Direct Regeneration of Layered Oxide Cathodes via Seawater

Direct Regeneration of Layered Oxide Cathodes via Seawater | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Direct Regeneration of Layered Oxide Cathodes via Seawater Wei Liu, Ning Xue, Yuanyuan Cui, Wu Xiaoyan, Yuyao Zhang, Luyao Wang, and 5 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9324161/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Lithium extraction from seawater can transform lithium resources and help ease upstream supply strain. Besides, the escalating deployment of lithium-ion batteries necessitates sustainable recycling strategies to address resource scarcity and environmental concerns. Herein, we report a green strategy on direct regenerating degraded layered oxide cathodes via a hydrothermal process using simulated seawater containing trace LiCl and high concentration of NaCl, followed by a mild annealing step. The regenerated cathode exhibits a fully restored layered structure, smooth surfaces, and no residual impurities. It delivers a high specific capacity of 146.6 mAh·g-1 at 0.5 C and excellent capacity retention of 91.67% after 100 cycles. We reveal that Li+ and Na+ play a synergistic effect on the regeneration mechanism. Na+ selectively adsorbs at surface defect sites without entering the bulk lattice as a dynamic coating layer to guide morphological repair, while Li⁺ achieves lithium replenishment via ion exchange with Na+ based on its high adsorption energy and excellent diffusion capability. This cost-effective approach bridges sustainable battery recycling with brine resource utilization, establishing a closed-loop "lithium extraction-cathode regeneration" paradigm. Physical sciences/Materials science/Materials for energy and catalysis/Electrochemistry Earth and environmental sciences/Ocean sciences/Marine chemistry Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1 Introduction The rapid proliferation of rechargeable lithium-ion batteries (LIBs) in modern electronics and electric vehicles has precipitated critical challenges in resource sustainability and environmental protection. 1,2 Conventional LIBs recycling paradigms (pyrometallurgy/hydrometallurgy) suffer from high energy intensity and secondary pollution during Li extraction, such as HF gas generated by electrolyte decomposition and heavy metal ash residues from incineration. 3,4 Increasing research groups have been exploring new methods of direct regeneration strategies, which enable in-situ repair of cathodes with the merits of time efficiency and minimal pollution. The molten salt regeneration method is currently the most widely used approach. It primarily reduces the eutectic point by using multiple salts to facilitate lithium salt intercalation, but it requires a large number of salts and often leads to insufficient or excessive lithium insertion at the applied temperature. 5,6 Besides, the solid-state method is the simplest one, which only involves blending lithium salts with spent cathodes followed by high temperature calcination; however, it demands strict implementation of homogeneous mixing of raw materials and precise control over lithium salt dosage. 7-9 In brief, they all face bottlenecks in low lithium replenishment efficiency and difficult structural restoration. 10,11 Particularly, the prevalent use of costly lithium salts (LiOH/Li 2 CO 3 ) in current direct recycling processes significantly compromises economic viability. For example, under ambient conditions, self-adsorbed amyloxylithium can directly regenerate spent LiNi 0.6 Co 0.2 Mn 0.2 O 2 via simple solid-liquid mixing. 12 Also, the organic lithium salt like 3,4-dihydroxybenzonitrile dilithium can repair spent LiFePO 4 cathodes. 13 Developing innovative lithium compensation approaches with cost-effective lithium sources therefore becomes imperative to achieve both high-performance cathode regeneration and sustainable closed-loop battery recycling. Notably, lithium resources in salt lakes and seawater hold substantial reserves, estimated globally at over 230 billion tons, yet remain underexploited due to lithium extraction complexities. 14,15 However, the predominant presence of Na + in seawater complicates Li + extraction. Recent studies have focused on extracting lithium from seawater using lithium-ion-sieving materials such as LiMn 2 O 4 , 16-18 LiNi x Co y Mn z O 2 (eg. Li 1-x Ni 0.33 Co 0.33 Mn 0.33 O 2) 19-21 and LiFePO 4 . 22,23 These materials exploit their spinel, layered or olivine frameworks to selectively adsorb Li + through electrochemical or ion-exchange mechanisms, achieving high extraction efficiency even in hypersaline environments. 24 The extracted lithium can be purified to obtain LiCl or converted through chemical reactions to commonly used compounds such as LiOH and Li 2 CO 3 . It should be noted that these lithium sieves are consistent in structure and composition to the cathode materials for LIBs. The only difference lies in that lithium-ion sieve materials are obtained by leaching lithium from them with acid to form lithium-depleted materials. When the cathode is in a Li-deficient state, it is very similar to the delithiated ion sieve material from which lithium is to be extracted. If the lithium in seawater can be directly used to replenish the lithium in the discarded lithium battery cathodes, the cost will be greatly reduced. Herein, we accomplish the direct regeneration of the spent LiNi 0.5 Co 0.2 Mn 0.3 O 2 (NCM523) cathode using a simulated brine. To incorporate lithium ions from brine into the degraded NCM523, we employed a hydrothermal reaction using a mixed Na/Li solution, which effectively restored its crystal structure and rectified structural disorders. Subsequently, a quick annealing process is employed to further stabilize the lattice of regenerated NCM523 cathode. Interestingly, compared to using pure LiCl solution for hydrothermal process, the resulting NCM523 processed with the mixed Na/Li solution exhibits a significantly more homogeneous morphology. The direct regeneratedNCM523 cathodes via the mixed Na/Li simulated brine deliver comparable specific capacity and cycling stability to the commercial counterpart. The superior electrochemical performances of the regeneratedNCM523 benefit from the dynamic adsorption shell formed by a large amount of Na under lithium-deficient conditions. As the reaction proceeds, a small amount of Li + with extremely strong adsorption energy will gradually and selectively replace the weakly bound Na + , dominating the regeneration of NCM. This method simultaneously enables the direct restoration of electrode materials and the extraction of lithium from seawater, which holds a highly promise for achieving a closed-loop of utilizing resources and energy, offering high economic benefits. 2 Results and discussions 2.1 Morphology and Structure of NCM After Repair using Brine Our research explored an effective approach on the direct regeneration of the spent NCM523 (SNCM) cathode using a Li-scarce and Na-rich brine, as is shown in Fig. 1 . The restoration process involves a hydrothermal reaction and short post-annealing. For hydrothermal process, LiCl serves as a lithium source to replenish the active lithium lost in SNCM. Meanwhile, abundant NaCl may form a dynamic adsorption layer on the surface of NCM during the restoration process, aiding defect repair and morphology homogenization without being incorporated into the NCM lattice, owing to the large ionic radius of Na + (1.02 Å vs. Li + : 0.76 Å). Additionally, some ethylene glycol was introduced to increase solution viscosity, thereby providing more sufficient reaction time for the above process. The sample after hydrothermal process is defined as RNCM-hydro. After further water washing and post-annealing, the repaired NCM (RNCM) is finally obtained. Specifically, the SNCM powder separated from electrode sheets through the process shown in Supplementary Fig. 1 was subjected to hydrothermal repair according to Fig. 1b . SNCM powder was dispersed in an aqueous solution of 3 mM LiCl and 3 M NaCl with 2 mL ethylene glycol, followed by hydrothermal reaction at 200 ℃ for 3 h. The product was then centrifuged and washed repeatedly, and annealed in air at 800 ℃ for 5 h. Firstly, we studied the structures of the layered SNCM, RNCM-hydro, RNCM, and CNCM cathodes by the XRD patterns ( Supplementary Fig. 2 ). The (003) peak of RNCM-hydro and RNCM shifts to a higher 2θ compared to SNCM, closely aligning with that of CNCM. This shift indicates successful Li + intercalation. The intercalated Li + generate electrostatic attraction with the transition metal layers, which in turn reduces the interlayer distance along the c-axis. 25 Furthermore, the XRD patterns reveal a significant expansion of the (108)/(110) peak separation in SNCM compared to CNCM, which suggests a contraction of the a-axis lattice parameter. Notably, the regenerated RNCM exhibits a restored (108)/(110) peak profile comparable to CNCM, providing strong evidence for the successful reconstruction of the original layered structure. As detailed in Supplementary Table 2 and visualized in Figs. 2a-c and Supplementary Fig. 3 , the Rietveld refinement results indicate that refined unit cell parameters show a marked decrease in c-axis length from 14.27 Å (SNCM) to 14.22 Å (RNCM), accompanied by an increased c/a ratio. These observations collectively point to an enhancement of the layered structure's integrity in the regenerated material. Concurrently, a substantial reduction in Li/Ni cation mixing was observed - from 10.82% in SNCM to 5.80% in RNCM-hydro, and further to 4.10% in the final RNCM product. This progressive decrease signifies a marked recovery of long-range crystallographic order and a reduction in cation disorder, directly attesting to the efficacy of the hydrothermal repair process. The element ratio of Li/transition metal (TM) in SNCM, RNCM-hydro, RNCM and CNCM through inductively coupled plasma-optical emission spectroscopy (ICP-OES) indicates that lithium has been successfully replenished into the NCM after hydrothermal process ( Fig. 2d and Supplementary Table 1 ). These combined results conclusively demonstrate that the spent NCM cathode was successfully replenished with Li + and its original crystal lattice effectively repaired through the hydrothermal process. SEM images in Fig. 2e trace the progressive morphological repair of degraded NCM during hydrothermal treatment with Na + /Li + brine. The spent cathode exhibits severe surface defects, including cracks and nanoflower-like phases characteristic of structural collapse. The nanoflower morphology corresponding to severe degradation basically disappears at 100 ℃. The pores and cracks are further eliminated with temperature; ultimately, the smooth morphology of the secondary particles is clearly visible when the temperature reaches 200 ℃.After hydrothermal process, damaged surface of particles becomes smooth, and the heterogeneous phases completely disappear, restoring the morphology of SNCM close to the state of commercial NCM (CNCM) ( Supplementary Fig. 4 ). To visually assess the repair effect from the cross-sectional morphology, we employed focused ion beam (FIB) to cut individual SNCM and RNCM-hydro particle ( Figs. 2f - g ). The latter particles show obvious disappearance of internal cracks and pores. And the Ni, Co, and Mn elements are uniformly distributed on the surfaces of SNCM and RNCM-hydro ( Supplementary Fig. 5 ). After a brief post-annealing treatment, the RNCM particles also exhibited a smooth surface and an indistinct distribution of Na signal ( Supplementary Fig. 6 ). This indicates that through a simple hydrothermal reaction, seawater with a relatively low lithium content as the lithium source can restore the morphology of the NCM cathode. Subsequently, due to the high content of Na + (39.3 wt%) in the Na/Li mixed solution simulating seawater, we systematically characterized whether Na was intercalated into the RNCM and its impact on the repair efficiency. Two-dimensional time-of-flight secondary ion mass spectrometry (TOF-SIMS) mapping confirms homogeneous Li, Ni, Co and Mn dispersion across the RNCM surface ( Supplementary Fig. 7 ). And three-dimensional TOF-SIMS profiling further validates uniform bulk distribution of Li, Ni, Co, Mn and O from surface to interior regions, along with the absence of Na ( Fig. 2h ). Intriguingly, Na + exhibits distinct localization behavior: while trace Na⁺ signals persist at the surface ( Supplementary Fig. 8 ), bulk concentrations remain below detection thresholds across complementary analytical techniques. And the surface Na signal detected by TOF-SIMS is most likely attributed to the high sensitivity of this characterization technique, which enables the detection of contaminant Na. Crucially, XPS depth profiling with prolonged Ar + sputtering (1300 s) confirms the absence of Na 1s (1071 eV) and Cl 2p (198.5 eV) emissions throughout the etching process ( Fig. 2i ), which indicates that Na has not entered the NCM lattice. The scanning transmission electron microscopy (STEM) image reveals a smooth surface morphology of RNCM, and energy-dispersive X-ray spectroscopy (EDS) detects no significant Na or Cl signals ( Supplementary Fig. 9) . Besides, spatially resolved Na L-edge electron energy-loss spectroscopy (EELS) detect no characteristic Na signals in subsurface or bulk phases ( Supplementary Fig. 10 ). It is evident that Li + intercalated into the NCM can fully replenish the deficient lithium content, without Na + incorporation, arising from structural constraints imposed by lithium vacancies in the cathode framework. The smaller ionic radius of Li + (0.76 Å) compared to Na + (1.02 Å) enables preferential occupancy of octahedral sites. Meanwhile, abundant Na + adsorbs at high-energy defect sites, forming dynamic coating layer on the NCM surface, while trace Li + exchanges with adsorbed Na + and diffuses into the lattice to replenish lithium. The phase structural evolution of RNCM was further analyzed using high-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED). The SNCM particle displays disordered lattice arrangements ( Fig. 3a ), indicative of irreversible phase degradation after cycling. SAED patterns acquired from three distinct regions of SNCM reveal coexisting bulk layered phases, spinel-type structures, and rock-salt phases at grain boundaries ( Fig. 3b ). In contrast, the RNCM particle features a smooth surface ( Supplementary Fig. 11 ) and exhibits uniformly clear and periodic lattice fringes ( Fig. 3c ), unambiguously assigned to the R-3m layered structure with restored long-range order. The HRTEM images and SAED pattern of RNCM ( Fig. 3d ) confirm a single-phase R-3m symmetry with no secondary phases, attesting to the full recovery of the layered lattice and the overall success of the structural repair. Furthermore, surface elemental valence states during regeneration were comprehensively characterized for RNCM demonstrates chemical reversibility indicated the absence via X-ray photoelectron spectroscopy (XPS). Comparative analysis of O 1s, Ni 2p, Co 2p, and Mn 2p spectra ( Fig. 3e ) demonstrates the absence of NiO signatures (530.1 eV) in RNCM-hydro and RNCM, confirming elimination of surface rock-salt impurities from SNCM, which aligns with TEM results ( Fig. 3c ). And Ni 2p spectra show normalized Ni 2+ /Ni 3+ ratios (34.8% Ni 2+ in RNCM vs. 57.7% in SNCM), indicating a transformation from the surface rock-salt phase to the layered phase ( Supplementary Fig. 12 ). The contents of different valence states, derived from XPS results ( Fig. 3e ), also reveals lattice oxygen recovery from 36.0% (SNCM) to 55.2% (RNCM-hydro) and 63.2% (RNCM), reducing oxygen vacancies and improving structural coherence through suppressed transition metal migration. The relatively high presence of Co 2+ on the surface of SNCM also implies a severe loss of Co 3+ in the bulk. In the repaired NCM, Co 3+ is the dominant species. Moreover, in the Mn 2p3/2 spectra, the Mn 4+ peak positions of RNCM-hydro and RNCM shift slightly towards the lower binding energy direction. This suggests that Mn 4+ , which underwent charge compensation due to lithium deficiency, has returned to its original state after hydrothermal treatment. Additionally, compared to SNCM, the 7 Li magic angle spinning solid state nuclear magnetic resonance spectrum ( 7 Li MAS NMR) of RNCM ( Supplementary Fig. 13 ) displays a narrower chemical shift distribution and sharper peaks. These features signify a more stable Li + environment with enhanced coordination uniformity, underscoring the successful recovery of structural order. Raman spectroscopy demonstrates that the RNCM-hydro and RNCM show diminished E g peak intensity and a red shift of the A 1g mode, indicative of phase transformation from disordered rock-salt/spinel phases to a layered structure ( Supplementary Fig. 14 ). This evolution confirms the hydrothermal process effectively repairs structural defects. As shown in Supplementary Fig. 15 , the fourier transform infrared spectrometer (FT-IR) spectrum of the hydrothermally treated NCM reveals a significant reduction in peak intensity at ~864 cm -1 , with complete disappearance after post-annealing. This confirms the effective removal of Li 2 CO 3 and related lithium-based impurities from the NCM surface. Consistently, the thermogravimetric (TG) curves of SNCM and RNCM ( Supplementary Fig. 16 ) demonstrate negligible mass loss up to 800 ℃ for the repaired NCM, validating the elimination of residual impurities by the hydrothermal and annealing treatments. Further supporting evidence is provided by the TG profile of SNCM mixed with binder and conductive carbon (SNCM-0), which exhibits two distinct mass losses at ~120 ℃ and 400 ℃, corresponding to the decomposition of residual binders and conductive additives. The absence of such losses in RNCM confirms the success of the 500 ℃ impurity-removal protocol. Overall, after hydrothermal treatment, the NCM cathode material can effectively recover its crystalline structure. The increase in temperature and the introduction of H 2 O and CO 2 can accelerate the degradation of NCM with residual lithium and heterogeneous phases on the surface. Ambient pressure X-ray photoelectron spectroscopy (APXPS) with heating up to 600 ℃ ( Fig. 3f ) observed that significant -CO 3 signals gradually emerge on the surface of SNCM with many defects, and there is severe lattice oxygen loss. This is mainly because when SNCM is attacked by CO 2 and H 2 O, the surface inactive Li + combines with CO 2 to form Li 2 CO 3 , and meanwhile, surface Ni 3+ is more likely to transform to Ni 2+ . In contrast, only a strong Ni 3+ signal can be observed on RNCM without obvious lattice oxygen loss ( Fig. 3g ), indicating that the interference of CO 2 and H 2 O does not cause a violent reaction on the surface of RNCM. Hence, the repaired NCM cathode has a cleaner surface and a more stable structure. 2.2 Electrochemical Performance of NCM After Repair using Brine Next, the electrochemical performances of direct regenerated NCM were systematically studied. Firstly, the charge transfer kinetics and cathode-electrolyte interphase (CEI) stability of Li||SNCM and Li||RNCM cells during the first charge-discharge cycle were investigated via in-situ electrochemical impedance spectroscopy (EIS). As illustrated in Figs. 4a-b , a complete CEI layer was not yet formed at the initial stage of charging, resulting in the impedance spectrum dominated by the semicircle corresponding to charge transfer resistance (R ct ). Notably, RNCM exhibited significantly lower interfacial resistance than SNCM, indicating a substantial reduction in Li + migration resistance after regeneration. Upon entering the high-voltage stage, R ct decreased and a second semicircle emerged, which was associated with CEI formation (R CEI ). SNCM still has a relatively high R ct , which corresponds to a slow kinetic process. In contrast, benefiting from its smooth surface and stable CEI, RNCM maintained a low R ct throughout the cycling process. Beyond 3.8 V, R ct of RNCM stabilized, verifying its excellent interfacial stability under high-voltage operation. As present in Fig. 4c , cyclic voltammetry (CV) curves of RNCM's indicated improved reversibility versus SNCM, with sharper redox peaks, higher currents, and a narrowed ΔE (200 mV, 17% lower than SNCM), evidencing reduced polarization due to its cleaner surface and faster charge transfer. Furthermore, CV curves of SNCM, RNCM and CNCM ( Supplementary Fig. 17a-c ) were obtained at different scan rates. As the scan rate increased, the polarization of SNCM greatly increased, and the redox peaks became less distinct, with no response in the peak current. This directly indicates that the electrochemical activity of the waste NCM is very low. The slope for the plot of the peak current I p and the scan rate v 1/2 represents the diffusion coefficient of Li + ( Fig. 4d ) by fitting with Equation (1) : where I p is the peak current, n is the number of electrons transferred in the electrode reaction, S is the effective area of the electrode, D is the diffusion coefficient, C is the bulk concentration of the reactant, and v is the scan rate. It can be seen that RNCM has an ion diffusion capacity comparable to that of CNCM, confirming the recovery of electrochemical activity. For SNCM, increasing scan rates induced severe polarization and attenuated redox peaks ( Supplementary Fig. 17a ), with negligible peak current response, underscoring its poor electrochemical activity. In contrast, as shown in Fig. 4d , RNCM exhibits a lithium-ion diffusion capability comparable to CNCM. Additionally,the voltage profiles of NCMs acquired from the galvanostatic intermittent titration technique (GITT) test were shown in Fig. 4e and Supplementary Fig. 18 . For SNCM, the voltage variation during the galvanostatic pulse (∆E t ) is extremely large (0.009 V), corresponding to significant polarization, which is attributed to its internal structure damage and the increased charge transfer resistance. The voltage change during the relaxation stage (∆E s ) after the galvanostatic pulse ends is also the largest (0.010 V) among the samples, indicating that ions face relatively greater difficulty diffusing within the cathode. Moreover, compared with SNCM (charging: 1.2×10 -12 cm 2 ·s -1 , discharging: 5×10 -13 cm²·s -1 ), RNCM and CNCM exhibit similar and relatively higher average lithium ion diffusion coefficients (D Li⁺ ) (charging: 8.0×10 -11 cm 2 ·s -1 , 6.5×10 -11 cm 2 ·s -1 ; discharging: 6.2×10 -11 cm 2 ·s -1 , 7.8×10 -11 cm 2 ·s -1 ). Both the GITT and CV results indicate that RNCM has restored good ion-diffusion capability and excellent Li + transport kinetics. The first-cycle charge-discharge profiles with the cutoff voltage of 2.7-4.3 V under the Crate of 0.2 C in Fig. 4f reveal that the SNCM exhibits severe polarization and the lowest specific capacity, whereas the RNCM demonstrates a reduced overpotential and a higher discharge capacity (153.8 mAh·g -1 ) comparable to commercial NCM (CNCM, 157.7 mAh·g -1 ). Fig. 4g and Supplementary Fig. 19 show that SNCM suffers rapid capacity fade during charge-discharge cycling, with its capacity approaching zero after 100 cycles. In contrast, the repaired NCM exhibits high capacity retention of 87.93% at 0.2 C and 93.89% at 0.5 C comparable to CNCM (99.42% and 91.67%). Moreover,RNCM also exhibits restored rate performance, and its performance is closely comparable to that of CNCM ( Fig. 4h ). Notably, it delivers a specific capacity of 100.64 mAh·g -1 even at a high rate of 1 C, which is comparable to that of CNCM (109.00 mAh·g -1 ). These results highlight the effectiveness of the hydrothermal repair method in a Na/Li mixed environment for restoring the electrochemical performance of spent NCM materials. In order to further to study the mechanism on enhanced electrochemical behavior for repaired NCM, we conducted in-situ XRD tests on the first charge-discharge at 0.2 C within the voltage range of 3.0-4.3 V. The XRD patterns with 2θ ranging from 17.9-18.7° and 36.2-37.6° were selected to observe the evolution process of the (003) and (101) peaks ( Fig. 4i and Supplementary Fig. 20 ). As the voltage increases, the (003) peaks of both materials first shift to a lower 2θ angle, indicating that an H1→H2 phase transition occurs with the expansion along the c axis. While the (101) peaks shift towards a higher angle, which means that the lattice parameter a gradually decreases, and the phase transition process of RNCM is even smoother than CNCM. Subsequently, at a higher voltage, the (003) peaks suddenly shift to a higher 2θ angle, reflecting the rapid contraction of the lattice along the c axis when lithium ions are further deintercalated, and an H2 to H3 phase transition occurs. This process is very brief, and after continuing to discharge, the (003) peak can return to its original position, indicating that RNCM and CNCM both have good reversibility of the H2/H3 phase transition. Simultaneously, in order to reduce the material cost during mass repairs, we attempted to reuse the original solution from the hydrothermal reaction three times consecutively to repair SNCM, yielding three repaired samples: RNCM-1 st , RNCM-2 nd , and RNCM-3 rd ( Supplementary Fig. 21 ). It was observed that all three RNCM samples exhibited smooth and intact secondary particles. Notably, RNCM-3 rd maintained a specific capacity of 146.5 mAh·g −1 at 0.2 C and 144.9 mAh·g −1 at 0.5 C. This demonstrates that a minimal lithium dose (3 mM Li + ) can fully regenerate SNCM about 3 times via repetitive Li + insertion. It is noted that this method significantly reduces raw material costs ( Fig. 4j ), which include the aqueous solution used for hydrothermal treatment, ethylene glycol, and water consumed for washing NCM after hydrothermal processes. Notably, concentrated seawater can be directly adopted as the lithium source for regeneration, thus only the marginal cost of seawater concentration needs to be considered. Compared with traditional hydrometallurgical and hydrothermal technologies, our method significantly reduces recycling costs by utilizing low-cost raw materials of seawater. 2.3 Mechanism of Regenerating NCM cathode using Brine Next, we first demonstrate that in the two steps of repairing the NCM cathode, hydrothermal process plays a decisive role rather than the subsequent annealing. We investigated the phase and morphology evolution of NCM during the post annealing step. An additional 3 wt% of Li 2 CO 3 was added to prevent lithium loss during the 800 ℃-annealing after the hydrothermal treatment. With progressive heating, the in-situ XRD patterns ( Fig. 5a ) reveal the gradual disappearance of the Li 2 CO 3 peaks, accompanied by a shift of the (003) and (104) peaks toward lower angles. And the peak positions return to their original positions when the temperature drops to room temperature. This indicates that during the annealing treatment, lattice thermal expansion is the main process occurring. Besides, the in-situ SEM images of the annealing process ( Fig. 5b ) show that when the temperature rises to 800 ℃, small Li 2 CO 3 particles on the surface of NCM particles completely disappears, and there is no obvious change in the particle morphology during the entire annealing process. This is closely consistent with the in-situ heating XRD results. To verify the effect of the hydrothermal step, we heat SNCM via controlled amount of Li 2 CO 3 and SNCM powder, yielding the solid-phase product (RNCM-S). The morphology of RNCM-S remained incompletely restored and there is no obvious recovery in electrochemical performance ( Supplementary Fig. 22 ). Unlike the solid-phase method, where lithiation is constrained by the inherently limited solid-solid contact between lithium salts and cathode particles, the hydrothermal process achieves uniform nanoscale interfacial interaction through Li + diffusion in aqueous media, eliminating compositional gradients and thus conferring distinct regeneration advantages. To further demonstrate the efficacy of a seawater-mimetic solution in regenerating SNCM cathodes, we systematically evaluate the individual and combined effects of key salts (Li + , Na + , K + , Ca 2+ ) during hydrothermal processes as follows. The LiCl-only system (RNCM-Li) produced particles with smoother surface than the degraded SNCM ( Supplementary Fig. 23 ), yet failed to achieve the morphological uniformity like RNCM. And the diffuse and elongated diffraction spots observed in the TEM results ( Supplementary Fig. 24 ) further confirm that the LiCl-repaired NCM still retains numerous structural defects. Although RNCM-Li delivered improved capacity (134.0 mAh·g -1 at 0.2 C) and cycling stability over SNCM, its performance lagged significantly behind RNCM, suggesting that particle uniformity critically influences Li + transport kinetics. Notably, treatment with NaCl alone (RNCM-Na) paradoxically yielded well-defined secondary particles with smooth surfaces ( Supplementary Fig. 25a-c ). However, neither the XRD patterns nor the electrochemical performance showed meaningful regeneration, as the lack of lithium supplementation left the lithium vacancies unaddressed. When used individually, either KCl or CaCl 2 partially restored the morphology, yet surface defects and lattice distortion still persisted. ( Supplementary Figs. 26-27 ). The RNCM-LNCK using 30 mM LiCl, 3 M NaCl, 0.1 M KCl and 0.1 M CaCl 2 system exhibited moderate electrochemical performance ( Supplementary Fig. 28 ). The capacity (143.5 mAh·g -1 at 0.2 C) failed to be fully recovered, indicating that the introduction of K + and Ca 2+ did not contribute to the structural repair of RNCM. Thereby, we propose this repair mechanism for hydrothermal process using brine: NaCl facilitates Li + insertion and morphological homogenization, LiCl enables lithium replenishment. The schematic diagram in Fig. 5c illustrates the role of Na + in the hydrothermal repair process. To elucidate the underlying mechanisms, we systematically investigated the adsorption and intercalation energies of Li + , Na + , K + and Ca 2+ on the NCM surface ( Figs. 5b - c ). Both Li + and Ca 2+ exhibit strong surface adsorption tendencies. However, Ca 2+ -rich regions generate localized electron-dense domains ( Fig. 5d ), which electrostatically repel approaching Li + and hinder their lattice insertion. Furthermore, smaller hydration radius of Ca 2+ (3.12 Å vs. Li + : 3.40 Å) may enable partial Ca 2+ insertion into interlayers. The negligible contribution of K + aligns with its lowest adsorption/insertion energies and highest diffusion barriers, rendering it ineffective for regeneration. In contrast, NaCl exerts beneficial effects chiefly through Na + selective adsorption behavior. With moderate adsorption energy and significant content, Na + preferentially occupies high-energy surface defects (e.g., Li vacancies, grain boundaries), forming a uniform dynamic adsorption layer under hydrothermal conditions. The high Na + concentration ensures complete defect coverage, and prevents the defects from undergoing uncontrollable dissolution and Ostwald ripening in the hydrothermal environment, 26,27 and thus guarantees morphological uniformity. As the reaction proceeds, a small amount of Li + with extremely strong adsorption energy will gradually and selectively replace those Na + with weaker bonding, dominating the regeneration of NCM. This explains why Na + alone could drive morphological reconstruction. Besides, DFT calculations show that when different ions enter NCM and diffuse along the path (from site 1 to site 3) shown in Fig. 5e , Li + exhibits the significantly lowest bulk diffusion barrier (0.826 eV, 0.802 eV and 0.426 eV) ( Fig. 5f ), ensuring efficient filling of internal vacancies in Na + -enriched systems. This may also stem from the fact that the hydration radius of Na + (3.58 Å) is larger than that of Li + (3.40 Å), resulting in greater steric hindrance when entering NCM. Meanwhile, ethylene glycol enhances this process by increasing solution viscosity, prolonging Na⁺-surface interactions to allow sufficient time for Li + diffusion. This explains why Li⁺ can still enter and achieve repair without the entry of Na⁺ in the presence of a large amount of Na + . In brief, Na + dominates the recovery of morphology, while Li + replenishes the Li loss in spent NCM for the hydrothermal process using Li + -Na + co-presence brine, which achieves superior electrochemical performance for RNCM cathodes. 3 Conclusion We proposed a sustainable and eco-friendly strategy for direct regeneration of spent NCM cathodes through hydrothermal treatment using simulated seawater containing high concentrations of NaCl and trace amount of LiCl, followed by a short annealing step. The regenerated cathode showed a restored layered structure within the secondary particles, featuring smooth surfaces and no detectable impurities. These structural improvements resulted in a high specific capacity of 146.6 mAh·g -1 at 0.5 C and excellent capacity retention of 91.67% after 100 cycles. During the hydrothermal process, owing to overwhelming abundance of NaCl, it preferentially covers the high surface-energy defects and acts as a dynamic coating layer to ensure morphological homogenization. The subsequent annealing further stabilized the crystal lattice that may have been damaged by washing. This work integrated lithium recovery from brines with cathode regeneration in a single low-carbon process, paving a sustainable pathway for battery recycling. 4 Experimental Section Direct Regeneration Process The process is illustrated in Fig. S1 . First, the waste LiNi 0.5 Co 0.2 Mn 0.3 O 2 (NCM523) electrode sheets are processed. They are cut into pieces and immersed in N-methyl-2-pyrrolidone (NMP), and then ultrasonic treatment is carried out to remove surface impurities and part of the weakly - bonded binders. Subsequently, the waste electrode sheets are placed in a 3 M sodium hydroxide aqueous solution and stirred to obtain electrodes separated from the aluminum foil. The electrode fragments are placed in an alumina crucible and heated in a muffle furnace at 500 ℃ for 3 h in air to remove residual binders, conductive carbon and other impurities. After thorough grinding, the spent NCM powder (SNCM) is obtained. Next, the hydrothermal repair stage begins. Place 0.1 g of SNCM powder into 40 ml of the prepared simulated seawater (3 M NaCl, 3 mM LiCl), and then add 2 ml of ethylene glycol. Subsequently, ultrasonically stir the mixture for 30 minutes to fully disperse the particles. After that, transfer the well-dispersed mixture into a reaction kettle and heat it at 200 ℃ for 3 h. After the reaction kettle has completely cooled down, take out the liquid in the inner liner and perform centrifugation. Wash the centrifuged product three times with deionized water and then dry it. Finally, put the dried powder into an alumina crucible and anneal it at 800 ℃ for 4 h in air atmosphere, thus obtaining the repaired NCM powder (RNCM). The hydrothermal solutions used in other experiments were aqueous solutions of 3 mM LiCl, 3 M NaCl, 3 M KCl, 3 M CaCl 2 , 3 mM LiCl and 3 M NaCl mixed with 0.1 M CaCl 2 /KCl. The obtained NCM materials were labeled as RNCM-Li, RNCM-Na, RNCM-Ca, RNCM-K, and RNCM-LNCK respectively. Structural Characterization Inductively coupled plasma-optical emission spectroscopy (ICP-OES) was conducted on an iCAP 7400 Radial. ICP is used to determine the content of each element in the powder. To improve the accuracy, the results are the average values of three sets of data. X-ray diffractometer (XRD) was performed on a Bruker D8 Advance equipped with Cu Kα radiation source. The scanning range of 2θ was 10°-80°. The XRD patterns of the samples were refined using the FullProf software. And the in situ XRD data were obtained by means of X-ray diffractometry in combination with an electrochemical workstation (Bio-Logic, VMP-300). Thermogravimetric analysis (TGA, Perkin - Elmer) was used to assess the regeneration effect from 20 to 800 ℃ with a heating rate of 5 ℃·min -1 . Fourier transform infrared (FT-IR) spectroscopy was used to carry out tests on a VERTEX 70 spectrophotometer, covering the range from 400 to 2000 cm⁻¹. The Raman spectrum was obtained by using an SR-500I-D2-1F1 Spectrograph. SEM (JEOL JSM-7800F) and TEM (JEM-2100 Plus, JEM-F200) with EDS attachments were employed to characterize the microstructure, morphology, and chemical compositions of the samples. XPS (ThermoFisher Scientific, ESCALAB 250Xi) was used to get the surface chemical composition data. APXPS was configured with a PHOIBOS hemispherical electron energy analyzer, a focused monochromatic X-ray source (Al Kα) with a spot size of approximately 300 μm, and an IQE-11A ion gun, all supplied by SPECS Corporation. The APXPS spectra were acquired under atmospheres of 0.5 mBar H₂O and 0.1 mBar CO₂. 7 Li MAS NMR was carried out using ADVANCE III HD 400MHz Solid NMR Spectrometer. The time-of-flight secondary ion mass spectroscopy (TOF-SIMS) measurements were conducted using an IONTOF M6 equipped with a Bi 3+ gun. Electrochemical Measurement First, the working electrode was prepared. The active material powder, conductive carbon Super-P, and binder PVDF were thoroughly mixed in a mass ratio of 8:1:1. Then, N-methyl-2-pyrrolidone (NMP) was added. After thorough stirring, the slurry was evenly coated on an aluminum foil. The coated foil was placed in a vacuum drying oven at 90 ℃ for 12 h. Finally, a positive electrode sheet with an active material loading of approximately 4-5 mg·cm⁻² was obtained. The CR2032 coin cells were fabricated in an argon-filled glove box (with the content of O₂ and H₂O less than 1 ppm). In this case, lithium metal (Adamas-beta) was used as both the reference electrode and the counter electrode, the polymer membrane (Celgard 2325) was used as the separator, and 1 mol/L LiPF₆ dissolved in ethylene carbonate/ethyl methyl carbonate (EC/EMC, v/v = 3/7) with the addition of 1% vinyl carbonate (VC) was used as the liquid electrolyte. The electrochemical behavior of the Li||NCM cells was evaluated by means of the Electrochemical Impedance Spectroscopy (EIS) technique and Cyclic Voltammetry (CV) using an electrochemical workstation (Bio-Logic, VMP-300). For the EIS test, the perturbation bias voltage was 10 mV, and the working frequency range was from 7 MHz to 0.1 Hz. The CV was carried out within a potential range of 2.75-4.30 V at a scanning rate of 0.1 mV·s -1 -0.4 mV·s -1 . The relationship between the peak current I p and the square root of the scan rate v 1/2 is described by Equation (1) . Galvanostatic charge-discharge cycle tests and rate performance tests (2.7-4.3 V) were conducted on a battery testing system (LAND, CT 2001 A) at room temperature. Galvanostatic Intermittent Titration Technique (GITT) was measured at 0.1 C for 5 min along with the relaxation time of 30 min. DFT calculations The DFT calculations were performed in the Vienna ab initio simulation package (VASP). The Perdew-Burke-Ernzerhof (PBE) form of generalized gradient approximation (GGA) was used as the exchange-correlation potential. The structural and electronic properties were calculated by the PBE+ U approach with spin polarization, with U values of 3.5 eV, 3.3 eV, and 6.4 eV for Mn, Co, and Ni, respectively. The atomic configurations and the U values of NCM523 were adopted from Pan’s work. 28 The cut-off energy was 450 eV for the plane-wave basis. The k -point mesh of 2 × 4 ×1 was used to sample the Brillouin zone of bulk NCM523. For the surfaces absorbed with Li, each slab had a vacuum thickness of 15 Å and the k-point was adapted to achieve a similar sampling density in reciprocal space. The convergence criteria were 10 -4 eV for the total energies during structure relaxation. Table S3 lists the details in calculation the atomic and electronic structures of NCM523 systems. The insertion energy ( E insert ) of M ( M = Li, Na, K and Ca ions) in NCM was defined as: E insert = E t - E M - E NCM , where E NCM , E M and E t are the energies of the NCM bulk, the M ions, and the combined system of them, respectively. The absorption energy ( E a ) of M ( M = Li, Na, K and Ca ions) on NCM surface was defined as: E absorb = E A - E M - E slab , where E slab , E M and E A are the energies of the NCM slab, the M ions, and the absorption system of them, respectively. The electron density difference (Δ ρ ) on the NCM surface is defined as Δ ρ = ρ A -ρ M -ρ slab , where ρ slab , ρ M and ρ A is the charge density of NCM slab, the M ions, and the absorption system of them, respectively. The energy barriers of M ( M = Li, Na, K and Ca ions) ion migration in NCM were calculated by the nudged elastic band method. Declarations Acknowledgements The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (92472119 to W.L., 52222311 to Y.Y.), the Natural Science Foundation of Shanghai (24ZR1451200 to W.L.) and Development Fund for Schools of ShanghaiTech University. This project is supported by State Key Laboratory of New Ceramic Materials Tsinghua University (No. KF202504 to W.L.). The electron microscopy experiments were supported by the Center for High-Resolution Electron Microscopy (CℏEM) at ShanghaiTech University. References Biswal, B. K., Zhang, B., Thi Minh Tran, P., Zhang, J. & Balasubramanian, R. Recycling of spent lithium-ion batteries for a sustainable future: recent advancements. Chem Soc Rev 53 , 5552-5592 (2024). https://doi.org/10.1039/d3cs00898c Ahuis, M. et al. Recycling of solid-state batteries. Nature Energy 9 , 373-385 (2024). https://doi.org/10.1038/s41560-024-01463-4 Doose, S., Mayer, J. K., Michalowski, P. & Kwade, A. Challenges in Ecofriendly Battery Recycling and Closed Material Cycles: A Perspective on Future Lithium Battery Generations. Metals 11 (2021). https://doi.org/10.3390/met11020291 Cheng, M. et al. H 2 O-balance-regulated cation-anion competitive coordination for selective elements extraction from spent lithium-ion batteries. eScience 4 (2024). https://doi.org/10.1016/j.esci.2024.100275 Ji, H. et al. Closed-Loop Direct Upcycling of Spent Ni-Rich Layered Cathodes into High-Voltage Cathode Materials. Adv Mater 36 , e2407029 (2024). https://doi.org/10.1002/adma.202407029 Li, Y.-M. et al. The Weakened Super-Exchange Interaction Realizes the Direct Regeneration of Spent Lithium-lon Battery Cathodes. Angewandte Chemie International Edition 64 , e202520448 (2025). https://doi.org/https://doi.org/10.1002/anie.202520448 Zheng, N. et al. Surface Catalytic Repair for the Efficient Regeneration of Spent Layered Oxide Cathodes. Journal of the American Chemical Society (2024). https://doi.org/10.1021/jacs.4c10107 Huang, Q. et al. Surface Engineering Enabling Efficient Upcycling of Highly Degraded Layered Cathodes. Advanced Materials 37 , 2419872 (2025). https://doi.org/https://doi.org/10.1002/adma.202419872 Lin, J. et al. A green repair pathway for spent spinel cathode material: Coupled mechanochemistry and solid-phase reactions. eScience 3 (2023). https://doi.org/10.1016/j.esci.2023.100110 Shen, J. et al. Advanced direct recycling technology enables a second life of spent lithium-ion battery. 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How to make lithium extraction cleaner, faster and cheaper - in six steps. Nature 616 , 245-248 (2023). Xue, N. et al. Single Lithium-Ion Conductor Decorated LiMn 2 O 4 with High Selectivity and Stability for Electrochemical Lithium Extraction. ACS Nano 18 , 33743-33753 (2024). https://doi.org/10.1021/acsnano.4c15473 Xue, N. et al. Stable Electrochemical Lithium Extraction Using LiMn 2 O 4 Coated With Lithiated Nafion. Small Methods , e2401972 (2025). https://doi.org/10.1002/smtd.202401972 Tan, G., Wan, S., Chen, J. J., Yu, H. Q. & Yu, Y. Reduced Lattice Constant in Al-Doped LiMn 2 O 4 Nanoparticles for Boosted Electrochemical Lithium Extraction. Adv Mater 36 , e2310657 (2024). https://doi.org/10.1002/adma.202310657 Lawagon, C. P. et al. Li1−Ni0.33Co1/3Mn1/3O2/Ag for electrochemical lithium recovery from brine. Chemical Engineering Journal 348 , 1000-1011 (2018). https://doi.org/10.1016/j.cej.2018.05.030 Lawagon, C. P. et al. Li Ni0.5Mn1.5O4/Ag for electrochemical lithium recovery from brine and its optimized performance via response surface methodology. Separation and Purification Technology 212 , 416-426 (2019). https://doi.org/10.1016/j.seppur.2018.11.046 Ahn, W. et al. High Lithium Ion Transport Through rGO-Wrapped LiNi 0.6 Co 0.2 Mn 0.2 O 2 Cathode Material for High-Rate Capable Lithium Ion Batteries. Front Chem 7 , 361 (2019). https://doi.org/10.3389/fchem.2019.00361 Pasta, M., Battistel, A. & La Mantia, F. Batteries for lithium recovery from brines. Energy & Environmental Science 5 (2012). https://doi.org/10.1039/c2ee22977c Kim, J. S. et al. An Electrochemical Cell for Selective Lithium Capture from Seawater. Environ Sci Technol 49 , 9415-9422 (2015). https://doi.org/10.1021/acs.est.5b00032 Zhang, G. et al. Spontaneous lithium extraction and enrichment from brine with net energy output driven by counter-ion gradients. Nature Water 2 , 1091-1101 (2024). https://doi.org/10.1038/s44221-024-00326-2 Goodenough, J. B. & Park, K. S. The Li-ion rechargeable battery: a perspective. J Am Chem Soc 135 , 1167-1176 (2013). https://doi.org/10.1021/ja3091438 Eiji Hosono, S. F., Keita Kakiuchi, and Hiroaki Imai. Growth of Submicrometer-Scale Rectangular Parallelepiped Rutile TiO 2 Films in Aqueous TiCl 3 Solutions under Hydrothermal Conditions. Journal of the American Chemical Society 126 , 7790-7791 (2004). Lai, L. et al. A facile hydrothermal synthesis and properties of TiO 2 nanosheet array films. Materials Research Express 7 (2020). https://doi.org/10.1088/2053-1591/ab638b Wei, Y. et al. Kinetics Tuning of Li-Ion Diffusion in Layered Li(Ni x Mn y Co z )O 2 . J Am Chem Soc 137 , 8364-8367 (2015). https://doi.org/10.1021/jacs.5b04040 Additional Declarations There is NO Competing Interest. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9324161","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":622538636,"identity":"87345d38-00db-4eb7-8231-1811250ab8dd","order_by":0,"name":"Wei Liu","email":"data:image/png;base64,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","orcid":"https://orcid.org/0000-0002-6206-8321","institution":"ShanghaiTech University","correspondingAuthor":true,"prefix":"","firstName":"Wei","middleName":"","lastName":"Liu","suffix":""},{"id":622538637,"identity":"a1b26c6b-4488-4e81-ba3e-d10a2d04c482","order_by":1,"name":"Ning Xue","email":"","orcid":"","institution":"ShanghaiTech University","correspondingAuthor":false,"prefix":"","firstName":"Ning","middleName":"","lastName":"Xue","suffix":""},{"id":622538638,"identity":"ae57966b-4ce9-4564-b8e7-755e6b976dac","order_by":2,"name":"Yuanyuan Cui","email":"","orcid":"","institution":"Shanghai University","correspondingAuthor":false,"prefix":"","firstName":"Yuanyuan","middleName":"","lastName":"Cui","suffix":""},{"id":622538639,"identity":"4e8a3f65-9578-4a06-a6bf-641677740ad4","order_by":3,"name":"Wu Xiaoyan","email":"","orcid":"","institution":"ShanghaiTech University","correspondingAuthor":false,"prefix":"","firstName":"Wu","middleName":"","lastName":"Xiaoyan","suffix":""},{"id":622538640,"identity":"884c0393-ded3-4580-9a27-c503d58817ed","order_by":4,"name":"Yuyao Zhang","email":"","orcid":"","institution":"ShanghaiTech University","correspondingAuthor":false,"prefix":"","firstName":"Yuyao","middleName":"","lastName":"Zhang","suffix":""},{"id":622538641,"identity":"e131554d-f9ad-4a55-8dc8-1f871c8e5b84","order_by":5,"name":"Luyao Wang","email":"","orcid":"","institution":"ShanghaiTech University","correspondingAuthor":false,"prefix":"","firstName":"Luyao","middleName":"","lastName":"Wang","suffix":""},{"id":622538642,"identity":"d24bb505-a4ba-4caa-98f2-1f239b68c98a","order_by":6,"name":"Ruixin Hao","email":"","orcid":"","institution":"ShanghaiTech University","correspondingAuthor":false,"prefix":"","firstName":"Ruixin","middleName":"","lastName":"Hao","suffix":""},{"id":622538643,"identity":"ba7e2012-171f-44dd-95e8-a27b42dbd636","order_by":7,"name":"Tianyi Gao","email":"","orcid":"","institution":"ShanghaiTech University","correspondingAuthor":false,"prefix":"","firstName":"Tianyi","middleName":"","lastName":"Gao","suffix":""},{"id":622538644,"identity":"85ce5403-0049-4d23-a336-b07f42565c67","order_by":8,"name":"Jinjiang Liang","email":"","orcid":"","institution":"ShanghaiTech University","correspondingAuthor":false,"prefix":"","firstName":"Jinjiang","middleName":"","lastName":"Liang","suffix":""},{"id":622538645,"identity":"045b10a6-9ba6-4eb8-ac3e-6f4a633d3258","order_by":9,"name":"Yihang Yang","email":"","orcid":"","institution":"ShanghaiTech University","correspondingAuthor":false,"prefix":"","firstName":"Yihang","middleName":"","lastName":"Yang","suffix":""},{"id":622538646,"identity":"b1359844-a57a-4871-b569-46e076799027","order_by":10,"name":"Yi Yu","email":"","orcid":"https://orcid.org/0000-0003-4326-5992","institution":"ShanghaiTech University","correspondingAuthor":false,"prefix":"","firstName":"Yi","middleName":"","lastName":"Yu","suffix":""}],"badges":[],"createdAt":"2026-04-05 05:20:17","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9324161/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9324161/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":106941052,"identity":"4c209d7a-24f5-468b-976c-6996b6bbf85e","added_by":"auto","created_at":"2026-04-15 05:23:13","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":420716,"visible":true,"origin":"","legend":"\u003cp\u003eThe cathode material NCM523 of LIBs was directly repaired through a simple hydrothermal reaction using brine with multiple ions (30 mM LiCl, 3 M NaCl, 0.1 M KCl and 0.1 M CaCl\u003csub\u003e2\u003c/sub\u003e) as a lithium source, with ethylene glycol added to modulate the ionic environment. After the repair, the defects of NCM disappeared, the surface heterophases vanished, and the morphology was restored. (b) The repair of NCM is achieved through a simple hydrothermal reaction and short-term post-annealing.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-9324161/v1/c32723b9e17efcefb2acb4ad.png"},{"id":106941051,"identity":"f389b69b-26da-4f01-a6e2-b816f9d17920","added_by":"auto","created_at":"2026-04-15 05:23:13","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":899681,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eStructure and morphological characterizations of NCMs before and after treatment and regeneration.\u003c/strong\u003e XRD Rietveld refinement of (a) SNCM, (b) RNCM-hydro and (c) CNCM. (d) Molar ratio of Li in SNCM, RNCM-hydro, RNCM and CNCM. (e) Ex-situ SEM images of the hydrothermal process. FIB-SEM images of (f) SNCM and (g) RNCM-hydro. (h) 3D TOF-SIMS reconstruction of the Li\u003csup\u003e+\u003c/sup\u003e, Ni\u003csup\u003e+\u003c/sup\u003e, Co\u003csup\u003e+\u003c/sup\u003e, Mn\u003csup\u003e+\u003c/sup\u003e, O\u003csup\u003e+\u003c/sup\u003e and Na\u003csup\u003e+\u003c/sup\u003e species at RNCM. (i) The Na 1s and Cl 2p XPS spectrum during the process of Ar etching on RNCM for 1300 s.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-9324161/v1/0d87850e668121521fa969d9.png"},{"id":106941050,"identity":"ddfa0603-c83a-4106-9e9e-b8095ac458e4","added_by":"auto","created_at":"2026-04-15 05:23:12","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1535223,"visible":true,"origin":"","legend":"\u003cp\u003eStructural and Compositional Insights on NCMs. (a) TEM images of SNCM and (b) the SAED patterns of region\u0026nbsp;Ⅰ, Ⅱ and Ⅲ in SNCM. (c) TEM images of RNCM and (d) the magnified lattice fringe images in RNCM and the SAED pattern of RNCM. (e) XPS spectra of O 1s, Ni 2p\u003csub\u003e3/2\u003c/sub\u003e, Co 2p\u003csub\u003e3/2\u003c/sub\u003e, and Mn 2p in SNCM, RNCM-hydro and RNCM. The measured Ni 2p and O 2p spectra of (f) SNCM and (g) RNCM during the APXPS test with heating at 300-600 ℃ in an environment of 0.5 mBar H\u003csub\u003e2\u003c/sub\u003eO and 0.1 mBar CO\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-9324161/v1/54310ce69a3963dc6af96fcc.png"},{"id":106961837,"identity":"c93a30ab-f912-40f4-8c24-e029e1614162","added_by":"auto","created_at":"2026-04-15 09:27:13","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":546373,"visible":true,"origin":"","legend":"\u003cp\u003eElectrochemical performance of the regenerated cathode material, compared with pristine and spent cathode. The in-situ EIS obtained during the 1\u003csup\u003est\u003c/sup\u003e cycle of (a) SNCM and (b) RNCM. (c) CV curves of the 2\u003csup\u003end\u003c/sup\u003e cycle of SNCM and RNCM. (d) The compared plots of \u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003ep\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e - v\u003c/em\u003e\u003csup\u003e\u003cem\u003e1/2\u003c/em\u003e\u003c/sup\u003e. (e) GITT curves of NCMs at the same cycle number. (f) The voltage profile in 1\u003csup\u003est\u003c/sup\u003e cycle at 0.2 C in Li||NCM half-cell. (g) Cycling stability at 0.5 C in the half cell at RT. (h) Rate performance of NCMs. (i) The selected in-situ XRD contour plots of RNCM and corresponding voltage curves within the voltage range of 3.0-4.3 V. (j) Cost comparison among the hydrothermal method, the hydrothermal method for direct seawater repair, and the hydrometallurgical method.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-9324161/v1/8f73e9c16d26fc49d18862fa.png"},{"id":106941055,"identity":"f074bfa1-30d4-424f-be6d-8ca80c84e742","added_by":"auto","created_at":"2026-04-15 05:23:14","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":913923,"visible":true,"origin":"","legend":"\u003cp\u003eVisualization of the annealing process and exploration of the repair mechanism. (a) In-situ XRD patterns and (b) in-situ SEM images of RNCM-hydro during short annealing with temperature rising to 800 ℃ and holding for 3 h. (Scale bar=1 μm) (c) The schematic illustration depicting the shaping effect of abundant Na⁺ on NCM morphology in hydrothermal process. Comparison of (d) the adsorption energies and (e) the insertion energies of Li, Na, K, and Ca on the surface of NCM523. (f) The schematic diagram of the surface ionic domain. (g)\u003cstrong\u003e \u003c/strong\u003eSchematic diagram of the ion migration path in NCM523. (h) Comparison of and the migration barriers in the lattice after they enter the NCM lattice.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-9324161/v1/d56f2d7831ddb3389b09df0c.png"},{"id":108804626,"identity":"aa20f2fa-81fe-45d9-8b4e-0c736830c9b7","added_by":"auto","created_at":"2026-05-08 15:22:25","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4430258,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9324161/v1/dacc4121-71cf-489a-b5d4-f2d33ab1251f.pdf"},{"id":106941053,"identity":"940955c0-28dc-409f-a3a8-74f9c2deadb7","added_by":"auto","created_at":"2026-04-15 05:23:14","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":7296368,"visible":true,"origin":"","legend":"Supporting information","description":"","filename":"Supportinginformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-9324161/v1/b92fc308e02c585bfb6b84eb.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Direct Regeneration of Layered Oxide Cathodes via Seawater","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eThe rapid proliferation of rechargeable lithium-ion batteries (LIBs) in modern electronics and electric vehicles has precipitated critical challenges in resource sustainability and environmental protection.\u003csup\u003e1,2\u003c/sup\u003e Conventional LIBs recycling paradigms (pyrometallurgy/hydrometallurgy) suffer from high energy intensity and secondary pollution during Li extraction, such as HF gas generated by electrolyte decomposition and heavy metal ash residues from incineration.\u003csup\u003e3,4\u003c/sup\u003e Increasing research groups have been exploring new methods of direct regeneration strategies, which enable in-situ repair of cathodes with the merits of time efficiency and minimal pollution. The molten salt regeneration method is currently the most widely used approach. It primarily reduces the eutectic point by using multiple salts to facilitate lithium salt intercalation, but it requires a large number of salts and often leads to insufficient or excessive lithium insertion at the applied temperature.\u003csup\u003e5,6\u003c/sup\u003e Besides, the solid-state method is the simplest one, which only involves blending lithium salts with spent cathodes followed by high temperature calcination; however, it demands strict implementation of homogeneous mixing of raw materials and precise control over lithium salt dosage.\u003csup\u003e7-9\u003c/sup\u003e In brief, they all face bottlenecks in low lithium replenishment efficiency and difficult structural restoration.\u003csup\u003e10,11\u003c/sup\u003e Particularly, the prevalent use of costly lithium salts (LiOH/Li\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e) in current direct recycling processes significantly compromises economic viability. For example, under ambient conditions, self-adsorbed amyloxylithium can directly regenerate spent LiNi\u003csub\u003e0.6\u003c/sub\u003eCo\u003csub\u003e0.2\u003c/sub\u003eMn\u003csub\u003e0.2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e via simple solid-liquid mixing.\u003csup\u003e12\u003c/sup\u003e Also, the organic lithium salt like 3,4-dihydroxybenzonitrile dilithium can repair spent LiFePO\u003csub\u003e4\u003c/sub\u003e cathodes.\u003csup\u003e13\u003c/sup\u003e Developing innovative lithium compensation approaches with cost-effective lithium sources therefore becomes imperative to achieve both high-performance cathode regeneration and sustainable closed-loop battery recycling.\u003c/p\u003e\n\u003cp\u003eNotably, lithium resources in salt lakes and seawater hold substantial reserves, estimated globally at over 230 billion tons, yet remain underexploited due to lithium extraction complexities.\u003csup\u003e14,15\u003c/sup\u003e However, the predominant presence of Na\u003csup\u003e+\u003c/sup\u003e in seawater complicates Li\u003csup\u003e+\u003c/sup\u003e extraction. Recent studies have focused on extracting lithium from seawater using lithium-ion-sieving materials such as LiMn\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e,\u003csup\u003e16-18\u003c/sup\u003e LiNi\u003csub\u003ex\u003c/sub\u003eCo\u003csub\u003ey\u003c/sub\u003eMn\u003csub\u003ez\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e(eg. Li\u003csub\u003e1-x\u003c/sub\u003eNi\u003csub\u003e0.33\u003c/sub\u003eCo\u003csub\u003e0.33\u003c/sub\u003eMn\u003csub\u003e0.33\u003c/sub\u003eO\u003csub\u003e2)\u003c/sub\u003e\u003csup\u003e19-21\u003c/sup\u003e and LiFePO\u003csub\u003e4\u003c/sub\u003e.\u003csup\u003e22,23\u003c/sup\u003e These materials exploit their spinel, layered or olivine frameworks to selectively adsorb Li\u003csup\u003e+\u003c/sup\u003e through electrochemical or ion-exchange mechanisms, achieving high extraction efficiency even in hypersaline environments.\u003csup\u003e24\u003c/sup\u003e The extracted lithium can be purified to obtain LiCl or converted through chemical reactions to commonly used compounds such as LiOH and Li\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e. It should be noted that these lithium sieves are consistent in structure and composition to the cathode materials for LIBs. The only difference lies in that lithium-ion sieve materials are obtained by leaching lithium from them with acid to form lithium-depleted materials. When the cathode is in a Li-deficient state, it is very similar to the delithiated ion sieve material from which lithium is to be extracted. If the lithium in seawater can be directly used to replenish the lithium in the discarded lithium battery cathodes, the cost will be greatly reduced.\u003c/p\u003e\n\u003cp\u003eHerein, we accomplish the direct regeneration of the spent\u0026nbsp;LiNi\u003csub\u003e0.5\u003c/sub\u003eCo\u003csub\u003e0.2\u003c/sub\u003eMn\u003csub\u003e0.3\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e (NCM523) cathode using a simulated brine. To incorporate lithium ions from brine into the degraded NCM523, we employed a hydrothermal reaction using a mixed Na/Li solution, which effectively restored its crystal structure and rectified structural disorders. Subsequently, a quick annealing process is employed to further stabilize the lattice of regenerated NCM523 cathode. Interestingly, compared to using pure LiCl solution for hydrothermal process, the resulting NCM523 processed with the mixed Na/Li solution exhibits a significantly more homogeneous morphology. The direct regeneratedNCM523 cathodes via the mixed Na/Li simulated brine deliver comparable specific capacity and cycling stability to the commercial counterpart. The superior electrochemical performances of the regeneratedNCM523 benefit from the dynamic adsorption shell formed by a large amount of Na under lithium-deficient conditions. As the reaction proceeds, a small amount of Li\u003csup\u003e+\u003c/sup\u003e with extremely strong adsorption energy will gradually and selectively replace the weakly bound Na\u003csup\u003e+\u003c/sup\u003e, dominating the regeneration of NCM. This method simultaneously enables the direct restoration of electrode materials and the extraction of lithium from seawater, which holds a highly promise for achieving a closed-loop of utilizing resources and energy, offering high economic benefits.\u003c/p\u003e"},{"header":"2 Results and discussions","content":"\u003ch2\u003e\u003cstrong\u003e2.1 Morphology and Structure of NCM After Repair using Brine\u003c/strong\u003e\u003c/h2\u003e\n\u003cp\u003eOur research explored an effective approach on the direct regeneration of the spent NCM523 (SNCM) cathode using a Li-scarce and Na-rich brine, as is shown in \u003cstrong\u003eFig. 1\u003c/strong\u003e. The restoration process involves a hydrothermal reaction and short post-annealing. For hydrothermal process, LiCl serves as a lithium source to replenish the active lithium lost in SNCM. Meanwhile, abundant NaCl may form a dynamic adsorption layer on the surface of NCM during the restoration process, aiding defect repair and morphology homogenization without being incorporated into the NCM lattice, owing to the large ionic radius of Na\u003csup\u003e+\u003c/sup\u003e (1.02 \u0026Aring; vs. Li\u003csup\u003e+\u003c/sup\u003e: 0.76 \u0026Aring;). Additionally, some ethylene glycol was introduced to increase solution viscosity, thereby providing more sufficient reaction time for the above process. The sample after hydrothermal process is defined as RNCM-hydro. After further water washing and post-annealing, the repaired NCM (RNCM) is finally obtained.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eSpecifically, the SNCM powder separated from electrode sheets through the process shown in \u003cstrong\u003eSupplementary Fig. 1\u003c/strong\u003e was subjected to hydrothermal repair according to \u003cstrong\u003eFig. 1b\u003c/strong\u003e. SNCM powder was dispersed in an aqueous solution of 3 mM LiCl and 3 M NaCl with 2 mL ethylene glycol, followed by hydrothermal reaction at 200 ℃ for 3 h. The product was then centrifuged and washed repeatedly, and annealed in air at 800 ℃ for 5 h.\u003c/p\u003e\n\u003cp\u003eFirstly, we studied the structures of the layered SNCM, RNCM-hydro, RNCM, and\u0026nbsp;CNCM cathodes by the XRD patterns (\u003cstrong\u003eSupplementary Fig. 2\u003c/strong\u003e). The (003) peak of RNCM-hydro and RNCM shifts to a higher 2\u0026theta; compared to SNCM, closely aligning with that of CNCM. This shift indicates successful Li\u003csup\u003e+\u003c/sup\u003e intercalation. The intercalated Li\u003csup\u003e+\u003c/sup\u003e generate electrostatic attraction with the transition metal layers, which in turn reduces the interlayer distance along the c-axis.\u003csup\u003e25\u003c/sup\u003e Furthermore, the XRD patterns reveal a significant expansion of the (108)/(110) peak separation in SNCM compared to CNCM, which suggests a contraction of the a-axis lattice parameter. Notably, the regenerated RNCM exhibits a restored (108)/(110) peak profile comparable to CNCM, providing strong evidence for the successful reconstruction of the original layered structure. As detailed in \u003cstrong\u003eSupplementary\u003c/strong\u003e \u003cstrong\u003eTable 2\u003c/strong\u003e and visualized in \u003cstrong\u003eFigs. 2a-c\u0026nbsp;\u003c/strong\u003eand\u003cstrong\u003e\u0026nbsp;Supplementary Fig. 3\u003c/strong\u003e, the \u003cem\u003eRietveld\u003c/em\u003e refinement results indicate that refined unit cell parameters show a marked decrease in c-axis length from 14.27 \u0026Aring; (SNCM) to 14.22 \u0026Aring; (RNCM), accompanied by an increased c/a ratio. These observations collectively point to an enhancement of the layered structure\u0026apos;s integrity in the regenerated material. Concurrently, a substantial reduction in Li/Ni cation mixing was observed - from 10.82% in SNCM to 5.80% in RNCM-hydro, and further to 4.10% in the final RNCM product. This progressive decrease signifies a marked recovery of long-range crystallographic order and a reduction in cation disorder, directly attesting to the efficacy of the hydrothermal repair process. The element ratio of Li/transition metal (TM) in SNCM, RNCM-hydro, RNCM and CNCM through inductively coupled plasma-optical emission spectroscopy (ICP-OES) indicates that lithium has been successfully replenished into the NCM after hydrothermal process (\u003cstrong\u003eFig. 2d\u0026nbsp;\u003c/strong\u003eand\u003cstrong\u003e\u0026nbsp;Supplementary Table 1\u003c/strong\u003e). These combined results conclusively demonstrate that the spent NCM cathode was successfully replenished with Li\u003csup\u003e+\u003c/sup\u003e and its original crystal lattice effectively repaired through the hydrothermal process.\u003c/p\u003e\n\u003cp\u003eSEM images in\u003cstrong\u003e\u0026nbsp;Fig. 2e\u003c/strong\u003e trace the progressive morphological repair of degraded NCM during hydrothermal treatment with Na\u003csup\u003e+\u003c/sup\u003e/Li\u003csup\u003e+\u003c/sup\u003e brine. The spent cathode exhibits severe surface defects, including cracks and nanoflower-like phases characteristic of structural collapse. The nanoflower morphology corresponding to severe degradation basically disappears at 100 ℃. The pores and cracks are further eliminated with temperature; ultimately, the smooth morphology of the secondary particles is clearly visible when the temperature reaches 200 ℃.After hydrothermal process, damaged surface of particles becomes smooth, and the heterogeneous phases completely disappear, restoring the morphology of SNCM close to the state of commercial NCM (CNCM) (\u003cstrong\u003eSupplementary Fig. 4\u003c/strong\u003e). To visually assess the repair effect from the cross-sectional morphology, we employed focused ion beam (FIB) to cut individual SNCM and RNCM-hydro particle (\u003cstrong\u003eFigs. 2f\u003c/strong\u003e-\u003cstrong\u003eg\u003c/strong\u003e). The latter particles show obvious disappearance of internal cracks and pores. And the Ni, Co, and Mn elements are uniformly distributed on the surfaces of SNCM and RNCM-hydro (\u003cstrong\u003eSupplementary Fig. 5\u003c/strong\u003e). After a brief post-annealing treatment, the RNCM particles also exhibited a smooth surface and an indistinct distribution of Na signal (\u003cstrong\u003eSupplementary Fig. 6\u003c/strong\u003e). This indicates that through a simple hydrothermal reaction, seawater with a relatively low lithium content as the lithium source can restore the morphology of the NCM cathode.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eSubsequently, due to the high content of Na\u003csup\u003e+\u003c/sup\u003e (39.3 wt%) in the Na/Li mixed solution simulating seawater, we systematically characterized whether Na was intercalated into the RNCM and its impact on the repair efficiency. Two-dimensional time-of-flight secondary ion mass spectrometry (TOF-SIMS) mapping confirms homogeneous Li, Ni, Co and Mn dispersion across the RNCM surface (\u003cstrong\u003eSupplementary\u003c/strong\u003e \u003cstrong\u003eFig. 7\u003c/strong\u003e). And three-dimensional TOF-SIMS profiling further validates uniform bulk distribution of Li, Ni, Co, Mn and O from surface to interior regions, along with the absence of Na (\u003cstrong\u003eFig. 2h\u003c/strong\u003e). Intriguingly, Na\u003csup\u003e+\u003c/sup\u003e exhibits distinct localization behavior: while trace Na⁺ signals persist at the surface (\u003cstrong\u003eSupplementary\u003c/strong\u003e \u003cstrong\u003eFig. 8\u003c/strong\u003e), bulk concentrations remain below detection thresholds across complementary analytical techniques. And\u0026nbsp;the surface Na signal detected by TOF-SIMS is most likely attributed to the high sensitivity of this characterization technique, which enables the detection of contaminant Na. Crucially, XPS depth profiling with prolonged Ar\u003csup\u003e+\u003c/sup\u003e sputtering (1300 s) confirms the absence of Na 1s (1071 eV) and Cl 2p (198.5 eV) emissions throughout the etching process (\u003cstrong\u003eFig. 2i\u003c/strong\u003e),\u0026nbsp;which indicates that Na has not entered the NCM lattice. The scanning transmission electron microscopy (STEM) image reveals a smooth surface morphology of RNCM, and energy-dispersive X-ray spectroscopy (EDS) detects no significant Na or Cl signals (\u003cstrong\u003eSupplementary Fig. 9)\u003c/strong\u003e. Besides, spatially resolved Na L-edge electron energy-loss spectroscopy (EELS) detect no characteristic Na signals in subsurface or bulk phases (\u003cstrong\u003eSupplementary Fig. 10\u003c/strong\u003e). It is evident that Li\u003csup\u003e+\u003c/sup\u003e intercalated into the NCM can fully replenish the deficient lithium content, without Na\u003csup\u003e+\u0026nbsp;\u003c/sup\u003eincorporation, arising from structural constraints imposed by lithium vacancies in the cathode framework. The smaller ionic radius of Li\u003csup\u003e+\u003c/sup\u003e (0.76 \u0026Aring;) compared to Na\u003csup\u003e+\u003c/sup\u003e (1.02 \u0026Aring;) enables preferential occupancy of octahedral sites. Meanwhile, abundant Na\u003csup\u003e+\u003c/sup\u003e adsorbs at high-energy defect sites, forming dynamic coating layer on the NCM surface, while trace Li\u003csup\u003e+\u003c/sup\u003e exchanges with adsorbed Na\u003csup\u003e+\u003c/sup\u003e and diffuses into the lattice to replenish lithium.\u003c/p\u003e\n\u003cp\u003eThe phase structural evolution of RNCM was further analyzed using high-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED). The SNCM particle displays disordered lattice arrangements (\u003cstrong\u003eFig. 3a\u003c/strong\u003e), indicative of irreversible phase degradation after cycling. SAED patterns acquired from three distinct regions of SNCM reveal coexisting bulk layered phases, spinel-type structures, and rock-salt phases at grain boundaries (\u003cstrong\u003eFig. 3b\u003c/strong\u003e). In contrast, the RNCM particle features a smooth surface (\u003cstrong\u003eSupplementary Fig. 11\u003c/strong\u003e) and exhibits uniformly clear and periodic lattice fringes (\u003cstrong\u003eFig. 3c\u003c/strong\u003e), unambiguously assigned to the\u0026nbsp;R-3m\u0026nbsp;layered structure with restored long-range order. The HRTEM images and SAED pattern of RNCM (\u003cstrong\u003eFig. 3d\u003c/strong\u003e) confirm a single-phase R-3m symmetry with no secondary phases, attesting to the full recovery of the layered lattice and the overall success of the structural repair.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFurthermore, surface elemental valence states during regeneration were comprehensively characterized for RNCM demonstrates chemical reversibility indicated\u0026nbsp;the absence via X-ray photoelectron spectroscopy (XPS). Comparative analysis of O 1s, Ni 2p, Co 2p, and Mn 2p spectra (\u003cstrong\u003eFig. 3e\u003c/strong\u003e) demonstrates\u0026nbsp;the absence of NiO signatures (530.1 eV) in RNCM-hydro and RNCM, confirming elimination of surface rock-salt impurities from SNCM, which aligns with TEM results (\u003cstrong\u003eFig. 3c\u003c/strong\u003e). And Ni 2p spectra show normalized Ni\u003csup\u003e2+\u003c/sup\u003e/Ni\u003csup\u003e3+\u003c/sup\u003e ratios (34.8% Ni\u003csup\u003e2+\u003c/sup\u003e in RNCM vs. 57.7% in SNCM), indicating a transformation from the surface rock-salt phase to the layered phase (\u003cstrong\u003eSupplementary\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;Fig.\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003e12\u003c/strong\u003e). The contents of different valence states, derived from XPS results (\u003cstrong\u003eFig. 3e\u003c/strong\u003e), also reveals lattice oxygen recovery from 36.0% (SNCM) to 55.2% (RNCM-hydro) and 63.2% (RNCM), reducing oxygen vacancies and improving structural coherence through suppressed transition metal migration.\u0026nbsp;The relatively high presence of Co\u003csup\u003e2+\u003c/sup\u003e on the surface of SNCM also implies a severe loss of Co\u003csup\u003e3+\u003c/sup\u003e in the bulk. In the repaired NCM, Co\u003csup\u003e3+\u003c/sup\u003e is the dominant species. Moreover, in the Mn 2p3/2 spectra, the Mn\u003csup\u003e4+\u003c/sup\u003e peak positions of RNCM-hydro and RNCM shift slightly towards the lower binding energy direction. This suggests that Mn\u003csup\u003e4+\u003c/sup\u003e, which underwent charge compensation due to lithium deficiency, has returned to its original state after hydrothermal treatment.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAdditionally, compared to SNCM, the \u003csup\u003e7\u003c/sup\u003eLi magic angle spinning solid state nuclear magnetic resonance spectrum (\u003csup\u003e7\u003c/sup\u003eLi MAS NMR) of RNCM (\u003cstrong\u003eSupplementary Fig. 13\u003c/strong\u003e) displays a narrower chemical shift distribution and sharper peaks. These features signify a more stable Li\u003csup\u003e+\u003c/sup\u003e environment with enhanced coordination uniformity, underscoring the successful recovery of structural order. Raman spectroscopy demonstrates that the RNCM-hydro and RNCM show diminished E\u003csub\u003eg\u003c/sub\u003e peak intensity and a red shift of the A\u003csub\u003e1g\u003c/sub\u003e mode, indicative of phase transformation from disordered rock-salt/spinel phases to a layered structure (\u003cstrong\u003eSupplementary Fig. 14\u003c/strong\u003e). This evolution confirms the hydrothermal process effectively repairs structural defects. As shown in \u003cstrong\u003eSupplementary\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;Fig. 15\u003c/strong\u003e, the fourier transform infrared spectrometer (FT-IR) spectrum of the hydrothermally treated NCM reveals a\u0026nbsp;significant reduction in peak intensity\u0026nbsp;at ~864 cm\u003csup\u003e-1\u003c/sup\u003e, with\u0026nbsp;complete disappearance\u0026nbsp;after post-annealing. This confirms the\u0026nbsp;effective removal of Li\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e and related lithium-based impurities from the NCM surface. Consistently, the thermogravimetric (TG) curves of SNCM and RNCM (\u003cstrong\u003eSupplementary Fig. 16\u003c/strong\u003e) demonstrate negligible mass loss up to 800 ℃ for the repaired NCM, validating the elimination of residual impurities by the hydrothermal and annealing treatments. Further supporting evidence is provided by the TG profile of SNCM mixed with binder and conductive carbon (SNCM-0), which exhibits two distinct mass losses at ~120 ℃ and 400 ℃, corresponding to the decomposition of residual binders and conductive additives. The absence of such losses in RNCM confirms the success of the 500 ℃ impurity-removal protocol. Overall, after hydrothermal treatment, the NCM cathode material can effectively recover its crystalline structure.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe increase in temperature and the introduction of H\u003csub\u003e2\u003c/sub\u003eO and CO\u003csub\u003e2\u003c/sub\u003e can accelerate the degradation of NCM with residual lithium and heterogeneous phases on the surface. Ambient pressure X-ray photoelectron spectroscopy (APXPS) with heating up to 600 ℃ (\u003cstrong\u003eFig. 3f\u003c/strong\u003e) observed that significant -CO\u003csub\u003e3\u003c/sub\u003e signals gradually emerge on the surface of SNCM with many defects, and there is severe lattice oxygen loss. This is mainly because when SNCM is attacked by CO\u003csub\u003e2\u003c/sub\u003e and H\u003csub\u003e2\u003c/sub\u003eO, the surface inactive Li\u003csup\u003e+\u003c/sup\u003e combines with CO\u003csub\u003e2\u003c/sub\u003e to form Li\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e, and meanwhile, surface Ni\u003csup\u003e3+\u003c/sup\u003e is more likely to transform to Ni\u003csup\u003e2+\u003c/sup\u003e. In contrast, only a strong Ni\u003csup\u003e3+\u003c/sup\u003e signal can be observed on RNCM without obvious lattice oxygen loss (\u003cstrong\u003eFig. 3g\u003c/strong\u003e), indicating that the interference of CO\u003csub\u003e2\u003c/sub\u003e and H\u003csub\u003e2\u003c/sub\u003eO does not cause a violent reaction on the surface of RNCM. Hence, the repaired NCM cathode has a cleaner surface and a more stable structure.\u0026nbsp;\u003c/p\u003e\n\u003ch2\u003e\u003cstrong\u003e2.2 Electrochemical Performance of NCM After Repair using Brine\u003c/strong\u003e\u003c/h2\u003e\n\u003cp\u003eNext, the electrochemical performances of direct regenerated NCM were systematically studied. Firstly, the charge transfer kinetics and cathode-electrolyte interphase (CEI) stability of Li||SNCM and Li||RNCM cells during the first charge-discharge cycle were investigated via in-situ electrochemical impedance spectroscopy (EIS). As illustrated in \u003cstrong\u003eFigs. 4a-b\u003c/strong\u003e, a complete CEI layer was not yet formed at the initial stage of charging, resulting in the impedance spectrum dominated by the semicircle corresponding to charge transfer resistance (R\u003csub\u003ect\u003c/sub\u003e). Notably, RNCM exhibited significantly lower interfacial resistance than SNCM, indicating a substantial reduction in Li\u003csup\u003e+\u003c/sup\u003e migration resistance after regeneration. Upon entering the high-voltage stage, R\u003csub\u003ect\u003c/sub\u003e decreased and a second semicircle emerged, which was associated with CEI formation (R\u003csub\u003eCEI\u003c/sub\u003e). SNCM still has a relatively high R\u003csub\u003ect\u003c/sub\u003e, which corresponds to a slow kinetic process. In contrast, benefiting from its smooth surface and stable CEI, RNCM maintained a low R\u003csub\u003ect\u003c/sub\u003e throughout the cycling process. Beyond 3.8 V, R\u003csub\u003ect\u003c/sub\u003e of RNCM stabilized, verifying its excellent interfacial stability under high-voltage operation.\u003c/p\u003e\n\u003cp\u003eAs present in\u003cstrong\u003e\u0026nbsp;Fig. 4c\u003c/strong\u003e, cyclic voltammetry (CV) curves of RNCM\u0026apos;s indicated improved reversibility versus SNCM, with sharper redox peaks, higher currents, and a narrowed \u0026Delta;E (200 mV, 17% lower than SNCM), evidencing reduced polarization due to its cleaner surface and faster charge transfer.\u0026nbsp;Furthermore, CV curves of SNCM, RNCM and CNCM (\u003cstrong\u003eSupplementary Fig. 17a-c\u003c/strong\u003e) were obtained at different scan rates. As the scan rate increased, the polarization of SNCM greatly increased, and the redox peaks became less distinct, with no response in the peak current. This directly indicates that the electrochemical activity of the waste NCM is very low. The slope for the plot of the peak current \u003cem\u003eI\u003csub\u003ep\u003c/sub\u003e\u003c/em\u003e and the scan rate \u003cem\u003ev\u003c/em\u003e\u003csup\u003e1/2\u003c/sup\u003e represents the diffusion coefficient of Li\u003csup\u003e+\u003c/sup\u003e (\u003cstrong\u003eFig. 4d\u003c/strong\u003e) by fitting with \u003cstrong\u003eEquation (1)\u003c/strong\u003e:\u003c/p\u003e\n\u003cp\u003e\u003cimg src=\"data:image/png;base64,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\" style=\"width: 490px;\"\u003e\u003c/p\u003e\n\u003cp\u003ewhere \u003cem\u003eI\u003csub\u003ep\u003c/sub\u003e\u003c/em\u003e is the peak current, \u003cem\u003en\u003c/em\u003e is the number of electrons transferred in the electrode reaction, \u003cem\u003eS\u003c/em\u003e is the effective area of the electrode, \u003cem\u003eD\u003c/em\u003e is the diffusion coefficient, \u003cem\u003eC\u003c/em\u003e is the bulk concentration of the reactant, and \u003cem\u003ev\u003c/em\u003e is the scan rate. It can be seen that RNCM has an ion diffusion capacity comparable to that of CNCM, confirming the recovery of electrochemical activity. For SNCM, increasing scan rates induced severe polarization and attenuated redox peaks (\u003cstrong\u003eSupplementary Fig. 17a\u003c/strong\u003e), with negligible peak current response, underscoring its poor electrochemical activity. In contrast, as shown in \u003cstrong\u003eFig. 4d\u003c/strong\u003e, RNCM exhibits a lithium-ion diffusion capability comparable to CNCM.\u003c/p\u003e\n\u003cp\u003eAdditionally,the voltage profiles of NCMs acquired from the galvanostatic intermittent titration technique (GITT) test were shown in \u003cstrong\u003eFig. 4e\u003c/strong\u003e and \u003cstrong\u003eSupplementary Fig. 18\u003c/strong\u003e. For SNCM, the voltage variation during the galvanostatic pulse (∆E\u003csub\u003et\u003c/sub\u003e) is extremely large (0.009 V), corresponding to significant polarization, which is attributed to its internal structure damage and the increased charge transfer resistance. The voltage change during the relaxation stage (∆E\u003csub\u003es\u003c/sub\u003e) after the galvanostatic pulse ends is also the largest (0.010 V) among the samples, indicating that ions face relatively greater difficulty diffusing within the cathode. Moreover, compared with SNCM (charging: 1.2\u0026times;10\u003csup\u003e-12\u003c/sup\u003e cm\u003csup\u003e2\u003c/sup\u003e\u0026middot;s\u003csup\u003e-1\u003c/sup\u003e, discharging: 5\u0026times;10\u003csup\u003e-13\u003c/sup\u003e cm\u0026sup2;\u0026middot;s\u003csup\u003e-1\u003c/sup\u003e), RNCM and CNCM exhibit similar and relatively higher average lithium ion diffusion coefficients (D\u003csub\u003eLi⁺\u003c/sub\u003e) (charging: 8.0\u0026times;10\u003csup\u003e-11\u003c/sup\u003e cm\u003csup\u003e2\u003c/sup\u003e\u0026middot;s\u003csup\u003e-1\u003c/sup\u003e, 6.5\u0026times;10\u003csup\u003e-11\u003c/sup\u003e cm\u003csup\u003e2\u003c/sup\u003e\u0026middot;s\u003csup\u003e-1\u003c/sup\u003e; discharging: 6.2\u0026times;10\u003csup\u003e-11\u003c/sup\u003e cm\u003csup\u003e2\u003c/sup\u003e\u0026middot;s\u003csup\u003e-1\u003c/sup\u003e, 7.8\u0026times;10\u003csup\u003e-11\u003c/sup\u003e cm\u003csup\u003e2\u003c/sup\u003e\u0026middot;s\u003csup\u003e-1\u003c/sup\u003e). Both the GITT and CV results indicate that RNCM has restored good ion-diffusion capability and excellent Li\u003csup\u003e+\u003c/sup\u003e transport kinetics.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe first-cycle charge-discharge profiles with the cutoff voltage of 2.7-4.3 V under the Crate of 0.2 C in \u003cstrong\u003eFig. 4f\u0026nbsp;\u003c/strong\u003ereveal that the SNCM exhibits severe polarization and the lowest specific capacity, whereas the RNCM demonstrates a reduced overpotential and a higher discharge capacity (153.8 mAh\u0026middot;g\u003csup\u003e-1\u003c/sup\u003e) comparable to commercial NCM (CNCM, 157.7 mAh\u0026middot;g\u003csup\u003e-1\u003c/sup\u003e). \u003cstrong\u003eFig.\u003c/strong\u003e\u003cstrong\u003e\u0026nbsp;4g\u003c/strong\u003e and \u003cstrong\u003eSupplementary Fig. 19\u003c/strong\u003e show that SNCM suffers rapid capacity fade during charge-discharge cycling, with its capacity approaching zero after 100 cycles. In contrast, the repaired NCM exhibits high capacity retention of 87.93% at 0.2 C and 93.89% at 0.5 C comparable to CNCM (99.42% and 91.67%). Moreover,RNCM also exhibits restored rate performance, and its performance is closely comparable to that of CNCM (\u003cstrong\u003eFig. 4h\u003c/strong\u003e). Notably, it delivers a specific capacity of 100.64 mAh\u0026middot;g\u003csup\u003e-1\u003c/sup\u003e even at a high rate of 1 C, which is comparable to that of CNCM (109.00 mAh\u0026middot;g\u003csup\u003e-1\u003c/sup\u003e). These results highlight the effectiveness of the hydrothermal repair method in a Na/Li mixed environment for restoring the electrochemical performance of spent NCM materials.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIn order to further to study the mechanism on enhanced electrochemical behavior for repaired NCM, we conducted in-situ XRD tests on the first charge-discharge at 0.2 C within the voltage range of 3.0-4.3 V. The XRD patterns with 2\u0026theta; ranging from 17.9-18.7\u0026deg; and 36.2-37.6\u0026deg; were selected to observe the evolution process of the (003) and (101) peaks (\u003cstrong\u003eFig. 4i\u0026nbsp;\u003c/strong\u003eand\u003cstrong\u003e\u0026nbsp;Supplementary Fig. 20\u003c/strong\u003e). As the voltage increases, the (003) peaks of both materials first shift to a lower 2\u0026theta; angle, indicating that an H1\u0026rarr;H2 phase transition occurs with the expansion along the c axis. While the (101) peaks shift towards a higher angle, which means that the lattice parameter a gradually decreases, and the phase transition process of RNCM is even smoother than CNCM. Subsequently, at a higher voltage, the (003) peaks suddenly shift to a higher 2\u0026theta; angle, reflecting the rapid contraction of the lattice along the c axis when lithium ions are further deintercalated, and an H2 to H3 phase transition occurs. This process is very brief, and after continuing to discharge, the (003) peak can return to its original position, indicating that RNCM and CNCM both have good reversibility of the H2/H3 phase transition.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eSimultaneously, in order to reduce the material cost during mass repairs, we attempted to reuse the original solution from the hydrothermal reaction three times consecutively to repair SNCM, yielding three repaired samples: RNCM-1\u003csup\u003est\u003c/sup\u003e, RNCM-2\u003csup\u003end\u003c/sup\u003e, and RNCM-3\u003csup\u003erd\u003c/sup\u003e (\u003cstrong\u003eSupplementary Fig. 21\u003c/strong\u003e). It was observed that all three RNCM samples exhibited smooth and intact secondary particles. Notably, RNCM-3\u003csup\u003erd\u003c/sup\u003e maintained a specific capacity of 146.5 mAh\u0026middot;g\u003csup\u003e\u0026minus;1\u003c/sup\u003e at 0.2 C and 144.9 mAh\u0026middot;g\u003csup\u003e\u0026minus;1\u003c/sup\u003e at 0.5 C. This demonstrates that a minimal lithium dose (3 mM Li\u003csup\u003e+\u003c/sup\u003e) can fully regenerate SNCM\u0026nbsp;about 3 times\u0026nbsp;via repetitive Li\u003csup\u003e+\u003c/sup\u003e insertion. It is noted that this method significantly reduces raw material costs (\u003cstrong\u003eFig. 4j\u003c/strong\u003e), which include the aqueous solution used for hydrothermal treatment, ethylene glycol, and water consumed for washing NCM after hydrothermal processes. Notably, concentrated seawater can be directly adopted as the lithium source for regeneration, thus only the marginal cost of seawater concentration needs to be considered. Compared with traditional hydrometallurgical and hydrothermal technologies, our method significantly reduces recycling costs by utilizing low-cost raw materials of seawater.\u003c/p\u003e\n\u003ch2\u003e\u003cstrong\u003e2.3 Mechanism of Regenerating NCM cathode using Brine\u003c/strong\u003e\u003c/h2\u003e\n\u003cp\u003eNext, we first demonstrate that in the two steps of repairing the NCM cathode, hydrothermal process plays a decisive role rather than the subsequent annealing. We investigated the phase and morphology evolution of NCM during the post annealing step. An additional 3 wt% of Li\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e was added to prevent lithium loss during the 800 ℃-annealing after the hydrothermal treatment. With progressive heating, the in-situ XRD patterns (\u003cstrong\u003eFig. 5a\u003c/strong\u003e) reveal the gradual disappearance of the Li\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e peaks, accompanied by a shift of the (003) and (104) peaks toward lower angles. And the peak positions return to their original positions when the temperature drops to room temperature. This indicates that during the annealing treatment, lattice thermal expansion is the main process occurring. Besides, the in-situ SEM images of the annealing process (\u003cstrong\u003eFig. 5b\u003c/strong\u003e) show that when the temperature rises to 800 ℃, small Li\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e particles on the surface of NCM particles completely disappears, and there is no obvious change in the particle morphology during the entire annealing process. This is closely consistent with the in-situ heating XRD results.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eTo verify the effect of the hydrothermal step, we heat SNCM via controlled amount of Li\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e and SNCM powder, yielding the solid-phase product (RNCM-S). The morphology of RNCM-S remained incompletely restored and there is no obvious recovery in electrochemical performance (\u003cstrong\u003eSupplementary Fig. 22\u003c/strong\u003e). Unlike the solid-phase method, where lithiation is constrained by the inherently limited solid-solid contact between lithium salts and cathode particles, the hydrothermal process achieves uniform nanoscale interfacial interaction through Li\u003csup\u003e+\u003c/sup\u003e diffusion in aqueous media, eliminating compositional gradients and thus conferring distinct regeneration advantages.\u003c/p\u003e\n\u003cp\u003eTo further demonstrate the efficacy of a seawater-mimetic solution in regenerating SNCM cathodes, we systematically evaluate the individual and combined effects of key salts (Li\u003csup\u003e+\u003c/sup\u003e, Na\u003csup\u003e+\u003c/sup\u003e, K\u003csup\u003e+\u003c/sup\u003e, Ca\u003csup\u003e2+\u003c/sup\u003e) during hydrothermal processes as follows. The LiCl-only system (RNCM-Li) produced particles with smoother surface than the degraded SNCM (\u003cstrong\u003eSupplementary Fig. 23\u003c/strong\u003e), yet failed to achieve the morphological uniformity like RNCM. And the diffuse and elongated diffraction spots observed in the TEM results (\u003cstrong\u003eSupplementary Fig. 24\u003c/strong\u003e) further confirm that the LiCl-repaired NCM still retains numerous structural defects. Although RNCM-Li delivered improved capacity (134.0 mAh\u0026middot;g\u003csup\u003e-1\u003c/sup\u003e at 0.2 C) and cycling stability over SNCM, its performance lagged significantly behind RNCM, suggesting that particle uniformity critically influences Li\u003csup\u003e+\u003c/sup\u003e transport kinetics. Notably, treatment with NaCl alone (RNCM-Na) paradoxically yielded well-defined secondary particles with smooth surfaces (\u003cstrong\u003eSupplementary\u003c/strong\u003e \u003cstrong\u003eFig. 25a-c\u003c/strong\u003e). However, neither the XRD patterns nor the electrochemical performance showed meaningful regeneration, as the lack of lithium supplementation left the lithium vacancies unaddressed. When used individually, either KCl or CaCl\u003csub\u003e2\u003c/sub\u003e partially restored the morphology, yet surface defects and lattice distortion still persisted. (\u003cstrong\u003eSupplementary Figs. 26-27\u003c/strong\u003e). The RNCM-LNCK using 30 mM LiCl, 3 M NaCl, 0.1 M KCl and 0.1 M CaCl\u003csub\u003e2\u003c/sub\u003e system exhibited moderate electrochemical performance (\u003cstrong\u003eSupplementary Fig. 28\u003c/strong\u003e). The capacity (143.5 mAh\u0026middot;g\u003csup\u003e-1\u0026nbsp;\u003c/sup\u003eat 0.2 C) failed to be fully recovered, indicating that the introduction of K\u003csup\u003e+\u003c/sup\u003e and Ca\u003csup\u003e2+\u003c/sup\u003e did not contribute to the structural repair of RNCM. Thereby, we propose this repair mechanism for hydrothermal process using brine: NaCl facilitates Li\u003csup\u003e+\u003c/sup\u003e insertion and morphological homogenization, LiCl enables lithium replenishment. The schematic diagram in \u003cstrong\u003eFig. 5c\u003c/strong\u003e illustrates the role of Na\u003csup\u003e+\u003c/sup\u003e in the hydrothermal repair process.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eTo elucidate the underlying mechanisms, we systematically investigated the adsorption and intercalation energies of Li\u003csup\u003e+\u003c/sup\u003e, Na\u003csup\u003e+\u003c/sup\u003e, K\u003csup\u003e+\u003c/sup\u003e and Ca\u003csup\u003e2+\u003c/sup\u003e on the NCM surface (\u003cstrong\u003eFigs. 5b\u003c/strong\u003e-\u003cstrong\u003ec\u003c/strong\u003e). Both Li\u003csup\u003e+\u003c/sup\u003e and Ca\u003csup\u003e2+\u003c/sup\u003e exhibit strong surface adsorption tendencies. However, Ca\u003csup\u003e2+\u003c/sup\u003e-rich regions generate localized electron-dense domains (\u003cstrong\u003eFig. 5d\u003c/strong\u003e), which electrostatically repel approaching Li\u003csup\u003e+\u003c/sup\u003e and hinder their lattice insertion. Furthermore, smaller hydration radius of Ca\u003csup\u003e2+\u003c/sup\u003e (3.12 \u0026Aring; vs. Li\u003csup\u003e+\u003c/sup\u003e: 3.40 \u0026Aring;) may enable partial Ca\u003csup\u003e2+\u003c/sup\u003e insertion into interlayers. The negligible contribution of K\u003csup\u003e+\u003c/sup\u003e aligns with its lowest adsorption/insertion energies and highest diffusion barriers, rendering it ineffective for regeneration. In contrast, NaCl exerts beneficial effects chiefly through Na\u003csup\u003e+\u003c/sup\u003e selective adsorption behavior. With moderate adsorption energy and significant content, Na\u003csup\u003e+\u003c/sup\u003e preferentially occupies high-energy surface defects (e.g., Li vacancies, grain boundaries), forming a uniform dynamic adsorption layer under hydrothermal conditions. The high Na\u003csup\u003e+\u003c/sup\u003e concentration ensures complete defect coverage, and prevents the defects from undergoing uncontrollable dissolution and Ostwald ripening in the hydrothermal environment, \u003csup\u003e26,27\u003c/sup\u003e and thus guarantees morphological uniformity. As the reaction proceeds, a small amount of Li\u003csup\u003e+\u003c/sup\u003e with extremely strong adsorption energy will gradually and selectively replace those Na\u003csup\u003e+\u003c/sup\u003e with weaker bonding, dominating the regeneration of NCM. This explains why Na\u003csup\u003e+\u003c/sup\u003e alone could drive morphological reconstruction. Besides, DFT calculations show that when different ions enter NCM and diffuse along the path (from site 1 to site 3) shown in \u003cstrong\u003eFig. 5e\u003c/strong\u003e, Li\u003csup\u003e+\u003c/sup\u003e exhibits the significantly lowest bulk diffusion barrier (0.826 eV, 0.802 eV and 0.426 eV) (\u003cstrong\u003eFig. 5f\u003c/strong\u003e), ensuring efficient filling of internal vacancies in Na\u003csup\u003e+\u003c/sup\u003e-enriched systems. This may also stem from the fact that the hydration radius of Na\u003csup\u003e+\u003c/sup\u003e (3.58 \u0026Aring;) is larger than that of Li\u003csup\u003e+\u003c/sup\u003e (3.40 \u0026Aring;), resulting in greater steric hindrance when entering NCM. Meanwhile, ethylene glycol enhances this process by increasing solution viscosity, prolonging Na⁺-surface interactions to allow sufficient time for Li\u003csup\u003e+\u003c/sup\u003e diffusion. This explains why Li⁺ can still enter and achieve repair without the entry of Na⁺ in the presence of a large amount of Na\u003csup\u003e+\u003c/sup\u003e. In brief, Na\u003csup\u003e+\u003c/sup\u003e dominates the recovery of morphology, while Li\u003csup\u003e+\u003c/sup\u003e replenishes the Li loss in spent NCM for the hydrothermal process using Li\u003csup\u003e+\u003c/sup\u003e-Na\u003csup\u003e+\u003c/sup\u003e co-presence brine, which achieves superior electrochemical performance for RNCM cathodes.\u0026nbsp;\u003c/p\u003e"},{"header":"3 Conclusion","content":"\u003cp\u003eWe proposed a sustainable and eco-friendly strategy for direct regeneration of spent NCM cathodes through hydrothermal treatment using simulated seawater containing high concentrations of NaCl and trace amount of LiCl, followed by a short annealing step. The regenerated cathode showed a restored layered structure within the secondary particles, featuring smooth surfaces and no detectable impurities. These structural improvements resulted in a high specific capacity of 146.6 mAh·g\u003csup\u003e-1\u003c/sup\u003e at 0.5 C and excellent capacity retention of 91.67% after 100 cycles. During the hydrothermal process, owing to overwhelming abundance of NaCl, it preferentially covers the high surface-energy defects and acts as a dynamic coating layer to ensure morphological homogenization. The subsequent annealing further stabilized the crystal lattice that may have been damaged by washing. This work integrated lithium recovery from brines with cathode regeneration in a single low-carbon process, paving a sustainable pathway for battery recycling.\u003c/p\u003e"},{"header":"4 Experimental Section","content":"\u003cp\u003e\u003cstrong\u003eDirect Regeneration Process\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe process is illustrated in \u003cstrong\u003eFig. S1\u003c/strong\u003e. First, the waste LiNi\u003csub\u003e0.5\u003c/sub\u003eCo\u003csub\u003e0.2\u003c/sub\u003eMn\u003csub\u003e0.3\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e (NCM523) electrode sheets are processed. They are cut into pieces and immersed in N-methyl-2-pyrrolidone (NMP), and then ultrasonic treatment is carried out to remove surface impurities and part of the weakly - bonded binders. Subsequently, the waste electrode sheets are placed in a 3 M sodium hydroxide aqueous solution and stirred to obtain electrodes separated from the aluminum foil. The electrode fragments are placed in an alumina crucible and heated in a muffle furnace at 500 ℃ for 3 h in air to remove residual binders, conductive carbon and other impurities. After thorough grinding, the spent NCM powder (SNCM) is obtained.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eNext, the hydrothermal repair stage begins. Place 0.1 g of SNCM powder into 40 ml of the prepared simulated seawater (3 M NaCl, 3 mM LiCl), and then add 2 ml of ethylene glycol. Subsequently, ultrasonically stir the mixture for 30 minutes to fully disperse the particles. After that, transfer the well-dispersed mixture into a reaction kettle and heat it at 200 ℃ for 3 h. After the reaction kettle has completely cooled down, take out the liquid in the inner liner and perform centrifugation. Wash the centrifuged product three times with deionized water and then dry it. Finally, put the dried powder into an alumina crucible and anneal it at 800 ℃ for 4 h in air atmosphere, thus obtaining the repaired NCM powder (RNCM). The hydrothermal solutions used in other experiments were aqueous solutions of 3 mM LiCl, 3 M NaCl, 3 M KCl, 3 M CaCl\u003csub\u003e2\u003c/sub\u003e, 3 mM LiCl and 3 M NaCl mixed with 0.1 M CaCl\u003csub\u003e2\u003c/sub\u003e/KCl. The obtained NCM materials were labeled as RNCM-Li, RNCM-Na, RNCM-Ca, RNCM-K, and RNCM-LNCK respectively.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eStructural Characterization\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eInductively coupled plasma-optical emission spectroscopy (ICP-OES) was conducted on an iCAP 7400 Radial. ICP is used to determine the content of each element in the powder. To improve the accuracy, the results are the average values of three sets of data. X-ray diffractometer (XRD) was performed on a Bruker D8 Advance equipped with Cu Kα radiation source. The scanning range of 2θ was 10°-80°. The XRD patterns of the samples were refined using the FullProf software. And the in situ XRD data were obtained by means of X-ray diffractometry in combination with an electrochemical workstation (Bio-Logic, VMP-300).\u0026nbsp;Thermogravimetric analysis (TGA, Perkin - Elmer) was used to assess the regeneration effect from 20 to 800 ℃ with a heating rate of 5 ℃·min\u003csup\u003e-1\u003c/sup\u003e. Fourier transform infrared (FT-IR) spectroscopy was used to carry out tests on a VERTEX 70 spectrophotometer, covering the range from 400 to 2000 cm⁻¹. The Raman spectrum was obtained by using an SR-500I-D2-1F1 Spectrograph. SEM (JEOL JSM-7800F) and TEM (JEM-2100 Plus, JEM-F200) with EDS attachments were employed to characterize the microstructure, morphology, and chemical compositions of the samples. XPS (ThermoFisher Scientific, ESCALAB 250Xi) was used to get the surface chemical composition data. APXPS was configured with a PHOIBOS hemispherical electron energy analyzer, a focused monochromatic X-ray source (Al Kα) with a spot size of approximately 300 μm, and an IQE-11A ion gun, all supplied by SPECS Corporation. The APXPS spectra were acquired under atmospheres of\u0026nbsp;0.5 mBar H₂O and 0.1 mBar CO₂. \u003csup\u003e7\u003c/sup\u003eLi MAS NMR was carried out using ADVANCE III HD 400MHz Solid NMR Spectrometer. The time-of-flight secondary ion mass spectroscopy (TOF-SIMS) measurements were conducted using an IONTOF M6 equipped with a Bi\u003csup\u003e3+\u003c/sup\u003e gun.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eElectrochemical Measurement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFirst, the working electrode was prepared. The active material powder, conductive carbon Super-P, and binder PVDF were thoroughly mixed in a mass ratio of 8:1:1. Then, N-methyl-2-pyrrolidone (NMP) was added. After thorough stirring, the slurry was evenly coated on an aluminum foil. The coated foil was placed in a vacuum drying oven at 90 ℃ for 12 h. Finally, a positive electrode sheet with an active material loading of approximately 4-5 mg·cm⁻² was obtained. The CR2032 coin cells were fabricated in an argon-filled glove box (with the content of O₂ and H₂O less than 1 ppm). In this case, lithium metal (Adamas-beta) was used as both the reference electrode and the counter electrode, the polymer membrane (Celgard 2325) was used as the separator, and 1 mol/L LiPF₆ dissolved in ethylene carbonate/ethyl methyl carbonate (EC/EMC, v/v = 3/7) with the addition of 1% vinyl carbonate (VC) was used as the liquid electrolyte. The electrochemical behavior of the Li||NCM cells was evaluated by means of the Electrochemical Impedance Spectroscopy (EIS) technique and Cyclic Voltammetry (CV) using an electrochemical workstation (Bio-Logic, VMP-300). For the EIS test, the perturbation bias voltage was 10 mV, and the working frequency range was from 7 MHz to 0.1 Hz. The CV was carried out within a potential range of 2.75-4.30 V at a scanning rate of 0.1 mV·s\u003csup\u003e-1\u003c/sup\u003e-0.4 mV·s\u003csup\u003e-1\u003c/sup\u003e. The relationship between the peak current\u003cem\u003e\u0026nbsp;I\u003csub\u003ep\u003c/sub\u003e\u003c/em\u003e and the square root of the scan rate \u003cem\u003ev\u003csup\u003e1/2\u003c/sup\u003e\u003c/em\u003e is described by \u003cstrong\u003eEquation (1)\u003c/strong\u003e. Galvanostatic charge-discharge cycle tests and rate performance tests (2.7-4.3 V) were conducted on a battery testing system (LAND, CT 2001 A) at room temperature. Galvanostatic Intermittent Titration Technique (GITT) was measured at 0.1 C for 5 min along with the relaxation time of 30 min.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDFT calculations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe DFT calculations were performed in the Vienna \u003cem\u003eab\u003c/em\u003e initio simulation package (VASP). The Perdew-Burke-Ernzerhof (PBE) form of generalized gradient approximation (GGA) was used as the exchange-correlation potential. The structural and electronic properties were calculated by the PBE+\u003cem\u003eU\u003c/em\u003e approach with spin polarization, with \u003cem\u003eU\u003c/em\u003e values of 3.5 eV, 3.3 eV, and 6.4 eV for Mn, Co, and Ni, respectively. The atomic configurations and the \u003cem\u003eU\u003c/em\u003e values of NCM523 were adopted from Pan’s work.\u003csup\u003e28\u003c/sup\u003e The cut-off energy was 450 eV for the plane-wave basis. The \u003cem\u003ek\u003c/em\u003e-point mesh of 2 × 4 ×1 was used to sample the Brillouin zone of bulk NCM523. For the surfaces absorbed with Li, each slab had a vacuum thickness of 15 Å and the k-point was adapted to achieve a similar sampling density in reciprocal space. The convergence criteria were 10\u003csup\u003e-4\u003c/sup\u003e eV for the total energies during structure relaxation. \u003cstrong\u003eTable S3\u003c/strong\u003e lists the details in calculation the atomic and electronic structures of NCM523 systems. The insertion energy (\u003cem\u003eE\u003c/em\u003e\u003csub\u003einsert\u003c/sub\u003e) of \u003cem\u003eM\u003c/em\u003e (\u003cem\u003eM\u003c/em\u003e = Li, Na, K and Ca ions) in NCM was defined as: \u003cem\u003eE\u003c/em\u003e\u003csub\u003einsert\u003c/sub\u003e=\u003cem\u003eE\u003c/em\u003e\u003csub\u003et\u003c/sub\u003e-\u003cem\u003eE\u003csub\u003eM\u003c/sub\u003e\u003c/em\u003e-\u003cem\u003eE\u003c/em\u003e\u003csub\u003eNCM\u003c/sub\u003e, where \u003cem\u003eE\u003c/em\u003e\u003csub\u003eNCM\u003c/sub\u003e, \u003cem\u003eE\u003csub\u003eM\u003c/sub\u003e\u003c/em\u003e and \u003cem\u003eE\u003c/em\u003e\u003csub\u003et\u003c/sub\u003e are the energies of the NCM bulk, the \u003cem\u003eM\u003c/em\u003e ions, and the combined system of them, respectively. The absorption energy (\u003cem\u003eE\u003c/em\u003e\u003csub\u003ea\u003c/sub\u003e) of \u003cem\u003eM\u003c/em\u003e (\u003cem\u003eM\u003c/em\u003e = Li, Na, K and Ca ions) on NCM surface was defined as: \u003cem\u003eE\u003c/em\u003e\u003csub\u003eabsorb\u003c/sub\u003e=\u003cem\u003eE\u003c/em\u003e\u003csub\u003eA\u003c/sub\u003e-\u003cem\u003eE\u003c/em\u003e\u003csub\u003eM\u003c/sub\u003e-\u003cem\u003eE\u003c/em\u003e\u003csub\u003eslab\u003c/sub\u003e, where \u003cem\u003eE\u003c/em\u003e\u003csub\u003eslab\u003c/sub\u003e, \u003cem\u003eE\u003csub\u003eM\u003c/sub\u003e\u003c/em\u003e and \u003cem\u003eE\u003c/em\u003e\u003csub\u003eA\u003c/sub\u003e are the energies of the NCM slab, the \u003cem\u003eM\u003c/em\u003e ions, and the absorption system of them, respectively. The electron density difference (Δ\u003cem\u003eρ\u003c/em\u003e) on the NCM surface is defined as Δ\u003cem\u003eρ\u003c/em\u003e =\u003cem\u003eρ\u003csub\u003eA\u003c/sub\u003e-ρ\u003csub\u003eM\u003c/sub\u003e-ρ\u003csub\u003eslab\u003c/sub\u003e\u003c/em\u003e, where \u003cem\u003eρ\u003csub\u003eslab\u003c/sub\u003e\u003c/em\u003e, \u003cem\u003eρ\u003csub\u003eM\u003c/sub\u003e\u003c/em\u003e and \u003cem\u003eρ\u003csub\u003eA\u003c/sub\u003e\u003c/em\u003e is the charge density of NCM slab, the \u003cem\u003eM\u003c/em\u003e ions, and the absorption system of them, respectively. The energy barriers of \u003cem\u003eM\u003c/em\u003e (\u003cem\u003eM\u003c/em\u003e = Li, Na, K and Ca ions) ion migration in NCM were calculated by the nudged elastic band method.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors gratefully acknowledge financial support from the National Natural Science Foundation of China (92472119 to W.L., 52222311 to Y.Y.), the Natural Science Foundation of Shanghai (24ZR1451200 to W.L.) and Development Fund for Schools of ShanghaiTech University. This project is supported by State Key Laboratory of New Ceramic Materials Tsinghua University (No. KF202504 to W.L.). The electron microscopy experiments were supported by the Center for High-Resolution Electron Microscopy (CℏEM) at ShanghaiTech University.\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eBiswal, B. 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Besides, the escalating deployment of lithium-ion batteries necessitates sustainable recycling strategies to address resource scarcity and environmental concerns. Herein, we report a green strategy on direct regenerating degraded layered oxide cathodes via a hydrothermal process using simulated seawater containing trace LiCl and high concentration of NaCl, followed by a mild annealing step. The regenerated cathode exhibits a fully restored layered structure, smooth surfaces, and no residual impurities. It delivers a high specific capacity of 146.6 mAh·g-1 at 0.5 C and excellent capacity retention of 91.67% after 100 cycles. We reveal that Li+ and Na+ play a synergistic effect on the regeneration mechanism. Na+ selectively adsorbs at surface defect sites without entering the bulk lattice as a dynamic coating layer to guide morphological repair, while Li⁺ achieves lithium replenishment via ion exchange with Na+ based on its high adsorption energy and excellent diffusion capability. This cost-effective approach bridges sustainable battery recycling with brine resource utilization, establishing a closed-loop \"lithium extraction-cathode regeneration\" paradigm.","manuscriptTitle":"Direct Regeneration of Layered Oxide Cathodes via Seawater","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-04-15 05:23:05","doi":"10.21203/rs.3.rs-9324161/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"f5a31980-5330-4d9e-a317-28364a420f9c","owner":[],"postedDate":"April 15th, 2026","published":true,"recentEditorialEvents":[{"type":"decision","content":"Reject after peer review","date":"2026-05-05T18:00:03+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"This content is not available.","date":"2026-05-04T18:31:45+00:00","index":1,"fulltext":"This content is not available."},{"type":"editorInvitedReview","content":"This content is not available.","date":"2026-05-03T13:40:44+00:00","index":2,"fulltext":"This content is not available."},{"type":"editorInvitedReview","content":"This content is not available.","date":"2026-04-29T09:54:45+00:00","index":3,"fulltext":"This content is not available."}],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":66248327,"name":"Physical sciences/Materials science/Materials for energy and catalysis/Electrochemistry"},{"id":66248328,"name":"Earth and environmental sciences/Ocean sciences/Marine chemistry"}],"tags":[],"updatedAt":"2026-05-05T18:00:50+00:00","versionOfRecord":[],"versionCreatedAt":"2026-04-15 05:23:05","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9324161","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9324161","identity":"rs-9324161","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}