Oxidation-reconstructed Li^+ transport enables high-tap-density single-crystal regeneration of spent LiNi_0.5Co_0.2Mn_0.3O_2 positive electrodes

Domain: chemistry
Published: 2026-04-14
Editorial outcome: minor_revision

Abstract
Direct regeneration offers a shortcut to close the material supply loop of lithium-ion batteries and is a promising recycling strategy. However, in spent LiNi0.5Co0.2Mn0.3O2 positive electrode, severe bulk cation disorder and surface rock salt phase hinder Li+ reinsertion. Moreover, the coexistence of single and poly-crystal particles in commercial batteries further complicates uniform re-lithiation and morphological regeneration. Herein, we propose an oxidation strategy to simultaneously regulate the structural reconstruction and morphological evolution of spent material. During oxidation, surface NiO transforms into NiOOH, while targeted oxidation of the anti-site Ni2+ to Ni3+ in the bulk reduces Li+/Ni2+ mixing. This reconstructs Li+ diffusion channels from surface to bulk, facilitating re-lithiation. Meanwhile, structural changes induce lattice expansion in secondary particles, causing their decomposition into primary particles and forming uniform precursor particles. These particles, with continuous Li+ transport channels and NiOOH surface, agglomerate into large single-crystal during calcination. The regenerated LiNi0.5Co0.2Mn0.3O2 achieves a high tap density of 2.57 g/cm3 and retains 80.2% capacity after 600 cycles. This work presents a concept for the direct regeneration of degradable positive materials.

Introduction
Lithium-ion batteries (LIBs), valued for their high energy density and long cycle life, are extensively used in electric vehicles and grid-scale energy storage, resulting in a rising volume of spent LIBs1-4. Accordingly, efficient recycling of spent batteries, driven by environmental protection and resource reuse, is now a primary focus in industry and academia5-7. Current LIBs recycling relies primarily on pyrometallurgy, hydrometallurgy, or their combinations. Metallurgy routes, which break the chemical bonds of electrode materials and thus release valuable lithium and transition metals, are recognized as “downcycling” strategies8, 9. Besides, metallurgy routes generally require high temperatures or strong acids/alkalis, leading to high energy costs and secondary pollution10, 11. In contrast, the direct regeneration route aims to repair defects in spent positive electrodes and restore their electrochemical performance without destroying the original crystal structure, which provides a shortcut to close the material supply loop of LIBs as a simpler, less polluting, and higher-value recycling method12-14.

However, the direct regeneration of spent LiNi0.5Co0.2Mn0.3O2 (NCM) positive electrodes is hampered by sluggish and heterogeneous Li+ diffusion and poor morphology controllability, arising from intrinsic structural degradation and particle heterogeneity. During cycling, intrinsic structural instability drives Ni migration from the transition-metal (TM) layer to the Li layer, leading to severe Li+/Ni2+ mixing and surface degradation, particularly the formation of rock salt phase15-17. These degraded phases severely hinder Li+ re-insertion and diffusion within the solid oxides, that is, the Li replenishment during regeneration9, 18. In addition, to enhance compacted density and cycling stability, commercial NCM positive electrodes typically combine primary (single-crystal) and secondary (poly-crystal) particles. This heterogeneity varies Li+ diffusion pathways among different particles, complicating uniform re-lithiation and morphologically homogeneous regeneration.

Currently, the direct regeneration of spent NCM primarily adopts hydrothermal or molten salt methods16, 19-22. In the recovery process, to promote Li+ insertion, high pressure and high lithium concentration are always required, which erodes the economic profit and elevates the burden of pollution control23, 24. Moreover, the hydrothermal re-lithiation can suffer from poor uniformity of Li replenishment due to the diffusion paths differing in primary/secondary particles, undermining structural stability. The molten salt method is likely to induce anomalous growth of the original primary particles due to the addition of at least twice the amount of lithium salt required for direct regeneration, intrinsically limiting morphology controllability. Thus, although both methods enable re-lithiation, they rely on the harsh conditions and struggle to balance structural repair with morphology regulation, making efficient regeneration of NCM materials challenging.

As shown in Fig. 1, we propose an oxidation strategy based on Na2S2O8 that can simultaneously address the above challenges under mild conditions. Strongly oxidizing ·SO4-/·OH radicals derived from Na2S2O8 oxidize the surficial rock-salt NiO into a layered β-NiOOH phase, thereby establishing two-dimensional Li+ diffusion channels. Meanwhile, in the bulk, the anti-site Ni2+ in LiO6 octahedron is oxidized to Ni3+, which reverts to the TMO6 octahedron due to its smaller radius and charge balance, significantly reducing Li+/Ni2+ mixing and repairing the Li+ diffusion pathway. Furthermore, the increase in Ni3+ in the bulk phase and the surface transition NiO to β-NiOOH jointly induce lattice expansion, driving secondary particles to disintegrate into uniform primary “precursors”. These “precursor” features continuous Li+ transport channels and a NiOOH-layered surface, enabling their coalescence into large single-crystal during subsequent calcination.

The regenerated single-crystal LiNi0.5Co0.2Mn0.3O2 (NCM523) exhibits a reversible discharge capacity of 144.2 mAh/g at 1C, with capacity retention as high as 80.2% after 600 cycles. Notably, the oxidation strategy is also applicable to the direct regeneration of spent LiNi0.6Co0.2Mn0.2O2 (S-NCM622) and LiNi0.8Co0.1Mn0.1O2 (S-NCM811), yielding uniform single-crystal particles with high structural order and electrochemical stability. We anticipate that this oxidation strategy will inspire future research on the recycling of spent positive electrode materials and deepen the understanding of the structure recovery mechanism in the spent NCM materials.

Results
Screening oxidants to achieve Ni2+ to Ni3+
To choose a proper oxidant, the Eh-pH phase diagram of the Ni-H2O system is constructed, as shown in Fig. 2a. An alkaline environment (pH = 14) is predefined to prevent transition metal dissolution. The diagram reveals that NiO transforms into the layered NiOOH phase when the redox potential exceeds 0.54 V at pH=14, whereas the Co3O4 phase remains stable (Supplementary Fig. 1a). Meanwhile, to oxidize the bulk Ni2+, an Eo of ~2.50 V is required25. Standard electrode potentials of the typical oxidizing agents, calculated using the Nernst equation26-28, are shown in Fig. 2b. Only ·SO4-/·OH radicals generated by the decomposition of S2O82- are available to satisfy the oxidative demands of both surface and bulk phases. The presence of ·SO4-/·OH radicals during oxidation is confirmed by ESR testing (Supplementary Fig. 1b).

As shown in Fig. 2c, with Na2S2O8 as an oxidizing agent, the surface NiO is converted to NiOOH, accompanied by a significant increase in the lattice spacing from 2.40 Å to 4.83 Å; the anti-site Ni2+ in the LiO6 octahedron, which has higher activity, is also oxidized to Ni3+ and hence returns to the TMO6 octahedron due to the radius variation and charge balance, causing c-axis expansion. The surficial and internal phase transitions induce lattice distortions, which increase the internal stresses within secondary particles, prompting their fragmentation into primary particles. The fragmentation unifies the morphology and homogenizes the Li+ diffusion channels, further paving the way for subsequent Li replenishment.

Oxidation mechanism of S-NCM523
With spent NCM523 as a paradigm, the regeneration process involves two steps: (1) oxidation of spent NCM523 (denoted as S-NCM523, consisting of single and poly-crystal particles), where the as-obtained product is denoted as O-NCM523; (2) the subsequent high-temperature calcination of O-NCM523 with LiOH·H2O as Li source, where the as-obtained product is denoted as R-NCM523. The initial pouch cell performance of S-NCM523 is shown in Supplementary Fig. 2. Additionally, regeneration via direct Li replenishment via a simple calcination route was conducted, and the product was used as a control sample (D-NCM523) to understand the role of the oxidation process.

As shown in Supplementary Table 1, R-NCM523 demonstrates a Li/TM molar ratio of 1.05, consistent with that of commercial NCM523. Neutron pair distribution function (NPDF) testing was employed to clarify the atomic-scale structural evolution from S-NCM523 to O-NCM523. As shown in Fig. 3a, d1-d5 correspond to the nearest in-plane TM-O and TM-TM atom pairs. NPDF refinement in Fig. 3b reveals that O-NCM523 and S-NCM523 have similar d1-d5 peaks in the 1-6 Å range, indicating a similar crystal framework. While meticulous comparisons reveal that the TM-O (d1, 2.055 Å) and TM-TM (d2, 2.925 Å) bonds of O-NCM523 are shorter than those of S-NCM523, reflecting stronger interatomic interactions associated with the oxidation of TM from lower to higher valence states. Additionally, the intensity of the d1 peak in O-NCM523 is lower than that in S-NCM523, which is due to the higher Ni3+ content in O-NCM523 (less Li+/Ni2+ mixing), enhancing the Jahn-Teller effect and weakening the symmetry of the TM-O bond29.

X-ray photoelectron spectroscopy (XPS) tests with different etching times were used to determine the oxidation of which specific TM elements in NCM. As shown in Supplementary Fig. 4, for S-NCM523, the Ni 2p3/2 peak shifts from 853.2 eV to 854.7 eV with increasing etching time, indicating the presence of surface degradation zones corresponding to NiO rock-salt phases. In contrast, for O-NCM523, the Ni 2p3/2 peak at 854.7 eV remains consistent from the surface to the near-surface region. Obviously, the oxidation process promotes the conversion of Ni (II) to Ni (III) as expected. Meanwhile, the valence states of Co and Mn remain unchanged (Supplementary Fig. 5a-d). Moreover, Raman spectra in Fig. 3c show only Ag signals in S-NCM523, confirming the presence of surface NiO phase30. In contrast, O-NCM523 exhibits both A1g and Eg signals, suggesting the formation of a layered structure on the surface. Subsequently, as shown in Fig. 3d, X-ray absorption near-edge structure (XANES) spectroscopy was used to probe Ni valence states in the bulk of S-NCM523 and O-NCM523. The Ni K-edge spectra of O-NCM523 exhibit a clear shift to higher energies. Meanwhile, the extended X-ray absorption fine structure (EXAFS) Fourier transform reveals that the Ni-O and Ni-TM bond lengths in O-NCM523 are shorter than those in S-NCM523, and the Ni-TM peak intensity in O-NCM523 is weaker than in S-NCM523 (Supplementary Fig. 5e), suggesting the elimination of the NiO phase31, 32. Collectively, these results confirm that the oxidation mechanism primarily targets the oxidation of Ni2+ in S-NCM523.

The evolution of the bulk Li+/Ni2+ mixing and surface rock salt phase in S-NCM523 after oxidation was determined by neutron powder diffraction (NPD) refinement. As shown in Fig. 3e, f, the Li+/Ni2+ mixing decreases from 8.33% in S-NCM523 to 4.64% in O-NCM523. This is attributed to the oxidation of Ni2+ in LiO6 to Ni3+, which has a smaller radius (0.56 Å) than Li+ (0.76 Å) and tends to migrate from LiO6 back to TMO6 33. Meanwhile, the NiO phase in S-NCM523 transforms into NiOOH during oxidation, and the NiOOH content in O-NCM523 is 5.90%, with both Rwp (5.87%) and RF (5.40%) values below 10% (Supplementary Table 12 and 13), indicating good reliability of the fitting. Moreover, after removing the NiOOH phase during refinement of NPD and NPDF, the Rwp values increase to 7.69% and 26.40%, respectively, further confirming the existence of the NiOOH phase in O-NCM523 (Supplementary Fig. 6). XPS analysis further confirms this transformation. The O1s XPS spectra in Supplementary Fig. 7a reveal the formation of NiOOH on the O-NCM523 surface. The Ni 2p XPS spectra in Supplementary Fig. 7b show a higher fraction of Ni (III) in O-NCM523 than that in S-NCM523, indicating the surface transformation from NiO to NiOOH.

All reactions related to the oxidation derived regeneration process are outlined in Supplementary Table 2. To further verify the transformation from the tough rock-phase NiO to layered NiOOH, Na2S2O8 solution was reacted with NiO. As shown in Supplementary Fig. 8, after 1h of reacting at 25℃, NiOOH diffraction peaks appear, confirming that the surface NiO in S-NCM523 could be directly transformed into NiOOH by Na2S2O8.

The Li+ loss during the oxidation process was quantified by ICP analysis. Comparable Li⁺ loss was observed across S-NCM523 samples subjected to different oxidation times. As shown in Supplementary Table 3, Li loss stabilizes after the reaction time reaches 1h. The final Li/TM ratio of O-NCM523 is slightly lower than that of S-NCM523 (0.5), indicating a limited Li loss during oxidation. These results indicate that the oxidation reaction proceeds rapidly and causes negligible structural damage.

The effects of Na2S2O8 concentration, pH, temperature and reaction time on the oxidation process are summarized in Supplementary Table 3, 25-29 and Supplementary Fig. 9. For spent samples subjected to the same oxidation treatment, those with lower anti-site Ni2+ content exhibit less Ni²⁺ oxidation and smaller changes in Li+/Ni2+ mixing and Li/TM ratio (Supplementary Fig. 10a-e and Table 3, 30-34). This indicates that Li loss during oxidation is positively correlated with the amount of anti-site Ni2+ (Supplementary Fig. 10f). This correlation arises because the anti-site Ni2+ occupies a distorted NiO6 octahedron with longer, weaker Ni-O bonds and an unstable coordination environment34, 35. Consequently, its electrons are more easily extracted by oxidizing agents, leading to oxidation to Ni3+. Samples with fewer anti-site Ni2+ are more structurally stable, reducing the likelihood of Ni2+ oxidation and consequently weakening the Li+ loss and structural changes.

The oxidation also induces fragmentation of the secondary particles of O-NCM523. As shown in Fig. 3g, the high-resolution transmission electron microscopy (HRTEM) image of S-NCM523 shows a lattice spacing of 2.39 Å in region I near the surface, corresponding to the rock salt phase, as confirmed by SAED. Fig. 3h shows a layered structure on the O-NCM523 surface (region Ⅲ) with a lattice fringe spacing of 4.83 Å, corresponding to the (001) plane of β-NiOOH (space group P3m1)36, 37. Region Ⅳ, with a lattice spacing of 2.40 Å, corresponds to the (101) plane of LiNiO2. These observations confirm the oxidation of surface NiO to NiOOH. Scanning electron microscopy (SEM) verifies that the secondary particles in S-NCM523 (Fig. 3i) fragment into primary particles after oxidation (Fig. 3j). Two main factors drive the fragmentation of secondary particles. First, the oxidation from Ni2+ to Ni3+ in the bulk causes partial Li+ extraction to maintain charge balance. The removal of Li+ increases the negative charge density of the oxygen layers, thereby strengthening the electrostatic repulsion between adjacent oxygen layers38. Consequently, the lattice parameter c of O-NCM523 increases to 14.292 Å, higher than the 14.246 Å of S-NCM523 (Fig. 3k). Second, the surface phase transition from NiO to layered β-NiOOH significantly expands the surface lattice spacing (2.40 Å to 4.83 Å). These changes increase stress between the primary particles, fragmenting secondary particles into primary particles. The regularized particle morphology homogenizes the Li+ diffusion paths and distances during subsequent re-lithiation. Subsequently, based on the density functional theory (DFT) calculation, the Li+ diffusion energy barrier in O-NCM523 (0.51 eV) is lower than that in S-NCM523 (0.55 eV), as shown in Fig. 3l, m.

Regeneration mechanism of O-NCM523
Subsequently, S-NCM523 and O-NCM523 were calcined at 900 °C for 10 h to obtain D-NCM523 and R-NCM523, respectively. As shown in Fig. 4a, as compared to the characteristic NPDF peaks before annealing, the r values of d1-d5 decrease after high-temperature Li replenishment, indicating that TM is oxidized to a higher valence state. Meanwhile, meticulous comparison demonstrates that R-NCM523 restores the order of the crystal structure more sufficiently than D-NCM523. The TM-O bond length in R-NCM523 is 1.945 Å, which is shorter than that in D-NCM523 (1.995 Å), suggesting more complete oxidation of Ni2+. Moreover, the d1 peak intensity of R-NCM523 is lower than that of D-NCM523, indicating a stronger Jahn-Teller effect and lower Li+/Ni2+ mixing. Therefore, the Li+ diffusion barrier of R-NCM523 (0.41 eV) is lower than that of D-NCM523 (0.48 eV), as shown in Fig. 4b. NSPD refinements reveal specific Li+/Ni2+ mixing, with R-NCM523 at 2.06%, which is less than the 3.50% in D-NCM523 (Fig. 4c, d and Supplementary Table 14 and 15). This is consistent with the NPDF and X-ray diffraction (XRD) results (Supplementary Fig. 11 and Table 20-24), indicating that R-NCM523 has a highly ordered layered structure.

Furthermore, the HRTEM image in Fig. 4e shows that the lattice spacing of R-NCM523 is 4.70 Å (regions Ⅴ and Ⅵ) in both the surface and bulk phases, corresponding to the (003) plane of LiNiO2, with the SAED pattern confirming the regeneration of the layered structure. These results demonstrate that high-temperature Li+ replenishment restores the degraded NCM523 crystal structure. XPS peak positions and intensities for Ni, Co, Mn and O remain constant from surface to bulk in R-NCM523 (Supplementary Fig. 12), and the A1g and Eg characteristic peaks of R-NCM523 in the Raman spectra are sharper than those of D-NCM523 (Supplementary Fig. 13), indicating more complete restoration of composition and layered ordering after calcination. This improvement arises from the reduced Li+/Ni2+ mixing in the bulk phase of O-NCM523 and the conversion of the surface rock salt phase to the layered NiOOH, which together facilitate Li+ re-insertion into the layered structure during subsequent calcination.

It is underscored that the pre-oxidation supported R-NCM523 obtains not only a unified single-crystal morphology but also a rare large size. To investigate the uniformity of elemental valence states within the particles, as shown in Supplementary Fig. 14, Electron energy loss spectroscopy (EELS) measurements were performed on different regions of the particles. The O K-edge and Ni L-edge spectra exhibit consistent peak positions and intensities, with no detectable Ni reduction or oxygen vacancy defects, indicating uniform elemental valence states within the particles. The SEM image in Fig. 4f clearly shows that R-NCM523 consists of primary particles with a flat, smooth surface, regular shape and a size of 3-7 μm. Meanwhile, R-NCM523 reaches a high tap density of 2.57 g/cm3 (Supplementary Table 5), significantly higher than that of commercial single-crystal NCM523 (CS-NCM523, ~1.62 g/cm3, ~2.57 μm, Supplementary Fig. 15a, b). This structural feature originates from the NiOOH-layered surface formed on O-NCM523. DFT calculations show that the energy barrier for Li+ migration from Li2O to NiO is 8.85 eV, whereas the barrier from Li2O to NiOOH is only 1.89 eV (Supplementary Fig. 17), indicating that the formation of NiOOH at the surface reduces the Li+ diffusion barrier during calcination. Moreover, during low-temperature lithiation, NiOOH readily converts to layered LiNiO2 39, 40, whose lattice compatibility facilitates grain boundary fusion and promotes particle growth. By contrast, Fig. 4g shows irregular and agglomerated particles in D-NCM523, with a particle size of 5-10 μm. Additionally, the lattice parameter c of 14.188 Å for R-NCM523 is smaller than that of D-NCM523 at 14.210 Å (Fig. 4h), indicating that the layered structure of R-NCM523 is more stable.

Microstructural evolution of O-NCM523 after high-temperature treatment
The subsequent high-temperature treatment also plays a key role in the structure recovery and the morphology control. Fig. 5a shows the NPDF patterns at different regeneration temperatures, reacting to the changes of bond lengths between TM-TM and TM-O atoms, which are used to determine the oxidation state of the Ni element. The TM-O and TM-TM bond lengths gradually shorten as the regeneration temperature increases from 600 to 1000 °C, indicating the continuous increase of the oxidation state of Ni, and finally, the TM-O and TM-TM bond lengths shorten to 1.945 Å and 2.815 Å, respectively, at 900 ℃. Moreover, an increased d4/d3 ratio and enhanced d5 intensity suggest higher TM occupancy in the second and third shells, correlated with reduced Li+/Ni2+ mixing41. Obviously, samples treated at 900 °C exhibit minimal Li+/Ni2+ mixing and maximal layered ordering (Fig. 5b).

Fig. 5c shows the cation ordering degree and the change of lattice parameter c obtained from the refinement of NPD patterns at different regeneration temperatures, corresponding to the layered structure ordering. Clearly, the degree of Li+/Ni2+ mixing decreases with increasing temperature, reaching a minimum of 2.06% at 900 °C, with a lattice parameter c of 14.188 Å. These results indicate that 900 °C is the optimal regeneration temperature, corresponding to the highest Ni oxidation state and the most ordered layered structure. For the 600 °C sample, there is still a residual 1.10% of the spinel phase, which fails to revert to a layered structure, ultimately leaving a 4.0% Li vacancy (Supplementary Fig. 20b and Table 16), and the above defects disappear when the regeneration temperature reaches 700 °C or above (Supplementary Fig. 20c, d and Table 17, 18). At 1000 °C, oxygen vacancies form, lowering the Li+/Ni2+ migration barrier and facilitating Li+ migration into the TM layer42, thus intensifying the Li+/Ni2+ mixing to 3.81% (Supplementary Fig. 20f and Table 19) and lengthening the TM-O and TM-TM bond lengths. In O-NCM523 and its 600°C regenerated samples, a multi-phase structure is observed, containing LiNiO2, Co3O4, and NiOOH phases. The existence of multiple phases greatly impacts the intensity of the short-range d1 peak in NPDF43, making it unreliable for evaluating Li+/Ni2+ mixing. In contrast, the intensity of the long-range d3, d4, and d5 peaks is relatively less impacted by multiple phases, making them suitable for analyzing Li+/Ni2+ mixing.

In-situ and ex-situ temperature XRD tests further confirmed that the most ordered layered structure, along with the lowest Li+/Ni2+ mixing, was achieved at a regeneration temperature of 900 °C. As shown in Fig. 5d, the (003) peak shifts to lower angles with increasing sintering temperature, confirming successful Li+ intercalation into the layered structure and layer spacing expansion, consistent with the NPDF and NPD results. But a clear Li2CO3 signal is observed below 600°C (Supplementary Fig. 21), which results from the incomplete insertion of Li+ into the layered structure and the consequent conversion of the remaining Li2CO3 on the surface of the particles.

The recovery of the layered structure in NCM523 is reflected in the intensity ratio of the characteristic peaks (003) and (104)44. Clearly, the I003/I104 ratio reaches a maximum at 900°C before decreasing at higher temperatures. SEM was employed to observe grain growth at different regeneration temperatures. As shown in Fig. 5f, at 600 °C, some Li2CO3 adheres to the particle surfaces, and the particle morphology does not change significantly compared to the precursor. As the temperature increases to 800 °C, fragmented particles fuse and grow into larger particles, approximately 1 μm in size. Optimal particle evolution occurs at 900 °C, where the particles are regularly shaped and range from 3-7 μm, exhibiting homogeneous dispersion. However, further heating to 1000 °C causes excessive grain boundary fusion, resulting in abnormal particle growth that severely hampers Li+ extraction and insertion.

Electrochemical performance
The electrochemical performances of S-NCM523 were systematically investigated. As shown in Fig. 6a, S-NCM523 has a discharge capacity of 42.8 mAh/g at 0.1 C, but after regeneration, the capacity of R-NCM523 is restored to 158.9 mAh/g, comparable to commercial NCM523 (C-NCM523). The galvanostatic intermittent titration (GITT) curve shows that S-NCM523 exhibits the highest polarization and overpotential during pulsed charge/discharge due to the severe structural degradation (Supplementary Fig. 22f). In contrast, the overpotential of R-NCM523 (14.0 mV) is much lower than that of S-NCM523 (129.0 mV) and similar to C-NCM523 (14.0 mV).

R-NCM523 exhibits an excellent rate performance. It delivers capacities of 132.7 and 112.8 mAh/g at 2 and 5C, respectively, close to that of C-NCM523 (Fig. 6b). At low rate of 0.1C, the capacity difference between D-NCM523 and R-NCM523 is ~17.0 mAh/g. As the rate increased, the capacity difference increases, too. At 5C rate, the capacity of D-NCM523 is ~35.0 mAh/g lower than that of R-NCM523. This can be attributed to the higher Li+/Ni2+ mixing and uneven particle size of D-NCM523, which blocks Li+ diffusion pathways and increases the Li+ diffusion distances. The electrochemical impedance spectroscopy (EIS) in Supplementary Fig. 23, 24 and the fitting data in Supplementary Table 7-10 show that the interfacial impedance (Rf) and charge transfer impedance (Rct) of R-NCM523 are significantly lower than those of S-NCM523 and D-NCM523, and close to C-NCM523. Cyclic voltammetry (CV) at a sweep rate of 0.1 mV/s demonstrates that the redox peak difference of R-NCM523 is similar to that of C-NCM523 (Supplementary Fig. 25), indicating excellent electrochemical reversibility. In contrast, S-NCM523 and D-NCM523 show larger redox peak differences. Analysis of GITT and CV data shows that R-NCM523 and C-NCM523 have higher Li+ diffusion coefficients, while S-NCM523 and D-NCM523 exhibit lower Li+ diffusion coefficients (Supplementary Fig. 26, 22a-e).

As shown in Fig. 6c, R-NCM523 has a reversible discharge capacity of 144.2 mAh/g at 4.3 V, 1C, with a high capacity retention of 80.2% after 600 cycles. R-NCM523 and C-NCM523 exhibit identical retention rates for the first 300 cycles, but C-NCM523 declines sharply due to secondary particle cracking (Supplementary Fig. 27), dropping to 67.8% after 500 cycles. CS-NCM523 maintains a capacity retention of 78.0% after 600 cycles, slightly lower than that of R-NCM523 (Supplementary Fig. 15d). This is attributed to the smaller particle size of CS-NCM523, which increases interfacial side reactions and undermines long-term structural stability45. D-NCM523 exhibits a capacity retention rate of only 59.3% after 400 cycles. As shown in Fig. 6d, XANES spectra identify the Ni chemical valence states of R-NCM523 and C-NCM523 after cycling. The Ni K-edge spectra of R-NCM523 shift to higher energies, suggesting a higher valence state of Ni in R-NCM523 than in C-NCM523 after cycling, with less Li+/Ni2+ mixing and rock salt. Meanwhile, EXAFS Fourier and wavelet transform analyses reveal shorter Ni-O and Ni-TM bonds in R-NCM523 than in C-NCM523 after cycling, suggesting better structural stability (Fig. 6e and Supplementary Fig. 29). In situ XRD analysis provides further insight into the excellent cycling stability of R-NCM523. As shown in Fig. 6f, for the (003) diffraction peak, the irreversible transition shift from H2 to H3 (Δθ1) in R-NCM523 is 0.13°, which is lower than 0.15° in C-NCM523. A similar trend is observed for the (101) and (104) diffraction peaks (Supplementary Fig. 30a, b). In addition, at the end of charging, the lattice parameter changes of R-NCM523 (Δc = 0.7%, Δa = 1.0%) are less than those of C-NCM523 (Δc = 1.0%, Δa = 1.2%), indicating smaller lattice distortions, thus reduced accumulation of internal stresses during long cycling (Supplementary Fig. 30c, d). Therefore, R-NCM523 has excellent structural stability during charge/discharge cycling.

In addition, to further investigate the structural stability of R-NCM523, its cycling performance at high voltage (4.5 V) and high rate (5C) was also tested. As shown in Fig. 6g, even after the voltage increases to 4.5 V, the retention rate is 81.6% after 500 cycles, which is much higher than the 59.9% of C-NCM523. R-NCM523 retains up to 72.5% of its capacity after 1000 cycles at 5C, with a degradation rate of only 0.0275% per cycle (Supplementary Fig. 31). The above electrochemical performance indicates stable cycling performance for directly regenerated NCM materials (Fig. 6h, Supplementary Table 11)19, 20, 24, 46-55. To validate the practical applicability of the regenerated positive electrode, a 1.0 Ah pouch cell was assembled using R-NCM523 as the positive electrode and graphite as the negative electrode. As shown in Fig. 6i, R-NCM523 exhibits stable cycling performance with a retention rate exceeding 78.3% after 600 cycles at 3.0-4.4 V, 1C.

In addition, it is underscored that the pre-oxidation route can also regenerate S-NCM622 and S-NCM811, achieving retention rates of 79.1% for R-NCM622 and 77.6% for R-NCM811 after 200 cycles at 0.5C (Supplementary Fig. 32). Moreover, the XRD refinement and ICP results for NCM622 and NCM811 samples before and after regeneration are shown in Supplementary Fig. 33 and Supplementary Table 35-40. Thus, by using this proposed strategy, we successfully regenerated three degradable materials from different sources, all demonstrating excellent performance. This highlights the broad applicability of the proposed method and its great potential for practical use.

Techno-Economic Analysis
The oxidation strategy offers several advantages. First, the regeneration process is concise, without requiring complex or hazardous devices. Second, it employs only conventional lithium salts like LiOH, reducing process costs. Third, Na2S2O8 serves as the reagent in this method, and subsequent addition of Ca(OH)2 enables SO42- to form CaSO4 precipitation, minimizing environmental impact. Additionally, to further verify that no sulfur oxide species are released during the high-temperature regeneration process, TG-MS testing was conducted. The corresponding TG and MS signals are shown in Supplementary Fig. 34a, b. Over the entire temperature range from 30 to 900 °C, only signals at m/z = 18 (H2O) and m/z = 44 (CO2) are detected, corresponding to water release and the decomposition of Li2CO3, respectively. No signals at m/z = 64 (SO2) or m/z = 80 (SO3) are detected. The TG-MS results clearly indicate that no SO2 or SO3 is emitted during the high-temperature sintering of O-NCSM523 regeneration. Therefore, the oxidation strategy offers advantages of high efficiency, low energy consumption, low emissions, and high profits.

The energy consumption of this method is 23.94 MJ/kg, and greenhouse gas (GHG) emissions are 3.11 kg/kg, both lower than those of the hydrothermal and molten salt methods, respectively (Supplementary Fig. 34c, d). Considering the comprehensive overall process costs, the oxidation method has a profit of $6.47/kg, which is 113.5% and 141.4% higher than the hydrothermal and molten salt methods, respectively (Supplementary Fig. 34e, f). In conclusion, compared to existing direct recycling technologies, the oxidation strategy has better capabilities in reducing GHG, lowering energy consumption, and enhancing profits. It provides an efficient, scalable solution for power battery recycling with broad industrial application potential.

Methods
Battery Disassembly Separation
Ten ~4Ah spent LiNi0.5Co0.2Mn0.3O2 (S-NCM523) pouch cells were discharged by immersing them in 2 mol/L sodium chloride (NaCl, 99%, Aladdin) solution for 24 h. Then, the cells were manually disassembled to obtain the positive electrode. The positive electrode was cleaned with dimethyl carbonate (DMC, 99%, Aladdin) and dried. The S-NCM622 and S-NCM811 were obtained using the same method with spent commercial NCM622 and NCM811 pouch cells. Spent pouch cells from Nanjing Tongning New Materials Research Institute Co., Ltd.

Direct regeneration process
S-NCM523 powder was placed in 0.5 mol/L sodium persulfate (Na2S2O8, 99%, Aladdin) and 1.0 mol/L sodium hydroxide (NaOH, 98%, Aladdin). The solution was stirred at 400 rpm for 1 h at 25℃ and filtered to obtain a filtered residue, which was then washed three times with deionized water. Subsequently, calcium hydroxide (Ca(OH)2, 95%, Macklin) was added to the filtrate to precipitate SO42- as CaSO4. The washed residue was dried in a vacuum drying oven to get a powder, which was recorded as the oxidized intermediate (O-NCM523). Then, O-NCM523 was uniformly ground with lithium hydroxide monohydrate (LiOH·H2O, Ganfeng Lithium Co., Ltd.) to achieve a molar ratio of Li/(Ni + Co + Mn) = 1.2: 1 and then calcined in a muffle furnace at 900 °C for 10 h (heating rate: 5 °C/min, atmosphere: air) to obtain the regenerated material, recorded as R-NCM523. Considering that the Li/TM molar ratio of O-NCM523 is 0.45, an additional 0.75 molar Li was added to reach the target ratio of Li/TM = 1.2. In addition, S-NCM523 powder was mixed with LiOH·H2O to achieve a molar ratio of Li/(Ni + Co + Mn) = 1.2: 1 and calcined in a muffle furnace at 900 °C for 10 h (heating rate: 5 °C/min, atmosphere: air). The regenerated material obtained in this way was named D-NCM523. The oxidation conditions for S-NCM622 and S-NCM811 were the same as those described above, with regeneration temperatures of 850 °C for S-NCM622 and 780 °C for S-NCM811. The commercial poly-crystal material was denoted as C-NCM523. The commercial single-crystal material was denoted as CS-NCM523 (Xinxiang Tianli Lithium Energy Co.), with a tap density of 1.62 g/cm3.

Material characterizations
The XRD measurements were carried out on an Empyrean model instrument from Panacor in the Netherlands under the following conditions: radiation source Cu Kα (λ = 1.54184 Å), and a measuring range of 10-80°. The in-situ XRD measurements used CR2032 coin cells with a glass window (Wuhan Congtical Technology Co., Ltd). The XRD data were characterized and analyzed using HighScore Plus and GSAS II software. The elemental contents of the materials and solutions were characterized by an inductively coupled plasma emission spectrometer (ICP-OES) from Leeman Labs, USA. The microstructure and morphology of the materials were analyzed by a Nova NanoSEM450 field emission scanning electron microscope (SEM) from FEI, USA, a Tecnai G2 20 and Talos F200X transmission electron microscope (TEM) from FEI, Netherlands. The elemental distribution analyses by EELS were performed using a Titan G2 60-300 high-resolution STEM microscope from Thermo Fisher Scientific (USA). To obtain the cross sections for the SEM, the particles were cut by the focused ion beam. X-ray photoelectron spectroscopy (XPS) of AXIS-ULTRA DLD-600W from Shimadzu-Kratos, Japan was utilized to detect the variation of elements and valence states on the surface of the materials and combined with Avantage software for analysis. The tap density of the material was tested using an AccuPyc 1340 fully automated true density tester from Anton Paar, Austria, with the vibration frequency set at 250 cycles per minute the total vibration count set to 8000 cycles. The variation of functional groups and bond lengths of materials was investigated by a LabRAM HR800 laser confocal Raman spectrometer (Raman) from Horiba Jobin Yvon. The local structural evolution of the material (atomic occupancy, bond lengths between atoms) was measured by the Multi-Physics Spectrometer (MPI) at the China Scattered Neutron Source (CNSNS) and combined with the GASA II and PDFgui software to analyze it. The X-ray absorption spectroscopy (XAS) experiments were performed at beamline P64 Advanced X-ray Absorption Spectroscopy at PETRA III (Germany) and RapidXAFS 2M X-ray Absorption Spectrometer (Anhui Absorption Spectroscopy Analysis Instrument Co., Ltd.). The obtained XAS data were processed using ATHENA software.

Theoretical calculations
Our computational framework implements the Vienna Ab Initio Simulation Package (VASP) for density functional theory (DFT). To describe the exchange-correlation function, the generalized gradient approximation (GGA) was utilized in the Perdew-Burke-Ernzerhof (PBE) scheme. The projector augmented wave (PAW) formalism was adopted to model ionic cores, with valence electrons represented through a plane-wave basis set featuring a 500 eV kinetic energy cutoff. Partial occupancies of the Kohn-Sham orbitals were allowed using the Gaussian smearing method with a width of 0.05 eV. For energy variations less than 10-5 eV, the electron energies were regarded as self-consistent. When the force variation was less than 0.05 eV/Å, the geometry optimization was regarded as convergent. The k-point sampling was performed using gamma points in the Brillouin zone and allowing all atoms to relax during the structure optimization process. Lastly, the nudged elastic band (NEB) approach was used to characterize the Li migration transition state at the path steps. In the NEB approach, pathways among reactant and product were discretized into a series of structural images.

Electrochemical measurements
For coin cell preparation, NCM powder, carbon black (Super P, 99.9%, TIMCAL), and polyvinylidene fluoride (PVDF900, 99%, Canrd Technology Co., Ltd.) were mixed in a weight ratio of 8:1:1 and dissolved in N-methyl-2-pyrrolidone (NMP, 99%, Aladdin). After stirring for 4 hours to achieve a uniform slurry, the slurry was spread onto carbon-coated Al foil (15 μm thick, 99.9%) using a doctor blade and dried in an oven at 80°C for 6 hours. The electrodes were punched into 8 mm discs (0.503 cm2), with a mass loading of 4-6 mg/cm2. CR2032 coin cells were assembled in an Ar-filled glovebox (25 ± 1°C, O2 and H2O < 0.01 ppm), using Celgard 2500 (25 μm thick, 19 mm diameter, 55% porosity, from Celgard, USA) as the separator and lithium metal (0.5 mm thick, 12 mm diameter, China Energy Lithium Co., Ltd.) as negative electrode. Each coin cell was filled with 40 μL of electrolyte (1.0 M LiPF6 in EC: DEC = 1:1 vol%, with 5.0% FEC). The positive and negative cases, along with the nickel foam mesh, were purchased from Canrd Technology Co., Ltd. Cycling was tested at 3.0-4.3 V and 3.0-4.5 V (1C=170 mA/g).

Pouch-type full cells with ~1 Ah capacity assembled using R-NCM523 as the positive electrode and graphite as the negative electrode. The positive electrode had a mass loading of ~29.8 mg/cm2 and consisted of 96.5 wt% active material, 1.5 wt% conductive additive, and 2.0 wt% PVDF binder, with an electrode size of 8.0 × 6.0 cm2. The negative electrode had a mass loading of ~16.0 mg cm2 and consisted of 95.4 wt% active material, 2.0 wt% conductive additive, and 2.6 wt% binders, including carboxymethyl cellulose (CMC, Canrd Technology Co., Ltd.) and styrene-butadiene rubber (SBR, Canrd Technology Co., Ltd.), with an electrode size of 8.4 × 6.3 cm2. In the pouch battery, the N/P ratio is 1.15. Approximately 3 mL of electrolyte was uniformly introduced into the pouch cell using a pipette. After sufficient wetting, the cell was transferred to a vacuum sealer. Before final heat sealing, a 5 s degassing step was performed under a vacuum of -98.0 kPa. During cycle testing, an external pressure of ~200 kPa was applied to the pouch cell to ensure stable electrode contact. Cycling performance was tested at 3.0-4.4 V, 1C. The compacted density, porosity and volumetric capacity of the positive electrode are shown in Supplementary Fig. 28.

The Princeton electrochemical workstation measured cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) with a voltage range of 3-4.3 V. The CV scan rate ranges from 0.1 to 1.0 mV/s. Additionally, EIS was performed over the frequency range 0.01-100,000 Hz at a signal amplitude of 10 mV. A total of 71 data points were collected within the test frequency range. The Galvanostatic Intermittent Titration Technique (GITT) measurement was conducted using a 10-minute constant-current charge and a 40-minute rest. All coin cells and pouch batteries were rested for 12 hours to ensure complete electrolyte wetting before electrochemical testing. All electrochemical measurements were conducted at 25±1°C using NEWARE and Land battery testing systems without a temperature-controlled chamber.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This research introduces an innovative pre-oxidation strategy for directly regenerating spent NCM cathode materials. The\nauthors achieve surface-to-bulk Li+ diffusion channel restoration by converting surface inert NiO to layered NiOOH and\nreducing Li+/Ni2+ mixing by oxidizing the anti-site Ni2+ in the bulk phase to Ni3+ via the one-pot pre-oxidation treatment. I\nfind that the engineering is very good which not only facilitates the decomposition of secondary particles into primary\nparticles, enabling subsequent homogeneous re-lithiation, but also facilitates the\ncoalescence of particles in subsequent calcination, enabling the formation of large\nsingle-crystal. As a result, the regenerated single-crystal NCM achieves a high\ntap density along with excellent electrochemical performance. In my opinion, the article\nmeets the standards of Nature Communications and can be accepted for publication\nafter minor revisions.\n1. The study demonstrates that a one-hour oxidation treatment of spent NCM523 effectively transforms the NiO rock-salt\nphase into layered NiOOH while simultaneously oxidizing Ni2+ to Ni3+. Why is one-hour? Experimental data for shorter\noxidation times should be supplemented to improve the reliability of the oxidation experiment.\n2. The authors suggest that spent NCM523 contains Ni2+ anti-site defects, which could be removed by targetedly oxidizing\nNi2+ to Ni3+. To better assess this approach, the change of anti-site defects before and after oxidation should be\nsystematically studied in spent NCM523 materials with different levels of degradation. Additionally, it remains to be seen\nwhether this oxidation process would also change the Ni2+ in the transition metal layers into Ni3+.\n3. The authors should include comparative experiments to show that the spent NCM523 after oxidation treatment forms\nlarger single-crystal particles during subsequent high-temperature repair.\n4. For spent NCM622 and NCM811 cathode materials, the transformation of crystal structure and Li+/Ni2+ mixing after\nregeneration should be supplemented.\n5. Fig. 6h demonstrates superior cycling stability of the regenerated NCM523 material compared to literature reports. Please\nspecify the cycle numbers and corresponding capacity retention rates from comparative literature reports.\n6. The tap density measurements should be accompanied by complete testing parameters, including vibration frequency\nand vibration cycles. Also, please provide the testing parameters for the EIS and GITT.\n7. The O 1s XPS peaks at ~529.5 eV in Supplementary Fig. 3 and 4 are inconsistently assigned as either lattice oxygen or\nTM-O bonds. A unified interpretation must be adopted throughout. Additionally, the sample labeling in Supplementary Fig.\n13 needs to match the corresponding figure legend.", "reviewer_2": "The submitted manuscript, “Oxidation-Reconstructed Li+ Transport Enables High-Tap-Density Single-Crystal Regeneration\nof Spent NCM Cathodes”, presents a timely study on the direct regeneration of spent Ni-rich NCM cathodes through an\n\"oxidation-reconstructed\" strategy. The core idea of using pre-oxidation to simultaneously reduce cation mixing, reconstruct\nLi transport pathways, and facilitate high-tap-density single-crystal formation addresses critical challenges in spent battery\nrecycling. The recovered electrochemical performance, particularly the impressive cycle stability, certainly represents a step\nforward in the field.\nHowever, some noticeable issues prevent this manuscript from being fully convincing and scientific. Therefore, the reviewer\nrecommends that this manuscript be transferred to a more suitable journal after addressing the following issues.\nComments:\n1. Previous studies (e.g., Ref. 1 and Ref. 2) have already proposed pre-oxidation-based strategies for regenerating spent\nNCM cathodes. Ref. 1 uses NH₄OH to convert surface rock-salt and spinel phases into a Ni(OH)₂ structure prior to direct\nregeneration, and Ref. 2 reports the use of the same oxidizing agent as in your study (Na₂S₂O₈) for recycling NCM622, along\nwith an explanation of the chemical mechanism involved.\nBased on prior research, it remains unclear what differentiates the major approach from those in the existing literature. The\nkey originality of the strategy should be clearly explained. Specifically, the strategy in the manuscript should provide a\ndistinct reaction mechanism, processing advantage, or practical improvement over the techniques described in previous\nstudies. Such well-structured comparisons are essential to establish the novelty and contribution of this work.\n2. The term \"High-Tap-Density\" appears in the title, suggesting that it represents a key outcome of the study. However, the\nmanuscript does not present any data demonstrating the advantages of high tap density in practical battery applications. To\nsupport the application potential as a battery cathode, the authors should include evidence such as electrode fabrication\nwith high compaction density and improved energy density performance compared with appropriate reference cathode\nelectrodes.\nMoreover, the experimental section does not report essential information, such as the electrode density, a critical parameter\ndirectly linked to energy density. Furthermore, the slurry used contains only 80% active material, which significantly deviates\nfrom typical commercial electrode formulations. These factors limit the evaluation of practical applicability and raise\nconcerns about the feasibility of implementing the developed material in real battery systems.\n3. After the oxidation treatment, the NiO phase underwent a phase transition into NiOOH, whereas the spinel phase\nremained unchanged. It would be helpful if the authors could clarify why the spinel phase did not undergo a phase transition.\nIn addition, the diffraction data for the O-NCM sample do not show any distinct peaks corresponding to the NiOOH phase.\nPlease explain why the NiOOH-related peak is absent in the diffraction analysis, despite the proposed formation of NiOOH.\n4. The description of the TM–TM (transition metal–transition metal) distance for O-NCM523 in lines 149–151 appears to\ncontradict the data presented in Fig. 3b. The authors state that O-NCM523 exhibits a shorter d2 distance compared to S-\nNCM523, whereas the graph shows that the d2 bond length for O-NCM523 is actually longer. This inconsistency between\nthe textual explanation and the presented data should be clarified.\nIn addition, the authors suggest that Li loss after oxidation treatment may originate from Li at antisite positions. However, this\ninterpretation lacks sufficient supporting evidence. Providing further data or analysis would help to improve the overall\nreliability of the structural interpretation.\n5. As shown in the SEM images in Fig. 3i and 3j, polycrystalline NCM particles undergo fragmentation after oxidation\ntreatment, whereas single-crystal NCM particles retain their original morphology. This indicates that the morphology change\nresponse to oxidation depends on the particle type. If such initial morphology-dependent behavior exists, it may present a\nlimitation to the complete homogenization of particles as discussed in the manuscript.\n6. The choice of appropriate control samples is critical for both material characterization and electrochemical performance\nevaluation. If the pristine (pre-degraded) S-NCM had been compared alongside both R-NCM and S-NCM, the novelty of the\nproposed regeneration strategy could have been more clearly demonstrated.\nIn addition, the electrochemical performance of R-NCM was mainly compared with that of C-NCM, which is based on\ncommercial secondary particles. However, considering the purpose of the evaluation, using a primary particle-based\ncommercial NCM would have provided a more meaningful and fair comparison. Such a design would have better\nhighlighted the practical advantages of R-NCM.\n7. The NPDF analysis presented in Fig. 3b reveals noticeable variations in the d1 (TM–O) peak. The authors note that the\nd1 (TM–O) bond lengths observed in O-NCM523 are shorter than those in S-NCM523, attributing this primarily to oxidation-\ninduced contraction of ionic radii as the transition metals adopt higher valence states. However, a more comprehensive\ndiscussion is necessary regarding how both cation mixing and changes in the valence states collectively influence the\nintensity of the d1 peak (symmetry of TM-O bonding), particularly in relation to Jahn–Teller distortions. (Ref. 3) Given that S-\nNCM523 exhibits a more reduced state compared to O-NCM523, it likely contains fewer Ni3+ cations, consequently\ndiminishing the Jahn–Teller effect.\n8. Although the XPS depth profiling presented in Fig. 3d effectively illustrates oxidation state changes at the surface, X-ray\nAbsorption Near Edge Structure (XANES) analysis is typically considered a more quantitative and reliable method for\nassessing the bulk oxidation states of transition metals. The authors suggest that anti-site Ni2+ in the bulk phase of O-\nNCM523 is selectively oxidized to Ni3+. Given the critical nature of this assertion, it would be beneficial for the authors to\ninclude Ni K-edge XANES spectra for both S-NCM523 and O-NCM523. Providing these spectra would directly confirm\nthe bulk oxidation state changes of Ni following pre-oxidation, thereby considerably reinforcing the mechanistic insights\npresented in the study.\n9. The manuscript reports the presence of a notable amount (6.14%) of NiO in S-NCM523 based on Rietveld refinement\n(Fig. 3e). However, accurately distinguishing and quantifying NiO using Rietveld refinement alone can be difficult, as the\nmain diffraction peaks of NiO substantially overlap with those of layered oxides such as LiNiO2. To more definitively confirm\nthe presence of NiO particularly in the bulk it is recommended that the authors consider complementary techniques capable\nof directly probing the local coordination environment and bond distances around Ni. For instance, Ni K-edge EXAFS could\nprovide valuable insight into the local structural configuration and help validate the existence of a distinct NiO phase within\nthe material. (Ref. 3)\n10. It is generally accepted in NCM research that the oxidation of nickel (Ni2+ → Ni3+) leads to a contraction of the Ni–O\nbond lengths, thereby reducing the overall lattice parameters. However, Fig. 3m reveals that the c-axis lattice parameter for\nO-NCM523 (14.293 Å) is greater than that for S-NCM523 (14.246 Å), which seems contradictory. Although the authors\nsuggest that this expansion arises from a combination of bulk Ni3+ formation and surface NiO-to-NiOOH conversion, a more\ncomprehensive theoretical explanation is necessary. Specifically, it would be beneficial for the authors to clarify how the\nproposed factors including reduced Li+/Ni2+ mixing and surface-phase transformations collectively result in c-axis\nexpansion despite the anticipated lattice contraction from nickel oxidation.\n11. In Figs. 4 and 5, the authors attribute the reduction in cation mixing primarily to changes observed in the d3, d4, and d5\npeaks of the NPDF patterns, highlighting an increased d4/d3 ratio and enhanced intensity of d5. While these peaks\ncorrespond to higher-order coordination shells (e.g., TM1st–O2nd and TM1st–TM2nd interactions), typically, the d1 (TM–O)\npeak is the direct and sensitive indicator of Ni2+ oxidation and associated local structural modifications. If Ni2+ oxidation is\nindeed significant, one would expect noticeable alterations in the d1 peak intensity due to variations in bond character and\nlocal atomic order. It would strengthen the manuscript if the authors could elaborate on why cation mixing reduction was\nprimarily correlated with changes in d3, d4, and d5, rather than a detailed assessment of the d1 peak.\n12. The novel pre-oxidation strategy is a key innovation in this study. However, the\nexperimental parameters described in the Methods section (Page 20, Lines 438–443), as well as Supplementary Table 2, are primarily limited to variations in reaction time.\nImportant variables such as oxidizing-agent concentration (e.g., Na2S2O8 molarity), the NaOH concentration above the fixed value of 1.0 mol/L, and reaction temperature were either maintained constant or only implicitly explored. (1)\nDid the authors systematically investigate how varying oxidizing-agent concentration impacts oxidation efficiency, the extent of Li+/Ni2+ mixing reduction, and lithium loss? (2) Although reactions were conducted at room temperature, was there any exploration of reaction temperature variations to better understand their influence on oxidation efficiency, lithium loss, and final material characteristics (e.g., morphology, crystallinity)?\n13. The manuscript highlights the successful formation of \"large single-crystal\" R-NCM523 particles with uniform\nmorphology. For materials of this type, ensuring uniform lithium distribution and insertion within the large crystals is critical.\nDid the authors perform any spatially resolved analyses (e.g., Li-sensitive EELS, lithium-ion microscopy, or other Li-sensitive\ncharacterization techniques) to verify uniform lithium insertion within these regenerated single-crystal particles? Confirming\nsuch uniformity would substantiate the effectiveness of the regeneration approach at the particle level.\n14. The authors set the Li/TM molar ratio at 1.2 during the re-lithiation step, acknowledging that excess lithium typically\ncompensates for lithium loss during high-temperature synthesis. However, previous research has indicated that excess\nlithium, particularly at elevated synthesis temperatures, often results in undesirable residual lithium compounds (e.g.,\nLi2CO3 or LiOH) forming on particle surfaces, potentially degrading electrochemical performance. Did the authors\nthoroughly verify the absence of such residual lithium compounds in the final R-NCM523 material? If detected, what\nmitigation strategies were employed, and how did residual lithium impact the reported electrochemical performance?\nAlthough Supplementary Figure 14 indicates Li2CO3 formation at lower temperatures, explicit confirmation of its absence at\noptimized synthesis temperatures in the final product is necessary.\n15. In Fig. 6d, the Ni K-edge XANES spectrum of C-NCM523 appears to shift toward higher energies compared to that of R-\nNCM523, indicating that C-NCM523 may retain a higher average Ni oxidation state after cycling. In contrast, Fig. 6e shows\nthat the EXAFS-derived bond lengths for both Ni–O and Ni–TM interactions in C-NCM523 are longer than those in R-\nNCM523. Since longer bond lengths are typically associated with a lower Ni oxidation state, this presents an apparent\ndiscrepancy between the XANES and EXAFS results. To ensure a coherent interpretation of the post-cycling Ni oxidation\nstate and local structure, a more detailed discussion is needed to reconcile these seemingly contradictory observations and\nclarify the relationship between electronic state and local bonding environment in C-NCM523.\n16. In Supplementary Table 13, the Li occupancy for the R-NCM523@600 sample is reported as 0.96, based on the sum of\noccupancies at the Li1 (3b) and Li2 (3a) sites (0.9252 + 0.0348 = 0.96). Similarly, Supplementary Table 12 shows a total Li\nsite occupancy of 1.00 for D-NCM523 (0.9650 + 0.0350), although it was referred to as \"0.99\" in the main text. To ensure\nclarity and reproducibility, the methodology used to obtain these specific Li occupancy values should be explicitly stated.\nWere these occupancies refined freely during the Rietveld refinement, or were they constrained to fixed values? If they were\nrefined, a clear explanation is needed for the observed deviation from full occupancy particularly the reduction to 0.96 in the\nR-NCM523@600 sample and how this relates to the “Li vacancy” discussed in Figure 5e and\nSupplementary Figure 13b. On the other hand, if the occupancies were fixed, the\nrationale behind choosing these specific values should be provided. A clear\ndocumentation of the refinement strategy for Li occupancy is crucial to support the\nvalidity and reproducibility of the structural interpretations presented.\n17. Regarding the Rietveld refinement results presented throughout the Supplementary Information (Tables 9–25), the\nauthors consistently report only the weighted-pattern R-factor (Rwp). Although Rwp is an important measure of overall fit\nquality, including additional refinement parameters such as the Bragg R-factor (RB or RF) would further enhance the\nstatistical robustness and reliability of the presented structural data, particularly for sensitive parameters such as cation\nmixing.\n18. It is recommended that the authors include isotropic thermal displacement parameters (Biso) for each atomic site in their\nRietveld refinement tables (Supplementary Tables 9–25). Biso values provide valuable insight into the thermal vibrations\nand degree of atomic disorder within the lattice, supporting discussions related to structural stability, improved atomic\nordering, and overall material quality.\n[Reference]\n1. J. Am. Chem. Soc. 2023, 145, 7288-7300.\n2. ACS Sustainable Chem. Eng. 2021, 9, 50, 16997–17006\n3. Chem. Eng. J. 2022, 448, 137685", "reviewer_3": "This work involves wet treatment of the cathode material with sodium persulfate to pre-oxidize Ni in the cathode, facilitating\nsubsequent solid-state sintering regeneration. Although the overall drawing level is relatively high, the innovation of the\nresearch is average, lacking a comparison of advantages with the traditional solid-state lithium replenishment process,\nsuch as lithium diffusion kinetics. Moreover, from an environmental perspective, this paper lacks an environmental\neconomic analysis, which raises doubts about the use of persulfate.\n1. The manuscript lacks a quantitative comparison of Li⁺ diffusion kinetics during solid-state sintering between pre-oxidized\nand conventional regeneration routes. It remains unclear whether the pre-oxidation strategy genuinely enhances Li+\ntransport kinetics or reduces Li salt consumption in subsequent sintering. Quantitative metrics, such as Li+ diffusion\ncoefficients, activation energies, or Li salt utilization efficiency, should be provided to substantiate the claimed kinetic\nadvantages of pre-oxidation over traditional methods.\n2. While direct regeneration is positioned as an eco-friendly alternative to pyro-/hydro-metallurgy, the environmental footprint\nof the pre-oxidation step (using substantial Na2S2O8) remains unaddressed. A life-cycle assessment comparing carbon\nemissions, energy consumption, and upstream chemical production impacts between this strategy and existing recycling\nmethods is essential. Furthermore, economic viability metrics (e.g., reagent cost per ton of regenerated cathode) are absent,\nundermining the practical relevance of the proposed approach.\n3. Wastewater management from Na2S2O8 rinsing and potential SO2 emissions during high-temperature calcination (due to\nsulfate residues) are not discussed. Residual sodium ions may degrade electrochemical performance, yet no data on Na\ncontent in regenerated materials (e.g., ICP analysis) is provided. Clarification is needed on: (1) wastewater treatment\nprotocols, (2) measures to suppress SO2 release, and (3) quantification of impurity retention (Na/S) and their effects on\ncycling stability.\n4. The schematic in Fig. 2c lacks a corresponding description in the main text or caption. This figure illustrates structural\nevolution from NiO rock-salt to layered NiOOH/LiNiO2 but omits labels for atomic rearrangements or stress-induced\nfragmentation. Enhanced annotation is required to align the visual with the proposed\nmechanism.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature\nCommunications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career\nResearchers who co-review manuscripts."}, "author_response": "Response to Referees\nReviewer #1\nGeneral Comment: This research introduces an innovative pre-oxidation strategy for\ndirectly regenerating spent NCM cathode materials. The authors achieve surface-to-\nbulk Li+ diffusion channel restoration by converting surface inert NiO to layered\nNiOOH and reducing Li+/Ni2+ mixing by oxidizing the anti-site Ni2+ in the bulk phase\nto Ni3+ via the one-pot pre-oxidation treatment. I find that the engineering is very good\nwhich not only facilitates the decomposition of secondary particles into primary\nparticles, enabling subsequent homogeneous re-lithiation, but also facilitates the\ncoalescence of particles in subsequent calcination, enabling the formation of large\nsingle-crystal. As a result, the regenerated single-crystal NCM achieves a high tap\ndensity along with excellent electrochemical performance. In my opinion, the article\nmeets the standards of Nature Communications and can be accepted for publication\nafter minor revisions.\nOverall Response: We sincerely thank the reviewer for the positive comments,\nparticularly the remarks that “the engineering is very good” and “the regenerated single-\ncrystal NCM achieves a high tap density and excellent electrochemical performance”.\nIndeed, as the reviewer commented, our pre-oxidation strategy effectively transforms\nthe surface NiO into layered NiOOH and oxidizes anti-site Ni2+ in the bulk into Ni3+,\nthereby reconstructing the Li+ diffusion channels from the surface to the bulk. The\noxidation-induced structural transitions facilitate the decomposition of secondary\nparticles into primary particles, enabling homogeneous re-lithiation. These synergistic\nprocesses collectively enable the restoration of structural order and morphological\nuniformity, which are essential for achieving high-performance regeneration.\nIn the response, we have supplemented experimental data and comprehensive\nanalyses to further elucidate the oxidation mechanism. We believe these experimental\nresults and extended discussions can well address your concerns. We hope these efforts\ncan further solidify the foundation of the pre-oxidation strategy and provide clarity on\nthe scientific contributions of our research.\n1. The study demonstrates that a one-hour oxidation treatment of spent NCM523\neffectively transforms the NiO rock-salt phase into layered NiOOH while\nsimultaneously oxidizing Ni2+ to Ni3+. Why is one-hour? Experimental data for shorter\noxidation times should be supplemented to improve the reliability of the oxidation\nexperiment.\nResponse: We appreciate the reviewer’s insightful question. In this study, various\noxidation durations are systematically investigated, and the one-hour condition is\nselected as the optimized treatment to present the best results. To address the reviewer’s\nconcern, we have included data for the 0.5-hour oxidation treatment to improve the\ncompleteness and reliability of the analysis.\nAs shown in Fig. R1 and Table R1, XRD refinement indicates that although the\n0.5-hour treatment removes the NiO rock-salt phase, the Li+/Ni2+ mixing remains\nhigher (5.67%) than that of the 1-hour sample (4.59%). Consistently, ICP analysis\n(Table R2) shows a Li/TM ratio of 0.47 for the 0.5-hour sample, compared with 0.45\nfor the 1- and 6-hour ones. These results suggest that shorter oxidation time leads to\nincomplete oxidation of anti-site Ni2+. In contrast, oxidation for ≥1 hour ensures\nsufficient Ni2+→Ni3+ conversion, confirming that a 1-hour treatment is essential for\nachieving material reconstruction.\nFig. R1 XRD Rietveld refinement results of O-NCM523-0.5h and O-NCM523.\nTable R1. Structural parameters obtained from Rietveld refinement of the XRD diffraction pattern\nof O-NCM523-0.5 h. Phase 1 LiNiO : Space group: R-3m, R = 4.16%, R =2.79%, a = 2.8545 Å,\n2 wp F\nb = 2.8545 Å, c = 14.2810 Å, α = β = 90°, γ =120°"}, {"round": 2, "reviewer_comments": {"reviewer_1": "I initially reviewed this work. In the revised manuscript, the authors have devoted substantial effort to addressing the\ncomments from me and the other reviewers. The comprehensive revisions have markedly improved the rigor, reliability, and\nclarity of the manuscript, reflecting the authors' significant work. I recommend the manuscript for publication in Nature\nCommunications.", "reviewer_2": "In this revised submission, the authors have addressed the main issues raised in the first round by expanding the process-\nparameter screening, including more relevant controls and application-level tests, and strengthening several key\ninterpretations with additional XANES/EXAFS analyses. The bulk Ni oxidation-state results, the wider screening of oxidation\nconditions, and the added electrode/cell data related to the “high tap density” claim are particularly useful. That said, a few\npoints still need clarification and tighter internal consistency across the manuscript\nand SI before the paper can be finalized. On balance, I recommend minor revision.\nComments:\n1. The pouch-electrode parameters and the reported electrode compaction density are helpful. Please make sure the\nthickness/areal-loading values and the compaction-density calculation are consistent and clearly defined across the\nmanuscript, the SI, and the Response to Referees. I noticed an apparent thickness-reporting inconsistency between R-\nNCM523 and C-NCM523 in the Response to Referees.\n2. Because “High-Tap-Density” is central here, please summarize electrode-level metrics (e.g., porosity and\ncalendering/compaction conditions) and include a simple volumetric capacity (or equivalent electrode-level) comparison,\nrather than relying on tap density alone.\n3. The argument that NiOOH peaks are difficult to resolve due to low fraction/overlap is reasonable, but the refinement\nassumptions and key fit indicators should be easier to find and interpret in the main text and SI. Please also ensure the\nNPDF d1-peak interpretation (including the direction of the intensity change) is consistent between the text and figures/SI.\n4. Please include a concise comparison table versus prior pre-oxidation studies to make the claimed novelty clearer\n(conditions, target compositions, defect/mixing metrics, morphology outcome, and electrode-level performance).\n5. While the added control samples are appreciated, please clearly state whether particle size distributions and electrode\nfabrication conditions are matched across all samples; if not, explicitly justify the differences in the manuscript (and\nindicate how they may affect the comparisons).\n6. Please document residual lithium species (e.g., Li₂CO₃/LiOH) in a way that is straightforward to reproduce. Along with\nXPS, please provide a quantitative method such as potentiometric titration (or an equivalent approach), and specify the\nprocedure and the criterion used to conclude that residual Li is minimal in the final material.", "reviewer_3": "Compared to the initial version, the authors have addressed some of my concerns, and the current manuscript shows\nnoticeable improvement, which is commendable. Nevertheless, several critical issues require further clarification and\nexperimental support, as detailed below:\n1. The crystal nature (single-crystal vs. polycrystalline) of the starting material is not specified. Furthermore, the manuscript\ndoes not explain why the regenerated cathode exhibits an exclusively single-crystal morphology. A clear discussion on the\ntransformation mechanism from polycrystalline components to single-crystals should be provided.\n2. The origin of the Li2CO3 species detected in Fig. 5(f) remains unclear, including the specific formation mechanism and\nthe source of carbon. The accuracy of the XPS fitting results needs further verification. Supplementary XRD characterization\nis recommended to conclusively rule out the presence of other lithium compounds such as Li2O or LiF.\n3. A description of the Life Cycle Assessment appears to be absent from the main text. The authors are requested to confirm\nwhether this analysis has been conducted and include it if available, or to provide a justification for its omission.\n4. The concern regarding potential SO2 emission has not been adequately resolved. To provide definitive evidence,\nsupplementary gas-phase analysis (e.g., TG-MS) of the regeneration process should be conducted to demonstrate the\nabsence of SO2 release during the high-temperature sintering stage.\n5. On Line 86, “can simultaneously addressing” is grammatically incorrect and should be corrected to “can simultaneously\naddress”."}, "author_response": "Response to Referees\nReviewer #2\nGeneral Comment: In this revised submission, the authors have addressed the main\nissues raised in the first round by expanding the process-parameter screening,\nincluding more relevant controls and application-level tests, and strengthening several\nkey interpretations with additional XANES/EXAFS analyses. The bulk Ni oxidation-\nstate results, the wider screening of oxidation conditions, and the added electrode/cell\ndata related to the “high tap density” claim are particularly useful. That said, a few\npoints still need clarification and tighter internal consistency across the manuscript\nand SI before the paper can be finalized. On balance, I recommend minor revision.\nOverall Response: We are grateful to the reviewer for the constructive evaluation of\nour manuscript and sincerely appreciate the positive feedback of the revisions made in\nthe previous round. In response to the reviewer’s further feedback, we have\nsystematically supplemented and revised the manuscript, including unifying electrode\nparameters, adding electrode porosity and volumetric capacity analyses, clarifying the\nstructural refinement and NPDF interpretations, and highlighting the novelty of this\nwork through comparison with previous oxidation studies. Additionally, we have\nquantitatively analyzed residual lithium by potentiometric titration. We believe that\nthese revisions significantly enhance the rigor and clarity of the manuscript, and we\nsincerely hope that the revised version addresses the reviewer’s concerns and meets the\nstandards for publication in Nature Communications.\nComment #1. The pouch-electrode parameters and the reported electrode compaction\ndensity are helpful. Please make sure the thickness/areal-loading values and the\ncompaction-density calculation are consistent and clearly defined across the\nmanuscript, the SI, and the Response to Referees. I noticed an apparent thickness-\nreporting inconsistency between R-NCM523 and C-NCM523 in the Response to\nReferees.\nResponse: We thank the reviewer for the careful check. We have thoroughly re-checked\nand unified the electrode thickness, areal loading, and compacted density of both R-\nNCM523 and C-NCM523 across the Manuscript, the Supplementary Information and\nthe Response.\nTo make an effective comparison, the first essential in battery design and\nmanufacturing is to control the active material areal loadings as consistently as possible\n(the actual loadings of the two electrodes are 29.8 and 29.2 mg/cm2, respectively). Due\nto the distinct crystallographic morphologies of the two materials, the electrode\nthickness and compacted density are different. C-NCM523 exhibits a poly-crystal\nmorphology, featuring intergranular voids and microcracks both within and between\nsecondary particles, which restrict effective densification during calendering. In\ncontrast, R-NCM523 exhibits a single-crystal morphology, enabling more efficient\nparticle packing and lower electrode porosity. At similar areal loading, R-NCM523\nexhibits an effective thickness of 80 μm and a compacted density of 3.73 g/cm3, while\nC-NCM523 has an effective thickness of 86 μm and a compacted density of 3.40 g/cm3.\nOur revision:\n1. Added detailed description. (pages 24-25, lines 560-562, lines 564-565)\nPouch-type full batteries with ~1 Ah capacity, using R-NCM523 as the cathode\nwith a loading of ~29.8 mg/cm2 on the cathode side (active material is 96.5%,\nconductive additives are 1.5 wt%, PVDF is 2.0 wt%, 8.0 × 6.0 cm2 in size) and anode\nloading of ~16.0 mg/cm2 (active material is 95.4%, 8.4 × 6.3 cm2 in size) were tested\nfor cycling performance at 1C, 3.0-4.4 V. The compacted density, porosity and\nvolumetric capacity of the cathode electrode are shown in Supplementary Fig. 27.\n2. Added data and figures.\nSupplementary Fig. 27 The area, weight and thickness of the electrode with C-NCM523 (a) and\nR-NCM523 (b) as cathode material. Compacted density: The areal loading of R-NCM523 is 29.8\nmg/cm2 and the commercial NCM523 (C-NCM523) is 29.2 mg/cm2; Effective electrode thickness\n(excluding aluminum foil) for R-NCM523 is 80 μm and C-NCM523 is 86 μm; The Al foil thickness\nis 12 μm (3.03 mg/cm2). The calculation formula for compaction density is: 𝜌 = Areal loading\nelectrode\n÷ Effective electrode thickness. The compacted density of R-NCM523 is 3.73 g/cm3, and C-\nNCM523 is 3.40 g/cm3.\nComment #2. Because “High-Tap-Density” is central here, please summarize\nelectrode-level metrics (e.g., porosity and calendaring/compaction conditions) and\ninclude a simple volumetric capacity (or equivalent electrode-level) comparison,\nrather than relying on tap density alone.\nResponse: We thank the reviewer for the valuable suggestion. As the reviewers pointed\nout, the advantage of a high tap density material should ultimately be reflected at the\nelectrode level. We have added electrode porosity and volumetric capacity, along with\nthe related calculation procedures. The electrode compacted density is obtained from\nexperimental measurements, determined from the mass and thickness of the electrodes\nafter calendaring under identical conditions. Based on the measured compacted density\n(R-NCM523: 3.73 g/cm3; C-NCM523: 3.40 g/cm3), the electrode porosity and\nvolumetric capacity were subsequently calculated.\nFirst, the electrode porosity is quantitatively assessed. The theoretical solid density\nof the electrode is calculated using the formula: 𝜌 = (∑𝑤 / 𝜌)-1. The cathode\n𝑠𝑜𝑙𝑖𝑑 𝑖 𝑖\nformulation for the pouch cell includes 96.5 wt% NCM523 (𝜌 = 4.78 g/cm3), 1.5 wt%\nconductive additive (𝜌 = 2.26 g/cm3), and 2.0 wt% PVDF (𝜌 = 1.77 g/cm3)1, 2. Based\non this composition, the theoretical solid density is calculated as 𝜌 = 4.55 g/cm³.\n𝑠𝑜𝑙𝑖𝑑\nThe electrode porosity (ε) is then calculated with: ε = 1 - 𝜌 / 𝜌 . The densities\nelectrode solid\nof the compacted electrodes for R-NCM523 and C-NCM523 are 3.73 and 3.40 g/cm3,\nrespectively, corresponding to porosities of 18.0% and 25.3%. These results show that,\nunder identical electrode formulations and calendaring conditions, R-NCM523\nproduces electrodes with lower porosity.\nSecondly, the volumetric capacity of the electrodes is evaluated using: C = C\nv g,\n× 𝜌 , where C indicates the specific capacity of the active material (the\nactive electrode g, active\ntheoretical specific capacity of NCM523 is 150 mAh/g). The densities of the compacted\nelectrodes for R-NCM523 and C-NCM523 are 3.73 and 3.40 g/cm3, corresponding to\nvolumetric capacities of 559.5 mAh/cm3 and 510.0 mAh/cm3.\nOverall, through the combined analysis of electrode porosity and volumetric\ncapacity (Fig. R1), we demonstrate the intrinsic advantage of high-tap-density\nmaterials when used in pouch-cell cathodes.\nFig. R1 The compacted density, porosity and volumetric capacity of the electrode with C-NCM523\nand R-NCM523 as cathode material.\nOur revision:\n1. Added detailed description. (pages 25 lines 564-565)\nThe compacted density, porosity and volumetric capacity of the cathode electrode\nare shown in Supplementary Fig. 27.\n2. Added data and figures.\nSupplementary Fig. 27 (c) The compacted density, porosity and volumetric capacity of the\nelectrode with C-NCM523 and R-NCM523 as cathode material.\nPorosity: The theoretical solid density of the electrode is calculated using the formula: 𝜌 = (∑𝑤\n𝑠𝑜𝑙𝑖𝑑 𝑖\n/ 𝜌)-1. The cathode formulation for the pouch cell includes 96.5 wt% NCM523 (𝜌 = 4.78 g/cm3),\n𝑖\n1.5 wt% conductive additive (𝜌 = 2.26 g/cm3), and 2.0 wt% PVDF (𝜌 = 1.77 g/cm3). Based on this\ncomposition, the theoretical solid density is calculated as 𝜌 = 4.55 g/cm3. The electrode porosity\n𝑠𝑜𝑙𝑖𝑑\n(ε) is then calculated with: ε = 1 - 𝜌 / 𝜌 . The densities of the compacted electrodes for R-\nelectrode solid\nNCM523 and C-NCM523 are 3.73 and 3.40 g/cm3, respectively, corresponding to porosities of 18.0%\nand 25.3%.\nVolumetric capacity: The volumetric capacity of the electrodes is evaluated using: C = C ×\nv g, active\n𝜌 , where C indicates the specific capacity of the active material (the theoretical specific\nelectrode g, active\ncapacity of NCM523 is 150 mAh/g). The densities of the compacted electrodes for R-NCM523 and\nC-NCM523 are 3.73 and 3.40 g/cm3, corresponding to volumetric capacities of 559.5 mAh/cm3 and\n510.0 mAh/cm3.\nComment #3. The argument that NiOOH peaks are difficult to resolve due to low\nfraction/overlap is reasonable, but the refinement assumptions and key fit indicators\nshould be easier to find and interpret in the main text and SI. Please also ensure the\nNPDF d1-peak interpretation (including the direction of the intensity change) is\nconsistent between the text and figures/SI.\nResponse: We thank the reviewer for this helpful suggestion. We have clarified the\nrefinement assumptions and fit indicators, and ensured consistency in interpreting the\nNPDF d1 peak.\nThe NPD data are analyzed using a three-phase model consisting of LiNiO , Co O ,\n2 3 4\nand NiOOH, with the constraint that the sum of the phase fractions equals 1.00. Under\nthis constraint, the refined phase fractions are 90.20% for LiNiO , 3.90% for Co O ,\n2 3 4\nand 5.90% for NiOOH, confirming the presence of NiOOH. The fit quality indicators,\nR = 5.87% and R = 5.40%, are both below 10%, indicating good reliability of the\nwp F\nfitting. Moreover, after removing the NiOOH phase during refinement of NPD and\nNPDF, the R values increased to 7.69% and 26.40%, respectively, confirming the\nwp\nexistence of the NiOOH phase in O-NCM523 (Fig. R2).\nAdditionally, the interpretation of the NPDF d1 peak has been unified throughout\nthe Manuscript and Supplementary Information. The intensity of the d1 peak in O-\nNCM523 is lower than that in S-NCM523, which is due to the higher Ni3+ content in\nO-NCM523 (less Li+/Ni2+ mixing)\nFig. R2 NPD (a) and NPDF (b) refinement patterns for O-NCM523 without NiOOH phase.\nOur revision:\n1. Added detailed description. (page 8, lines 159-161, page 9, lines 187-193)\nAdditionally, the intensity of the d1 peak in O-NCM523 is lower than that in S-\nNCM523, which is due to the higher Ni3+ content in O-NCM523 (less Li+/Ni2+ mixing),\nenhancing the Jahn-Teller effect and weakening the symmetry of the TM-O bond.\nMeanwhile, the NiO phase in S-NCM523 transforms into NiOOH during\noxidation, and the NiOOH content in O-NCM523 is 5.90%, with both R (5.87%) and\nwp\nR (5.40%) values below 10% (Supplementary Table 12 and 13), indicating good\nF\nreliability of the fitting. Moreover, after removing the NiOOH phase during refinement\nof NPD and NPDF, the R values increase to 7.69% and 26.40%, respectively, further\nwp\nconfirming the existence of the NiOOH phase in O-NCM523 (Supplementary Fig. 5).\n2. Added data and figures.\nSupplementary Fig. 5 NPD (a) and NPDF (b) refinement patterns for O-NCM523 without NiOOH\nphase.\nComment #4. Please include a concise comparison table versus prior pre-oxidation\nstudies to make the claimed novelty clearer (conditions, target compositions,\ndefect/mixing metrics, morphology outcome, and electrode-level performance).\nResponse: We thank the reviewer for this valuable suggestion. To clearly highlight the\nnovelty of this work compared to previous pre-oxidation strategies, we have added a\nconcise comparison table (Table R1) that systematically summarizes representative\nliterature reports and our study.\nAs shown in Table R1, previous pre-oxidation methods mainly included high-\ntemperature hydrothermal, wet chemical treatment and mechanical milling. These\nmethods often lead to aggregation of regenerated single-crystal particles, fail to upgrade\npoly-crystal to single-crystal, and still suffer from the relatively high Li+/Ni2+ mixing.\nIn contrast, this study employs a Na S O (0.5M)/NaOH (1M) solution system. During\n2 2 8\nthe oxidation treatment, highly oxidative ·SO -/·OH radicals selectively oxidize anti-\n4\nsite Ni2+ to Ni3+, while surface NiO phase transforms into NiOOH phase, thereby\nopening Li+ diffusion pathways. The regenerated cathode exhibits the lowest Li+/Ni2+\nmixing (2.06%), enabling the reconstruction of poly-crystal and single-crystal into a\nuniform single-crystal morphology, and markedly enhances long-term cycling stability\n(80.2% capacity retention after 600 cycles).\nTable R1. Pre-oxidation strategies for regenerated NCM cathodes: literature vs. this work3-5.\nSpent cathode NCM523 single-crystal NCM622 poly-crystal NCM523 single-crystal NCM523 single-/ poly-crystal\nStirred with spent Milling of spent cathode Stirred with spent\nHydrothermal reaction at\nReaction conditions cathode/Na2S2O8/NaOH at and hydroxide precursor cathode/Na2S2O8 (0.5 M)/NaOH\n180°C for 6 h\n25°C for 0.5 h for 20 min (1.0 M) at 25°C for 1 h\nRegenerated cathode\n4.76% 2.70% 3.90% 2.06%\nLi+/Ni2+ mixing\nRegenerated cathode\nAggregated single-crystal Poly-crystal Aggregated single-crystal Single-crystal\nmorphology\nCycling performance 76.0%@200cycles 94.9%@100cycles 81.1%@250cycles 80.2%@600cycles\nReferences 3 4 5 This work\nOur revision:\n1. Added data and table.\nTable 6. Pre-oxidation strategies for regenerated NCM cathodes: literature vs. this work.\nSpent cathode NCM523 single-crystal NCM622 poly-crystal NCM523 single-crystal NCM523 single-/ poly-crystal\nStirred with spent Milling of spent cathode Stirred with spent\nHydrothermal reaction at\nReaction conditions cathode/Na2S2O8/NaOH at and hydroxide precursor cathode/Na2S2O8 (0.5 M)/NaOH\n180°C for 6 h\n25°C for 0.5 h for 20 min (1.0 M) at 25°C for 1 h\nRegenerated cathode\n4.76% 2.70% 3.90% 2.06%\nLi+/Ni2+ mixing\nRegenerated cathode\nAggregated single-crystal Poly-crystal Aggregated single-crystal Single-crystal\nmorphology\nCycling performance 76.0%@200cycles 94.9%@100cycles 81.1%@250cycles 80.2%@600cycles\nReferences 3 4 5 This work\nComment #5. While the added control samples are appreciated, please clearly state\nwhether particle size distributions and electrode fabrication conditions are matched\nacross all samples; if not, explicitly justify the differences in the manuscript (and\nindicate how they may affect the comparisons).\nResponse: We appreciate the reviewer’s insightful suggestion. We clarify the particle-\nsize distribution and electrode fabrication conditions to ensure the validity and\ntransparency of the comparisons.\nFirst, all electrodes and coin cells are prepared using an identical protocol across\nall samples. The NCM powder, polyvinylidene fluoride (5 wt% dissolved in N-methyl-\n2-pyrrolidone), and carbon black are blended in a weight ratio of 8:1:1 and stirred for\n4 h. The slurry was covered with carbon-coated Al foil and dried in an oven at 80 °C\nfor 6 h. The electrodes were punched into 8 mm discs (0.503 cm2), with a mass loading\nwithin 4~6 mg/cm2.\nSecondly, the particle size distribution depends on both the crystal morphology\n(single-crystal or poly-crystal) and the synthesis conditions. In this study, R-NCM523\ndenotes the regenerated single-crystal particles. For comparison, commercial poly-\ncrystal (C-NCM523) and single-crystal (CS-NCM523) materials are employed as\nreferences to evaluate the structural stability and electrochemical performance of the\nregenerated R-NCM523.\nAs shown in Fig. R3e, under comparable electrode mass loadings (R-NCM523:\n5.54 mg/cm2; CS-NCM523: 5.47 mg/cm2; C-NCM523: 5.41 mg/cm2), single-crystal\nNCM generally exhibits superior structural stability upon cycling compared with a\npoly-crystal sample. Accordingly, both R-NCM523 (80.2% retention after 600 cycles)\nand CS-NCM523 (78.0% retention after 600 cycles) exhibit improved cycling stability\ncompared to poly-crystal C-NCM523 (67.8% retention after 500 cycles), which is\nprimarily attributed to crack formation in the poly-crystal particles, thereby accelerating\nstructural degradation. We further focus on the comparison between the two single-\ncrystal samples, R-NCM523 and CS-NCM523. These samples exhibit significant\ndifferences in particle sizes (CS-NCM523: 2.57 μm; R-NCM523: 4.62 μm; Fig. R3a-\nd), which is attributed to NiOOH on the O-NCM523 surface readily converting to\nlayered LiNiO . The lattice compatibility facilitates grain boundary fusion and\n2\npromotes particle growth, resulting in a larger R-NCM523 particle size.\nThis particle size variation impacts performance: smaller particles exhibit a larger\nspecific surface area and stronger electrode-electrolyte interfacial reactivity, which may\naccelerate interfacial side reactions and degrade long-term structural stability6. This\ntrend is consistent with the electrochemical results (Fig. R3f). During the initial 300\ncycles, CS-NCM523 exhibits slightly higher capacity retention. In contrast, beyond 300\ncycles, its capacity retention gradually becomes inferior to that of R-NCM523.\nTherefore, we conclude that the differences in interfacial reaction kinetics and structural\nevolution, arising from the differing particle size, lead to the difference in long-term\ncycling stability between the two samples.\nBased on these considerations, we revised the manuscript to clearly state the\nconsistency of electrode fabrication conditions and to discuss how particle-size\ndifferences may impact comparisons, thereby enhancing the rigor of our conclusions.\nFig. R3 (a, c) SEM images of CS-NCM523 and R-NCM523. (b, d) Particle size distribution of CS-\nNCM523 and R-NCM523. (e) Cycling performance of all samples. (f) Cycling performance of CS-\nNCM523 and R-NCM523.\nOur revision:\n1. Added detailed description. (page 24, lines 553-557, page 19, lines 397-400)\nFor the preparation of coin cells, NCM powder, polyvinylidene fluoride (5 wt%\ndissolved in N-methyl-2-pyrrolidone), and carbon black were blended in a weight ratio\nof 8:1:1 and stirred for 4 h. The slurry was covered with carbon-coated Al foil and dried\nin an oven at 80 °C for 6 h. The electrodes were punched into 8 mm discs (0.503 cm2),\nwith a mass loading within 4~6 mg/cm2. Cycling was performed on assembled 2032-\ncoin cells at 3.0-4.3 V and 3.0-4.5 V (1C=170 mA/g). The electrolyte was 1.0 M LiPF\n6\nin EC: DEC=1:1 Vol% with 5.0% FEC.\nCS-NCM523 maintains a capacity retention of 78.0% after 600 cycles, slightly\nlower than that of R-NCM523 (Supplementary Fig. 15d). This is attributed to the\nsmaller particle size of CS-NCM523, which increases interfacial side reactions and\nundermines long-term structural stability.\n2. Added data and figure.\nSupplementary Fig. 15 (d) Cycling performance of CS-NCM523 and R-NCM523.\nComment #6. Please document residual lithium species (e.g., Li CO /LiOH) in a way\n2 3\nthat is straightforward to reproduce. Along with XPS, please provide a quantitative\nmethod such as potentiometric titration (or an equivalent approach), and specify the\nprocedure and the criterion used to conclude that residual Li is minimal in the final\nmaterial.\nResponse: We thank the reviewer for the valuable suggestion regarding the\nquantification of residual lithium and the reproducibility of the analytical method. In\nthe revision, in addition to XPS analysis, potentiometric titration is used to\nquantitatively determine water-soluble residual lithium species, providing a simple and\nreproducible approach. R-NCM523-115%Li, R-NCM523-120%Li (corresponding to\nthe optimized Li addition amount) and R-NCM523-125%Li are prepared for lithium-\nratio optimization. By varying the lithium addition, the optimal lithium content was\nidentified, and residual lithium was subsequently quantified to evaluate the impact of\nexcess lithium on surface species.\nThe detailed procedure is as follows: 5 g of the sample is dispersed in 50 mL of\ndeionized water and stirred at 600 rpm for 5 min at room temperature to dissolve the\nwater-soluble residual lithium species from the surface. Subsequently, vacuum filtration\nis performed to separate the solid and liquid, and the filtrate is collected. A 10 mL\nsample of the filtrate is analyzed using an automatic potentiometric titrator (SCHOTT\nTitroLine Easy, Germany). A standard 0.1 mol/L HCl solution is used as the titrant, with\nthe endpoint defined at pH = 7.0.\nAs shown in, Table R2, the soluble residual lithium contents of R-NCM523-\n115%Li, R-NCM523-120%Li (R-NCM523), and C-NCM523 are comparable and\nlower than that of R-NCM523-125%Li. Notably, the R-NCM523-125%Li sample\nshows a much higher residual lithium content, indicating more surface residual lithium\nspecies. This quantitative trend aligns with the variation of the Li CO signal observed\n2 3\nin the XPS analysis (Fig. R4), further confirming the reliability of the results. Therefore,\nwe adopt the standard that a soluble lithium content similar to that of the commercial\nreference material indicates a low residual lithium level, demonstrating that the R-\nNCM523 material contains only minimal residual lithium species.\nFig. R4 Li 1s XPS spectra of R-NCM523-115%, R-NCM523-120%, and R-NCM523-125%.\nTable R2. Quantification of soluble residual lithium for R-NCM523-115%, R-NCM523-120%, R-\nNCM523-125% and C-NCM523.\nSoluble residual lithium (ppm) Initial pH HCl consumption (ml)\nR-NCM523-115%Li 1211.2 11.72 17.45\nR-NCM523-120%Li 1223.7 11.73 17.63\nR-NCM523-125%Li 2465.5 12.14 32.64\nC-NCM523 1208.0 11.72 17.72\nOur revision:\n1. Added data and figure.\nSupplementary Fig. 18 (e) Soluble residual lithium of R-NCM523-115%, R-NCM523-120%, R-\nNCM523-125% and C-NCM523. The procedure for residual lithium testing is as follows: 5 g of the\nsample is dispersed in 50 mL of deionized water and stirred at 600 rpm for 5 min at room\ntemperature to dissolve the water-soluble residual lithium species from the surface. Subsequently,\nvacuum filtration is performed to separate the solid and liquid, and the filtrate is collected. A 10 mL\nsample of the filtrate is analyzed using an automatic potentiometric titrator (SCHOTT TitroLine\nEasy, Germany). A standard 0.1 mol/L HCl solution is used as the titrant, with the endpoint defined\nat pH = 7.0.\nBased on the results obtained with this method (Fig. 18e), the soluble residual lithium contents of\nR-NCM523-115%Li, R-NCM523-120%Li (R-NCM523), and C-NCM523 are comparable and\nlower than that of R-NCM523-125%Li. Therefore, we adopt the standard that a soluble lithium\ncontent similar to that of the commercial reference material indicates a low residual lithium level,\ndemonstrating that the R-NCM523 material contains only minimal residual lithium species.\nReviewer #3\nReviewer Comment: Compared to the initial version, the authors have addressed some\nof my concerns, and the current manuscript shows noticeable improvement, which is\ncommendable. Nevertheless, several critical issues require further clarification and\nexperimental support, as detailed below.\nOverall Response: We thank the reviewer for the careful evaluation and positive\nassessment of the revised manuscript. In response to the reviewer’s further comments,\nwe have systematically supplemented and revised the manuscript, including clarifying\nthe crystalline morphology of spent cathode and the mechanism underlying the\ntransformation from poly-crystal to single-crystal, identifying the residual lithium\nspecies as Li CO through XRD characterization, incorporating key descriptions of the\n2 3\ntechno-economic analysis into the manuscript and directly verifying the absence of SO\n2\nemission during the high-temperature regeneration by TG-MS measurement. In\naddition, grammatical issues in the manuscript have been corrected. We sincerely hope\nthat these changes adequately address your concerns and further strengthen the\nmanuscript for publication.\nComment #1. The crystal nature (single-crystal vs. polycrystalline) of the starting\nmaterial is not specified. Furthermore, the manuscript does not explain why the\nregenerated cathode exhibits an exclusively single-crystal morphology. A clear\ndiscussion on the transformation mechanism from polycrystalline components to\nsingle-crystals should be provided.\nResponse: We sincerely thank the reviewer for this thoughtful comment. First, it is\nnecessary to clarify that in commercial battery manufacturing, a particle blending\nstrategy (single-crystal and poly-crystal particles) is commonly adopted to maximize\nthe electrode's compaction density. The spent cathode materials used in this study are\ncollected from commercial pouch batteries provided by the Nanjing Tongning Research\nInstitute. Accordingly, the experimental samples exhibit a mixed morphology\nconsisting of single-crystal particles and poly-crystal secondary particles (Fig. R1a).\nThe single-crystal morphology observed after regeneration arises from a continuous\nstructural evolution involving oxidation-induced deconstruction of poly-crystal\nparticles followed by high-temperature single-crystal reconstruction (Fig. R1b and 1c).\nSpecifically, during the oxidation treatment in this work, highly oxidative ·SO -\n4\n/·OH radicals (oxidation potential: 2.6-2.8 V) selectively oxidize anti-site Ni2+ to Ni3+\n(oxidation potential: 2.5 V), accompanied by partial Li+ removal to maintain charge\nbalance (Fig. R1d, 1e and Table R1). This results in an increase in the negative charge\ndensity of the oxygen layers, thereby strengthening the electrostatic repulsion between\nadjacent oxygen layers. It is this enhanced interlayer repulsion that drives the expansion\nalong the c-axis (Fig. R1f). Simultaneously, the surface NiO phase (~2.4 Å interlayer\nspacing) transforms into the NiOOH phase (~4.8 Å interlayer spacing), causing\nsignificant surface-layer expansion. (Fig. R1g and 1h). The synergistic interaction\nbetween the bulk and surface lattice expansion facilitates the decomposition of\nsecondary particles into primary particles. During the subsequent high-temperature\nsintering at 900 °C, these primary particles undergo melting-assisted growth in the\npresence of a lithium source, accompanied by gradual grain-boundary elimination and\nthe formation of single-crystal (Fig. 1c).\nFig. R1 (a-c) SEM images of S-NCM523, O-NCM523 and R-NCM523. (d) Eh-pH diagrams of\nthe Ni-H O system (273.15 K, 1 mol/kg). (e) ESR spectra during the oxidation process of\n2 2 8\nHRTEM images of S-NCM523 and O-NCM523.\nTable R1. Elemental analysis of S-NCM523 and O-NCM523.\nElement contents (ppm) Molar ratio of\nLi 670.783 Ni 221.648 Co 228.615 Mn 257.372 Li/TM\nS-NCM523 3.23 27.07 10.79 15.71 0.50\nO-NCM523 2.86 26.58 10.58 15.53 0.45\nChanges: In the revised manuscript, we have supplemented on page 6, lines 126-134\nand lines 137-138.\nOur revision:\n1. Added detailed description. (page 6, lines 126-134 and lines 137-138)\nAs shown in Fig 2c, with Na S O as an oxidizing agent, the surface NiO is\n2 2 8\nconverted to NiOOH, accompanied by a significant increase in the lattice spacing from\n2.40 Å to 4.83 Å; the anti-site Ni2+ in the LiO octahedron, which has higher activity,\n6\nis also oxidized to Ni3+ and hence returns to the TMO octahedron due to the radius\n6\nvariation and charge balance, causing c-axis expansion. The surficial and internal phase\ntransitions induce lattice distortions, which increase the internal stresses within\nsecondary particles, prompting their fragmentation into primary particles. The\nfragmentation unifies the morphology and homogenizes the Li+ diffusion channels,\nfurther paving the way for subsequent Li replenishment.\nWith spent NCM523 as a paradigm, the regeneration process involves two steps:\n(1) oxidation of spent NCM523 (denoted as S-NCM523, consisting of single and poly-\ncrystal particles), where the as-obtained product is denoted as O-NCM523.\nComment #2. The origin of the Li CO species detected in Fig. 5(f) remains unclear,\n2 3\nincluding the specific formation mechanism and the source of carbon. The accuracy of\nthe XPS fitting results needs further verification. Supplementary XRD characterization\nis recommended to conclusively rule out the presence of other lithium compounds such\nas Li O or LiF.\n2 3\ndetected in Fig. 5(f) mainly results from surface carbonation of residual lithium species\nafter lithium-compensated sintering, which inevitably reacts with atmospheric CO\n2\nduring sample cooling and transfer. Therefore, the carbon in Li CO comes from\n2 3\nenvironmental CO rather than from the material itself or any external carbon source.\nthat although excess lithium could theoretically form Li O at high temperatures, Li O\n2 2\nis thermodynamically unstable in air and quickly reacts with CO during cooling and\n2\ntransfer, converting into Li CO (Li O + CO → Li CO )7. Additionally, since no\n2 3 2 2 2 3\nfluorine source is introduced during the regeneration process, the formation of LiF is\nprevented. This is further confirmed by XPS analysis, which detects no F element signal\n(Fig. R2c), thus excluding the presence of LiF.\nFollowing the reviewer’s suggestion, supplementary XRD analyses at different\nregeneration temperatures are carried out to verify lithium-related phases. As shown in\nFig. R2a, the XRD pattern of the R-NCM523@600 °C sample displays peaks\ncorresponding to LiNiO , Co O , and Li CO , while no peaks associated with Li O, LiF,\n2 3 4 2 3 2\nor other lithium compounds are observed. Similarly, in the R-NCM523@550 °C sample,\nthe Li CO peak becomes more prominent, yet no diffraction features of Li O or LiF\n2 3 2\nare present (Fig. R2b). Overall, these findings confirm that the lithium-related species\nidentified in Fig. 5(f) are mainly Li CO rather than Li O or LiF. XRD patterns of\n2 3 2\nsamples regenerated at different temperatures are added to the Supplementary\nInformation (Fig. R2d).\nFig. R2 (a, b) XRD pattern of R-NCM523@600°C and R-NCM523@550°C. (c) F 1s XPS spectra\nof R-NCM523@600°C. (d) Ex-situ XRD patterns at different regeneration temperatures.\nOur revision:\n1. Added detailed description. (page 15, lines 342-344)\nBut a clear Li CO signal is observed below 600°C (Supplementary Fig. 21),\n2 3\nwhich results from the incomplete insertion of Li+ into the layered structure and the\nconsequent conversion of the remaining Li CO on the surface of the particles.\n2 3\n2. Added data and figures.\nSupplementary Fig. 21 Ex-situ XRD patterns at different regeneration temperatures.\nComment #3. A description of the Life Cycle Assessment appears to be absent from the\nmain text. The authors are requested to confirm whether this analysis has been\nconducted and include it if available, or to provide a justification for its omission.\nResponse: We apologize for the confusion caused to the reviewer. A life cycle\nassessment is indeed conducted in this study. However, in the original manuscript, the\nrelevant description is mainly placed in the Supplementary Information, which led to\nits absence from the manuscript. In response to the reviewer’s suggestion, we have\nrevised the manuscript by including the key description of the life cycle assessment in\nthe manuscript, while keeping the related figures and tables in the Supplementary\nInformation to ensure clarity and conciseness.\nOur revision:\n1. Added detailed description. (pages 20-21, lines 435-458)\nTechno-Economic Analysis\nThe oxidation strategy offers several advantages. First, the regeneration process is\nconcise, without requiring complex or hazardous devices. Second, it employs only\nconventional lithium salts like LiOH, reducing process costs. Third, Na S O serves as\n2 2 8\nthe reagent in this method, and subsequent addition of Ca(OH) enables SO 2- to form\n2 4\nCaSO precipitation, minimizing environmental impact. Additionally, to further verify\n4\nthat no sulfur oxide species are released during the high-temperature regeneration\nprocess, TG-MS testing was conducted. The corresponding TG and MS signals are\nshown in Supplementary Fig. 34e, f. Over the entire temperature range from 30 to\n900 °C, only signals at m/z = 18 (H O) and m/z = 44 (CO ) are detected, corresponding\n2 2\nto water release and the decomposition of Li CO , respectively. No signals at m/z = 64\n2 3\n(SO ) or m/z = 80 (SO ) are detected. The TG-MS results clearly indicate that no SO\n2 3 2\nor SO is emitted during the high-temperature sintering of O-NCM523 regeneration.\nconsumption, low emissions, and high profits.\nThe energy consumption of this method is 23.94 MJ/kg, and greenhouse gas (GHG)\nemissions are 3.11 kg/kg, both lower than those of the hydrothermal and molten salt\nmethods, respectively (Supplementary Fig. 34a, b). Considering the comprehensive\noverall process costs, the oxidation method has a profit of $6.47/kg, which is 113.5%\nand 141.4% higher than the hydrothermal and molten salt methods, respectively\n(Supplementary Fig. 34c, d). In conclusion, compared to existing direct recycling\ntechnologies, the oxidation strategy has superior capabilities in reducing GHG,\nlowering energy consumption, and enhancing profits. It provides an efficient, scalable\nsolution for power battery recycling with broad industrial application potential.\n2. Added data, figures and tables.\nSupplementary Fig. 34 (a) Energy consumption. (b) Greenhouse gas (GHG) emission, (c) profit\nand (d) cost comparison of direct regeneration technologies. (e, f) TG and MS curves of O-NCM523\nduring the high-temperature regeneration.\nSupplementary Table 41. The material loss rate of each step\nStep device Material loss rate (%) Note\nMixing Mixing tank 1 A-1\nSintering Calciner 10 B-10\nWashing Filter press 5 C-5\nDrying Dryer 5 D-5\nCrushing Crusher 5 E-5\nHydrothermal Hydrothermal reactor 1 F-1\nPre-oxidation Pre-oxidation 1 G-1\nSupplementary Table 42. The regeneration process of molten salt\nStep Item Dosage Unit Note\nSpent NCM523 10.00 kg\nEnergy 10.80 MJ\nIn put LiOH 0.71 kg\nMixing LiNO 1.02 kg A-1\n2 3\nNCM-MS 1.28 kg\nOut put\nGHG 1.40 kg\nSpent NCM523 12.69 kg\nIn put Energy 234.00 MJ\nOxygen 6.00 L\nSintering B-10\nNCM-MSL 11.42 kg\nOut put GHG 27.17 kg\nWaste gas 6.00 L\nNCM-H 11.42 kg\nIn put\nEnergy 14.40 MJ\nCrushing E-5\nNCM-HC 10.85 kg\nOut put\nGHG 1.87 kg\nNotes: During the production of regenerated NCM, 1MJ of electricity produces 0.13 kg GHG and\n0.67 L of water according to the Everbatt 2020 database. During the mixing procedure, the mixing\ntank is working for 1 hour. The first sintering process was carried out at ~773 K for 5 hours. The\nsecond sintering process was carried out at ~1223 K for 10 hours under an oxygen atmosphere. The\ncrusher worked for 0.5 hours.\nSupplementary Table 43. Cost analysis of molten salt\nDosage Unit Unit price Cost ($) Data source\nEnergy 259.20 MJ 0.07 $/kWh 18.14 Everbatt 2020\nSpent NCM523 10.00 kg 4517.04 $/t 45.17 SMM\nLi CO 1.09 kg 10988.78 $/t 11.98 SMM\n2 3\nLiNO 1.02 kg 65.57 $/kg 66.88 CGB\nO 6.00 L 25.23 $/40L 3.79 B2B\nNotes: SMM: https://www.smm.cn. CGB: https://www.caigoubang.com. B2B:\nhttps://b2b.baidu.com.\nSupplementary Table 44. The regeneration process of hydrothermal\nStep Item Dosage Unit Note\nSpent NCM523 10.00 kg\nEnergy 79.20 MJ\nIn put\nLiOH 2.87 kg\nHydrothermal F-1\nH O 30.00 L\nOut put\nGHG 10.30 kg\nH-NCM523 9.90 kg\nIn put Energy 2.66 MJ\nWater 44.55 L\nWashing C-5\nH-NCM523 9.41 kg\nOut put Waste water 74.55 L\nGHG 0.35 kg\nH-NCM523 9.41 kg\nIn put\nEnergy 28.80 MJ\nDrying D-5\nH-NCM523 8.94 kg\nOut put\nGHG 3.74 kg\nH-NCM523 8.94 kg\nIn put Energy 10.80 MJ\nMixing Li CO 0.34 kg A-1\n2 3\nH-NCM523 9.19 kg\nOut put\nGHG 1.40 kg\nH-NCM523 9.19 kg\nIn put Energy 108.00 MJ\nOxygen 3.00 L\nSintering B-10\nNCM-H 8.27 kg\nOut put GHG 14.04 kg\nWaste gas 3.00 L\nNCM-H 8.27 kg\nIn put\nEnergy 14.40 MJ\nCrushing E-5\nNCM-HC 7.86 kg\nOut put\nGHG 1.87 kg\nNotes: During the production of regenerated NCM, 1MJ of electricity produces 0.13 kg GHG and\n0.67 L of water according to the Everbatt 2020 database. The hydrothermal reactor was carried out\nat ~493 K for 4 hours, respectively. During the washing process, the products were washed three\ntimes, with a solid-to-liquid ratio of 1.5. The drying procedure was applied at 353 K for 10 hours.\nDuring the mixing procedure, the mixing tank is working for 1 hour. The sintering process was\ncarried out at ~1123 K for 6 hours under an oxygen atmosphere. And the crusher worked for 0.5\nhours.\nSupplementary Table 45. Cost analysis of hydrothermal\nDosage Unit Unit price Cost ($) Data source\nEnergy 243.86 MJ 0.07 $/kWh 17.07 Everbatt 2020\nWater 74.55 L 0.11 $/m3 0.01 Everbatt 2020\nBlack mass 10.00 kg 4517.04 $/t 45.17 SMM\nLi CO 0.34 kg 10988.78 $/t 3.74 SMM\n2 3\nLiOH 2.87 kg 11440.83 $/t 32.83 SMM\nO 3 L 25.23 $/40L 1.89 B2B\nNotes: SMM: https://www.smm.cn. CGB: https://www.caigoubang.com. B2B:\nhttps://b2b.baidu.com.\nSupplementary Table 46. The regeneration process of the pre-oxidation strategy\nStep Item Dosage Unit Note\nSpent NCM523 10.00 kg\nEnergy 2.70 MJ\nIn put NaOH 1.60 kg\nPre-oxidation Na S O 4.76 kg G-1\n2 2 8\nH O 40.00 L\nOut put\nGHG 0.35 kg\nO-NCM523 9.90 kg\nIn put Energy 2.66 MJ\nWater 44.55 L\nWashing C-5\nO-NCM523 9.41 kg\nOut put Waste water 84.55 L\nGHG 0.35 kg\nO-NCM523 9.41 kg\nIn put\nEnergy 28.80 MJ\nDrying D-5\nO-NCM523 8.94 kg\nOut put\nGHG 3.74 kg\nO-NCM523 8.94 kg\nIn put Energy 10.80 MJ\nMixing LiOH 1.66 kg A-1\nH-NCM523 10.50 kg\nOut put\nGHG 1.40 kg\nSpent NCM523 10.50 kg\nIn put\nEnergy 180.00 MJ\nSintering B-10\nNCM-MSL 9.45 kg\nOut put\nGHG 23.40 kg\nCrushing In put NCM-H 9.45 kg E-5\nEnergy 14.40 MJ\nNCM-HC 8.98 kg\nOut put\nGHG 1.87 kg\nNotes: During the production of regenerated NCM, 1MJ of electricity produces 0.13 kg GHG and\n0.67 L of water according to the Everbatt 2020 database. During the pre-oxidation procedure, the\nreaction is working for 1 hour. Adding 3.55 kg of Ca(OH) forms a CaSO precipitate, removing\n2 4\nSO 2-. The sintering process was carried out at ~1173 K for 10 hours. The crusher worked for 0.5\n4\nhours.\nSupplementary Table 47. Cost analysis of the pre-oxidation strategy\nDosage Unit Unit price Cost ($) Data source\nEnergy 239.36 MJ 0.07 $/kWh 16.76 Everbatt 2020\nWater 84.55 L 0.11 $/m3 0.01 Everbatt 2020\nSpent NCM523 10.00 kg 4517.04 $/t 45.17 SMM\nNaOH 1.60 kg 350.43 $/t 0.56 B2B\nNa S O 4.76 kg 700.86 $/t 3.34 B2B\n2 2 8\nCa (OH) 3.55 kg 63.08 $/t 0.22 B2B\nAverage cost ($/kg) 8.50\nNotes: SMM: https://www.smm.cn. B2B: https://b2b.baidu.com.\nChanges: We have added the energy consumption, greenhouse gas (GHG) emission,\nprofit and cost comparison of direct regeneration technologies (oxidation, molten salt\nand hydrothermal) in the supporting information (Supplementary page 1, Fig. 33 and\nTable 40-46). In the revised manuscript, we have added the description on pages 21,\nlines 458-459.\n4. The concern regarding potential SO emission has not been adequately\n2\nresolved. To provide definitive evidence, supplementary gas-phase analysis (e.g., TG-\nMS) of the regeneration process should be conducted to demonstrate the absence of\nSO release during the high-temperature sintering stage.\n2\nemission. To obtain direct evidence of gas-phase behavior, TG-MS measurements are\nperformed to monitor in real-time the possible release of sulfur-containing gases during\nregeneration.\nThe TG-MS tests cover a temperature range of 30-900 °C with a heating rate of\n20 °C min-1. As shown in Fig. R3a, the total mass loss of the sample is 7.46%,\nwhich can be divided into three stages: Stage I, associated with the removal of\ncrystallization water from LiOH·H O and adsorbed water; Stage II, linked to the\n2\nmelting of LiOH; and Stage III, caused by dehydration"}]

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