they are not uniformly categorized nor quantified across reports, which\nchallenges comparison and reproducibility. Our contribution to the literature is a clear, consistent\ndelineation between extraction, transport, and refinement steps in both the conventional and\ncircular supply chains. In particular, our addition of granular industrial recycling data provides\nadditional insights that cannot be captured by simply modeling processes, particularly for mixed-\nstream feedstocks and mixed-salt products. More broadly, critical insights arise from attributing\nand comparing the relative environmental impacts to each of these steps across circular and\nconventional supply chains. We anticipate that our rigorous, systematic approach can help\nstandardize future literature especially as battery recycling scales rapidly in the near future.\nReviewer: Lines 153 – 154, 286-287, ...: Some of the major study findings, such as that\n“recycling exhibit lower environmental impacts than primary production” or that “electricity\nconsumption dominates recycling environmental impacts” have already been established in\nprevious studies. I think the authors need to make a clearer case of what their results add to\nour current understanding. For instance, are they providing an important update to the\nmagnitude of the comparative differences, or greater clarity about relative impacts, or a more\ngranular level of detail not available in previous studies but which are potentially important?\nFor example, something I don’t remember in other studies is an analysis like that shown in\nFigure 5b, which seems like an outcome that could be elevated and used as a basis for the\nbroader discussion that incorporates consideration for (grid) energy transitions.\nOur response: We thank the reviewer for raising the issue of novelty. Below we first explain the\nimportant new insights provided by the electricity analysis, along with further revisions made to\nthe manuscript to highlight the novelty.\nWe acknowledge that it can be intuitive that electricity affects the environmental impacts of LIB\nsupply chains, but point out that our study is the first to assess comprehensive industrial recycling\n18\npathways developed outside of East Asia, and provides important findings in addition to the\nelectricity-carbon intensity relationship.\nFirst, the majority of previous studies that investigated the influence of electricity on battery\nrecycling reference specific patent applications (e.g., Umicore1 and Brunp2 for hydrometallurgy),\nmodel data gaps with assumptions (i.e., using data from similar industrial processes outside of\nbattery manufacturing3), or receive incomplete industry data through surveys4. In particular, we\nnote that many existing analyses refer to a 2015 study by Xie et al.2, where the input/output of\nrecycled batteries was adapted from a Chinese company but recycling technologies were not\nexplicitly reported nor explicitly based on industrial-scale operations. Therefore, for the first time\nthe current work employs comprehensive industrial-scale lithium-ion battery recycling data and\nprocesses developed in the U.S. to quantitatively identify the roles of input electricity at a\nbalancing area specific for comprehensive environmental impact metrics (i.e., CO -eq, water\n2\nconsumption, and criteria air pollutants). Our results provide benchmark electricity contributions\nto environmental impacts for state-of-the-art lithium-ion battery recycling technologies.\nSecond, our in-depth analysis of environmental impacts of energy sources indicates a tradeoff\nrelationship between CO -eq emissions and water consumption, which are important contributions\n2\nto the broader audience of Nature Communications. Importantly, water consumption’s connection\nin the broader context of LIB supply chains has not been interrogated nor summarized for a broad\naudience.\nWe are also grateful to the reviewer for highlighting our important contributions. We added lines\n125–133, 133–135, 135–137, 138–140, and 140–152 to the main manuscript to better clarify the\nnovelty of the findings:\n“First, we quantify quantified and compare…Industrial-scale operational data provided by\nRedwood Materials are analyzed from a more granular level and compared to conventional LIB\nsupply chain values based on Argonne National Laboratory’s Greenhouse Gases, Regulated\nEmissions, and Energy use in Technologies (GREET 2021) model12.”\n“Second, for the first time we quantitatively identified the roles of input electricity at the balancing\narea level on comprehensive environmental impact metrics based on industrial-scale LIB recycling\ndata.”\n“SecondThird, influences of the product formats in the refinement pathways on environmental\nimpacts are were examined by varying the industrial-scale refinement pathways.”\n“ThirdFourth, we assess assessed the environmental impacts of upstream processes before gate-\nto-gate refinement based on modeling.”\n“With the methodologies and results reported herein, researchers can prioritize major\nopportunities to improve process efficiencies, practitioners can benchmark their environmental\nimpacts, and policymakers can incentivize best environmental practices in LIB supply chain\nmanagement. Granular Iinsights provided by this study based on practical data can also help\nrecyclers optimize the environmental impacts of their refinement processes, and spark more\nacademic-industrial collaborations to further advance the field.\nReferences\n19\ncom/patent/US7169206B2/en, 2007).\n2 谢英豪, 余海军, 欧彦楠 & 李长东. 废旧动力电池回收的环境影响评价研究. 无机盐工业 47,\n43-46 (2015).\nion batteries. Procedia Cirp 48, 188-193 (2016).\nbattery manufacturing: Challenges and outlook. J. Power Sources 593, 233955 (2024).\nhydrometallurgical or direct recycling? Electrochem. Soc. Interface 30, 51 (2021).\nReviewer: 201 – 204, Figure 3, Figure 5,..: While it is interesting that the authors include\nRecycled scrap in the analysis, I would expect some additional context for how to interpret its\nrelevance. For instance, what are the relative material flows of scrap vs recycled batteries in\nsecondary supply chains (currently or projected for the future)? For example, if scrap-to-\nrecycled battery stream ratio is 1:100, then the additional reduction in water consumption\nfrom 74% for battery streams to 88.4% for scrap does not really amount to much, and is not\nparticularly important to designing strategies for water efficiency and resilience in circular\nsupply chains.\nOur response: We thank the reviewer for raising the valuable point of the relevance of battery\nscrap. Battery scrap is generated during battery manufacturing and assembly as byproducts,\nwastes, and defective batteries. While advances in manufacturing are expected to reduce the scrap rate to\n<10%, the rapid growth of new production lines by rising gigafactories will produce a large amount\nof scraps in the next decade5,6. Battery scrap is now considered as a primary and ideal proximate\nsource for recycling, and is projected to account for approximately half of the total recycling source\nby 20305,6. Therefore, considering both recycled scrap and batteries as major sources is necessary\n(particularly in the near term), and is an important contribution to the field from our paper.\nThe manuscript has been revised to better clarify the considerations and significance of battery\nscrap (lines 159–163 and 543–549):\n“Battery scrap generated from manufacturing and assembly is considered a primary recycling\nsource today, and is projected to account for approximately half of the recycling source material\nin the next decade as battery production outpaces disposal of energized batteries 8,14. Therefore,\ncircular refinement was analyzed starts starting with either end-of-life batteries (1 and 2) or battery\nscrap (5).”\n“Battery scrap is currently the primary recycling source with the rising gigafactories, but will\ndecrease in the future with quality control advances in manufacturing11. While refining recycled\nbatteries is more environmentally intensive than refining scrap (Fig. 3), it is critical to improve the\ntechnologies for recycling energized batteries for future challenges. Our findings also\ndemonstrated the benefits of mixed metal sulfates as refinement products over single salts in the\ncircular supply chain, indicating that the further separations between Ni and Co salts can be\navoided.”\nReviewer: Additional Comments:\ncircular “recycling facility” in Figure 1b?\nOur response: We thank the reviewer for pointing out the ambiguity. In general, the conventional\n“material refinery facility” and circular “recycling facility” are both refinery facilities. However,\ncompared to a conventional material refinery facility, a circular recycling facility differs in the 1)\nfeedstocks received and 2) refining technologies employed. The ore concentrate from mining\nenters material refinery facilities, whereas energized batteries or battery scraps are the feedstocks\nfor a circular recycling facility. Conventional facilities employ multistep refining methods, and a\ncircular facility utilizes technologies described in this study, including pyrometallurgy, mechanical\nprocessing, hydrometallurgy, etc.\nTo avoid potential confusion, we renamed the “material refinery” and “recycling facility” to\n“mining refinery” and “recycling refinery”, respectively, and have revised Fig. 1b to:\nWe also added lines 83–90 to the caption of Fig. 1b to further clarify:\n“Cradle-to-gate steps of manufacturing battery-grade LIB materials (i.e., salts) from conventional\nand circular supply chains, both of which include three steps: extraction, transport, and refinement.\nConventional extraction refers to natural mining, and the circular counterpart is battery collection.\nTransport in the conventional and circular supply chains move ore concentrate and batteries,\nrespectively. Conventional mining refineries and circular recycling refineries receive ore\nconcentrates and batteries, respectively, and employ different refining technologies. Extraction and\ntransport are considered “upstream” steps relative to gate-to-gate refinement…\nReviewer: Figure 2 – Process descriptions: It would help if the authors included (perhaps as\nsupplemental material) the details of the underlying process descriptions for the different\npathways illustrated, specifically to help with distinguishing between Redwood process and\n“Representative Circular (RC)” pathway. For instance, do they share common/similar process\n21\nsteps and are the assumptions about these overlapping steps identical? Do they use the same\nchemicals? What degree of chemicals/solvent recovery or reuse is assumed? Does the RC Hy*\nprocess (in Figure 3) also include mechanical processing - which one would expect, but which\nis not obvious from the figure?\nOur response: The reductive calcination (RC) process at Redwood falls under the comprehensive\ncategory of “pyrometallurgical technologies”. RC at Redwood is the industrial-scale application\nof the emerging carbothermal reduction-based pyrometallurgy, and employs optimized operating\nconditions. We have revised the manuscript to better distinguish the technology (lines 247–255),\nbut cannot share operating details like chemical recovery or reuse due to non-disclosure\nagreements with Redwood. We aim to achieve a compromise in this academic-industrial\ncollaboration between academic transparency and industrial intellectual property. We consider this\ncompromise critical to provide access to data the facilitate the granular insights we report in this\nstudy, and as an opportunity to model academic-industrial collaborations in the rapidly developing\nbattery recycling field.\n“The RC process is an innovative exothermic pyrometallurgical process that converts reduces the\ncathode metal oxide compounds under the oxygen-free condition energized battery feedstock under\ncertain conditions that leverage heat from exothermic processes and inhibit graphite combustion\nfor subsequent refinement. Unlike dominant industrial pyrometallurgical processes (e.g., direct\nroasting or smelting) that require high temperature >1400 ℃, This the RC process does not use\noptimizes the working conditions, favoring the carbothermal reduction without using graphite\ncarbon, thus avoiding direct fossil fuel inputs onsite and facilitates subsequent hydrometallurgical\nrefinement into battery-grade materials and graphite combustion.”\nWe confirm that the representative hydrometallurgy (Hy* in Fig. 3) includes a mechanical\nprocessing step. Thus, an analogous Me+Hy process is modeled to analyze the environmental\nimpacts of producing discrete salts in Fig. 4. We have revised the manuscript to describe this point\nmore explicitly (lines 257–260 and 310–313):\n“Energy consumption and CO -eq emissions of representative existing recycling pathways from\n2\nthe literature, including pyrometallurgy (Py*), hydrometallurgy (Hy*), and direct recycling\n(Direct*), are were also presented in Fig. 3 for comparison. Note that a mechanical processing\nstep is often included in existing Hy* recycling.”\n“Circular pathways refining batteries to different products are were analyzed using the Redwood\ndata by the RC+Me+Hy process and the modeling of a representative battery recycling method\ncombining mechanical and hydrometallurgy (Me+Hy, analogous to the Hy* in Fig. 3) refinement\n(Method (3) in Fig. 2).”\nReviewer: Figure 3 caption: Please correct the b and c captions of Figure 3 – it appears\nthey are switched.\nOur response: We thank the reviewer for pointing out the error and have revised Fig. 3 with data\ncorrection:\npredicted LCA impacts between studies, even if just for reference. However, how exactly\nshould we interpret the results from the current work against the backdrop of those other\nresults? Are there obvious limitations in other studies that is corrected for here? Can we have\ngreater transparency in the underlying process and data for the current work?\nOur response: We thank the reviewer for favorable comments on our findings and raising the\nissue of transparency in the field of battery recycling.\nFirst, we have substantially validified our analytical methodology by comparing the findings to\nthe existing literature data of both conventional and circular supply chains. According to the\ncomparison, our results are reliable (lines 197–199 and 260–261):\n“The values of energy consumption and greenhouse gas emissions are were comparable with\nprevious studies based on GREET datasets12,15 (Fig. S3b).”\n23\n“In general, the RC+Me+Hy pathway at Redwood exhibits exhibited comparable energy\nconsumption and CO -eq emissions with Hy and Direct literature values11”\nprocessing conditions, and the utility of primary industrial data access over modeling processes\nfrom literature sources, and have mentioned the corresponding limitations in the main manuscript\n(lines 278–282, 282–285, and 285–288). We further note that the differences highlight the novelty\nand importance of the current work, i.e., collating real operating data with the model-based results.\n“However, direct comparison can be inexact due to varying underlying assumptions and data\nsources. For example, Argonne National Laboratory’s GREET and EverBatt models leverage a\ncombination of technology descriptions from patent applications, literature data on process flow\nconsumptions, industry site visits and surveys, expert advice solicitation, and stated assumptions\nto form complete pathways.”\n“Further, Ciez and Whitacre quantified environmental impacts using output products represented\nas “metal offsets” for pyrometallurgy or with metals in solution for hydrometallurgy11 (Note 3 in\nSupplementary Information), rather than cathode salts in this study.”\n“In addition, the previous studies included a portion of recycled metal materials in its conventional\nsupply chain analysis, whereas this work references only mined natural deposits in conventional\nsupply chains to fully deconvolute the environmental impacts12.”\nThird, to overcome the limitations of previous studies and maximize the transparency of the current\nwork, we have clearly described our method and provided a full set of data in Supplementary\nInformation and Supplementary Data File. We note that the refinement technologies by Redwood\nMaterials in this paper is innovative, and some working details are not explicitly published, to\nbalance between novelty and transparency.\nLastly, we acknowledge that standardization of methodology and communications is critically\nneeded to advance the whole research field of battery recycling. We have added lines XX for\nfurther discussion, and highlighted the idea in the introduction (lines 536–541, 145–146, and 150–\n152):\n“We note the inconsistencies among existing studies on assessing conventional LIB refinement due\nto different product compositions, which underscores the need to standardize the functional unit in\nreporting environmental analyses for future studies in the field. The deviations between our results\nof circular refinement and model-based literature data highlight the importance of reconciling\nmodels with industrial operating data.”\n“Findings of this study provide important guidance to advancing comprehensive refinement\ntechnologies.”\n“Granular Iinsights provided by this study based on practical data can also help recyclers optimize\nthe environmental impacts of their refinement processes, and spark more academic-industrial\ncollaborations to further advance the field.”\nReviewer: Lines 258 – 260: The authors suggest that in conventional supply chains, producing\nmixed salts has higher energy and carbon footprint compared to producing discrete salts. This\nresult is counter-intuitive, since I'd expect that producing individual salts would theoretically\n24\ninclude an additional separation step, which adds energy, and consequently, CO2 emissions.\nCould the authors explain this non-intuitive trend, and how come it is different from that seen\nfor the circular pathway? It would help if they also provide the underlying process description\nas well.\nOur response: We are grateful to the reviewer for raising the confusion. According to the GREET\nmodel, producing discrete salts is not achieved from post-treatment of mixed salts, but starts from\nores distinct from the laterite refined for mixed salts. In conventional refinement, the discrete salts,\nNiSO and CoSO , are refined from Ni-rich and Co-rich ores separately, generating little mixed\n4 4\nsalts except impurities. On the other hand, producing the mixed hydroxide precipitate\n(Ni,Co)(OH) starts from refining the Ni laterite, where the concentration of Co is very low (3.6%),\n2\nthus significantly elevating the energy consumption.\nWe have revised the manuscript to provide a clearer explanation (lines 302–309):\n“While the discrete products NiSO and CoSO are produced from the mixed hydroxide precipitates\n4 4\nthrough additional post-treatment Ni-rich and Co-rich ores separately, generating mixed\nhydroxide precipitates starts from the Ni laterite where, the very low composition of Co (3.6%) in\nthe latter limits the NCA stoichiometry, thus increasing the total energy cost to generate 1 kg NCA-\nequivalent materials. On the other hand, water consumption of refining mixed hydroxides is slightly\nlower (−6.6%) than that in producing discrete salts. In general, discrete salts-based pathway is\nwas favorable in conventional refinement to reduce environmental impacts.”\nReviewer: Figure 6: I am not sure why Figure 6 is presented as a result, and not as\nsupplemental information illustrating the approach used to model the transport logistics.\nOur response: The contents have been combined with the original Fig. 7 in the revised manuscript\n(see next page). Data in the bar graphs of original Fig. 7 have also been updated based on earlier\ncorrections that were not carried through from earlier figures.\n“Fig. 6 | A logistics model for assessing upstream environmental impacts of extraction and\ntransport in circular and conventional lithium-ion battery supply chains. Cradle-to-gate\nenvironmental impacts of different supply chains. A, Modeled circular extraction of LCO-based\nsmartphones from every census-block group based on population to the closest, existing private or\nmunicipal collection facility (CF) using a shortest-route algorithm. Inset details modeled circular\ntransport of smartphones aggregated at CFs and then transported to a central recycling facility at\nthe center (gravity point) of the California population by the shortest route (red lines). Colors of\nblock groups indicate the catchment area of a specific CF, where CF size shows the relative number\nof smartphones collected in 2019. B, A weighted distribution estimate of international transport\nlogistics for conventional supply chains between mining and refining countries based on cobalt\nproductivity in the top Sankey diagram. C, An example of transport logistics for cobalt mined and\naggregated in the Democratic Republic of the Congo (DRC) and then shipped via primary road,\n26\ntrain rail, and maritime routes using a shortest-distance path to major refinery locations, with\ninsets showing the degree of detail considered. Similar analyses were performed for Li, Ni, Co, and\nAl. Inserts present more detailed transit routes in DRC and Canada. Ad, Energy consumption (left),\nbe, CO -eq emissions (middle), and cf, water consumption (right) of conventional (conv.) and\n2\ncircular (cir.) supply chains by step including material extraction, transport, and refinement. NCA-\neq cathode used in electric vehicles (EV-NCA, left panels) and LCO-eq cathode material used in\nsmartphones (Phone-LCO, right panels) are provided. Environmental impacts of refinement are\nanalyzed based on electricity generated from balancing grid authority CISO and upstream supply\nchain steps (extraction and transport) are based on data from GREET and transport models\ndeveloped in the preceding section and depicted in Fig. 6panels a–c. Specific environmental\nimpacts of each step are detailed in Tables S5–S7.”\nReviewer: Lines 354 – 355, Figure 6: I think there is a very important element that is missing,\nwhich is scale.\nTo compare primary and secondary logistics fairly, it would be good to try to make the scale\nmore comparable. For example, expanding the collection radius from just California to say,\nthe entire U.S. significantly changes the associated impact footprints. While I expect this to\nstill be smaller than conventional, the comparison at this scale will be more justifiable.\nThe converse argument is that for a much smaller scale of primary manufacturing, it is possible\nto identify a domestic supply chain that ends up with a much smaller footprint than the\ntransnational supply chain considered in the model in Figure 6.\nOur response: We thank the reviewer for the suggestion. We agree that scale may influence our\nresults, but given that battery recycling is not as widespread as battery manufacturing, there is an\ninherent mismatch in current scale. Our study of California represents a localized domestic\nprocessing strategy with lower environmental impacts instead of inter-state transport. Collecting\nand transporting LIBs to recycling centers inside California minimizes the distances and the\nenvironmental impacts, and can benefit safer handling LIBs. We also note that this study is focused\non refinement, and analyzing the upstream steps at the scale of one state is a first step. Considering\nLIB recycling in the regions of multiple states can be beneficial, but warrants further investigation\noutside the scope of this study. We have added lines 582–587 to highlight the point:\n“Upstream process optimization of environmental impacts warrants further investigation, such as\nthe active area of high-throughput automation of LIB extraction from non-standardized devices\nand EV battery packs or rapid assessment of LIBS for second life uses. While the current study\nmodeled localized collection and transport inside the state of California, extending the scale to\nregions combining multiple states is critical to further investigate the upstream environmental\nimpacts.”\nReviewer: Figure 7: Can the authors clarify the reason for the difference in transport energy\nand CO2 footprint between EV and smartphone batteries? Does the difference arise from\nrelative battery compositions, or transport distances? In addition, are there any key differences\nin their respective recycling steps, and how do these translate to differences in overall lifecycle\nimpacts?\nOur response: Due to different packing densities, the mass of active materials per kg of battery\nfor LCO (0.162) is much lower than that of NCA (37.4). Therefore, the environmental impacts of\n27\ntransport LCO normalized by kg active materials are correspondingly higher. Information of the\npacking densities was detailed in Table S10 in the Supplementary Information, and we have\nrevised the main manuscript to better clarify (lines 479–482):\n“Circular production of LCO-grade materials leads to higher environmental impacts than that of\nNCA-grade materials based on the mixed-stream feedstock composition processed by Redwood\nMaterials. Note that LCO has relatively lower packing densities of active materials compared with\nNCA (detailed in Table S10, thus elevating the environmental impacts of transporting LCO).”\nReviewer: Lines 485 – 487 – Can the authors say more/ be a bit more specific here? Anyone\ncan mention this trade- off without analysis. I would expect that the current analysis can supply\nenough information to recommend which of the two options – disassembly before transport vs\ntransport before disassembly - is potentially a more impactful approach?\nOur response: We appreciate the suggestion from the reviewer, but note that concluding remarks\nof the environmental impacts of the two disassembling strategies are beyond the scope of the\ncurrent study and warrant additional investigation. In addition, transport plays a minor role in the\noverall environmental impacts of circular battery supply chains (Fig. d–f), and the disassembling\nstrategies will not have significantly different contributions to the overall environmental impacts.\nCurrently there is no standardization for LIB design (packs, modules, and cells) or integration into\nelectric vehicles (e.g., cell-to-chassis, module-to-chassis, cell-to-pack), challenging the evaluation\nof environmental impacts of the disassembling. Distance-to-facility and economies-of-scale for\ndifferent operations would need to be considered. In addition, pre-processing materials into black\nmass prior to shipping to a recycling facility is also a consideration to reduce both shipping costs\nand lower safety concerns present for energized LIBs.\nWe have changed lines 572–578 to better clarify:\n“Practical battery collection operations will likely vary based on route selection and preprocessing\nstrategy further influencing environmental impacts6. For example, the Further investigating the\nenvironmental impacts of the disassembly of collected EV battery packs or removal of smartphone\nbatteries from devices prior to transport can help balance between to a recycling facility can\nincrease extraction energy usage through extraction but reduce and transport environmental\nimpacts by lowering transportation weight (Table S10).”\nReviewer: Lines 489 – 490: A number of studies mention the potential of rail transport to\nreduce energy and environmental impacts of secondary battery supply chain logistics relative\nto trucking, but how practical is this, especially for this analysis which is restricted to\nCalifornia? I imagine there are major limitations that make this nearly impossible, such as\ninadequate local rail coverage, competition with other high value cargo, etc.\nOur response: We acknowledge the inadequacy of local rail coverage, but note that a rail line can\nbe used as an aggregation point for long transport combined with other vehicles. Determination of\nthe optimal transport strategy for a certain collection/transport task warrants investigations of\nspecific available vehicles. Implementing policy recommendations of collection points to train\ndepots will also facilitate the utilization of available rail transport. The manuscript has been revised\nto reflect this consideration (lines 578–582):\n28\n“Trucks are used as the primary vehicle for transport analysis given regulatory concerns that\nconsider LIBs hazardous material in many transportation scenarios39. However, alternative\ntransport like railway can further lower environmental impacts by approximately four times versus\ntrucking (Tables S12), and can be explored for use as aggregation points for long transport\ncombined with trucking.”\nReviewer: Impact allocation method: Could the authors clarify what type of impact allocation\nmethod they used. For instance, was it based on the embodied economic value of the target\nproducts in the recycled or primary pathways?\nOur response: Among commonly tracked sustainability metrics, including environmental metrics,\nfinancial metrics, social metrics, and governance metrics, this study is focused on understanding\nthe environmental impact metrics of LIB recycling. We follow the environmental metrics criteria\nproposed by United Nations’ Sustainable Development Goals and widely accepted by the\ncommunity of life cycle assessment studies, i.e., energy consumption, gas emissions, and water\nconsumption. Target products in the conventional and circular supply chains were based on the\nrecycling operations in Redwood Materials and the GREET model, respectively.\nWe added lines 163–166 and 180–183to better clarify our impact allocation method:\n“Ni and Co in refinement products for subsequent manufacturing can be discrete salts ((1) and (3))\nor mixed compounds (2, 4, and 5). Target products of the conventional and circular pathways were\nbased on the GREET model and practical recycling operations, respectively.”\n“Environmental impacts including eEnergy consumption, greenhouse gas emissions (CO -\n2\nequivalents, CO -eq; additional criteria air pollutants are detailed in Table S1) and water\n2\nconsumption were chosen as the key metrics to analyze environmental impacts of LIB supply chains\nin this study11,16,17.”\nReviewer #2 (Remarks to the Author):\nThe authors have taken a major step to revise the manuscript and some of the previous concerns\nhave been (partially addressed), however, this reviewer remains conservative of this work at this\ncurrent format. A few key questions are summarized for the authors’ consideration:\n1. It is of great concern to keep emphasizing Redwood RC technology and respectively exaggerate one specific company. It is known that RC is not exclusive for Redwood and they are not even the first company doing this. This reviewer understood that some of the data cited in this study are from Redwood, but the more data used for comparative analysis are cited from other literature, sometimes 2019, sometimes 2021, and 2023 crossing different segments, which causes inconsistency and confusion. We must realize that keeping a neutral opinion and be fair is the fundamental importance for academic integrity. Through the whole manuscript, this reviewer has a feeling that the authors are mainly praising a specific tech company which is very much concerned, especially for such a journal.\n2. The first part of the manuscript is more recycling analysis/comparison, and the other half of this\nwork is on other less coherent topics such as electricity. It is understandable that the entire supply\nchain could be compared, however, each segment needs to be tied with the specific process.\nEspecially, the electricity part is something quite loosely attached to the main topic and the\nrelatively random geographic effect on the cost and environmental impact on recycling may not be\ndirectly translated into any useful judgement as the recycling innovation has little to do with the\npower company (their electricity generation process).\n3. It should be noted that mixed hydroxide product provides more value and embedded energy (as\nwell as CO2 emission) compared with mixed metal sulfate. To close the recycling loop, sulfates will\nneed to be processed further with consuming energy and CO2 emission to produced hydroxide\nbefore cathode materials can be made. So the overall analysis of environmental impacts of\n“conventional” and circular refining technologies should be more fairly compared, otherwise the\nconclusion is misleading.\n4. The authors claimed that shipping is a minor factor for cost and CO2, but transport of spent\nbatteries is a different story. The authors still needs to address this point.\n5. The current manuscript, after revision, is an assembly of many pieces and some of them are a\nlittle out of scope for recycling and supply chain of batteries. It gives an impression like a working\nreport instead of a tight/coherent analysis around a central topic. Some of the less relevant context\nmight not be necessary and can be removed to make this manuscript tight and lean.\nReviewer #3 (Remarks to the Author):\nPlease see attached document for comments\nReview Summary\nThis paper presents a comparative life cycle analysis for primary and secondary supply chains for\nlithium-ion batteries. As stated in my previous review, the paper remains well written, the topic is of\ninterest, the core methodology is sound, and outcomes relevant for advancing the conversation\naround designing circular supply chains for lithium-ion batteries. After reviewing the author\nresponses to my comments and updates to the manuscript, I think that:\n1) The authors have adequately addressed the issues and questions I raised and have\nconsequently updated the manuscript.\n2) The updates to the manuscript have added clarity and context for the presented results,\nand clarified the nature and uniqueness of the main contributions from this work.\n3) While I understand the sensitivities around industrial data, it remains a pity that this\nconstrains transparency and limits the ability for benchmarking across different analyses,\nas well as drawing certain broader conclusions from reported outcomes. That said, I agree\nthat this represents a major step in the right direction.\n4) I believe this study makes an important contribution to foundational conversations around\ncircular supply chains for battery. There is certainly more work required in terms of\ninforming policy and strategy for incentivizing circular battery supply chains. It will be good\nto see analyses that incorporate adequate projected scale of circular supply chains, as well\nas economics of battery recycling, and quantitatively assess the potential impact\n(environmental, economic, social) of any number of policy interventions (e.g., producer\nresponsibility, collection programs) and strategies (e.g. design for efficient disassembly,\ndisaggregated siting of facilities for different recycling steps).\n5) I recommend this submission for publication.\nMinor comment\nLines 524 – 527: “… optimal environmental impacts and product formats will elevate costs.”\n1) Is this demonstrably the case, or simply intuition based on likely design/operational\ninterventions? I imagine certain adjustments to process conditions imply\ncost/environmental impact trade-offs, but it is not necessarily general.\nResponses to Reviewers\nManuscript Title: Life cycle comparison of industrial-scale lithium-ion battery recycling and\nmining supply chains\nManuscript ID: NCOMMS-23-46463A\nIn this document, we respond point-by-point to comments from the reviewers and the editor on\nour original manuscript submission. We thank the reviewers for their insightful feedback, and have\nincorporated several of their suggestions to further improve the manuscript before publication. The\nlegend for this document is as follows:\nReviewer comments are in italics.\nAuthor responses are in standard type and indented.\nDeletion and addition to the original manuscript are marked in red and blue texts, respectively.\nLine numbers are specified for either the original manuscript or the revised manuscript.\n\n REVIEWER 2\nReviewer: The authors have taken a major step to revise the manuscript and some of the\nprevious concerns have been (partially addressed), however, this reviewer remains\nconservative of this work at this current format. A few key questions are summarized for the\nauthors’ consideration:\nOur response: We are grateful to the reviewer for the favorable comments on our revision\nefforts, and have provided further improvements to address the questions raised. Specific\nresponses are provided below and major changes are:\ni. updating the data for environmental impacts of electricity to enhance the reference\nconsistency, and inclusion of a summary of types of data use;\nii. clarifying that the RC technology is not exclusive to Redwood, but has comprehensive\nimplications for carbothermal reduction-based pyrometallurgy at industrial scale;\niii. highlighting the focus and structure of result presentation, sharpening the discussion,\nand enhancing the readability of the manuscript.\nReviewer: 1. It is of great concern to keep emphasizing Redwood RC technology and\nrespectively exaggerate one specific company. It is known that RC is not exclusive for Redwood\nand they are not even the first company doing this. This reviewer understood that some of the\ndata cited in this study are from Redwood, but the more data used for comparative analysis\nare cited from other literature, sometimes 2019, sometimes 2021, and 2023 crossing different\nsegments, which causes inconsistency and confusion. We must realize that keeping a neutral\nopinion and be fair is the fundamental importance for academic integrity. Through the whole\nmanuscript, this reviewer has a feeling that the authors are mainly praising a specific tech\ncompany which is very much concerned, especially for such a journal.\nnot intend, nor feel obliged to, specially endorse the RC technology of Redwood. RC was studied\nas a representative carbothermal reduction-based pyrometallurgy with critical industrial-scale data.\nWe are well aware of the development and applications of carbothermal reduction elsewhere, and\nintentionally did not claim RC is exclusive to Redwood. However, to the best of our knowledge,\nwe did notice that 1) most of the existing studies on carbothermal reduction pyrometallurgy were\nbased on lab scale, and 2) only very few companies reported the practical installation and operation,\nand often without accessible data for quantitative evaluations. We politely point out that analyzing\nthe RC process at Redwood is of critical importance for the whole area of battery recycling. In\naddition, we find it most rigorous and accurate to only comment on the industrial data that we\ncould access, which was from Redwood’s RC process. Otherwise, we would risk over-generalizing\nour LCA results to other pyrometallurgical installations that may exhibit different environmental\nimpacts.\nTherefore, we highlighted the insights from the analysis for the environmental feasibility of\nindustrial-scale carbothermal reduction-based pyrometallurgy (lines 269–271 and 279–283),\nrather than the RC of Redwood.\n“Similar to RC, emerging oxygen-free pyrometallurgical processes rely on carbothermic and\nthermite reduction for recycling cathode metal compounds at moderate temperatures (600–\n1000℃)19-21.”\n“While this analysis was focused on Redwood Materials refinement pathways, the methodology\ncan be used to evaluate additional refinement pathways (e.g., analysis of the a representative\nhydrometallurgy was detailed in Fig. S3c), or others that use different material feedstocks,\nrefinement processes, and energy supplies.”\nTo reinforce the comprehensive importance of our study, we have better highlighted the category\nof pyrometallurgy that the “reductive calcination” (RC) process belongs to. We also clarified our\nuse of “Redwood” as the data source, instead of a specific name of the methods (e.g., RC+Me+Hy)\n(lines 254–258, 267–269, 271–276, 320–327, 355–358, 485–487, 511–513, 528–530):\n“To produce battery-grade cathode materials, Redwood Materials used a combination of reductive\ncalcination (RC) pyrometallurgical, mechanical (Me), and hydrometallurgical (Hy) LIB refinement\nprocesses (pathways detailed in Fig. S2). RC is an innovative industrial-scale exothermic\npyrometallurgical process that reduces the cathode metal oxide compounds under oxygen-free\nconditions for subsequent refinement.”\n“In general, the RC+Me+Hy pathway at Redwood exhibited comparable energy consumption and\nCO -eq emissions with Hy and Direct literature values11, and substantially lower environmental\n2\nimpacts than Py*.”\n“While most of the carbothermic and thermite reduction processes were investigated at the lab\nscale24-26, Our our results showed that the RC step accounted for 5.5%–7.47.5% of the total\nenvironmental impacts in Redwood the circular refinement (Fig. 3), demonstrating the\nenvironmental feasibility of industrial-scale carbothermal reduction-based pyrometallurgy. Note\nthat traditional pyrometallurgy and Redwood Material’s reductive calcination RC pyrometallurgy\ncan process energized batteries of varying states of charge, health, and formats with minimal\nmodification…”\n2\n“Circular pathways refining batteries to different products were analyzed using the Redwood data\nby the RC+Me+Hy process data and the modeling of a representative battery recycling method\ncombining mechanical and hydrometallurgy (Me+Hy, analogous to the Hy* in Fig. 3) refinement\n(Method (3) in Fig. 2). The Redwood process RC+Me+Hy pathway refines recycled batteries to\nmixed metal sulfate, (Ni,Co)SO , whereas the representative Me+Hy produces discrete NiSO and\n4 4\nCoSO . The RC pathway (RC+Me+Hy) Refining into mixed metal sulfate exhibited lower energy\n4\nconsumption (−72.3%), CO -eq emissions (−39.5 41.4%), and water consumption (−12 8.0%) than\n2\nthe Me+Hy pathway (Fig. 4)…”\n“Unlike conventional pyrometallurgical processes that require external energy sources11，23, the\nRC process pyrometallurgy is primarily autothermic because it leverages process heat released\nfrom exothermic reactions of the LIB materials30,31.”\n“Circular production of LCO-grade materials led to higher environmental impacts than that of\nNCA-grade materials based on the mixed-stream feedstock composition processed by Redwood\nMaterials analyzed in this study.”\n“Disaggregated analysis of LIB refinement pathways at Redwood Materials using industrial data\nprovided important insights into the performance and potential of different refinement processes.”\n“The alternative direct recycling technology is reported to exhibit comparable environmental\nimpacts to Redwood Materials circular refining methods in this study12, but warrants further\nassessment after industrial-scale implementation.”\nWe appreciate the reviewer for pointing out the possible confusion due to the different reference\nyears of data. The reference years were chosen based on the accessibility of prioritized data (i.e.,\nthe operational data at Redwood) and information from the literature. Note that the varied years of\ndatasets further supports the utility of the submitted manuscript in standardizing and updating the\nbattery recycling LCA literature. Overall, all data for analysis presented in the main manuscript\nwere based on 2021. 2019 data were obtained from the literature for comparison, and 2023-based\nresults were presented as Supplementary Data as requested by the reviewers in the previous\nsubmission. We have updated the data of electricity-embedded environmental impacts and\nmodeling information to 2021 in the revised manuscript to enhance the consistency. Note that no\nsubstantial changes were observed to the conclusive remarks. Thus, the only datasets in the revised\nmanuscript that are not 2021 are 1) the literature data, which we cannot control and 2) the 2023\nGREET model as requested by the reviewer (referenced in supplemental information). To help\nreaders catalogue these differences, we also added an additional Table S16 to summarize the\nsources and use of data from different reference years, mentioned in lines 174–175.\n“Unless specifically noted, all major analyses were based on 2021 data (data reference years were\nsummarized in Table S16).”\n“Table S16. Summary of sources from different reference years and how they were used in this\nstudy. Data uses were categorized into three types to represent analysis result in the main\nmanuscript (type I), literature data comparison in the main manuscript (type II), and supplementary\ninformation for additional reference (type III).”\n# Supply Data Source for Presentation Reference Presentation Type\nChain Step Year of Data of Use\nRefinement\nanalyzing circular refinement\n3\nfor analyzing circular refinement\nconventional refinement\nconventional refinement\nrefinement technologies, for\ncomparison\nFigs. 3–6 in the revised main manuscript were updated with 2021-based environmental impacts of\nelectricity:\nFig. 4\nReviewer: 2. The first part of the manuscript is more recycling analysis/comparison, and the\nother half of this work is on other less coherent topics such as electricity. It is understandable\nthat the entire supply chain could be compared, however, each segment needs to be tied with\nthe specific process. Especially, the electricity part is something quite loosely attached to the\nmain topic and the relatively random geographic effect on the cost and environmental impact\non recycling may not be directly translated into any useful judgement as the recycling\ninnovation has little to do with the power company (their electricity generation process).\nOur response: We politely note that the whole manuscript is explicitly centered around\nunderstanding environmental impacts of battery recycling and conventional mining, with the\nrefinement step as our focus. In the Results section, analyses of different aspects of refinement\nwere first presented, followed by modeling on the upstream steps and overall considerations of the\nwhole supply chain. Centering around the environmental impacts of refinement, we presented 1)\n7\nanalysis pathways (Fig. 2), 2) overall environmental impacts of the refinement step (Fig. 3), 3)\nimpacts of product formats on the refinement step (Fig. 4), and 4) contributions of different input\nmaterials (including electricity) to the refinement environment impacts. All the four aspects are\ncritical to understanding the environmental impacts of the refinement step, placed logically, and\nexplicitly reflected by the subsection titles in the main manuscript (lines 185–186, 301, and 342–\n343):\n“Refining lithium-ion batteries into battery-grade materials exhibited lower environmental\nimpacts than production from mined natural materials.”\n“Formats of refinement products influenced environmental impacts.”\n“Electricity consumption dominated the environmental impacts of lithium-ion battery circular\nrefinement.”\nThe electricity section served as a further identification of the major contributor to our main topic,\nrefinement environmental impacts, and provided a systematic investigation of the influences of\npower sources (lines 348–351). We also politely note that varying the electricity sources is not\npresenting the relatively random geographic effects, but used representative electricity grids to\nform a gradient of CO -eq intensities (lines 389–391).\n2\n“Electricity consumption was found to be a principal factor dominating the environmental impacts.\nFor both LIB feedstock pathways (Methods (4) and (5) in Fig. 2), electricity accounted for 70.3–\n91.0% of the total energy consumption, 71.8–79.1 70.5–83.4% of the total CO -eq emissions,\n2\nand 54.3–63.6 56.9–66.1% of water consumption (Fig. 5a).”\n“Because electricity dominated the environmental impacts of LIB recycling processes, we\ncompared several electricity grid balancing areas that emit a range of CO -eq emissions per MWh\n2\n(averaged for 2019 2021)32-34 in Fig. 5b…”\nBelow we tabulate the electricity grid areas used in this study following an order of increasing\nembedded CO -eq, for the reviewer’s reference. While NEVP is where Redwood based on\n2\n(highlighted), electricity grid areas of both lower (BPAT and CISO) and higher (WACM)\nembedded emissions were used in our methodology.\nElectricity Embedded CO -eq Normalized\nBPAT 92.8 0.2\nCISO 275.8 0.6\nNEVP 455.9 1.0\nWACM 682.0 1.5\nOur analyses on electricity were not only closely attached to the main topic, but also provided\nimportant actionable guidance to battery recycling companies. Although advice on power\ngeneration is beyond the scope of this study, our results showed that locating the recycling facility\nand choosing electricity sources is critical to reducing and balancing environmental impacts\nof the circular refinement step in battery recycling (lines 392–396, 545–549, and 585–588).\nelectricity to electricity from Nevada renewable energy tariff (NV*) to optimize their CO -eq\n2\nemission. Further, our results also inform other recycling companies about which part of their\nenvironmental impacts are based on electricity source versus other consumables (both for RC\npyrometallurgy and for hydrometallurgy for energized, end-of-life batteries). By segmenting\nemissions sources, recycling companies can focus on both electricity source and innovate on\ntechnologies and inputs to further reduce their environmental impacts.\n“Substituting NEVP electricity with other balancing areas including Bonneville Power Administration\nTransmission (BPAT), California Independent System Operator (CISO), Western Area Power\nAdministration of Colorado-Missouri (WACM), and a 100% renewable energy tariff in Nevada (NV*),\nyielded a significant reduction in CO -eq emissions of up to 93.3% (recycled scrap) and 87.4% (recycled\n2\nbattery) relative to conventional refinement (Fig. 5b).”\n“Electricity greatly influenced environmental impacts in LIB circular refinement, and the\nvariability among grid electricity sources elucidated a tradeoff between CO -eq emissions and\n2\nwater consumption (Fig. 5). Therefore, considering water consumption and CO -eq emissions is\n2\nnecessary for selecting recycling facility locations, particularly in water-sensitive or emissions-\nsensitive scenarios.”\n“As the prevalence of LIBs grows in the mobility sector and beyond, strategic placement of\ndomestic LIB collection, refinement and manufacturing facilities can further minimize future\nenvironmental impacts by considering heterogenous LIB growth by location, collection approach,\ntransportation distance, and electricity source for refinement processes.”\nOverall, we believe the analyses of electricity were highly necessary and germane to the main\ntopic. We have added lines 122, 132–134, 158–159, 171–175, 301–302, and 343–344 to the\nrevised Introduction to better overview the coherence and organization of the results:\n“In this study we quantified the cradle-to-gate environmental impacts of battery-grade cathode\nmaterial salts manufactured in conventional and circular supply chains across three major steps:\nmaterial extraction, transport, and refinement (Fig. 1b), focusing on the refinement step.”\n“Second, for the first time we This use of industrial-scale recycling quantitatively identified the\nroles dominant role of input grid electricity in circular refinement at the balancing area level on\ncomprehensive environmental impact metrics based on industrial-scale LIB recycling data.”\n“In this study, analyses of environmental impacts were presented with a focus on the refinement\nstep, followed by analysis of the upstream steps in the complete LIB supply chains. In LIB supply\nchains, the refinement step converts the collected feedstocks into battery-grade salts for further\nmanufacturing (Fig. 2a).”\n“Target products of the conventional and circular pathways were based on the GREET model and\npractical recycling operations, respectively. In the following sections, the overall refining\nenvironmental impacts were first analyzed, followed by influences of product formats on the\nrefinement step and key contributors to the refining environmental impacts. Lastly, upstream\nenvironmental impacts were analyzed and compared to the refinement step. Unless specifically\nnoted, all major analyses were based on 2021 data (data reference years were summarized in Table\nS16).”\n“Product format is an important factor in understanding and properly comparing LIB refinement\npathways (Fig. 2). Ni and Co are key elements for battery manufacturing, and can be traded in the\nformat of mixed metal salts or discrete salt products between battery refiners and battery\nmanufacturers28,29. ”\n“To further understand the performance limiting factors in the refinement step, The the relative\nenvironmental impacts of input consumables (e.g., energy, water, commodity chemicals) in the\ngate-to-gate refinement processes were disaggregated in Fig. 5 (additional criteria air pollutants\nin Tables S2–S3, Figs. S4–S5).”\n“Before the refinement step, LIBs undergo the Upstream of gate-to-gate refinement are upstream\nare material extraction and transport to refinement facilities (Fig. 1b).”\n“Various important aspects of the environmental impacts in the refinement step were focused on\nand Practical LIB feedstock and refinement pathways were analyzed using practical data from a\nrecycling company (Redwood Materials), and modeling was employed to examine the\nenvironmental impacts of upstream material extraction and transport steps.”\nWhile the current cradle-to-gate study is focused on Li, Ni, and Co as the major output materials,\nthe potential benefits of extracting additional LIB constitutive elements from ore (e.g., Cu and Co\nin Cu-Co sulfides) or from LIBs (e.g., Cu or Mn) warrants further investigation. Additionally, the\nsame mixed-stream LIB feedstocks consumed at Redwood Materials were used to quantify NCA-\nand LCO-equivalent values, and results would vary for single-stream LIB feedstocks. Generally,\nthe incremental benefits of extracting additional critical materials from concentrated sources like\nLIBs can offset the environmental impacts of both supply chains.”\nReviewer: 3. It should be noted that mixed hydroxide product provides more value and\nembedded energy (as well as CO emission) compared with mixed metal sulfate. To close the\n2\nrecycling loop, sulfates will need to be processed further with consuming energy and CO\n2\nemission to produced hydroxide before cathode materials can be made. So the overall analysis\nof environmental impacts of “conventional” and circular refining technologies should be more\nfairly compared, otherwise the conclusion is misleading.\nOur response: We politely note that our analysis on the refinement step is focused on production\nof battery-grade cathode materials, i.e., the precursors, for the supply chains, instead of the ready-\nto-use materials for direct use battery manufacturing. We have pointed this in lines 140–144 and\n159–161 of the manuscript:\n“The upstream assessment includes the extraction of LIB material from conventional (i.e., mined\nore) or circular (i.e., collected batteries) sources and the transport of extracted material to relevant\nrefinement facilities for production of battery-grade cathode materials as Li, Co, and Ni sulfate\nor carbonate salts.\n“In LIB supply chains, the refinement step converts the collected feedstocks into battery-grade salts\nfor further manufacturing (Fig. 2a).\nBattery-grade metal sulfates were produced at industrial scale and sold at Redwood Materials, and\nwere also reported to be commonly produced and collected in the emerging market of battery\nrecycling1-5. We added lines 249–250 in the revised manuscript to further highlight the important\nroles of battery-grade metal sulfates in the battery supply chains:\n“Note that while the elemental stoichiometry was identical, the output battery-grade materials\nvaried slightly between conventional (Li CO , NiSO , CoSO ) and circular (Li SO , (Ni, Co)SO )\n2 3 4 4 2 4 4\nsupply chains (detailed in Methods). Metal sulfates are commonly produced and traded in the\nbattery recycling market20-23.”\nIn our comparison, both circular and conventional refinements were based on producing sulfates,\nand this has been summarized in Fig. 2 in the main manuscript (pathway 1, 4, and 5). We have\nclearly stated our methods, and we believe that the comparison is overall fair and rigorous.\nfocus on producing battery-grade precursors instead of ready-to-use manufacturing materials:\n“The chemical formats of output products differ between the conventional and circular supply\nchains, but converting them to the same products would not substantially change the results due to\nthe similarity between the cathode salts of the two supply chains (Note 3 in Supplementary\nInformation). Battery-grade cathode metal sulfates were chosen as the refinement products for the\nmajor comparison, differing from the ready-to-use materials for manufacturing, but not\nsubstantially influencing the fairness of the comparison.”\nReferences\nComparison between Pyrometallurgical and Hydrometallurgical Battery Recycling Flowsheets. Metals\n13, 1915 (2023).\nProcess. RecycLiCo Battery Materials (2021).\nReviewer: 4. The authors claimed that shipping is a minor factor for cost and CO , but\n2\ntransport of spent batteries is a different story. The authors still needs to address this point.\nOur response: In our last round of revision we have incorporated the suggestions from the\nreviewer (now lines 590–593 and 598–599), emphasizing the importance of the considerations\nconcerning safety and additional costs in transporting spent batteries:\n“Business models for collection of all LIB types and sizes will likely vary from manufacturer-led\nto municipal or private collection programs, and can be influenced by the safety costs while\ncollecting used end-of-life batteries as hazardous wastes.”\n“Recyclable battery packing Employing reusable battery packs can save reduce refinement\nenergy and chemical inputs used in refinement, and manufacturing designing battery cells and\n11\nmodules favorable for extraction and integration will lower the environmental impacts of the\nupstream steps.”\nWe well understood the reviewer’s concern, but note that the current study is focused on environmental impacts,\nwith emphasis on the refinement step. Examining cost considerations in the circular battery supply chain warrants\nadditional investigations and is beyond the scope of the current study.\nWe also note that the referenced transport embedded CO -eq can significantly vary due to the\n2\ndifferent power sources employed, as we analyzed in Fig. 5c in the manuscript, which likely\npartially explains the different results from other ongoing studies.\nReviewer: 5. The current manuscript, after revision, is an assembly of many pieces and some\nof them are a little out of scope for recycling and supply chain of batteries. It gives an\nimpression like a working report instead of a tight/coherent analysis around a central topic.\nSome of the less relevant context might not be necessary and can be removed to make this\nmanuscript tight and lean.\nOur response: We thank the reviewer for suggestions on improving the readability of the\nmanuscript, and provide reconciled responses and revisions based on the similar Question 2 by the\nreviewer. We politely argue that the presentations were organized around our central topic. In our\nstudy, we conducted life cycle comparison of industrial-scale lithium-ion battery recycling and\nmining supply chains. While the whole supply chains for battery manufacturing include refinement,\nextraction, and transport, we focused on the environmental impacts of the refinement step and\npresented our major analyses around it, investigating different important aspects of refinement.\nWe subsequently presented studies on the complementary upstream steps (i.e., extraction and\ntransport), and considerations on the supply chains.\nTo better outline our narrative, we summarized the organization of Results section in the\nmanuscript for the reference of the reviewer:\nParagraph Title or main contents Data Relevance to the main topic\nnumber presentation\nanalysis description of analysis pathways\n2–5 “Refining lithium-ion batteries Fig. 3 Refinement step: presenting\ninto battery-grade materials overall environmental impacts\nexhibited lower environmental\nimpacts than production from\nmined natural materials”\n6–8 “Formats of refinement products Fig. 4 Refinement step: comparing\ninfluenced environmental product formats, informing the\nimpacts” fair product formats for\nfollowing analyses\n12\n9–10 “Electricity consumption Fig. 5 Refinement step: prioritizing\ndominated the environmental contributions to the refinement\nimpacts of lithium-ion battery step\ncircular refinement”\n11–12 “Environmental impacts of Fig. 6 Upstream steps (beyond\nmaterial extraction and transport refinement): modeling the\nwere significantly lower in extraction and transport steps\ncircular lithium-ion battery supply\nchains than in conventional supply\nchains”\n13 “The refinement step dominated Fig. 6 Refinement + upstream steps:\nenvironmental impacts of circular further analyzing the roles of\nand conventional supply chains” refinement in the whole supply\nchains\nTo further improve the conciseness and coherence, we revised Introduction (lines 54–67, 121–122,\nand 132–134), Results (lines 158–159, 171–175, 301–302, 343–344, and 413–414), and\nDiscussion (lines 501–503, and 578–584) in the new manuscript:\n“Factors central to the success of recycling include the ease cost of collecting products, the cost of\nrecycling processes, and the economic value of recovered materials. The average embodied\neconomic values of representative LIBs between 2018–2021 are shown in Fig. 1a (complete\nreferences are listed in Supplementary Information). Considering in LIBs LIB prices between\n2018–2021, Li, Ni, and Co comprise the highest embodied economic value (Fig. 1a, complete\nreferences are listed in Supplementary Information), and Al and Cu account for a significant\nweight percentage of EV battery packs (approximately 25%)5. While 99% of the lead-acid batteries\nare recycled in the U.S., LIBs exhibit Despite an embodied economic value that is 2–10 times higher\neconomic values but are compared to the lead in lead-acid batteries, only recycled 2–47% of LIBs\nare recycled globally6, compared to 99% for lead-acid batteries in the U.S. Regardless, the\nuntapped potential of The environmental benefits of circularity also strongly motivate LIB\nrecycling constitutes a significant economic and given the vast LIB production and emission-\nintensive mining refinement for key constituent metals. There is a critical need to evaluate the\nenvironmental opportunity that requires evaluation across several application scales…”\n“In this study we quantified the cradle-to-gate environmental impacts of battery-grade cathode\nmaterial salts manufactured in conventional and circular supply chains across three major steps:\nmaterial extraction, transport, and refinement (Fig. 1b), focusing on the refinement step.”\n“Second, for the first time we This use of industrial-scale recycling quantitatively identified the\nroles dominant role of input grid electricity in circular refinement at the balancing area level on\ncomprehensive environmental impact metrics based on industrial-scale LIB recycling data.”\n“In this study, analyses of environmental impacts were presented with a focus on the refinement\nstep, followed by analysis of the upstream material extraction and transport steps. In LIB supply\nchains, the refinement step converts the collected feedstocks into battery-grade salts for further\nmanufacturing (Fig. 2a).”\n13\n“Target products of the conventional and circular pathways were based on the GREET model and\npractical recycling operations, respectively. In the following sections, the overall refining\nenvironmental impacts were first analyzed, followed by influences of product formats on the\nrefinement step and key contributors to the refining environmental impacts. Finally, upstream\nenvironmental impacts were analyzed, and compared to the refinement step. Unless specifically\nnoted, all major analyses were based on 2021 data (data reference years were summarized in Table\nS16).”\n“Product format is an important factor in understanding and properly comparing LIB refinement\npathways (Fig. 2). Ni and Co are key elements for battery manufacturing, and can be traded in the\nformat of mixed metal salts or discrete salt products between battery refiners and battery\nmanufacturers28,29. ”\n“To further understand the performance limiting factors in the refinement step, The the relative\nenvironmental impacts of input consumables (e.g., energy, water, commodity chemicals) in the\ngate-to-gate refinement processes were disaggregated in Fig. 5 (additional criteria air pollutants\nin Tables S2–S3, Figs. S4–S5).”\n“Before the refinement step, LIBs undergo the Upstream of gate-to-gate refinement are upstream\nare material extraction and transport to refinement facilities (Fig. 1b).”\n“Various important aspects of the environmental impacts in the refinement step were focused on\nand Practical LIB feedstock and refinement pathways were analyzed using practical data from a\nrecycling company (Redwood Materials), and modeling was employed to examine the\nenvironmental impacts of upstream material extraction and transport steps.”\nWhile the current cradle-to-gate study is focused on Li, Ni, and Co as the major output materials,\nthe potential benefits of extracting additional LIB constitutive elements from ore (e.g., Cu and Co\nin Cu-Co sulfides) or from LIBs (e.g., Cu or Mn) warrants further investigation. Additionally, the\nsame mixed-stream LIB feedstocks consumed at Redwood Materials were used to quantify NCA-\nand LCO-equivalent values, and results would vary for single-stream LIB feedstocks. Generally,\nthe incremental benefits of extracting additional critical materials from concentrated sources like\nLIBs can offset the environmental impacts of both supply chains.”\n\n REVIEWER 3\nReviewer: Review Summary: This paper presents a comparative life cycle analysis for\nprimary and secondary supply chains for lithium-ion batteries. As stated in my previous review,\nthe paper remains well written, the topic is of interest, the core methodology is sound, and\noutcomes relevant for advancing the conversation around designing circular supply chains for\nlithium-ion batteries.\nOur response: We thank the reviewer for the favorable reception of the revised manuscript.\nReviewer: After reviewing the author responses to my comments and updates to the\nmanuscript, I think that:\n1) The authors have adequately addressed the issues and questions I raised and have\nconsequently updated the manuscript.\n2) The updates to the manuscript have added clarity and context for the presented results, and\nclarified the nature and uniqueness of the main contributions from this work.\n3) While I understand the sensitivities around industrial data, it remains a pity that this\nconstrains transparency and limits the ability for benchmarking across different analyses, as\nwell as drawing certain broader conclusions from reported outcomes. That said, I agree that\nthis represents a major step in the right direction.\n4) I believe this study makes an important contribution to foundational conversations around\ncircular supply chains for battery. There is certainly more work required in terms of informing\npolicy and strategy for incentivizing circular battery supply chains. It will be good to see\nanalyses that incorporate adequate projected scale of circular supply chains, as well as\neconomics of battery recycling, and quantitatively assess the potential impact (environmental,\neconomic, social) of any number of policy interventions (e.g., producer responsibility,\ncollection programs) and strategies (e.g. design for efficient disassembly, disaggregated siting\nof facilities for different recycling steps).\n5) I recommend this submission for publication.\nOur response: We are grateful to the reviewer for the positive evaluations and recommendation.\nWe also agree that additional work on policy and strategies are warranted to facilitate circular\nbattery supply chains, and hope that our study can pave the path for more comprehensive and in-\ndepth analyses for the rapidly growing battery recycling sector.\nReviewer: Minor comment: Lines 524 – 527: “... optimal environmental impacts and product formats will elevate costs.”\n15\n1) Is this demonstrably the case, or simply intuition based on likely design/operational\ninterventions? I imagine certain adjustments to process conditions imply\ncost/environmental impact trade-offs, but it is not necessarily general.\nOur response: We thank the reviewer for helping examine the rigorousness of the expression.\nWhile most of the existing literature on carbothermal reduction is based on lab-scale,\nunderstanding of the potential costs for optimizing process conditions is still lacking. We have\naccordingly revised our remarks (now lines 519–522) to:\n“Our findings showed the promise of carbothermal reduction-based pyrometallurgy for\npretreatment of LIB recycling; however, we note that additional work is warranted in controlling\nprocess conditions to achieve optimal environmental impacts and product formats without willl\nelevate elevating the costs.”\nReviewer #2 (Remarks to the Author):\nThe authors have done a decent job in addressing my concerns and I would support the manuscript\nto be accepted at this version.\nReviewer #3 (Remarks to the Author):\nMy earlier review comments had highlighted perceived limitations in the original draft which I\nbelieve were adequately addressed in the revision. I have provided a minor comment below for\nupdating the abstract.\nMinor Comments - Abstract: instead of stating in general terms what this study contributes - \"..\nprovides important insights for advancing sustainable LIB supply chains.... informs optimization....\",\nthe Abstract should be explicit on what those key contributions are - e.g. what insights does it\nprovide? what are the big-picture implications of the study outcomes.\n5) I recommend this submission for publication."}]

Decision letter