Hygroscopicity-driven spontaneous sustainable direct lithium extraction

Domain: biology
Published: 2026-03-17
Editorial outcome: major_revision

Abstract
Current lithium extraction processes demand high inputs of energy, chemicals, and freshwater because they rely on non‐spontaneous separation steps. To address these limitations, we introduce a spontaneous, hygroscopicity-driven direct lithium extraction method that leverages the unique entropy-increasing deliquescent behavior of lithium chloride hydrate (LHT) to extract and enrich lithium from solid deposits such as brine mining slags. At controlled relative humidities (12–30% RH), crystalline LHT selectively uptakes water moisture to form a lithium-enriched concentrate, while co‐occurring salts (halite, sylvite, bischofite, gypsum) remain in the solid phase. This phase‐selective deliquescence enables rapid solid-liquid separation without external water, reagents, or heating. By optimizing humidity and moisture flux, we achieve swift lithium recovery of up to 96% with concentrations reaching 97,000 ppm, far exceeding industrial grade requirements. We further validate the method’s robustness across binary and multicomponent mixtures, as well as actual slag samples under mining conditions. The results demonstrate competitive performances over a wide range of feeding ratios and seasonal changes, and the extraction times only take minutes to hours as compared to months. This entropy-increasing, ambient-temperature modular process minimizes energy, chemical, and water use, and it offers broad resource tolerance for lithium extraction and mining waste valorization.

Introduction
Lithium (Li) is an essential element for batteries in electric vehicles, renewable energy storage, robots and many electronics1-5. The global Li demand is projected to triple by 2030, relative to the recent two years (2023-2025)6,7, reaching above 2.5 million metric tons (Mt) of Li CO equivalent (LCE)7,8; while current production remains below 0.6 Mt per year9, creating a significant supply shortfall10. This gap cannot be offset by battery recycling alone but will be further exacerbated by the rising demand for large-scale energy storage, which is estimated to exceed 1.5 terawatt-hours by 2030 11-13. Current commercial Li production relies on solar evaporation of brine or mineral processing of hard rock ores. Brine evaporation is low-cost but slow (typically > 12 months) and water-intensive14,15, while ore mining is faster but at the expense of high energy intensity, CO emission, and tailing generation16. Emerging direct Li extraction (DLE) technologies promise high efficiency and low environmental impact17,18, but to date they have struggled to scale from laboratory to field applications due to high capital and operational costs, as well as technical uncertainties19,20.

The high cost and environmental impact associated with intensive energy, water, and chemical use by current technologies are largely due to the complexity of natural Li sources, which contain high concentrations of competing elements such as Na, K, Mg and Ca15. The essence of Li extraction is to harness the unique properties of Li against other elements to achieve separation. For example, the solvation energy21, ionic size22, charge density23, and solubility24 of Li have been widely explored to extract Li through ion exchange25, ion sieving26,27, and phase transformation mechanisms28. However, most of these methods are limited to Li separation from one specific kind of competing ions, such as tailored separation from either Na or Mg, which limits their applicability when multiple cations co-exist. The reason is rooted from the divergence in the trend of Li properties compared to different competing ions. For example, the hydration free energy of Li follows Mg > Ca > Li in divalent elements but Li > Na > K among the monovalent ions. Hence, adsorbents or membranes optimized for one pair often underperform against others20.

Current mainstream Li extraction relies on either non-phase-transformation approaches such as electrochemical methods29, adsorption30, nanofiltration26, solvent extraction31, or aqueous-to-solid transformations such as evaporitic precipitation32. The fundamental principle of these techniques lies in redistributing the originally disordered ion mixtures to more ordered states so that Li can be selectively transported or retained for separation. Consequently, these processes are inherently non-spontaneous and entropy-reducing. Overcoming these thermodynamic barriers demands intensive inputs of water, energy, or chemicals and therefore leads to long processing times, high operational costs, and heavy reliance on specialized materials. When producing one metric ton of Li CO , conventional mining consumes up to 60 m3 of freshwater, 2.9 tons of chemicals and 1.8 MWh of electricity33. To surmount these limitations, technologies must both harness inherently spontaneous behaviors of Li and deliver robust separation performance across the diverse chemistries of real‐world Li resources.

In this work, we introduce a sustainable, spontaneous method that has potential for extracting and enriching Li from a broad spectrum of solid resources, such as slag deposits from both brine- and ore-based mining as well as unconventional feedstocks such as spent battery electrodes and pharmaceutical wastes. In particular, industrial brine mining generates Li-bearing slags containing up to ~1 wt% Li (Fig. 1), yet these residues remain unexploited. We show that among the suite of constituent minerals that include LiCl hydrate (LHT), halite (NaCl), sylvite (KCl), bischofite (MgCl ·6H O), and gypsum (CaSO ·2H O), LHT stands out for its uniquely strong hygroscopicity. CaCl -based minerals are absent from the initial mineral mixtures because Ca occurs primarily as gypsum due to its low solubility and thermodynamic stability34-36. However, to assess the broader applicability of the hygroscopicity-driven separation mechanism, additional experiments involving CaCl are performed and discussed in Supplementary Method 1, Supplementary Note 1 and Supplementary Fig. 1. By selectively leveraging rapid moisture uptake, our process enables LHT to deliquesce under mild conditions, allowing facile separation of Li from less hygroscopic phases and thereby driving an energy-efficient, water-minimal extraction route. Different from other processes, this presented method is entropy-increasing and can effectively reduce year-long passive leaching into a few-hour extraction, producing a Li-concentrated solution that can be collected via simple filtration, while less hygroscopic phases remain solid. Here, we use “entropy-increasing” to describe the deliquescence-driven LiCl extraction step and do not imply a net entropy increase over downstream Li upgrading from LiCl to Li CO or LiOH, which would involve entropy-reducing crystallization and precipitation. Even from very low-grade feedstocks, this approach achieves over three orders of magnitude increase in Li purity. This process has low consumptions of freshwater, energy, and chemicals for Li extraction. It enables circular upgrading and valorization of industrial residues, mitigates the environmental impact of Li‐bearing wastes to landfills and waterways, and provides a low-carbon complement to conventional extraction methods.

Results
Humidity-controlled Li hygroscopic deliquescence recovery
To examine the hypothesis that Li can be effectively separated and recovered from mixed mineral deposits using hygroscopic deliquescence, we first quantify the stepwise moisture uptake in LHT and four commonly co-existing minerals, including bischofite, halite, sylvite and gypsum, by exposing each solid mineral at controlled relative humidity (RH) from 12% to 96% in 6% increments (1 h per step). As shown in Fig. 2a, LHT exhibits measurable moisture uptake at RH as low as 12%, and the accumulative adsorption increases from 0.006 to 3.11 g H O g-1 LHT. In contrast, bischofite loses water at low RH (-0.0097 g H O g-1 bischofite at RH 12%) and only begins to adsorb moisture above RH ~30%, with the total adsorption amount around 1.09 g H O g-1 bischofite across the entire moisture spectrum. Halite, sylvite, and gypsum remain essentially inert until reaching their limiting RH values of ~78%, ~90%, and >96%, respectively. From these data, we identify each mineral’s limiting RH and order them as LHT < bischofite << halite < sylvite < gypsum (Fig. 2b).

Because selective Li extraction requires LHT to dissolve while co-minerals remain solid, we designate RH 12-30% as the effective separation zone (Fig. 2bS). To account for practical RH control variability, this zone is subdivided into three subzonesS: 12-18%, 18-24%, and 24-30%. In batch experiments of individual minerals, LHT yields progressively larger solution volumes in the higher RH subzone of 24-30% (Fig. 2c and 2d). All other minerals remain solid across these subzones. This demonstrates that ambient RH alone can drive selective dissolution of LHT, while preserving co-minerals in solids.

The essence of moisture uptake is aggregating free moisture from the ambient into clusters, which then dissolve LHT into collectable liquids. Built on this validated hypothesis, we evaluate the thermodynamics of moisture uptake in LHT by calculating the instantaneous free‐energy change ΔG=θRTlna , where θ is the equilibrium moisture uptake, a is the water activity (equal to the numerical value of RH), and RTlna is the chemical potential of moisture equilibrated between the liquid and gaseous phases. More details can be found in Methods. Fig. 2e shows the LHT moisture uptake isotherm, and Fig. 2f illustrates ΔG against θ. Increasing RH from the lowest RH subzone (12-18%) through the intermediate (18-24%) to the highest subzone (24-30%) produces a ~16-fold increase in ΔG, indicating that higher RH substantially enhances the thermodynamic driving force for moisture uptake in LHT. Although ΔG plateaus overtime, these combined larger amounts of adsorption (Fig. 2e) and thermodynamic considerations identify RH 24-30% as the optimal control condition for selective Li extraction.

We discover that the moisture uptake behavior of LHT can be fitted to the D’Arcy-Watt model, which captures three terms that represent Langmuir adsorption (𝐾𝐾′𝑎 /(1+𝐾𝑎 ) ), linear adsorption (𝐶𝑎 ) and multi-molecular adsorption (𝑘𝑘′𝑎 /(1−𝑘𝑎 ) ) (Fig. 2f-g), respectively. Model parameters indicate that multi-molecular adsorption dominates at RH 12-18%, implying the surficial moisture clustering according to the D’Arcy-Watt’s theory37. This stage represents the onset of mineral deliquescence, where surface hydration swiftly transitions into a dissolution phase38-40. Gradually Langmuir and linear adsorption increasingly contribute, signifying stronger chemical binding and further hygroscopic growth towards complete mineral dissolution at higher RH subzones41,42. More details of modeling can be found in Methods and Supplementary Table 1 and 2.

Fast-flow hygroscopic uptake inhibits co-mineral dissolution
According to deliquescence theory, LHT in mixtures is expected to dissolve earlier than its pure form, because the inevitable eutonic effect would lower its critical humidity for deliquescence40,41,43. To probe the interactions of LHT’s rapid moisture uptake with neighboring minerals, we create artificial contacts between LHT and each co‐mineral, and image their interfaces in-situ by environmental SEM‐EDS. As shown in Fig. 3a, pristine contacts exhibit sharp, water‐free boundaries between LHT and halite, sylvite, bischofite or gypsum. After only 30 s at RH 24-30% (Fig. 3b), however, LHT aggregates moisture molecules to locally concentrate water, and the advancing liquid front visibly intrudes into adjacent minerals, suggesting that sustaining moisture uptake in LHT could gradually dissolve those co‐minerals as a side effect. More time-series images during moisture uptake can be found in Supplementary Fig. 2-10.

To harvest the Li‐rich solution as soon as it forms and mitigate its re‐adsorption or mixing, we built a custom humidity‐controlled chamber equipped with a dehumidifier, temperature controller, and vacuum‐filtration configuration for timely solid-liquid separation (Fig. 3c). By drawing moisture through the mineral layer at controlled flow rates, we are able to regulate the moisture uptake kinetics. Fig. 3d illustrates the normalized uptake rate (R ) against moisture flow rate (R ), revealing a clear Power-Law relationship. As R increases from 4 to 20 m min-1, R accelerates by ~9.4 times from 5.4×10-3 to 5.1×10-2 g H O g-1 min-1, with a marked boost occurring above 10 m min-1. A higher flow at 20 m min-1 not only facilitates moisture uptake but also strengthens vacuum‐driven solid-liquid separation, cutting the time to collect the first batch of Li‐rich solution from hours to mere minutes. At desired high R , controlled packing density for salt loading would also impact R by reconciling its structural features. With equivalent LHT being tapped to half its bulk layer thickness (see Supplementary Method 2 for packing methods), the packing density rises from 0.41 to 0.84 g cm-3 by 2.05-fold (Supplementary Table 3). Under enhanced packing compressibility (Carr’s Index climbs from 0-51.5), the theoretical porosity and flowability (Hausner ratio) are diminished by 25.8% and 106% correspondingly. Such structural densification attenuates moisture accumulation in LHT, with 58.2% reduction in R observed relative to the untapped loading (Supplementary Fig. 11). In this regard, the loose bulk without external tapping preserves structural merits that secure a more competitive kinetic profile for deliquescence.

Li extraction performance from binary mixtures
The performance of Li extraction from binary mineral mixtures is evaluated under optimal control conditions of RH 24-30% and moisture flow rate 20 m min-1. For each mixture, LHT is first mixed with one co‐mineral (halite, sylvite, bischofite or gypsum) at broad initial Li/M (“M” is Na, K, Mg or Ca) feeding ratios spanning from 1:100 to 50:1 in weight. Extraction proceeds via vacuum‐driven batch-wise dripping for no more than four sequential batches (Fig. 4a-d). Lower initial Li/M feeding ratios typically exhaust in fewer batches. After each batch, the Li-rich solution is collected and the remaining solids advance to the next cycle, progressively depleting Li out of the solid phase (Supplementary Fig. 12).

We report four common performance metrics in Fig. 4a-d, including Li selectivity, solution Li/M ratio (purity), solution Li concentration, and accumulative recovery. The measurements and calculation details can be found in Methods. We can find for Li-predominant feeds (initial Li/M > 1:1), peak solution purities reach Li/Na ratio = 172.9, Li/K = 9128.1, Li/Mg = 347.5 and Li/Ca = 4742.3, respectively, demonstrating high Li purity in the collected solution. For M-predominant feeds (initial Li/M < 1:1), on the other hand, Li/NM ratios in the collected solutions remain at high levels of ≥ 12-17 (Na), 3-12 (K), 0.1-0.7 (Mg) and 16.7-22.9 (Ca), respectively. Even for very low-grade feeds (initial Li/M ≤ 1:20), Li selectivity, defined by batch Li/M ratio in the collected solution relative to that in remaining solids from the previous batch, peaks at 2204.5 (Li/Na), 1436.1 (Li/K), 20.6 (Li/Mg) and 1621.3 (Li/Ca), illustrating strong performance against co‐minerals (Fig. 4a-d).

In Li/Na, Li/K and Li/Mg systems, the solution Li purities decline over successive batches, reflecting Li depletion in the solids. By contrast, Li/Ca mixtures display increasing solution Li/Ca ratios over the first three batches, which is a result of temporal gypsum viscous slurry formation upon contacting water in LHT (Supplementary Fig. 13-16). When more moisture is adsorbed by LHT, the slurry becomes fluidic, which in turn promotes Li solution drainage. In addition, we notice a lower Li/Mg selectivity relative to other mixtures, reflecting bischofite is more sensitive to the hygroscopic growth of LHT. We therefore hypothesize that applying stronger vacuum filtration for faster solid-liquid separation, or testing on more hydrophilic filters for accelerated liquid adsorption could further suppress competing effects especially for low-grade feeds and unleash extraction performances.

Li concentrations in collected solutions span 72.4-97.8 g L-1 (Li/Na), 72.6-95.2 g L-1 (Li/K), 11.6-88.7 g L-1 (Li/Mg) and 56.1-95.5 g L-1 (Li/Ca) across different initial feeding ratios. For the highest-grade Li/M feed (50:1), 95.1-96.9% of Li is recovered within 0.8-1.2 h, and even for low-grade feeds, the process still achieves 81.3-90.3% (Li/Na), 80.9-92.7% (Li/K), 46.0-80.6% (Li/Mg) and 64.6-73.3% (Li/Ca) recovery within 3.1 h. Importantly, most Li is harvested at peak purities. For example, in Li/Na systems, 53.1-93.5% of Li is recovered within just 0.1-0.6 h, and the collected solutions exhibit Li/Na ratio of 114.2-172.9 (Fig. 4a). Likewise, in Li/K systems, 81.6-92.9% of Li is recovered in 0.3-0.5 h at Li/K purities of 330.2-7934.0; in Li/Mg systems, 86.1-96.2% is recovered in 0.1-0.6 h at Li/Mg = 29.3-347.5; and in Li/Ca systems, 54.1-69.6% is recovered in 0.4-0.9 h at Li/Ca = 428.3-4742.3 (Fig. 4b-d).

Because strong hygroscopicity is an intrinsic property of mineral LHT, our humidity‐controlled extraction principle, and the associated configuration design, can be directly extended to more complex, multi-component Li resources.

Li extraction from mining slags in environmental conditions
Industrial Li mining operations produce large volumes of Li-bearing solid slags that are challenging to recycle because of diverse components and chemistries. For example, brine-based extraction generates slags typically contain ~1 wt% Li among all species and impurities, which are currently left unexploited. To demonstrate industrial applicability, we process a multi-component Li-bearing slag from a brine operation , which contains 6.4 wt% Li, 8.3 wt% Na, 2.6 wt% K, 82.6 wt% Mg, and 0.1 wt% Ca relatively across these five elements (mixed as LHT, halite, sylvite, bischofite, gypsum, and complex mutual phases such as bischofite-sylvite dual-salts; Supplementary Fig. 17), along with minor components such as boron and insoluble matters.

Under control condition (RH 24-30C%, moisture flow 20 m min-1), three successive vacuum-driven batches recover 83.5 % of total Li in just 1 h (Fig. 5a). Notably, Batch 1 alone recovers 71.2% of Li in only 0.08 h, where Li purity is 91.9, 117.6, 0.34 and 261.3 times that of Na, K, Mg and Ca in the collected solution. Subsequent Batch 2 and 3 further enrich Li/Na to 137.6, Li/K to 127.5, Li/Mg to 13.1, and Li/Ca to 12.8, with solution Li concentrations of 24.7, 23.1 and 19.3 g L-1. These numbers are several times higher than typical preconcentrated Li concentrates for industrial Li CO production15,44.

Most Li brine operations are located in hyper-arid basins around the world, for example, Salar de Atacama in Chile, Silver Peak in the U.S., and Zabuye Salt Lake in China (Supplementary Fig. 18). Because Li hygroscopicity drives extraction at low RHs, we exploit the operational performances under natural conditions in Atacama, where the averaged seasonal RH (13.3-23.8%) and temperature (11.7-20.9°C) both fall below our lab control, although higher perturbations are observed in nature (Fig. 5b and Methods). Beyond complex components, these environmental settings also represent unfavorable yet realistic operating conditions, in which low temperature and fluctuating humidity impose a more rigorous assessment on the general applicability of this hygroscopicity-driven approach. It is found that the total Li recovery across the four seasons remain as high as 84.5%, 84.0%, 90.2% and 87.7%, with processing time below 3.0, 1.2, 1.3 and 1.6 h, respectively. Moreover, even a single Batch 1 is able to recover 65.3%, 57.3%, 67.1% and 53.8% of Li, in around 0.9, 0.3, 0.4, and 0.6 h (Fig. 5c-f). These results confirm that our humidity‐controlled extraction performs robustly under real‐world, low‐humidity field conditions.

Under spring, fall and winter profiles (Fig. 5c-f), the cooler temperatures lead to reduced sylvite solubility and as a result elevate Li/K separation performance: Li/K ratios in solution climb to 220.5, 265.6 and 704.1 (up to 87.5%, 126.0% and 498.7% vs. control), translating into higher Li/K selectivity of 289.3, 342.3 and 449.5. Solution Li/Mg ratios show similar trends, rising by 164.0% (spring) and 147.0% (winter) related to the control. In terms of humidity, a low moisture content may restrain the amount of moisture uptake but can lead to higher Li concentrations in collected solutions, which is favorable for Li enrichment though only slightly extends the processing time. For example, under spring condition (Fig. 5c), recovered Li concentration ranges in 25.8-35.2 g L-1, which is ~40% higher than the control group (Fig. 5a). Overall, these results indicate this hygroscopicity approach can be harnessed under natural low-humidity conditions to achieve rapid, high-purity Li extraction from complex industrial mixed resources.

Modular scale-up demonstration
We further evaluate the scale-up potential of the hygroscopicity-driven extraction using a vertical shell-side filtration design (Fig. 6a). Industrial Li-bearing slags are packed into the hollow interlayer of an inward double-frame reactor. Application of vacuum to the inner chamber draws moisture-laden air through the slag bed, where hygroscopic uptake induces deliquescence and generates Li concentrates that drain spontaneously into the bottom collector under gravity (Fig. 6b). Detailed information on modular configuration and dimensions is provided in Supplementary Fig. 19 and Supplementary Table 4. This modular setup exhibits rapid processing especially under favorable conditions (RH 24-30%). Increasing the cross-slag air flow from 50 to 100 L min-1 raises the slag processing rate from 3.65 to 8.01 kg m-1 d-1 when normalized by module height (Fig. 6c, see Methods for calculation details), yielding up to 0.28 L d-1 of Li concentrates. Across this range of air flows, the module achieves high area-normalized Li extraction rates (0.08-0.25 kg m-2 d-1) and elevated Li concentrations in collected solutions (14,220-22,730 ppm), outperforming reported Li extraction technologies by 1-3 orders of magnitude (Fig. 6d). Notably, the yield LiCl obtained in the scale-up module remains > 2 times more concentrated than those of typical preconcentrated LiCl solutions qualified for further industrial Li CO upgrading15,44.

The hygroscopicity‐driven extraction establishes a rapid, low-carbon and resource-efficient route of high‐purity Li recovery with minimal energy, water, or chemical uses. Distinct from current chemical and energy intensive processes that use specially synthesized materials, harsh reagents and large amounts of freshwater, this method exploits the intrinsic spontaneous deliquescence of Li to selectively dissolve LHT in a few hours without external reagents or heat throughout operation. Its simple and modular setup can be retrofitted to existing workflows or paired with humidity-controlled adsorption or nanofiltration to further improve purity. This hygroscopicity-driven strategy can also extend beyond slags to sulfate-type salt lakes, seawater, and various Li-bearing solids. Notably, Li SO exhibits a limiting RH (~60%) in mixtures that can be well separated from MgSO (> 80%)45,46, providing an even larger RH window than chloride systems and highlighting sulfate resources’ particular favorability for this approach. Looking forward, we envision this approach to be beneficial for both controlled factory setup and direct deployment under natural climate conditions. To address foreseeable challenges relative to large-scale efficiency or environmental perturbations, research on humidity tuning, flow dynamics, pretreatment, multi-component mutual interactions, modular encapsulation, and reactor development is needed and more feedstocks such as hard rock ores, spent battery electrodes, and tailings can be tested under controlled environments or onsite field conditions. For a clear prospect, we also provide a shortlist on potential process controls that could be further investigated (see Supplementary Table 5) . Together, the findings in this first study point to a versatile and sustainable paradigm for mineral recovery and derisked Li supply in the most water‐stressed mining regions worldwide.

Methods
Materials and reagents
All salts employed in current setup as laboratory systems are analytical grade with conservative control margins to ensure reproducibility, including LHT in ≥ 99.99%, NaCl in ≥ 99%, KCl in 99-100.5%, MgCl ·6H O in 99-102%, CaSO ·2H O in ≥ 99%, and CaCl ·2H O in ≥ 99%, as stated by the suppliers. Multi-component Li-bearing slag mixtures used for industrial and scale-up modular demonstration are sourced from a brine operation with compositions and concentrations disclosed above in the main text.

Isothermal moisture uptake measurements
Isothermal moisture uptake in minerals is quantified using an analytical Dynamic Vapor Sorption (DVS, DSA-0184@MFG, TA Instrument) system with an accuracy of 10 ppm in mass change. Before measurement, ~25 mg of LHT, halite, sylvite, bischofite and gypsum are loaded onto separate sample pans within the DVS system, with the background environmental condition set to temperature 25°C and RH 6% for initialization. For each measurement, we take an enforced pre-drying procedure to eliminate the influence of system background condition on mineral moisture uptake. Each mineral is dried at 60°C until the mass fluctuation is stabilized within ± 1% for at least 1 h.

To determine each mineral’s limiting RH (Fig. 2a), we perform a stepped RH sweeping from 12% up to 96% in 6% increments. At each RH step, the sample is held for 1 h and the mass change (Δm) is recorded as the corresponding amount of water adsorbed during that interval. To identify the equilibrium moisture uptake in LHT across three RH subzones (Fig. 2e), we measure the full adsorption isotherm of LHT over the effective extraction window (RH 12-30%; Fig. 2e) by incrementing RH 1%. At each step, the sample is held until mass change stabilized within ± 1% for at least 1 h, ensuring true equilibrium uptake.

Thermodynamic analysis and mechanistic modeling
The thermodynamics of LHT moisture uptake involves the investigation of adsorption free energy and adsorption mechanisms, both of which are derived from the isothermal adsorption equilibrium. According to thermodynamic equilibrium, the instantaneous free energy change (ΔG) of moisture uptake at each RH step should be represented by the product of moisture chemical potential and the amount of equilibrium moisture uptake:
Δ𝐺 = 𝜃𝑅𝑇ln𝑎 (1)
moisture uptake w
where the amount of moisture uptake per unit LHT (θ) is determined from isothermal adsorption equilibrium, and water activity (a ) refers to the equilibrium ratio between the partial vapor pressure of atmospheric moisture and the saturation vapor pressure of liquid water adsorbed in the mineral, which is equal to the numerical value of RH. A D’Arcy-Watt model is established to investigate the binding mechanisms between LHT and moisture molecules51,52:
𝐾𝐾′𝑎 𝑘𝑘′𝑎
w w
𝜃 = +𝐶𝑎 + (2)
1+𝐾𝑎 w 1−𝑘𝑎
w w
This model is employed as a mechanistic framework across the full humidity range to capture the net outcome of ionic dissociation, salt dissolution and solution formation through hygroscopic moisture uptake. Based on thermodynamic principles, the θ in LHT is made up of three terms in sequence: 1) Langmuir adsorption that implies strong chemical bindings where K and K' represent the binding affinity and the number of strong binding sites; 2) linear adsorption that implies weak bindings and mineral dissolution where C is a combined measure of the binding affinity and the number of weak binding sites; and 3) multi-molecular adsorption that implies bulk free moisture clustering where k and k' represent the binding affinity and the number of multi-molecular binding sites. Because deliquescence manifests as a continuous solution growth profile rather than a sharp phase transition, by fitting the model with equilibrium isothermal adsorption, the potential dominant contribution of these three adsorption can be attained. Nonlinear regression (Supplementary Tables 1 and 2) yields excellent fittings (R2 > 0.996), allowing us to quantify each term’s contribution across RH subzones.

Kinetic analysis and moisture flow rate
The simplified binary mineral contact (Fig. 3a and b) is designed to illustrate the necessity of rapid solid-liquid separation in mitigating the mixing between LHT hygroscopic growth and co-minerals. The contacts are prepared by gentle mechanical mixing of finely ground mineral powders (typically <75 µm) to ensure close solid-solid interfaces without excessive tapping, structural modification, or partial dissolution. Co-precipitation or solution equilibration is excluded as the objective is to emulate the physical adjacency and interfacial behavior of representative mineral phases rather than chemical homogenization.

The purpose of moisture uptake kinetics is to examine the moisture flow rate that can accelerate the uptake in minerals and facilitate solid-liquid separation for Li extraction. The LHT is placed atop the vacuum filter, and the moisture flow rate across the LHT is regulated by the outlet rate of vacuum system. A flowmeter with an accuracy to ± 0.01 L min-1 ensures precise control of the outlet rate. The moisture uptake rate is measured at the optimal RH subzone 24-30% and the moisture flow rate across LHT is varied from 4 to 20 m min-1 (i.e. 4, 5, 6, 8, 10, 12, 14, 16, 18 and 20 m min-1). A flow rate below 4 m min-1 fails to achieve sufficient solid-liquid separation due to inadequate negative pressure. At each flow rate, the mass change of LHT, representing the amount of moisture uptake, is recorded every 5 min until the first droplet of Li solution obtained. A pseudo-first-order kinetic adsorption model based on mass change over time is developed to derive the moisture uptake rate. The moisture uptake rate is normalized by the amount of LHT involved. The attained trend is well identified to showcase the Power-Law behavior, which follows the exponential relationship as:
𝑅 = 𝐴exp(𝑁𝑅 ) (3)
uptake flow
where R and R denote the normalized moisture uptake rate, and the moisture feeding flow rate, respectively. A is the proportionality coefficient and N is the Power-Law exponent that describes the sensitivity of adsorption to feeding rate.

The in-situ imaging of the micro-boundaries between LHT and other minerals is enabled by an environmental SEM (ESEM, Quanta 200 FEG). The ESEM features a Peltier stage, allowing the imaging of real-time morphology of minerals when interacting with moisture. The internal temperature and RH are fixed at 25°C and 27% (the medium value between 24% and 30%). Due to the low-conductive nature of minerals, a low vacuum mode is adopted with an internal pressure maintained between 0.1-1.0 torr.

Li extraction and recovery
Li extraction and recovery is first carried out in binary mineral mixtures with varied initial feeding ratios under control conditions (RH 24-30% and flow rate 20 m min-1). The potential of applications in arid regions are then demonstrated using a real-world industrial slag with complex multi-components. The setup for Li extraction is displayed in Fig. 3c, and Li is recovered in the form of Li-rich solution in sequential batches (Fig. 1).

For binary mineral mixtures, LHT is individually mixed with halite, sylvite, bischofite or gypsum at the initial Li:M feeding ratio of 50:1 to 10:1, 5:1, 1:1, 1:5, 1:10, 1:20, 1:50 and 1:100 in weight (M = Na, K, Mg or Ca). At the end of each batch (n), the Li/M ratio in the collected liquid solution (LRn ), the Li/M ratio of the solids remaining atop the filter (SRn ), and the Li concentration in the collected solution (𝑐n) are measured. Li selectivity at Batch n is defined as:
Li
LRn E
Li/M
Li selectivity = (4)
SRnR−1
Li/M
Li recovery of batch n is calculated by:
Li in the collected solution
Li recovery = ×100% (5)
Total Li in initial solid mixture
The accumulative Li recovery corresponds to the summation of Li recovery up to the current batch.

For the industrial slag demonstration, XRD analysis (Supplementary Fig. 17) is used to characterize the major compositions of the sample, a mixture of LHT, halite, sylvite, bischofite and gypsum. Extractions are performed first under control conditions (25°C, RH 24-30%, flow 20 m min-1), and then under simulated climates of a brine mining site in Salar de Atacama. Based on two‐year meteorological records at the evaporation ponds, seasonal parameters are set as follows. Spring (Dec. –Feb.): RH 13.3 ± 4.5 %, temperature 16.7 ± 2.9 °C, wind 2.3 ± 1.2 m s-1. Summer (Mar. –May): RH 23.2 % ± 9.3 %, temperature 20.9 ± 1.7 °C, wind 3.3 ± 1.3 m s-1. Fall (Jun.–Aug.): RH 23.8 % ± 9.6 %, temperature 17.6 ± 3.0 °C, wind 2.1 ± 1.2 m s-1. Winter (Sep.–Nov.): RH 15.3 % ± 4.5 %, temperature 11.7 ± 2.1 °C, wind 1.4 ± 0.9 m s-1. Two commercial fans provide advection over the mineral bed and within the chamber. Wind speed is adjusted via fan orientation and position and calibrated with an anemometer (± 0.01 m s-1).

For the scale-up modular demonstration, the slag processing rate is defined as the mass of fully packed slag normalized by the module height and the duration of a single operation:
Loading mass
Slag processing rate = (6)
Module height×Operation time
The Li concentrate yield is calculated as the total volume of Li concentrate produced per unit operation duration:
Total volume of produced Li concentrates
Li concentrate yield = (7)
Operation time
The Li extraction rate is calculated as the mass of recovered Li normalized by the filtration area (i.e. side area of the inner frame) in the module, and the operation duration.
Mass of recovered Li
Li extraction rate = (8)
Module filtration are×Operation time

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript presents a spontaneous direct lithium extraction method to harvest lithium-rich saline from solid lithium resources driven by hygroscopicity. Given the simple principle and processing convenience, it is of certain interest to the industrial community. However, given the critical scientific concerns, I cannot recommend its possible publication in current version. Comments are listed as follows:\n1. This lithium extraction proposal is based on the hygroscopicity difference among solid salts. In short, there are three main prerequisites enabling this lithium extraction besides rapid solid-liquid separation, namely, solid lithium resources (e.g., mining slag deposits, spent battery electrodes, and pharmaceutical wastes), LiCl hydrate (LHT)-containing (i.e., lithium species with higher hygroscopicity than other salts), suitable relative humidity (RH) (should be controlled). These requirements make this presented method not a mainstream lithium extraction choice for lithium supply, but a slag/waste valorization alternative, in that most of the lithium resources are stored in aqueous sources. The Abstract and Introduction parts should be re-organized correspondingly (focusing on upgrading industrial residues and minimizing the environmental impact of Li‐bearing wastes on landfills and waterways).\n2. Author used a selected mineral suite containing LiCl hydrate (LHT), halite (NaCl), sylvite (KCl), bischofite (MgCl2·6H2O), and gypsum (CaSO4·2H2O) to demonstrate the feasibility of this hygroscopicity-driven lithium extraction idea. It is quite strange and tricky that sulfate-type gypsum was chosen alongside a series of chloride salts. Typically, CaCl2 hydrate possesses a higher hygroscopicity than LiCl, which breaks the working principle of this proposal, thus sparingly soluble CaSO4 rather than hygroscopic CaCl2 was used. Author should provide related explanation on not choosing CaCl2 hydrate and discuss the limiting conditions of this method in main text. Moreover, please test the lithium extraction performance using CaCl2 as a control sample for fair comparison.\n3. Following last comment, please disclose the critical working conditions and/or infrastructure requirements for proceeding this lithium extraction method. LiCl-containing mining slags/spent electrodes/natural minerals (with other salts sharing lower hygroscopicity) are not easy to obtain, for example, the Li-bearing solid slags from sulfate-type salt-lake brines. Producing LiCl-containing solids requires huge energy and chemical consumption, which increases the cost, not to mention the requirement to supply humidity-controlled vacuum-filtration setup.\n4. How about the influence of temperature and acidic/alkaline chemicals on this lithium extraction approach? Regarding the former one, temperature may alter the reaction kinetics and moisture absorption/evaporation balance. For the latter one, given that natural brines and seawater are often not neutral, some common acidic (e.g., hygroscopic P2O5 or H2C2O4 hydrate) and alkaline (e.g., hygroscopic CaO, NaOH/KOH, Na2CO3 hydrate) chemicals (some are involved in the common lithium production/recycling procedures), may also break this hygroscopic-driven method, not to mention other common salts with high hygroscopicity.\n5. How about the scale-up processing feasibility and life cycle assessment analysis for practical attempts?\n6. Author assessed the performance of Li extraction from actual brine mining slag examples by simulating certain environmental conditions. However, compared with the large amount of Li-containing brines with low concentration but huge transportation difficulties as well as cost, the transportation of Li-containing solids (e.g., mining slag deposits, spent battery electrodes, and pharmaceutical wastes) to processing plants with optimal processing conditions is much easier and feasible. Author should devote more efforts to exploring the optimal operation conditions as well as the scale-up feasibility for practices.\n7. As shown in the in-situ SEM-EDS analysis (Figs. 3a,b and related supplementary pictures), considerable co-mineral dissolution was observed alongside lithium extraction by rapid moisture uptake, which degrades the lithium level of harvested brines. Author then developed a humidity-controlled vacuum-filtration setup to achieve timely solid-liquid separation. Other material-scope strategy can be explored, such as particle size, tap density, operation temperature, etc.\n8. As shown in Fig. 4, the lithium extraction efficacy is not so good with a lower Li/M feed ratio, which is a critical challenge in upgrading the mining slags/wastes with low lithium level. Strategies should be raising to deal with this. Moreover, current minimum Li/M ratio of 1:100 is not small enough to simulate the salt-lake brine deposits, a lower set of 1:500 or 1:1000 (or even lower) is suggested.\n9. Author used a homemade humidity-controlled vacuum-filtration setup to achieve timely solid-liquid separation, which greatly increases the cost. Infrastructure design by taking full advantage of gravity may be favorable for this process, such as tilting or spiral substrate.\n10. The detailed patent application No. in Competing interests should be disclosed.", "reviewer_2": "This manuscript presents an innovative hygroscopicity-driven route for lithium extraction that is both conceptually novel and potentially impactful for sustainable resource recovery. The proof-of-concept results are compelling, but some claims, particularly regarding universal selectivity and entropy-driven mechanisms, are overstated, and the industrial relevance remains underdeveloped. With clearer discussion of selectivity, thermodynamics, and scalability, the work could make a strong contribution, but substantial revision is needed before it is suitable for publication.\nMajor Comments\n1. The abstract claims “consistent high selectivity over a wide range of feed ratios.” However, the results clearly show that separation against Mg²⁺ is much weaker compared to Na⁺, K⁺, and Ca²⁺. The claim should be moderated or explicitly qualified with discussion of Mg limitations.\n2. The manuscript refers to the process as “entropy-driven,” but this remains vague. While it is true that deliquescence generally increases entropy (as ordered crystalline ions dissolve into a more disordered aqueous phase), the free-energy analysis presented in Fig. 2f does not directly separate enthalpic and entropic contributions. I suggest the authors either (i) clarify the thermodynamic basis by explicitly discussing how deliquescence enhances entropy relative to competing ions, or (ii) reframe the language to standard terminology (e.g., “spontaneous deliquescence and hygroscopic growth”) to avoid overinterpretation.\n3. The batch-scale and controlled-chamber demonstrations are convincing. However, the industrial application remains unclear. Is this process envisioned for a controlled factory setup or for direct deployment in natural field environments? The manuscript should discuss scalability challenges, including process control under variable climate conditions, as well as potential engineering solutions.\n4. It is well known that the deliquescence relative humidity (DRH) of a mixture is typically lower than that of any pure component. Since slags are inherently mixtures, this should be explicitly acknowledged and discussed in the context of selective dissolution.\n5. The two “stages” in Fig. 2e resemble standard deliquescence (sharp uptake) and hygroscopic growth (gradual increase). The initial uptake (assigned as multimolecular adsorption, Term 3) could also be interpreted as pre-deliquescence. The mechanistic schematic in Fig. 2g is confusing and does not clearly map onto these interpretations.\n6. Established activity models (e.g., E-AIM, AIOMFAC) could be applied to describe deliquescence and hygroscopic growth. Such modeling would provide stronger support for the proposed mechanism.\n7. About the SEM-EDS experiments (Fig. 3), the preparation of binary mineral contacts is unclear. Were they prepared by mechanical mixing, co-precipitation, or equilibration in solution? The microstructure of the mixtures strongly influences dissolution, and clarification is essential. The authors should discuss to what extent these simplified binary contacts represent the heterogeneous mineralogy and microstructure of real slag systems.\n8. The normalized uptake curve depends on microstructural parameters such as particle size, thickness, porosity, and packing density. Since real slags are highly variable, the sensitivity of kinetics to these factors should be discussed to assess scalability and reproducibility.\n9. The definition of “Li selectivity” is only briefly given in the Methods, but its physical meaning is unclear in the main text. I recommend adding a short explanation, e.g., posing the question of why selectivity is very high vs. Na⁺/K⁺ but much weaker vs. Mg²⁺, to help readers intuitively grasp what the metric implies and to better align the claims of “consistent high selectivity” with the data.\nMinor Comments\n1. Page 2, Line 33: “The global lithium demand is projected to triple by 2030 …” Relative to which baseline year? Please specify.\n2. Figures 2c and 2d: Photographs are not quantitative and do not clearly convey volume differences.\n3. Figures S1–S9: The RH values at which SEM/EDS images were taken should be explicitly stated.\n4. Fig. S15: Not all diffraction peaks are assigned. What additional phases are present? Minor components could influence hygroscopicity and selectivity.\n5. Reproducibility of cycling: The authors could comment on whether repeated RH cycling could cause recrystallization or structural changes in LiCl hydrate, potentially altering performance.\n6. Environmental robustness: The demonstrations in Atacama are promising, but the manuscript should discuss performance under broader humidity and temperature fluctuations, and whether diurnal/seasonal variability could enhance or hinder the process."}, "author_response": "Manuscript NCOMMS-25-54106B\nResponse to Reviewers:\nReviewer 2:\nThe authors have addressed my questions satisfactorily, and their responses have clarified the\npoints I raised.\nResponse: We sincerely thank the reviewer for the positive evaluation. We appreciate the\nreviewer’s time and constructive comments in earlier rounds of revision."}, {"round": 2, "reviewer_comments": {"reviewer_2": "The authors have addressed my questions satisfactorily, and their responses have clarified the points I raised."}, "author_response": "Manuscript NCOMMS-25-54106B\nResponse to Reviewers:\nReviewer 2:\nThe authors have addressed my questions satisfactorily, and their responses have clarified the\npoints I raised.\nResponse: We sincerely thank the reviewer for the positive evaluation. We appreciate the\nreviewer’s time and constructive comments in earlier rounds of revision."}]

Decision letter