Process for stabilizing cathode interface and minimizing water usage in direct recycling of spent lithium-ion batteries

Abstract

Spent cathode powder from a lithium-ion battery is subjected to a hydrothermal relithiation process to recover lithium within the bulk structure. After low volume water washing of the relithiated cathode powder, the powder is separated from Li-bearing salt / solution and mixed with a scavenging agent to neutralize excessive alkaline residue on the surface of the cathode material. A mild annealing step induces in-situ formation of a Li+-conductive LiBO2 (LBO) glass layer coating to enhance the interfacial stability and electrochemical performance of the recycled cathode materials

Description

[0001] PROCESS FOR STABILIZING CATHODE INTERFACE AND MINIMIZING WATER USAGE IN DIRECT RECYCLING OF SPENT LITHIUM-ION BATTERIES

[0002] RELATED APPLICATIONS

[0003] This application claims the benefit of the priority of U.S. Provisional Application No. 63 / 682,671, filed August 13, 2024, which is incorporated herein by reference in its entirety.

[0004] GOVERNMENT RIGHTS

[0005] This invention was made with government support under a ReCell Center grant awarded by the Department of Energy, and Grant No. CBET-1805570 awarded by the National Science Foundation. The government has certain rights in the invention.

[0006] BACKGROUND

[0007] Over the last several decades, the burgeoning demand for lithium-ion batteries (LIBs) has been driven by expanding applications from portable electronics to electric vehicles (EVs) and large-scale energy storage systems. Direct LIB recycling is a promising strategy for addressing disposal issues of massive spent LIBs with better economic and environmental benefits compared with conventional pyrometallurgical and hydrometallurgical methods. This approach is preferred for its ability to retain the embedded energy within the structure of cathode active material (CAM) coupled with its lower energy consumption and potential for reduced greenhouse gas (GHG) emission. Some efforts have also been made to develop a closed-loop direct recycling system for spent LIB cathodes that operates on an improved scale under moderate temperature and pressure conditions. Direct recycling methods to restore Li+into the Li+-deficient cathode structures include solid-state, hydrothermal, ionothermal, and redox-mediated relithiation processes. While a variety of processes have been demonstrated to recover cathode materials to achieve the same electrochemical performance as their pristine counterparts, there remain two major obstacles to large-scale recycling implementation.

[0008] The first challenge lies in the removal of residual impurities on the surface of the recycled cathode products. Direct recycling of cathode materials in Li-rich salt / solution environments often leads to heterophases on the surface of the material, including lithium hydroxide (LiOH) and carbonates (Li2CO3). Failure to completely remove these surface residues can lead not only to the absorption of moisture from the air but also induce electrolyte decomposition and cathode-electrolyte-interphase (CEI) growth from the decomposition of these residues, potentially leading to rapid performance degradation of recycled cathodes for the practical use of LIBs. While this may be avoided through improved storage conditions to limit exposure to ambient atmosphere, the need for such storage facilities adds to the overall cost of the process. Additionally, the quantity of lithium residues escalates with the increase of nickel (Ni) content in the cathode materials and the batch production size.

[0009] With the increasing demand for high energy density LIBs, regulating the impurity residues is a critical step in recycling as well as production of Ni-rich layered cathode materials for industrial applications. A water washing process to remove impurity residues has been widely adopted by battery manufacturers as an essential step in processing. However, multiple washing cycles produces significant amounts of wastewater. In addition, during the water washing process, the exchange of protons and Li+leads to the formation of additional surface defects, adversely affecting the electrochemical performance and structural stability of the cathode material. To mitigate these drawbacks, some studies have explored using alternative solvents such as methanol, ethanol, sodium chloride aqueous solution, sodium hydroxide aqueous solution, etc., during the washing process. However, the increased process costs related to solvent substitution render this approach less commercially viable. These complexities further increase the challenge of effectively controlling the removal of surface impurities from the surface of Ni-rich layered cathode materials.

[0010] The second challenge relates to control of the product interface stability - a critical function in both newly synthesized and recycled materials. The surface of Ni-rich layered cathodes inherently exhibits interfacial instability, which can trigger chemical reactions that form residual lithium compounds on the cathode surface, even with limited exposure to ambient air and moisture. This phenomenon is due to the active oxygen species formed at the particle surface when high-Ni oxides are exposed to air, as the trivalent Ni3+is reduced to Ni2+. The lithium residue species readily react with trace amounts of CO2 and H2O in the air, forming a surface layer of residual lithium (primarily consisting of Li2COs, LiHCCh, and LiOH), which can cause gas evolution from electrolyte reaction. To mitigate residual lithium build-up associated with Ni-rich cathodes, various coating materials have been explored. Materials employed for such coating(s) include aluminum oxide (AI2O3), cobalt oxide (CO3O4), magnesium oxide (MgO), zirconium dioxide (ZrCh), and cerium dioxide (CeCh). For each of these coating materials, the singular stable valence state of the metal ions exhibits considerable chemical robustness within the electrochemical window but may have a negative impact on Li+transport. Alternatively, Li+-containing coating materials, including lithium zirconate (Li2ZrOs), lithium phosphate (LisPCh), and lithium borate (LiBCh), have garnered considerable interest in improving Li+transport kinetics. These materials are distinguished by their good ionic conductivity and the ability to preserve electrochemical stability. However, current research exhibits a notable gap between primary production and direct recycling of cathode materials for closed-loop manufacturing. Also, since airtight material handling systems are not commonly available in recycling facilities, the regenerated materials must be able to preserve both their structural integrity and electrochemical property during storage for the successful implementation on a large scale.

[0011] SUMMARY

[0012] According to the inventive approach to recycling of LIB s, a refined direct recycling process improves cathode interface stability by leveraging in-situ reaction between surface residual lithium species and a scavenging agent, e.g., a weak inorganic acid, to form a conformal Li+conductive coating that stabilizes the regenerated Ni-rich cathodes with significantly reduced water footprint. The conductive coating also prevents direct contact between contaminants and the cathode surface, thus improving the ambient storage stability. Since an extensive washing step is no longer necessary to remove the cathode surface residuals, this intensified recycling process significantly reduces water consumption and generation of contaminated wastewater. The inventive approach provides advancements toward transitioning direct recycling from laboratory to industrial-scale applications with improved product quality and environmental sustainability.

[0013] According to the inventive process, referred to as “scavenging agent - hydrothermal regeneration” (“SAHR”), a multifunctional scavenging agent (SA) is coupled into the direct regeneration process to simultaneously remove the surface alkaline residual and form an in- situ protective layer on the surface of regenerated Ni-rich LIB cathodes (i.e., LiNio.6Coo.2Mno.2O2 or NCM622). This modification of the direct recycling process addresses interfacial stability issues. Specifically, a hydrothermal relithiation process is first applied on spent cathode to recover the lithium inventory of the bulk structure. Without repeating the washing step with an excess consumption of water, the relithiated Ni-rich cathode powder separated from Li-bearing salt / solution is mixed and reacted with the SA, which may be a weak inorganic acid (H3BO3), to neutralize the excessive alkaline residue on the surface of the cathode material. A following mild sintering step induces the in-situ formation of a Li+-conductive LiBCh (LBO) glass layer coating to enhance the interfacial stability and electrochemical performance of the recycled materials. This modified process provides a promising pathway to transit and scale up direct recycling technology with high product quality and low water consumption.

[0014] DESCRIPTION OF THE DRAWINGS

[0015] FIG. 1A is a diagram illustrating a limited washing complemented by the application of a scavenging agent (SA); FIG. IB shows the inventive washing process and the SAHR process with an SA application. This illustration serves to elucidate the differential outcomes of each washing approach, highlighting their respective efficacies and limitations in preserving the quality and interfacial stability of the NCMs over time.

[0016] FIGs. 2A-2H illustrate results of investigation of structure and surface degradation upon shelf-life and aging test, where FIG. 2A shows XRD patterns of cathodes from different washing processes to remove alkaline residues, comparing excessive wash, limited wash, and the application of a SA during the aging process for regenerated NCMs; FIG. 2B is a plot of unit cell parameters of a and c for various NCM622 samples; FIGs. 2C and 2D show XRD refinement for regenerated NCM622 without and with the SA applied after aging process, respectively; FIG. 2E provides pH values of regenerated cathode samples immersed in water, both with and without the SA, as well as control samples (T-NCM622), before and after the aging process; FIGs. 2F and 2G show XPS of regenerated cathode samples without and with the SA applied as well as control samples (T-NCM622) before and after aging process, respectively; FIG. 2H provides a comparison of carbonate impurity content based on C Is spectra.

[0017] FIGs. 3A-3M illustrate microstructure characterization of different NCM622 particles, where FIGs. 3A-3D show morphology examination via SEM of HS-NCM622, aged HS-NCM622, HS-NCM622-SA, and aged HS-NCM622-SA, respectively; FIGs. 3E- 3H provide TEM and FFT images of HS-NCM622, aged HS-NCM622, HS-NCM622-SA, and aged HS-NCM622-SA, respectively; FIG. 31 shows STEM-EELS elemental mapping of HS-NCM622-SAat B K-edges and Ni K-edges; FIGs. 3J and 3K plot B Is spectra from XPS of regenerated cathode samples without and with SA applied as well as control samples before aging and after aging, respectively; FIGs. 3L and 3M respectively provide B K-edge, O K-edge, Mn L-edge, Co L-edge and Ni L-edge spectra of the aged HS-NCM622-SA and aged HS-NCM622 particle from the surface (line 1) to the inner side (line 6) with a step interval of 3 nm acquired alone the line scan in HAADF-STEM images.

[0018] FIGs. 4A-4F are plots of electrochemical performance results for regenerated NCM622 upon shelf-life test, where FIGs. 4A and 4B show voltage profiles and cycling stability, respectively, of regenerated NCM622 samples without and with SA and their comparison with the pristine T-NCM622; FIG. 4C shows full cell test of regenerated NCM622 samples with SA as well as the pristine NCM622 cycled at C / 2 C for 200 cycles; FIGs. 4D and 4E provide voltage profiles and cycling stability, respectively, of regenerated NCM622 samples without and with SA their comparison with the pristine T-NCM622 after aging process; and FIG. 4F shows impedance investigation upon aging for regenerated NCM622 samples without and with SA before and after aging process.

[0019] FIGs. 5A-5F provides results for the SAHR process for Cycled NCM622-CB, where FIGs. 5A and 5B, respectively, are SEM images of cycled NCM622-CB before and after hydrothermal, wash and after annealing (HS-NCM622-BM-SA); FIGs. 5C and 5D plot XPS data of fluorine for cycled NCM622-CB and HS-NCM622-BM-SA, respectively, to indicate removal of the binder from the recovered cathode through HRSA process; FIGs. 5E and 5F are voltage profiles and cycling stability, respectively, for HS-NCM622-BM-SA compared with pristine T-NCM622.

[0020] FIG. 6 diagrammatically illustrates an exemplary process flow for the inventive SAHR process.

[0021] FIG. 7 is a graphical comparison of wastewater production for direct recycling without and with the inventive approach.

[0022] DETAILED DESCRIPTION OF EMBODIMENTS

[0023] According to the inventive approach, interfacial stability in a LIB is enhanced via the “scavenging agent - hydrothermal regeneration,” or SAHR, strategy. Hydrothermal relithiation followed by a short annealing process provides excellent potential for scaling up among direct recycling methods. It has proven effective in reviving various spent cathode materials, including LiCoO? (LCO), LiNio.33Coo.33Mno.33O2 (NCM111), LiNio.5Coo.2Mno.3O2 (NCM523), LiNio.6Coo.2Mno.2O2 (NCM622), and LiFePO4(LFP). In addition, previously-reported work on “purification and regeneration integrated materials engineering,” the “PRIME' process, disclosed in International Patent Publication WO 2024 / 123811, which is incorporated herein by reference. The PRIME process uses a combined approach of purification and relithiation for the cathode black mass, and has been demonstrated in a greater than 100g batch of spent NCM111. This approach paves the way toward cost-effective and scalable production, however, the impurity residues and interfacial stability risk remains high after hydrothermal treatment of NCM622 cathodes, as the spent cathode must be immersed in a Li-rich aqueous solution for relithiation. Careful removal of surface contaminants is vital for full recovery of electrochemical performance, which is often achieved by multiple water washing steps that generate excess amounts of wastewater.

[0024] In a typical hydrothermal direct recycling process, the post-relithiation washing process and surface remedy treatment in Li-rich salt / solution mediums start with two steps. The first is a washing step, which aims to separate the NCMs from the Li-rich solution and remove other impurities such as conductive carbon and degraded polyvinylidene fluoride (PVDF) binder.

[0025] To provide a reference for assessing the efficacy of the inventive approach compared to existing water-washing practices for removal of alkaline residues, chemically delithiated NCM111 (D-NCM111) and spent NCM622 (C-NCM622) were used as model systems. Following hydrothermal treatment, these NCM samples were subjected to washing with deionized water at varied ratios, followed by an annealing process, for further evaluation. The resulting regenerated NCM is denoted as HS-X-W, with “X” representing the type of spent cathode material and “W” indicating the water ratio used.

[0026] In the context of low-nickel NCM materials (NCM111), the electrochemical performance of regenerated D-NCM111 under different washing conditions was evaluated through half-cell tests, compared alongside the pristine Toda NCM111 (T-NCM111). Samples HS-NCMl l l-10ml / gx2, HS-NCMl l l-10ml / gx6, and HS-NCMl l l-10ml / gx8 manifested initial capacities of 147.7mAh / g, 147.5mAh / g, and 147.4mAh / g at a C / 3 rate, with capacity retentions of 82.7%, 82.3%, and 78.0% after 100 cycles, respectively. However, the HS-NCMl l l-10ml / gx4 sample exhibited an initial capacity of 149.7 mAh / g at C / 3, with a capacity retention of 93.3%, paralleling the performance of T-NCM111 (93.2%). This demonstrates that a water ratio of approximately 40ml to wash 1g of HT- NCM111 can effectively regenerate D-NCM111 through the hydrothermal treatment methodology, emphasizing the detrimental impact of surface residues on electrochemical performance and the benefits of water washing for mitigating such effects.

[0027] The pH of water solution after HT-NCM622 immersion decreased from 11.9 to 11.4 with water ratios increased from 10 ml / gcathode to 60 ml / gcathode, which is comparable to that of T-NCM622 (11.7). Increasing the water ratio beyond 40ml / g led to the disintegration of NCM622 particles — NCM622 has a known chemical sensitivity to water. A washing ratio of 10 ml / gcathode of HT-NCM622 achieved an initial capacity of 170.0 mAh / g at C / 3, with a capacity retention of 93.8%, matching the performance of T-NCM622 (94.2%).

[0028] This test confirms that water washing is a viable technique for removing the bulk of alkaline residues, where effective washing correlates with enhanced electrochemical performance in regenerated NCMs. Nonetheless, given the water sensitivity of high-nickel NCM materials, even at a 40 ml / g ratio, an extensive washing approach does not address the specific challenges associated with these materials, indicating the need for tailored strategies in their regeneration.

[0029] Incorporation of extensive washing steps (e.g., 10 ml of water per gcathode, repeated four or more times) causes the overall process to be lengthy and tedious, while producing substantial amounts of wastewater (> 40mL / gcathode). Both excessive and inadequate washing intensify the interfacial air instability, ultimately leading to the accumulation of residual lithium compounds on the cathode surface. This accumulation typically has adverse implications for subsequent battery manufacturing processes. The subsequent short annealing step serves to repair the defective crystal structure and eliminate trace amounts of residual hydroxide and carbon species.

[0030] A process that adds scavenging agent to hydrothermally cycled material is illustrated in FIG. 1A. This sequence employs lean water washing with a low water-to-solid (w / s) ratio of about 1.6mL / gcathode to 2.5 ml / gcathode after cycling with an added step of applying a scavenging agent (SA), (boric acid (H3BO3) is shown), to remove alkaline residues and cleanse the unstable / unclean surface resulting from the water washing during a mild thermal treatment (300°C). The mechanism by which the SA removes residual lithium compounds from the cathode material is further illustrated in FIG. IB, depicting the in-situ reaction with the scavenging agent. As indicated by the following equations, such an in-situ reaction can effectively repair the unstable or unclean surface associated with alkaline residue compounds: LiOH + H3BO3-> LiBO2+ 2H2O (1)

[0031] Li2CO3+ 2H3BO3^ 2LiBO2+ CO2+ 3H2O (2)

[0032] LBO is a stable electrical insulator characterized by a wide bandgap of 6.4 eV, which contributes to its chemical and structural stability. Additionally, LBO acts as a fast Li+conductor with a low migration energy barrier (15 meV), leading to efficient Li+transport to the NCM particles. Accordingly, using the SAHR process, a significant improvement of product quality and environmental sustainability can be achieved for direct cathode recycling.

[0033] Pivotal considerations for the practical industrial application of the hydrothermal relithiation process encompass include: 1) the implementation of a meticulous washing procedure after relithiation for NCMs to eliminate surface residues during their separation from Li-rich salts / solutions; and 2) a comprehensive assessment of the shelf-life for the regenerated materials. As discussed above, surface residues deteriorate the electrochemical performance of layered cathodes, and water washing can remove surface residues to provide improved electrochemical performance. The sample must be washed with a ratio of total 40 ml / gcathode of hydrothermal-relithiated NCM622 (HT-NCM622) to achieve an initial capacity of 170.0 mAh / g at C / 3 with a capacity retention of 93.8%, similar to pristine NCM622 (T-NCM622) (94.2%). To promote scalability, the amount of water (40 mL / gcathode) used in the washing step must be significantly reduced.

[0034] To evaluate the effectiveness of the SAHR method, we started with the direct regeneration of degraded NCM622 cathode materials using a typical hydrothermal relithiation with repeated washing followed by a post-annealing process. The resulting sample was then compared with recycled NCM622 processed using the SAHR process. Both samples, as well as the pristine NCM622, were stored side by side in open air for 7 days, referred to as “aging”, with average temperature and relative humidity of 23.7 °C and 55.4%, respectively. Temperature and relative humidity were monitored every 12 hours.

[0035] Scanning Electron Microscope (SEM) images were taken for morphological comparisons. After application of the SA, no noticeable morphological changes were observed. Additionally, the secondary spherical shape was well-preserved. To verify the enhanced structural stability of recycled NCM622 via the SAHR process, X-ray diffraction (XRD) analysis was conducted, as shown in FIG 2A. After ambient aging for 7 days, all samples retained the a-NaFeCh bulk structure. However, for samples without SA layer, a left shift of the (003) peak was observed, which is attributed to Li loss during reaction with moisture and CO2 in air. This peak shift in aged HS-NCM622-SA is associated with the expansion of the c-axis caused by the Li+extraction, leading to an increase in cation mixing in NCM622 materials from 6.2% to 14.7%. Table 1 provides XRD refinement results of regenerated NCM622 particles before and after aging process.

[0036] Table 1

[0037] Sample a / A c / A Li / Ni mixing / %

[0038] T-NCM 622 2.87205 14.25025 4.552 1.09 0.73

[0039] HS-NCM 622 2.87227 14.24806 6.164 1.61 0.80

[0040] HS-NCM 622-SA 2.87235 14.24883 3.824 1.27 0.97

[0041] Aged T-NCM 622 2.87097 14.25516 11.441 1.42 0.89

[0042] Aged HS-NCM 622 2.86996 14.26384 14.724 1.32 0.87

[0043] Aged HS-NCM 622-SA 2.87227 14.24836 4.788 1.38 0.96

[0044] The a and c lattice parameters for the unit cell were determined via Rietveld refinement (see FIGs. 2B-2D) Without an in-situ protection layer on the cathode surface, the a lattice parameter (FIG. 2B, left vertical axis) decreased from 2.872A (T-NCM622) and 2.872 A (HS-NCM622) to 2.871A (Aged T-NCM622) and 2.870 A (Aged HS-NCM622), respectively. This decrease is attributed to the reduction in average metal-metal distance due to the smaller effective ionic radii of Ni3+compared with Ni2+upon Li loss from the bulk structure. The c lattice parameter (FIG. 2B, right vertical axis) increased from 14.250 A (T-NCM622) and 14.248 A (HS-NCM622) to 14.255 A (Aged T-NCM622) and 14.264 A (Aged HS-NCM622). In contrast, the sample with a SA exhibited a stable structure, with no (003) peak shift and almost no changes in the a and c lattice parameters. FIGs. 2C and 2D compare XRD refinement for regenerated NCM622 without and with the SA applied after aging process, respectively. Thus, the regenerated NCM622 exhibits atmospheric instability, primarily due to surface alkaline residues and air exposure, which triggers a delithiation reaction from proton-exchange. In contrast, the use of a SA effectively mitigates this delithiation phenomenon, thereby enhancing the air stability of the regenerated cathode.

[0045] Heterophase residue on the recycled cathodes increases interfacial instability. To evaluate the impact of surface alkaline residues with aging, and to assess the efficacy of the SA in the SAHR process, we examined the presence of surface impurities (Li2COs) on pristine NCM622 (Aged T-NCM622) and regenerated NCM622 (Aged HS-NCM622) samples after aging. These samples were analyzed for alkaline residue amounts via pH testing. As shown in FIG. 2E, HS-NCM622 exhibited a slightly higher initial pH of 12.1 compared to T-NCM622 (11.7), which, after aging, changed to 11.7 and 11.1, respectively. The introduction of an SA significantly reduced the pH to 9.9 for HS-NCM622-SA, a value that remained constant after aging, indicating effective neutralization of residual alkalinity by the SA. Fourier-transform infrared (FT-IR) spectroscopy of aged T-NCM622 and aged HS-NCM622-SA revealed absorption peaks associated with the vibration of the CO32group at 1496, 1425, and 861 confirming the presence of carbonates. Notably, the aged HS-NCM622-SA, protected by the SA layer, showed no discernible peaks for Li2COs or LiOH after 7 days of exposure to moist air, suggesting the role of SA layer as an effective barrier against undesired side reactions.

[0046] X-ray photoelectron spectroscopy (XPS) further quantifies carbon impurities on the surfaces of the NCM samples. As shown in FIG. 2F-2G, for the C Is spectrum, the carbon impurity content increased from 25.2% (T-NCM622) and 24.6% (HS-NCM622) to 34.9% (Aged T-NCM622) and 43.5% (Aged HS-NCM622), respectively.

[0047] FIG. 2H provides a comparison of carbonate impurity content before and after aging based on C Is spectra. The minor change from 21.5% to 21.7% in aged HS-NCM622- SA highlights the scavenging layer's ability to minimize carbonate formation, a prevalent degradation product from the reaction of LiOH with CO2. These findings affirm that the SA layer not only mitigates alkaline residue impacts but also enhances interfacial stability by acting as an inert barrier against air-induced side reactions, thereby enhancing the durability and efficacy of the SAHR process in the direct recycling of Ni-rich LIB cathodes.

[0048] Upon confirming the efficacy of surface alkaline residue removal and interfacial stability enhancement with SA, microstructure characterization was carried out for degraded NCM622. Referring to FIGs. 3A-3D, comparisons between the regenerated samples and the control sample reveal that the aging process has a negligible impact on particle morphology, a consequence attributed to the mild conditions of the regeneration process. To obtain more insights into the microstructure, high-resolution transmission electron microscopy (HRTEM) was employed to examine regenerated NCM622, with and without the SA layer (FIGs. 3E and 3G). The interplanar spacings of regenerated NCM622, measured at 0.243 nm prior to aging, corresponded to the (101) atom plane orientation typical in NCM structures. After air exposure, amorphous regions were discernible in the aged HS-NCM622, indicating the formation of amorphous or poorly-crystalline carbonates (FIGs. 3F and 3H). Conversely, aged HS-NCM622-SA maintained a distinct layered structure in the near-surface region. Scanning Transmission Electron Microscopy (STEM) images coupled with Electron Energy Loss Spectroscopy (EELS) mapping of the aged HS- NCM622, shown in FIG. 31, reveal a boron-containing layer on the surface of the NCM622 particles, originating from the SA layer LBO, denoting uniform distribution of the LBO coating on the cathode particle surface. B ls spectrum analysis detects a B-related peak for HS-NCM622-SA, exhibiting little change in the aged HS-NCM622-SA (FIGs. 3J and 3K). These findings confirm that carbonates are the predominant degradation byproducts, largely due to the reaction between LiOH and CO2 in ambient air.

[0049] Referring to FIG. 3L, an EELS experiment was further conducted to probe the valence states of B, O, Mn, Co, and Ni from the particle surface to its interior. In a representative particle, the B K-edge, O K-edge, Mn L-edge, CoL-edge and Ni L-edge spectra of the aged HS-NCM622-SA and the aged HS-NCM622 were compared across six distinct points from surface (line 1) to interior (line 6), with a 3 nm step interval). According to FIG. 3K, the boron coating layer in the aged HS-NCM622-SA was measured to be approximately 12 nm. As shown in FIG. 3M, for the aged HS-NCM622, an emerging O pre-peak from surface to bulk indicated substantial oxygen vacancies in the particle due to oxygen release after aging. In contrast, a less pronounced O pre-peak in the aged HS- NCM622-SA implies fewer oxygen vacancies. The Ni L-edge exhibited a shift from 857.4 to 856.0 eV from bulk to surface in the aged HS-NCM622, suggesting surface reduction of Ni. However, in the aged HS-NCM622-SA, the Ni state remained relatively consistent. Minimal differences were observed in Mn and Co L-edges between the two samples. Overall, the protective layer effectively maintains the O and Ni states, both inside and on the surface of the cathode particle, thus preventing the phase transition from the preferred layered structure to an undesired rock-salt phase.

[0050] The electrochemical performance of the regenerated HS-NCM622 and HS- NCM622-SA was examined by coin cells with cathode mass loading of ~10 mg cm'2and 1M LiPFe in EC / EMC=50 / 50 (v / v) electrolyte and compared with T-NCM622. As shown in FIG. 4A, after coating, the capacity of HS-NCM622-SA increased to 177 mAh / g from 175 mAh / g (HS-NCM622) at C / 10 after 4 activation cycles. Referring to FIG. 4B, at C / 3, HS-NCM622-SA exhibited an initial capacity of 171 mAh / g with a retention of 92.3% after 100 cycles, similar to the performance of pristine NCM622, which exhibited a capacity retention of 92.8%. This confirms that the SA layer enhances cycling performance compared to the sample without the SA layer, which only managed a capacity retention of 78.6%. Fullcell testing, pairing these cathodes (loading: 14 mg / cm2) with a commercial graphite anode, was also conducted. The results are shown in FIG. 4C, where the HS-NCM622-SA showed an initial capacity of 160 mAh g1at C / 2 and maintained the capacity of 128 mAh g1after 200 cycles in the full-cell test, comparable to the pristine T-NCM622 under the same conditions. Rate testing revealed identical electrochemical performances for all HS- NCM622-SA cells compared to T-NCM622, confirming efficacy of the inventive regeneration method. FIG. 4D and 4E demonstrate that without the protective SA layer, the aged HS-NCM622 only achieves an initial capacity of 124 mAh / g, significantly lower than its freshly regenerated counterpart at 174 mAh / g. At a higher cycling rate of C / 3, the initial specific capacity of the aged HS-NCM622 was only 78 mAh / g without the SA layer. In contrast, with the protective SA layer, the aged HS-NCM622-SA cathode managed to maintain a capacity of 173 mAh / g at C / 10, comparable to that of its freshly regenerated counterpart.

[0051] Referring to Table 2, electrochemical impedance spectroscopy (EIS) further validates the reduced charge-transfer resistance (Act) resulting from minimized surface impurities. HS-NCM622-SA exhibited a decreased Act from 128.5 to 101.1 after SA coating. As shown in FIG. 4F, the reduction in charge-transfer resistance indicates surfaceside reactions with electrolytes, leading to improved surface stability. After air exposure, the Act increased from 128.5 to 787.6 for HS-NCM622, while for HS-NCM622-SA, the increase was from 101.1 to 157.0

[0052] Table 2

[0053] HS-NCM622 HS-NCM622- Aged HS- Aged HS-

[0054] SA NCM622 NCM622-SA

[0055] Rs / Q 0.718 0.860 1.737 1.479 Rct / Q 128.5 101.1 787.6 157.0

[0056] The average rate of resistance increase after 7 days of aging is much lower for HS-NCM622- SA at 8.0 Q / day compared to 94.2 Q / day for HS-NCM622.

[0057] Thickness studies were also conducted for the SA layer. Various boric acid to NCM ratios in regenerated NCM622, ranging from 0 wt% to 2 wt%, were compared. The corresponding NCM622 samples are referred to as HS-NCM622-BZ, where Z denotes the boric acid to NCM mixing ratios in ascending order. Table 3 lists the 1stCycle specific discharge capacity at C / 10 and boric acid to NCM mixing ratio (wt%) of regenerated NCM622. Table 3

[0058] Sample 1stCycle specific Boric acid to NCM mixing discharge capacity at ratio (wt%)

[0059] C / 10

[0060] C-NCM622 122

[0061] HS-NCM622-B4 111 2.0

[0062] HS-NCM622-B3 104 1.5

[0063] HS-NCM622-B2 175 1.0

[0064] HS-NCM622-B1 176 0.5

[0065] HS-NCM622 175 0

[0066] T-NCM622 175

[0067] The reduced capacity of HS-NCM622-B3, relative to HS-NCM622-B2, is attributable to a thicker coating layer, resulting in increased charge transfer resistance within the cell. Conversely, the decreased capacity of LBO B 1 can be attributed to a too thin or non- continuous SA layer, which increases the likelihood of incomplete coverage, thereby failing to mitigate all undesirable reactions.

[0068] More practically, real NCM622 black mass (“Cycled NCM622-BM”) obtained from spent cells is composed of degraded cathode particles, PVDF binder, conductive carbon, and residual electrolyte. To show the versatility of the inventive process in treating such a practical feedstock, the process was applied to Cycled NCM622-BM for evaluation. Due to the existence of PVDF binder and conductive carbon impurities in Cycled NCM622-BM, the material underwent a short washing step using deionized water at a water-to-material ratio of about 1.6 ml / gcathode to 2.5 ml / gcathode each time, repeated for two times to separate the NCMs from the Li-rich aqueous solution and remove impurities.

[0069] An exemplary sequence for the overall inventive process is schematically shown in FIG. 6. In the illustrated sample process flow, the electrochemically degraded cathode black mass was agitated in 4 M LiOH aqueous solution at 220°C for hydrothermal relithiation in step 60. Subsequently, in step 62, the material underwent a washing step (2x) using deionized water to separate the NCMs from the Li-rich aqueous solution. Following drying, the relithiated powders were blended with an additional 5mol% of Li2COs (to compensate for the Li loss during the post-annealing process) and subjected to annealing in an oxygen environment at 850°C for a duration of 4 hours (step 64), with a heating rate of 5°C / min (first annealing). A scavenging agent was used in step 66 to apply a dry chemical coating process to neutralize excessive alkaline residues on the surface of the regenerated materials, which repairs the unstable / unclean surface after water washing. After a short annealing step at 300°C for 5 hours (step 68) with a ramping rate of 5°C / min, the coating materials formed an ionic conductive glass layer on NCMs (second annealing).

[0070] To provide a direct comparison, FIGs. 5A and 5B are backscattering mode SEM images of the cycled NCM622-BM and regenerated NCM622 after SA application (HS- NCM622-BM-SA), respectively. The SEM image of the cycled NCM622-BM (FIG. 5A) clearly shows that the binder and carbon predominantly coat the surface of the cathode. The HS-NCM622-BM-SA in FIG. 5B shows very clean surface, confirming removal of binder and carbon from the cathode surface. This was further corroborated by XPS analysis of FIs, as shown in FIGs. 5C and 5D, indicating that the PVDF binder and conductive carbon can be eliminated through limited water washing. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) results in Table 4 below demonstrate successful recovery of the lithium content (xu) in Cycled NCM622-BM from xu =0.91 to xu =1.03 following the regeneration process with the SA layer. Table 4

[0071] Sample Li Ni Co Mn

[0072] Cycled NCM622-BM 0.91 0.60 0.19 0.21

[0073] HT-NCM622-BM 0.98 0.60 0.19 0.21

[0074] HS-NCM622-BM 1.03 0.60 0.19 0.21

[0075] HS-NCM622-BM-SA 1.03 0.60 0.19 0.21

[0076] XRD results reveal that the (003), (108), and (110) peak positions shifted back after regeneration of the black mass. Referring to Table 5 below, a decreased cation mixing ratio from 5.9% to 3.7%, indicative of reduced cation mixing in NCM materials post-SA layer formation, aligns with previous findings. Half-cells composed of HS-NCM622-BM-SA demonstrated an Initial Coulombic Efficiency (ICE) of 88% with a discharge capacity of 179 mAh / g, close to that of T-NCM622. Referring to FIGs. 5E and 5F, long-term cycling data for these half-cells showed a commendable 95% capacity retention after 100 cycles, underscoring the high quality of the regenerated cathode materials. Rate capability testing conducted with HS-NCM622-BM-SA achieved performance levels comparable to T- NCM622, further validating the quality of the recycled cathode materials and versatility of our SAHR process in treating real cathode black mass.

[0077] Table 5

[0078] Sample a / A c / A Li / Ni Rwpl° / o R / ° / o

[0079] HS-NCM622-BM 2.87203 14.24755 5.899 1.32 0.67

[0080] HS-NCM622-BM-SA 2.87233 14.24803 3.758 1.54 0.94

[0081] T-NCM622 2.87205 14.25025 4.552 1.09 0.73

[0082] Economic and environmental analyses were conducted to determine the benefits provided by the inventive recycling process. There are three primary recycling methods for LIBs: pyrometallurgical, hydrometallurgical, and direct recycling. Pyrometallurgical and hydrometallurgical methods are currently in industrial use, while direct recycling remains predominantly at the laboratory scale, with efforts underway to upscale for real applications. The pyrometallurgical process involves high-temperature smelting to segregate battery components into elemental forms, typically featuring the burning of mixed LIB materials followed by chemical separation. This method generates gas, acid, and alkaline waste, i.e., significant environmental implications. The hydrometallurgical method includes acid or alkali leaching followed by lithium extraction, separation, and conversion to reclaim various metal compounds, also producing acid waste and alkaline waste, again with significant environmental impact. In contrast, direct recycling employs a physical separation process for recovering cathode and anode materials from the current collector surfaces, followed by a mild post-treatment to address the compositional and structural deficiencies within the electrode particles. This method maintains the original compound structures and preserves the embedded energy, producing Li-containing wastewater, with ongoing studies investigating the reuse of such waste solutions.

[0083] Technically, the inventive process marks a critical advancement that bridges the gap between laboratory-scale direct recycling and industrial applications. Considering the impracticality of air-tight material handling systems in industrial recycling settings, deploying this method at scale ensures that materials preserve their structural integrity and electrochemical potential. Integrating economic and ecological considerations is essential for holistic sustainability.

[0084] Typical descriptions of hydrothermal relithiation processes pay scant attention to the washing step, leading to a failure to recognize the significant economic and environmental impact that water washing could have in scaled up recycling operations. To study this impact, we conducted a comparative analysis between traditional pyrometallurgy, hydrometallurgy, direct recycling with regular washing, and SAHR-based direct recycling. FIG. 7 graphically illustrates how the inventive SA-assisted, minimal-washing process significantly reduces wastewater production by about 23.3kg per kg of cell compared to traditional direct recycling with extensive washing during the cathode material recovery. Considering the entire closed-loop recycling process, involving transportation, disassembly, recycling, material conversion, and cathode production, it is clear that the direct recycling process employing extensive washing at scale could result in massive water consumption and costs, approaching those of current pyrometallurgical and hydrometallurgical methods. The primary water use in the direct recovery method occurs during the recycling step. However, with methodological improvements, water consumption can be significantly reduced - to just one-fifth of its original demand. Furthermore, the inventive SAHR process not only maintains the cost-efficiency of direct recycling compared to traditional techniques but also removes a significant technical barrier for scaling direct recycling. All the above analysis delineates that this SAHR-based direct recycling paradigm is aligned with the sustainable LIB recycling practices.

[0085] EXAMPLES

[0086] The following non-limiting examples illustrate procedures and applications of the inventive recycling process.

[0087] Example 1 : Chemically Delithiated NCM111 and Electrochemically Degraded NCM622

[0088] Chemically delithiated NCM111, with approximately 10% lithium loss and labeled as "D-NCM111," was produced by the Materials Engineering Research Facility (MERF) at Argonne National Laboratory. Pristine NCM111, supplied by Toda America Inc., was reacted with an aqueous potassium persulfate solution to extract lithium. The material was then washed with water, followed by acetonitrile, and dried under vacuum at room temperature. This delithiated NCM111 served as the starting material for additive screening and was produced in 1 kg batches.

[0089] Electrochemically cycled NCM622 (C-NCM622) exhibiting substantial capacity degradation was provided by Argonne National Laboratory. To recover the cycled NCM622 materials, cathode strips were initially cleansed using dimethyl carbonate (DMC) to eliminate any remaining electrolytes. After drying, the cathode strips were immersed in N- Methyl-2-pyrrolidone (NMP) and subjected to 20 minutes of sonication to detach the NCM powders, binder, and carbon black (CB) from the aluminum substrates. The resulting solution was then subjected to centrifugation at 3,500 rpm for 5 minutes to isolate the cycled NCM622 (C-NCM622) powders. These precipitates were washed multiple times with NMP before being collected and dried in preparation for the regeneration experiment. It is worth noting that CB, having a significantly lower density compared to NCM622, was effectively separated during the precipitation process by gravity.

[0090] Example 2: Direct Regeneration

[0091] 100 g D-NCM111 was first relithiated in 4M LiOH solution at 220°C. The resulting NCM111 (HT-NCM111-100 g) was divided into several 4g batches and then mixed with different amounts of deionized water, with the ratio of 20 ml, 40 ml, 60 ml, 80 ml water, respectively, to wash 1 g HT-NCM111-100 g, under constant stirring at room temperature. Afterwards, washed material was separated and dried under vacuum for 4 hours at 100°C, followed by annealing at 850°C in a pure oxygen atmosphere. The samples obtained are denoted as “HS-NCMl l l-20ml / g”, “HS-NCMl l l-40ml / g”, “HS-NCMl l l-60ml / g”, and ‘HS-NCM11 l-80ml / g’, respectively.

[0092] For the relithiation process, degraded cathode powder (chemically delithiated NCM111, and electrochemically degraded NCM111 and NCM622) was agitated in 4 M LiOH aqueous solution at 220 °C for relithiation. Subsequently, the material underwent washing using deionized water at a water-to-material ratio of 1.6 mL / g to separate the NCMs from the Li-rich aqueous solution. Following the drying phase, the relithiated powders were blended with an additional 5mol% of Li2COs (to compensate for the Li loss during the postannealing process) and subjected to annealing in an oxygen environment at 850°C for a duration of 4 hours, with a heating rate of 5 °C / min.

[0093] Example 3: SA layer application

[0094] A scavenging agent (SA) was used to apply a dry chemical coating process to neutralize excessive alkaline residues on the surface of the regenerated materials, which repairs the unstable / unclean surface after water washing. For testing described herein, the scavenging agent was H3BO3 (boric acid), however, it will be apparent to those in the art that other inorganic acids or scavenging agents that may be used including, but not limited to, H3PO4 (phosphoric acid), (NFLjEEPCh (ammonium dihydrogen phosphate or ADP), H2SO3 (sulfuric acid), and HF (hydrofluoric acid). After a short annealing process at 300°C for 5 hours with a ramping rate of 5°C / min, the coating materials formed an ionic conductive glass layer on NCMs.

[0095] Example 4: Materials Characterization

[0096] The chemical composition of different cathode powders was assessed through inductively coupled plasma mass spectrometry (ICP-MS) using the Thermo Scientific iCAP RQ model. Surface composition analysis of the samples was carried out using X-ray photoelectron spectroscopy (XPS) with data collected using the PHI 5000 VersaProbe II system (Physical Electronics) employing Al Ka radiation at 1486.6 eV. Crystal structure determination was performed using X-ray powder diffraction (XRD) with Cu Ka radiation ( = 1.5406 A), utilizing the Bruker D2 Phaser, and Rietveld refinement against XRD results was conducted using the General Structure Analysis System (GSAS) software with the FullProf Suite interface. Surface and bulk morphology of various NCM cathode particles were examined using a scanning electron microscope (SEM), specifically the FEI XL30. High-resolution transmission electron microscopy (HRTEM) was recorded on a TEM (ThermoFisher Talos 200X TEM operating at 200 kV) with the CETA Camera. The TALOS microscope was equipped with a high-resolution Gatan imaging filter (Gatan Continuum 1069) for EELS mapping. STEM-EELS was performed on primary particles in high-angle annular dark-field imaging (HAADF) mode using the same instrument.

[0097] Example 5: Electrochemical Characterization

[0098] The electrochemical performance of all specimens was evaluated using coin cells (half-cell configuration) with a cathode mass loading of approximately 10 mg cm2. To prepare electrode slurries, the pristine, aged, or regenerated NCM cathode material was combined with a conductive agent (Super P65) and a polyvinylidene fluoride (PVDF) binder in a mass ratio of 8: 1 : 1 in N-methyl-2-pyrrolidone (NMP) solvent. Subsequently, the slurries were applied to aluminum foil using a doctor blade and then dried at 120°C for 12 hours in a vacuum oven. The resulting dry laminate was cut into disc shapes and calendared. Coin cells were assembled inside a glovebox, with a Li metal disc (1.1 mm thick) serving as the counter electrode, LP40 (IM LiPFe in ethylene carb onate / di ethyl carbonate = 50:50 (v / v)) or Gen2 (1 ,2M LiPFe in ECZEMC = 3 :7) as the electrolyte, and a tri-layer membrane (Celgard 2320) as the separator. Galvanostatic charge-discharge tests were conducted using a Neware battery cycler within the potential range of 3.0-4.3 V. The testing included 4 activation cycles at a rate of C / 10, followed by 50 cycles at a rate of C / 3.

[0099] Example 6: Economic and environmental analysis

[0100] To examine variations in techno-economic aspects and life cycle evaluations between pyrometallurgical, hydrometallurgical recycling methods, and direct cathode recycling approaches, we employed the EverBatt model. The EverBatt model, developed at Argonne National Laboratory, is a closed-loop battery recycling model. In this model, it is assumed that all recycling methods can handle an annual capacity of 10,000 metric tons of battery cells within the United States.

[0101] The inventive processes described herein employ an effective strategy of coupling a multifunctional scavenging material into the direct regeneration process to remove the surface alkaline residue to simultaneously address interfacial stability issues and minimize water consumption and wastewater generation in the recycling process. During this process, residual lithium will be simultaneously removed and an ionic conductive and protective coating will form on regenerated cathodes. The SAHR process removes heterophase residue in the recycled products through neutralization reactions between the inorganic acid and residual lithium compounds, effectively preventing direct contact between ambient air and the Ni-rich cathode surface. Furthermore, the application of the SA layer substantially increases the interfacial stability of the regenerated NCM622, as evidenced by half-cell and full-cell tests, compared to uncoated, pristine samples. The inventive recycling process has been successfully employed to recover cathode materials from spent LIBs, minimizing water consumption, and facilitating the production of new LIBs utilizing the regenerated materials. These newly fabricated LIBs exhibit comparable electrochemical performance to their freshly manufactured counterparts. Thus, the inventive process provides a viable pathway for the regeneration of cathode materials from end-of-life LIBs and can be extended for use in cathode manufacturing, offering a practical and scalable solution to production of high-performance LIBs.

Claims

CLAIMS:

1. A method for regenerating cathode material from a spent lithium-ion (LI) battery, the method comprising: coupling a scavenging agent into a direct regeneration process to simultaneously remove a surface alkaline residual and form an in-situ protective layer on a surface of a regenerated cathode material.

2. The method of claim 1, wherein the cathode material comprises lithium nickel cathode materials comprising LiNio.6Coo.2Mno.2O2 (NCM622).

3. The method of claim 1, wherein the scavenging agent is a weak inorganic acid.

4. The method of claim 3, wherein the weak inorganic acid is H3BO3 and the in- situ protective layer is a Li+-conductive LiBO2 (LBO) glass.

5. The method of claim 1, wherein the scavenging agent is selected from H3BO3, H3PO4, (NH4)H2PO4, H2SO3, and HF.

6. The method of claim 1, wherein the in-situ protective layer comprises a Li+- conductive glass layer coating.

7. The method of claim 6, wherein the glass layer coating comprises LiBO2 (LBO).

8. The method of claim 1, further comprising annealing the regenerated cathode material after mixing with the scavenging agent.

9. The method of claim 7, wherein annealing is performed at approximately 300°C.

10. A method for regenerating cathode material from a spent lithium-ion (LI) battery, the method comprising: performing hydrothermal relithiation in a lithium-bearing solution on a spent cathode to recover lithium within the cathode material and generate a relithiated cathode material; separating the relithiated cathode material from the lithium-bearing solution with a limited washing in water; mixing and reacting the relithiated cathode material with a scavenging agent to neutralize alkaline residue on a surface of the cathode material; andannealing the cathode material to induce in-situ formation of a Li+-conductive glass layer coating.

11. The method of claim 10, wherein the scavenging agent is a weak inorganic acid.

12. The method of claim 10, wherein the scavenging agent is selected fromH3BO3, H3PO4, (NH4)H2PO4, H2SO3, and HF.

13. The method of claim 10, wherein the scavenging agent is H3BO3.

14. The method of claim 13, wherein the Li+-conductive glass layer coating comprises LiBCh (LBO).

15. The method of claim 10, wherein annealing is performed at approximately300°C.

16. The method of claim 10, wherein limited washing comprising comprises using a water-to-material ratio of about 1.6 ml / gcathode to 2.5 ml / gcathode.

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