Hydrothermal relithiation using residual lithium

Abstract

A method for relithiating cathode material from a spent Li-ion cell includes hydrothermally treating separated cathode material in a KOH-based aqueous solution for a predetermined time to provide a regenerated cathode material using residual lithium on surfaces of the separated cathode material as the source of lithium for relithiation.

Description

[0001] HYDROTHERMAL RELITHIATION USING RESIDUAL LITHIUM

[0002] RELATED APPLICATIONS

[0003] This application claims the benefit of the priority of U. S. Provisional Application No.

[0004] 63 / 714,091, filed October 30, 2024, which is incorporated herein by reference in its entirety.

[0005] BACKGROUND

[0006] An unintended consequence of rapid electrification, lithium-ion battery (LIB) waste is expected to exponentially increase, nearly doubling before 2030. Projections show that most of this waste will continue to be from spent batteries which have been used in products and depleted of their capacities. In addition, a large portion of this waste will come from manufacturing scraps, notwithstanding improvements to manufacturing quality control. Since battery components rely on critical elements from increasingly geopolitically turbulent and scarcer reserves, governments and companies throughout the battery supply chain have begun making battery recycling a priority. Initial large-scale LIB recycling techniques involve traditional recycling methods adopted from other industries, including smelting or leaching, which aim to isolate precursors as products through a series of targeted steps. Alternatively, direct recycling restores defects that accumulate while the LIB is in use without requiring the complete deconstruction of battery materials.

[0007] Despite the limited adoption at a large scale, direct recycling remains significant as an alternative to traditional methods as it is forecasted to minimize cost, processing time, and environmental corollaries.

[0008] Regardless of its source, LIB waste consists of a variety of chemistries, commonly distinguished by their cathodes: lithium nickel cobalt aluminum oxide (NCA), lithium manganese oxide (LMO), lithium iron phosphate (LFP), lithium cobalt oxide (LCO), and lithium nickel cobalt manganese oxide (NCM). As the highest weight and cost component, LIB cathodes are the initial focus of direct recycling.

[0009] Defects differ depending on the battery type and chemistry, but all must be resolved through direct recycling. Cathode waste originates either from degraded batteries (spent) or from production waste including off-spec, unused, or excess cathode materials (scrap). Spent cathode black mass (CBM) has varying amounts of bulk Li loss due to Li trapping in cathode-electrolyte interphase (CEI) and solid electrolyte interphase (SEI) accumulation during cycling. In cases of extreme use or in high-Ni transition metal oxides, severe Li loss can lead to structural deterioration on the particle surface. Specifically, Ni existing in the transition metal layer can decrease, promoted by reduced entropy, and the resultant Ni2+ion can move into the Li layer due to its similar ionic radius and similar bonding environment. Furthermore, thick CEIs may develop from impurities or electrolyte deterioration containing substantial amounts of electrochemically resistive components which must be removed to reclaim a pure cathode. Many of these compounds within the CEI contain Li. In contrast, scrap materials typically do not have bulk delithiation but can contain CEI surface species due to formation cycles. All CBM also contains varying amounts of impurities which are either inherent ingredients in cathode black-mass, e.g., conductive additives and binders, or contaminants from the other battery components.

[0010] Regardless of the degree of degradation in the CBM, the objective of direct recycling is to repair the aforementioned deficiencies and remove impurities to produce a rejuvenated cathode material. Many approaches to direct recycling exist, typically involving two main steps: relithiation and a brief annealing step. The relithiation step involves reinsertion of Li-ions into the bulk cathode structure, using some additive Li salt in excess to induce phase change or reintercalation. The short annealing step resolves remaining surface deficiencies. Chemical solution relithiation (e.g. hydrothermal) is often favored for scalability since it does not require predetermination of lithium deficiency, is cost-effective relative to other relithiation methods, has been proven on a variety of cathode chemistries, and has been demonstrated to extract impurities.

[0011] SUMMARY

[0012] The inventive approach employs self-saturating direct regeneration techniques such as hydrothermal relithiation to achieve scalable lithium-ion battery recycling while reducing resource demands and environmental impact compared to conventional recycling methods. In some embodiments, direct resubmerging the CBM is performed in a solution containing excess Li+and promoting reinsertion of Li-ions by treating it at elevated temperatures and pressures in a solution that discourages further degradation. Currently, all efforts reported in the literature involve the addition of some Li salt within the solution. A key improvement of the inventive approach is that the solution composition is selected to minimize the need for added Li.

[0013] According to embodiments, the inventive regeneration process follows similar steps to those used in previously reported hydrothermal regeneration processes: spent material is treated in a constant-volume aqueous solution at elevated temperature and pressure, washed to remove solution compounds, and then annealed to resolve surface compounds. In some embodiment, rather than a batch-style reaction. In other implementations, in lieu of using a batch-style reaction, the process may be carried out at predetermined temperatures and pressures determined according to the materials being treated. Previous studies have demonstrated solution optimization for hydrothermal regeneration of cathode materials, mainly focusing on the impact of the solution concentration of hydroxides toward full regeneration. To inhibit further decomposition of NCM in water, a hydrothermal solution of 4 M KOH was used. Washing was optimized for removal of KOH using pristine NCM111, P-NCM111, and no morphological damage or leaching with hydrothermal treatment was observed.

[0014] In one aspect, a method for relithiating a spent Li-ion cell includes: discharging the cell; disassembling the cell to separate a cathode material; hydrothermally treating the separated cathode material in a KOH-based aqueous solution for a predetermined time to provide a regenerated cathode material; and annealing the regenerated cathode material; wherein residual lithium on surfaces of the separated cathode material provide sufficient lithium for relithiation. The KOH-based aqueous solution may have a concentration of 2 M KOH or more. In some embodiments, the concentration is 4 M KOH. The method may further include, after hydrothermally treating and prior to annealing, separating the regenerated cathode material from the solution; washing the regenerated cathode material; and drying the regenerated cathode material to provide a cathode precipitate. In some embodiments, a lithium salt may be added to the cathode precipitate in an amount sufficient to compensate for lithium loss during annealing. The lithium salt may be selected from Li₂CO₃, LiCO3, LiOH, and Li2O2.

[0015] The hydrothermally treating may include adding the separated cathode material to a reactor at an elevated temperature and pressure, wherein the elevated temperature is within a range of 120 °C to 220 °C and the elevated pressure is within a range of 2 psi to 200 psi. The cathode material may be selected from LiNio.33Coo.33Mno.33O2 (NCM 111), LiCoO2 (LCO), LiNio.6Coo.2Mno.2O2 (NCM622), and LiNi0.87Co0.05Mn0.08Al0.03O2 (NCMA). Where the cathode material is LiNio.33Coo.33Mno.33O2 (NCM 111) or LiCoO2 (LCO), hydrothermally treating includes heating the separated cathode material and the aqueous solution to 220 °C for 4 hours. Where the cathode material is LiNio.6Coo.2Mno.2O2 (NCM622), hydrothermally treating includes heating the separated cathode material and the aqueous solution to 160 °C for 4 hours. Where the cathode material is LiNi0.87Co0.05Mn0.08Al0.03O2 (NCMA), hydrothermally treating includes heating the separated cathode material and the aqueous solution to 120 °C for 6 hours.

[0016] In another aspect, a method for relithiating cathode material from a spent Li-ion cell includes: hydrothermally treating a separated cathode material from the spent Li-ion cell in a KOH-based aqueous solution for a predetermined time to provide a regenerated cathode material; separating the regenerated cathode material from the solution; washing the regenerated cathode material; drying the regenerated cathode material to provide a cathode precipitate; and annealing the regenerated cathode material; wherein residual lithium on surfaces of the separated cathode material provide sufficient lithium for relithiation. The KOH-based aqueous solution may have a concentration of 2 M KOH or more. In some embodiments, the concentration is 4 M KOH. In some embodiments, a lithium salt may be added to the cathode precipitate in an amount sufficient to compensate for lithium loss during annealing. The lithium salt may be selected from Li₂CO₃, LiCO3, LiOH, and Li2O2.

[0017] The hydrothermally treating may include adding the separated cathode material to a reactor at an elevated temperature and pressure, wherein the elevated temperature is within a range of 120 °C to 220 °C and the elevated pressure is within a range of 2 psi to 200 psi. The cathode material may be selected from LiNio.33Coo.33Mno.33O2 (NCM 111), LiCoO2 (LCO), LiNio.6Coo.2Mno.2O2 (NCM622), and LiNi0.87Co0.05Mn0.08Al0.03O2 (NCMA). Where the cathode material is LiNio.33Coo.33Mno.33O2 (NCM 111) or LiCoO2 (LCO), hydrothermally treating includes heating the separated cathode material and the aqueous solution to 220 °C for 4 hours. Where the cathode material is LiNio.6Coo.2Mno.2O2 (NCM622), hydrothermally treating includes heating the separated cathode material and the aqueous solution to 160 °C for 4 hours. Where the cathode material is LiNi0.87Co0.05Mn0.08Al0.03O2 (NCMA), hydrothermally treating includes heating the separated cathode material and the aqueous solution to 120 °C for 6 hours.

[0018] Although bulk Li within the cathode structure may be depleted, Li-containing components are often still present in the CBM. Prior to disassembly, batteries are discharged for safety, either electrochemically or in salt-containing solution. In any electrochemical discharge step, Li-ions return to the cathode and are either reintegrated into the damaged cathode structure or thicken the CEI. It is broadly understood that the CEI consists of an inner layer of polymers / polycarbonates surrounded by LiF (outer layer) and LixPOyFz. Guo, et al. (“Dynamic Evolution of a Cathode Interphase Layer at the Surface of LiNio.5Coo.2Mno.3O2 in Quasi-Solid-State Lithium Batteries” J. Am. Chem. Soc. 142, 20752-20762 (2020)) have shown that upon cell discharge, inorganic CEI components such as LiF and Li2CO3 overwhelm organic compounds. Due to CEI evolution during over-discharge, it is likely that cell disassembly aids in the supply of surface Li components which may be used for relithiation. However, following disassembly, much of the residual lithium originates from the electrolyte salt, which is highly reactive with ambient water and is typically present in various decomposition states.

[0019] Residual Li is electrochemically resistive therefore reducing electrochemical capacity and presents challenges for removal. Despite the emphasis on maximizing lithium recovery, it is typically washed or discarded prior to direct recycling treatment. In this work, the inventive process for hydrothermal relithiation of layered transition metal oxide cathode materials uses only the existing Li remaining within the CBM after disassembly. Despite Li loss in the particle bulk, the total Li in the CBM can exceed the amount needed for full rejuvenation due to residual Li salts present after discharge. As a result, even when the bulk crystal is delithiated, stoichiometric measurements of Li for the CBM can be greater than 1.00 after discharge and disassembly. Using spent LiNio.33Coo.33Mno.33O2 (NCM 111) produced without prior impurity separation of NMP washing, conductive carbon and binder impurities can be removed while simultaneously achieving relithiation with a KOH-based alkaline solution at elevated temperature. To demonstrate how Li within residual species is available for relithiation by using the inventive scheme, LiF, Li2CO3, and LiPF6are similarly subjected to hydrothermal treatment, with corresponding behaviors facilitate lithium release and reinsertion into delithiated NCM 111. Relithiation using residual Li salt from CBMs is also validated on other spent cathode materials, namely LiCoCh (LCO), LiNio.6Coo.2Mno.2O2 (NCM622), and LiNi0.87Co0.05Mn0.08Al0.03O2 (NCMA), demonstrating its applicability to a range of LIB cathode materials.

[0020] BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 diagrammatically illustrates an exemplary disassembly process showing Li-containing species remaining in CBM.

[0021] FIGs. 2A-2F illustrate recovery and surface characterization of spent NCM 111 (S-NCM111), where FIG. 2A shows evidence of Li residue, i.e., the light color on dried cathode, FIG. 2B is a SEM image of cracked spent particles, FIGs. 2C-2E respectively show elemental XPS spectra of Ols, FIs, and P2p for spent NCM 111 black mass to identify surface Li composition; FIG. 2F provides a comparison of washing capability via surface content from ICP-MS measurement (left axis) and wide spectrum XPS analysis (right axis).

[0022] FIGs. 3A-3C demonstrate impurity removal during hydrothermal treatment comparing LiOH solution and pure KOH solution, where FIG. 3A shows TGA data of the spent NCM 111 material compared to black mass after hydrothermal treatment with either L; OH solution or KOH solution; FIG. 3B plots ISE data demonstrating the removal of fluoride ions after regeneration with either solution; and FIG. 3C shows O1s spectra of spent NCM 111 compared to black mass after hydrothermal treatment with either LiOH solution or KOH solution and fully recycled using KOH hydrothermal solution FIGs. 4A-4C illustrate structural characterization of spent and regenerated CBM, where FIG. 4A provides the XRD patterns at each state of the recycling process with P-NCM111 as a reference; FIGs. 4B and 4C are HR-TEM images of the particle surface of S-NCM111 and KOH-FullRec., respectively.

[0023] FIGs. 5A-5D illustrate electrochemical performance of NCM before and after treatment, where FIG. 5A demonstrates the first cycle voltage profile; FIG. 5B plots capacity retention for 100 cycles; FIG. 5C provides a rate-study for the pristine and KOH-treated samples; and FIG. 5D provides economic and environmental modeling results using EverBatt demonstrating impact of the process improvement on cost of cathode production ($ / kg cathode produced) and GHG emissions (g emissions / kg cathode produced). Error bars reflect model sensitivity to solid-to-solution mass ratios during hydrothermal treatment between 15% and 45%.

[0024] FIGs. 6A-6D demonstrate the mechanism of Li reinsertion using D-NCM111 without impurities or surface Li and individually added lithium salts that appear on the CEI into the KOH solution during hydrothermal treatment, where FIG. 6A provides XRD patterns for D-NCM111 before and after treatment; FIG. 6B shows XRD patterns of spent and regenerated materials treated in various concentrations of KOH, FIG. 6C is a graph of molarity of Li within the KOH solution after treatment as measured by ICP-MS, and FIG.

[0025] 6D shows stoichiometric Li measured by ICP-MS within the cathode black mass after hydrothermal treatment.

[0026] FIG. 7 A shows solubility of each Li salt in various concentrations of KOH (left y-axis) compared to the effectiveness of relithiation for each condition (right y-axis), FIG.

[0027] 7B plots TGA data demonstrating the effectiveness impurity removal of various concentrations of KOH during hydrothermal relithiation; and FIG. 7C illustrates the overall mechanism suggesting effect of hydrothermal treatment freeing Li from residue compounds to be used during relithiation.

[0028] FIGs. 8A-8C demonstrate regeneration of various layered transition-metal oxide cathode materials, where each figure provides backscattering SEM images from spent materials of LCO, NCMA, and NCM622, respectively, before and after regeneration, and plots initial discharge capacities for regenerated LCO, NCMA, and NCM622, respectively,

[0029] DETAILED DESCRIPTION OF EMBODIMENTS

[0030] The inventive method employs hydrothermal regeneration. In some embodiments, the spent material is treated in a constant-volume aqueous solution at elevated temperature and pressure, washed to remove solution compounds, and then annealed to resolve surface compounds. In other implementations, in lieu of using a batch-style reaction, the process may be carried out at predetermined temperatures and pressures selected according to the materials being treated. The process is optimized to inhibit further decomposition of NCM in water by using a KOH-based alkaline solution.

[0031] Materials and Methods

[0032] Cathode Black Mass Recovery

[0033] Spent NCM111 (S-NCM111) was recovered from end-of-life prismatic cells (20 Ah) provided by American HONDA Motor Company. The cells were discharged (to an OCV of <1 V) and manually disassembled so that cathode strips were collected separately from the separator and anode. The cathode strips were dried in a vacuum oven at 80 °C for 2 days. Once dry, the cathode was cut into strips and mechanically ground to agitate and strip the cathode black mass (CBM) from the current collector. Using a series of sieves, the aluminum current collector was separated from the CBM. The resultant powder was used for cathode regeneration.

[0034] Spent LCO was recovered similarly from end-of-life laptop cells. For both scrap and spent NCM622, 1 Ah unfilled pouch cells were purchased from Canrd. For scrap, cathode strips were manually disassembled and ground prior to electrolyte filling. For spent NCM622, pouch cells were filled with Gen2 electrolyte (1.2 M LiPF6in ECZEMC = 3:7, Gotion, U. S.) and cycled at 1 C until the capacity retention was 90% (not including formation cycles). Cathode Regeneration

[0035] For the relithiation of S-NCM111, 10g of CBM were added to a 100 mL autoclave reactor containing 60 mL of KOH solution with various concentrations and heated at 220 °C for 4 hours. After the hydrothermal treatment, the KOH solution was separated, and the resultant sample was washed four times with DI water (lOmL / g). The cathode precipitate was collected and dried overnight at 80 °C. Once dried, the product was triturated and mixed with 5 mol% of Li₂CO₃ to compensate for the loss of Li during the annealing process. It was then annealed in oxygen at 850 °C for 4 hours using a ramping rate of 3 °C / min. Once samples are sintered, they are considered fully regenerated and assembled into cells for electrochemical testing.

[0036] To evaluate the broader applicability of the inventive method, additional cathode materials (NCMA, LCO, and NCM622) were subjected to similar hydrothermal treatment with adjusted temperatures and durations to optimize relithiation. NCMA was treated at 120 °C for 6 hours, LCO at 220 °C for 4 hours, and NCM622 at 160 °C for 4 hours. Following hydrothermal treatment, these materials were annealed under oxygen with tailored sintering conditions. NCMA and NCM622 were mixed with 5 mol% LiOH and annealed at 725 °C for 6 hours, LCO was mixed with 10 mol% of Li2CO3 and annealed at 850 °C for 4 hours. A higher Li salt additive was used since spent LCO was acquired in powder form and the battery could not be completely discharged to return Li to the cathode.

[0037] Materials Characterization

[0038] CBM was evaluated prior to treatment, after hydrothermal treatment and washing, and after sintering. To measure the impurity content of the CBM, thermogravimetric analysis (TGA) data were collected using a Discovery SDT 650 (TA Instruments). All X-ray diffraction (XRD) samples were measured in a Rigaku Miniflex XRD (Cu Ka radiation, 2 = 1.5406 A) with a scanning rate of 2 deg min '. Stoichiometric composition and ratios were calculated using ICP-MS (Thermo Scientific iCAP RQ model). To further evaluate surface contents before and after hydrothermal treatment, XPS was performed using the AXIS Supra XPS from Kratos Analytical using a monochromatized Al Ka source at 20kV under a 10-9Torr vacuum. Survey scans were conducted using a step size of 1.0 eV, and high-resolution scans were conducted using a step size of 0.1 eV. All spectra were analyzed using CasaXPS software and calibrated using the C1s C-C peak at 284.8 eV.

[0039] To investigate the surface structure and morphology of the cathode particles, a scanning electron microscope (SEM, FEI Apreo LoVac) was used to examine the surface structure and morphology of various cathode particles. Using the SEM, an Oxford Instruments XMax 80 EDS detector aided in EDS spectra for all cathode materials. The surface microstructure was further verified before and after regeneration using high-resolution transmission electron microscopy (HR-TEM) images which were collected on a JEOL-2800 at 20kV with a Gatan One View Camera (25 fps, full 4 K resolution). The FFT structure information was confirmed by DigitalMicrograph.

[0040] . Electrochemical Characterization

[0041] Electrochemical performance was evaluated by half-cells in galvanostatic chargedischarge cycles. Pristine NCM 111 (Toda, U. S.) served as a reference for regenerated samples. For homogeneous slurries, cathode powder was mixed with polyvinylidene fluoride (PVDF, KYNAR 2800) and carbon black (Super P65) in N-methylpyrrolidone (NMP, Sigma- Aldrich, anhydrous 99.5%) at a mass ratio of 8:1:1. The resulting slurries were cast onto aluminum foil and dried under vacuum at 120°C overnight. Cathode disks were cut, calendared, and assembled into CR2032 coin cells. Cathode disk loading was approximately 5 mg / cm2. Cells were assembled using Gen2 electrolyte (1.2 M LiPF6in ECZEMC = 3:7, Gotion, U. S.), a polypropylene (PP) separator membrane (Celgard 2500), and Li-metal as the counter electrode (thickness 1.1 mm). Coin cells were tested within the voltage potential range of 3.0 - 4.3 V at room temperature using a Neware battery cycler. NCM 111 samples were cycled at a rate of C / 10 for three activation cycles followed by extended cycling at C / 3 (1 C = 155 mA / g).

[0042] Slurries for NCMA, LCO, and NCM622 were prepared using the same method. NCM622 and NCMA samples cycled at a rate of C / 10 for three activation cycles followed by extended cycling at C / 3 (1 C = 180 mA / g). LCO samples were cycled at a rate of C / 10 for three activation cycles followed by extended cycling at C / 3 (1 C = 155 mA / g).

[0043] Mechanistic Study

[0044] Delithiated NCM 111 was used as cathode black mass for regeneration to isolate impact of reintercalation of various Li salts contributing to the CEI. Chemically delithiated NCM 111 (D-NCM111), with approximately 10% lithium loss was created by the Materials Engineering Research Facility (MERF) at Argonne National Laboratory. Delithiation was conducted by adding pristine NCM 111 (Toda, U. S.) in an aqueous solution of potassium persulfate to leach Li. Once removed, the leached material was washed with water, acetonitrile, and dried under vacuum.

[0045] To verify that relithiation is possible using other various Li salts, Li₂CO₃, LiF, and LiPF6were separately added to 100 mL autoclave reactors containing 60 mL of 4 M KOH solution. The amounts of the Li compounds that were added were based on the amount of Li loss in 10g of D-NCM111 at the stoichiometric ratio 2 Li[salt]: 1 Li loss[D-NCM111], After the hydrothermal reaction at 220 °C for 4 hours, the KOH solution was also collected to measure suspended and therefore dissolved Li in the solution at the elevated temperature and pressure. The cathode product was washed using the same protocol above, and both the solution and cathode material were measured to determine whether relithiation occurred. An Orion™ Star™ A214 pH / ISE Benchtop Meter was used to measure the pH of hydrothermal solutions before and after treatment and / or addition of various Li salts. Solubility measurements of the Li salts in various solution compositions of KOH were measured via ICP-MS (Thermo Scientific iCAP RQ model) after mixing excess of each salt in solutions for 24 hours followed by centrifugation.

[0046] The following non-limiting examples provide tests, results, and discussion to further illustrate the inventive approach:

[0047] Example 1: Decomposition of cathode electrolyte interphase (CEI)

[0048] To understand the context of lithium sources available after battery use and disassembly, it is necessary to study the decomposition of the cathode electrolyte interphase (CEI). The CEI is formed from impurities from exposure during the manufacturing and the breakdown of the electrolyte. Since it is the most widely used salt for Li transport in LIBs, hydrolysis of LiPF6 in various solvents is a topic of study, although the basic degradation is consistent30 32. Eqs. 1-2 show the initial decomposition of LiPF6 upon exposure to moisture.

[0049]

[0050] These highly reactive products can undergo further reactions to form various anions which join easily with Li+to form the CEI (Eq. 3).

[0051]

[0052] This evolution results in the products of LiF and LixPOyFz. These species also exist in the CBM in a smaller quantity on the CEI as the same breakdown of electrolyte occurs within batteries from impurities introduced in trace levels during manufacturing. Within the CEI, lithium alkyl carbonates, ROCO2Li, are also formed on the surface due to the electrolyte solvent decomposition, but with water contamination will further breakdown to Li2CC>3.

[0053] In addition to these CEI-derived products, other Li-containing salts are also found on the particle surface. Li2CC>3 and LiOH can form independently from air exposure during cathode pre-processing and storage, while electrolyte decomposition further contributes surface species such as LiF, Li2CC>3, and LiOH. The goal is to liberate the lithium present on the particle surface into the solution, making it available for relithiation within the hydrothermal solution, and subsequently reintegrating it into the transition metal lattice during the hydrothermal process.

[0054] Example 2: Relithiation Using Residual Li

[0055] Referring to FIG. 1, spent NCM 111 (S-NCM111) was recovered from 20 Ah prismatic cells retired from electric vehicles supplied by American HONDA Motor Company. After discharging, cells were manually disassembled to isolate cathode strips, cut and mechanically ground, and CBM was separated from the current collector via sieving. Unlike other preprocessing techniques which wash potential impurities and Li residue, this process bypassed an electrode washing step, allowing Li-containing species to remain throughout. Once dried, the Li salt residue is clearly visible on the electrode surface, as shown in FIG. 2A.

[0056] S-NCM111 was first studied to confirm the existence of surface Li species upon electrode deconstruction. Evidence of particle use is clear by particle cracking, clearly visible in the backscattered Scanning Electron Microscopy (SEM) image in FIG. 2B.

[0057] Energy -Dispersive X-ray Spectroscopy (EDS) data were collected for S-NCM111 sample to first scan for surface species. The presence of oxygen around the particle surface was further evaluated by X-ray photoelectron spectroscopy (XPS). A table of binding energies used for XPS peak assignment are shown in Table 1. FIG. 2C confirms the presence of likely surface lithium components including LiOH, Li2CC>3, and LixPOyFz. Since the particle surface is covered by these surface species, the signal from oxygen bonded to transition metals is small, indicated by the lattice oxygen binding energy. Further analysis confirmed presence of P and F which were subsequently measured with XPS. The FIs spectra in FIG. 2D confirm the presence of LiF and PVDF, as well as some contribution from the LiPF6electrolyte. Comparing the FIs to the P2p curve aided in deconvolving the form of the LiPF6. In FIG. 2E, a pronounced CEI is confirmed by the clear presence of LixPOyFz, while LiPF6is barely detectable as it is mostly present in its decomposed form. These findings establish that the S-NCM111 surface is coated with lithium salts and electrolyte-derived species, motivating their targeted removal and use for relithiation in subsequent processing.

[0058] Table 1

[0059] XPS Signal Binding Energy (eV) Assignment

[0060] P2p 134.1 LixPOyFz

[0061] P2p 137 LiPF6

[0062] Ols 529.2 Lattice-0

[0063] Ols 531 LiOH

[0064] Ols 531.6 Li2CO3

[0065] Ols 533.4 LixPOyFz

[0066] Ols 535.1 Adsorbed H2O

[0067] FIs 685.3 LiF

[0068] FIs 687.2 LixPOyFz

[0069] FIs 687.6 LiPF6

[0070] FIs 688.1 PVDF

[0071] FIs 690 _ PF5_

[0072] Inductively coupled plasma mass spectrometry (ICP-MS) was used in combination with XPS to differentiate between Li in surface species and bulk by comparing samples before and after washing with various solvents. Since ICP-MS is a bulk measurement, distinguishing between bulk Li and surface Li is not feasible for the spent CBM sample alone. Therefore, S-NCM111 was washed with ethanol, NMP, and an alkaline solution (0.05M LiOH in DI water, pH>12.5) and analyzed to quantify the remaining Li relative to the removal of the F- and P-containing compounds. This mildly alkaline solution contained a small amount of hydroxide, rather than pure water, to suppress protonation of the damaged S-CBM and to avoid overestimating lithium content due to excess Li removal from the CBM. Any lithium potentially adsorbed from within the solution is presumed negligible in subsequent measurements. The remaining Li of the bulk, represented by the molar ratio between Li and transition metals (Li / [TM]), and relative surface content of F and P determined by XPS are shown in in FIG.2F. The alkaline solution was deemed best at removing the surface Li. From these data, the bulk composition of S-NCM111 after removing impurities was measured as Lio.9Nio.35Coo.35Mno.35O2.

[0073] The hydrothermal regeneration process followed similar steps to those described by Y. Shi, et al. (“Resolving the Compositional and Structural Defects of Degraded LiNixCoyMnzO2 Particles to Directly Regenerate High-Performance Lithium-Ion Battery Cathodes”, ACS Energy Lett. 3 (2018) 1683-1692). In an exemplary implementation, spent material was treated in a constant-volume aqueous solution at elevated temperature and pressure, washed to remove solution compounds, and then annealed to resolve surface compounds. As is known to those of skill in the art, in a constant-volume (“isochoric”) process, the pressure is directly proportional to the temperature, i.e., the ratio of pressure to temperature is constant. Thus, different temperature settings will inherently result in different pressures. In other implementations, in lieu of using a batch-style reaction, the process may be carried out at predetermined temperatures and pressures. As discussed above, the specific materials being treated determine the hydrothermal processing conditions, with elevated temperatures ranging from about 120 °C to 220 °C, and the elevated pressure within a range of about 2 psi to 200 psi. Previous studies have demonstrated solution optimization for hydrothermal regeneration of cathode materials, mainly focusing on the impact of the solution concentration of hydroxides toward full regeneration. These studies emphasize that high concentration of hydroxide groups (pH > 14) prevent protonation and further delithiation. To ensure effective treatment, a hydrothermal solution of 4A / KOH was used, providing a high concentration of hydroxide ions. At this concentration, the ionic activity, though moderated by non-ideal behavior, is sufficient to maintain high pH, suppress protonation, and promote lithium reintegration.

[0074] Hydrothermal treatment of S-CBM in KOH solution (KOH-Tre) followed by washing restored bulk Li by ~5% without observable morphological damage or leaching. For comparison, results are benchmarked against the conventional 4 M LiOH process. To measure impurity removal, the thermogravimetric analysis (TGA) data and fluoride ion selective electrode (ISE) before and after hydrothermal treatment are shown in Error! Reference source not found.. Independent TGA testing confirmed that PVDF begins to decomposing at -350 °C, and the conductive carbon additive (e.g. ‘Super P’) typically combusts at 600 °C, consistent with literature. Weight loss curves for the spent hydrothermal treated samples are shown in FIG. 3A. Untreated S-NCM111 contains -3% conductive carbon and -6% PVDF impurities. After hydrothermal treatment in KOH solution and washing, -1% of impurities remain, mostly attributed to conductive carbon. This result is much better than the traditional hydrothermal solution, LiOH-Tre., in which some of the impurities remain (i.e., -6% of the total mass, details later confirmed by electrochemical measurements). The improvement of binder removal in the KOH solution suggests it is successful at depolymerization of PVDF. The removal of PVDF is essential, as residual PVDF can thermally decompose during annealing, leading to the formation of LiF, reducing electrochemical performance. Fluoride content in fully recovered samples after annealing (-FullRec) was compared to that of the spent material using ISE measurements, which quantify inorganic fluoride ions. To assess the contribution of PVDF decomposition to total fluoride content, S-NCM111 was pretreated at 500 °C for 4 hours to convert organic fluorine to an inorganic form, followed by acid digestion of all samples. As shown in FIG. 3B, fluoride content decreased from 23360 ppm in S-NCM111 to 1600 ppm in LiOH-FullRec and 320 ppm KOH-FullRec, confirming KOH more effectively facilitates PVDF removal.

[0075] Referring to FIG. 3C, the composition of surface species is shown after hydrothermal treatment using XPS. All results were normalized to their highest-intensity peaks. An increased signal corresponding to lattice oxygen was observed following treatment, indicating partial removal of surface species and improved exposure of the underlying crystal structure. In addition, after treatment the signal coming from LixPOyFzis relatively smaller than the other surface Li compounds, suggesting that it may have been used for relithiation or solvated in the hydrothermal solution. Encouragingly, XPS results of KOH and LiOH post-treatment are closely comparable. The Ols signal is also shown for the fully recovered sample, indicated as KOH-FullRec. After the annealing step, the remaining CEI composites appear to be further diminished and lattice oxygen is more apparent. Collectively, these results demonstrate that under identical pretreatment and washing conditions, hydrothermal treatment in KOH is superior in removing surface impurities, particularly PVDF-derived fluorine species, thereby uncovering the active cathode surface establishing favorable conditions for electrochemical recovery.

[0076] The structural recovery after hydrothermal regeneration was also analyzed. FIG. 4A provides XRD patterns for spent, pristine, and recovered CBM. The (003) peak shift indicates an increase in the c lattice parameter due to Li deficiency causing an increased repulsion between the oxygen layers. The spacing between the (108) / (l 10) doublet, and (006) / (012) doublet, increases after deintercalation of lithium ions due to the electrostatic repulsion within the transition metal layer as lithium is extracted, consistent with delithiation. Evidence of relithiation after treatment is clear, suggesting Li reinsertion and correctly coordinated TM layer, and is comparable to the LiOH sample. High-Resolution Transmission Electron Microscopy (HR-TEM) was used to assess the surface structure before and after KOH regeneration. FIGs. 4C shows that the regenerated material exhibits CEI removal and restored surface structure, respectively. Bas seen in FIG. 4B, before regeneration, the surface has visible deposited CEI on the surface with degraded rocksalt and spinel phases inside the CEI layer. However, after regeneration, the layered R3m structure is returned as is confirmed by the FFT. The interplanar distance demonstrates a spacing of 0.240 nm, matching with the (101) plane. To confirm the structure, the d-spacing corresponding to (104) was calculated as 0.201, consistent with expected electron diffraction for R3m structures.

[0077] Cathode half-cells were fabricated to evaluate electrochemical performance (FIGs.

[0078] 5A-5C). Before regeneration, the S-NCM111 sample exhibits a discharge capacity of 96 mAh / g after washing to remove impurities and residual Li. In addition, an observed overpotential is consistent with cation mixing and a delithiated bulk. Following regeneration, KOH-FullRec. sample exhibited an initial capacity of -157 mAh / g at C / 10 in Gen2 electrolyte, comparable to the LiOH-FullRec. sample (-156 mAh / g). Both values slightly exceed the P-NCM111 reference (-155 mAh / g), which can be attributed to a slightly higher Ni content in the spent Honda CBM compared to the control sample (stoichiometric Ni ratio of 0.35 in spent vs. 0.33 in the pristine Toda cathode). A pristine sample with identical stoichiometry as the Honda battery was not available for direct comparison.

[0079] During 100 cycles at a C / 3 rate, the S-NCM111 cell showed a slight performance improvement before stabilizing, likely due to partial bulk relithiation facilitated by lithium supplied from the Li-metal counter electrode. However, the discharge capacity remains much lower than that of the pristine sample. Both the P-NCM111 and KOH-FullRec. cells exhibit excellent capacity retention of 96% after 100 cycles. The sample relithiated with LiOH has a lower capacity retention rate - 89% - potentially due to incomplete impurity removal after the hydrothermal treatment on CBM without purification. Residual LiF generally contributes only to initial capacity loss as it is electrochemically and chemically inactive during cycling, but residual Li₂CO₃ has been demonstrated to chemically decompose when interacting with electrolyte, causing a delayed capacity fade. The limited washing applied in both KOH and LiOH treatments may have left remaining Li surface species that transformed into Li2CO3 during subsequent annealing and storage. This interpretation is consistent with TGA results (FIG. 3A) as Li₂CO₃ exhibits onset of decomposition at 500 °C and is fully decomposed by -700 °C. These results highlight the advantage of the KOH process with minimal washing, where the use of KOH ensures more complete removal of surface species from the hydrothermal solution, preserving long-term electrochemical performance.

[0080] Referring to FIG. 5C, the rate performance of the regenerated KOH-FullRec. was evaluated and compared to the pristine reference. After formation cycles at C / 10, increasing the rate to C / 3 resulted in a modest capacity drop of 5% for both samples. Subsequent cycling at 1C and 2C lead to capacity retention reaching 93% and 87% for both samples, relative to C / 3. At 5C, KOH-FullRec. n P-NCMlll had a marginal difference in capacity corresponding to 74% and 73% compared to C / 3, respectively. Upon returning to C / 3, capacity recovery reached 99% for KOH-FullRec. and 98% for the pristine electrode, indicating good structural stability and rate reversibility.

[0081] While the successful structural and electrochemical repair with minimal preprocessing and effective impurity removal is inherently beneficial, a quantitative evaluation of the environmental and economic impacts of avoiding additional LiOH from the direct regeneration process was performed using the EverBatt model developed by Argonne National Laboratory. An analysis was performed estimating the annual processing of 10,000 tons of spent LiBs in the United States. The direct recycling scenario followed constraints consistent with previous studies, however in contrast to prior work, this analysis considered the implications of substituting LiOH with KOH in the hydrothermal relithiation step. Although KOH has been proposed as a substitutional hydroxide to maintain 4M hydroxide concentration during hydrothermal treatment, its environmental and economic impact on the hydrothermal process has not yet been reported. KOH is not included as a default material in EverBatt or GREET and therefore was manually added for consideration.

[0082] Because impurities are expected to accumulate in the hydrothermal solution over time, a conservative replacement frequency of once every 10 treatment cycles was assumed. Comparisons of the direct recycling process using LiOH versus KOH are presented in FIG. 5D. A base case solid-to-solution mass ratio is assumed to be 30%, with error bars reflecting the variation model outputs when the solid-to-solution mass ratio is adjusted between 15% and 45%. Replacing the hydrothermal solution with KOH led to a reduction of over 20% in the cost per kilogram of cathode material produced. Notably, this cost improvement is relatively insensitive to the solid-to-solution mass ratio, as the hydrothermal step is no longer the primary cost driver. The substitution also results in a -11% reduction in GHG emissions per kg cathode produced due to the lower manufacturing burden and supply chain complexity of KOH synthesis. Direct recycling with KOH solution according to the inventive approach is projected to reduce cost by 72% compared to virgin manufacture, and 29% of hydrometallurgical recycling, making this process promising for scaling.

[0083] Example 3: Mechanistic Study

[0084] Cumulatively, the above results show clear evidence of impurity removal and Li recovery of a hydrothermal regeneration process without adding external Li source during the hydrothermal treatment step. Further evaluation was conducted to determine how relithiation is occurring from the existing residual Li content. To replicate the process by isolating each Li salt, chemically delithiated NCM 111 (D-NCM111) was separately combined with Li₂CO₃, LiF, and LiPF6and added to 4 M, 2 M, and 0.1 M KOH solution for relithiation. To compensate for different molecular weights, the added moles of Li were kept constant at 0.2 moles of Li, which is twice the amount of Li required for full relithiation. Since the D-NCM111 contains no impurities, the specific impact of each Li salt is isolated. LiOH is well-recognized as a Li source for hydrothermal relithiation from prior studies, so it is not repeated here. Due to the decomposition of LiPF6in water mimicking the LixPOyFzfound on the CEI, it was used as a Li salt. After the hydrothermal step was complete, the black mass was separated from the hydrothermal solution and washed to completely remove any remaining salts. FIG.6A shows the XRD patterns of the D-NCM111 material before and after 4 M KOH hydrothermal treatment for each of the Li salts added. For the samples with Li₂CO₃ and LiF additives, the diffraction patterns appear to return to pristine, observed most closely with the (003) peak shift and space between the (108) and (110) doublets, as mentioned above. As seen in FIG. 6A, the LiPF6treated sample appears to contain additional phases. Detection of KPO3 is consistent with PF6⁻ hydrolysis to phosphate under hydrothermal conditions. As discussed in Eqs. 1-3, anion hydrolysis consumes water and forms various neutral and anionic oxyfluorophosphates (POxFy) which can further hydrolyze to form of phosphoric acid (Eq. 4) below.

[0085] POF3+ 3H20 - H3P04+ 3HF (4)

[0086] In a concentrated base, the HF is neutralized and the POF3 undergoes hydrolysis with KOH forming K3PO4 (Eq. 5).

[0087] POF3+ 6K0H -► K3PO4+ 3KF + 3H2O (5)

[0088] In the presence of phosphoric acid generated within this process, K3PO4 may partially protonate (Eq. 6), which dehydrates upon drying hydrothermal-treated CBM, resulting in KPO3 (Eq. 7).

[0089] K3PO4+ 2H3PO4- 3KH2P04(6)

[0090] KH2P04-► KPO3+ H2O (7)

[0091] Since the hydrolysis of LiPF6scavenges the cathode surface to promote transition metal redistribution, mixed Ni / Co intermediates may oxidize to form the spinel phases of NiCo2O4. Despite these side reactions, diffraction peak shifts indicate bulk regeneration, thus decomposition is likely localized and does not dominate relithiation. XRD patterns for other KOH solution concentrations are shown in FIG. 6B, demonstrating how a less concentrated base results in further cathode decomposition via LiPF6hydrolysis.

[0092] After the hydrothermal treatment the supernatant for each sample was collected and the contents were measured via ICP-MS. The concentrations of remaining Li in the KOH solution for each added salt are shown in FIG. 6C. These concentrations can be compared to the supernatant of the S-NCM111 sample, indicated in the right-most bar. Comparing the contribution of solvated Li from the salts to the spent material, it is reasonable to assume the Li within the S-NCM111 solution is due to the solvated residual Li salts. Of the moles of Li ions added, 1%, 3%, and 6% remain in the solution for LiPF6, LiF, and Li2CO3, respectively. Assuming a density of 1.174 g / mL for 4 M KOH, the concentration of LiPF6, LiF, and Li2CO3 within solution after the hydrothermal treatment is calculated to be 0.039%, 0.022%, and 0.12%, respectively. The bulk Li / [TM] ratio within the CBM before and after washing is shown in FIG. 6D. Based in the ICP-MS content, 73% of the moles of Li added via LiPF6reintercalated in the NCM material after washing, whereas for LiF and Li2CC>3, 24% was consumed. These data show that despite the relatively small amount of solvated Li remained in the solution, the substantial amount of the LiPF6(or LixPOyFz) was used in the relithiation process, consistent with the takeaway from the CBM study. The Li from the salt is not accounted for may be loosely adsorbed to the cathode particles during the initial solution-liquid separation but then removed after subsequent water washing. The data suggests that as lithium is removed from the solution, the solubility equilibrium shifts, enabling further relithiation. To isolate the behavior of each lithium salt and better understand its role in the relithiation process, pH and stoichiometric concentration of Li measurements were taken before and after hydrothermal treatment of D-NCM111 across varying KOH concentrations.

[0093] Table 2

[0094] Li / [TM]

[0095] after Hydrothermal Treatment Li / [TM]

[0096] Before

[0097] Hydrothermal

[0098] Sample Treatment + LiF + LiPF6+ Li₂CO₃ 0.1 M KOH ±0.02 M 0.90 0.88 0.81 0.84 2 M KOH ±0.07 M 0.90 0.97 0.89 0.99 4 M KOH ±0.13 M 0.90 1.05 0.95 0.97

[0099] The relithiation effectiveness of each condition is evaluated by bulk Li as measured by ICP-MS. Table 2 above lists the stoichiometric Li amount of delithiated NCM 111 after hydrothermal treatment with each Li salt. At low KOH concentrations (i.e. 0.1 M KOH) it is possible that protonation competes with relithiation, and therefore bulk relithiation does not occur. Protonation changes XRD peaks similarly to relithiation, and therefore inspecting the XRD patterns in FIG. 6B, doublet expansion and (003) peak shifting do not worsen after the hydrothermal step. Contrasting with the 0.1 M KOH solution, relithiation does occur in 2 M KOH solution with LiF and Li2CO3salts, suggesting that relithiation overwhelms protonation as there are fewer protons available, and there is some mechanism that frees the Li from the Li salts. For 4 M KOH solution, all solutions demonstrate an increase in bulk lithium, suggesting that Li ions have the capability to be freed from these surface salts to perform relithiation.

[0100] A closer look at the pH change in each step of the process can aid in determining the chemical process of relithiation. The pH of the solutions exhibiting relithiation (2 M and 4 M KOH) was measured at three stages: before additives were introduced, after 30 minutes of mixing following the addition of the salt, and after the hydrothermal treatment at 220°C for 4 hours, using the supernatant of the solution. These measurements are shown in Table, which lists pH of KOH solution after Li salts were added during the hydrothermal treatment process using delithiated NCM 111.

[0101] Table 3

[0102] pH pH pH after Li Salt addition after Hydrothermal without Additives Li Salt Added + 30 min mix Treatment ± LiF 14.29 14.16 2 M KOH

[0103] ±0.07 AT 14.32 + LiPF613.95 7.34

[0104] ± Li2CO313.92 13.42 4 M KOH + LiF 14.67 14.60 ±0.13 14.67 ±LiPF614.35 14.32

[0105] ± Li2CO314.20 14.21

[0106] The pH of each KOH solution without additives is in good agreement with theoretical expectations (14.60 for 4 M and 14.30 for 2 M). Adding LiF to either concentration results in negligible pH changes, indicating that LiF dissociates without consuming hydroxide ions. However, after hydrothermal treatment, the pH changes measurably, suggesting the involvement of OH ions for relithiation. LiF may contribute Li ions via sink-driven dissolution, by which continuous removal or consumption of dissolved species maintains solution under saturation and therefore promotes additional dissolution. As Li ions are consumed by relithiation, the solubility equilibrium allows for more LiF into solution without materially altering OH.

[0107] For Li₂CO₃, there is a change in the solution pH once added to the solution suggesting the hydroxides participate in dissolving the salt. In the 4 M KOH solution, negligible pH change suggests that the carbonate does not decompose, and Li ion dissolution occurs via ion-exchange (Eq. 8) Li2CO3+ 2KOH K2CO3+ 2LiOH (8)

[0108] Elevated temperature accelerates the ion-exchange of Li₂CO₃, effectively improving Li availability. For the 2 M condition, a small but measurable pH change indicates some consumption of OH' perhaps in addition to ion-exchange described above. Such a pH change is consistent with measured reactions of CO2 uptake and the carbonatebicarbonate equilibria in alkaline solutions. Still, moderate relithiation for 4 M and 2 M treatments suggests that as Li is consumed, more Li may be dissolved (sink-driven dissolution) and intercalated.

[0109] The most complicated mechanism appears to be that of LiPF6, which typically hydrolyzes quickly in organic solutions with trace water, but slower in aqueous solutions. Spotte-Smith et al. (“Elementary Decomposition Mechanisms of Lithium Hexafluorophosphate in Battery Electrolytes and Interphases”, ACS Energy Lett. 8 (2023) 347-355.) confirm using first-principles analysis that hydrolysis will not happen at room temperature in water, but may become viable with thermal activation, consistent with the observation of the largest pH change after the hydrothermal step. As previously discussed, hydrolysis of POxFyconverts LixPOyFxto phosphate, leaving displaced Li ions within solution as Li OH. At high concentrations of KOH, decomposition products are neutralized and likely do not have a large effect on relithiation. As Li is consumed and more LiPF6may dissolve, the acid products are rapidly neutralized and deprotonated, so the bulk pH remains measurably unchanged. However, at 2 M KOH, the smaller OH' reservoir allows thermally activated LiPF6and LixPOyFxhydrolysis to outpace neutralization, and decomposition of LiPF6overwhelms the solution pH. This pushes the solution toward more acidic phosphate products and produces fluorinated byproducts, with concurrent HF release. These data suggest that unique mechanisms occur to relithiate CBM for each salt, and the most reliable KOH concentration to mobilize Li for use during relithiation and prevent counterproductive decomposition is 4 M.

[0110] To illustrate the impact of KOH concentration on solubility, FIG. 7 A compares the lithium concentration at varying KOH levels: 0.1 M, 2 M, and 4 M. Consistent with earlier discussions, LiF does not exhibit a significant increase in solubility after hydrothermal treatment, indicating that additional LiF dissolves as lithium is consumed during relithiation. The dissolution of Li₂CO₃ (square) increases with higher KOH concentrations, consistent with the Li mobility mechanism described above. In contrast, the solubility of LiPF6(circle) decreases sharply with increasing KOH concentration, suggesting that its insolubility arises from reduced proton availability, which is necessary for LiPF6breakdown. Nevertheless, at elevated temperatures and lower KOH concentrations, this relationship changes, as indicated in Table 4, which lists the pH of KOH solution of various concentrations before and after hydrothermal treatment with spent NCM 111 cathode material. The 4 M KOH solution optimizes relithiation by utilizing surface lithium species while minimizing degradation caused by Li salt dissociation.

[0111] The effectiveness of relithiation suggests that despite the low solubility at 4 M KOH concentration, more Li may be released once consumed. Still, the relithiation process remains effective, as lithium continues to be released upon consumption. The observed relithiation from sparingly soluble Li salts is consistent with sink-driven dissolution behavior governed by Le Chatelier’s Principle. As Li is consumed from the solution by intercalation into the delithiated NCM lattice, the local depletion of Li shifts the solubility equilibrium, enabling continued dissolution of the solid Li salts. This dynamic feedback mechanism facilitates sustained relithiation despite low Li concentrations in the bulk solution.

[0112] Not only does 4 M KOH concentration improve relithiation, but also impurity removal. Other concentrations (0.1 M and 2 M) of KOH in DI water were also tested to regenerate S-NCM111. The TGA data shown in FIG. 7B reveals incomplete removal of CBM impurities (i.e., PVDF and Super P conductive carbon) during hydrothermal treatment. This is consistent with discoloration of supernatant after hydrothermal treatment due to PVDF decomposition. A recent publication by P. Xu, et al. (Proton-exchange induced reactivity in layered oxides for lithium-ion batteries, Nat. Commun. 15 (2024) 9842), demonstrated the strong relationship between pH and relithiation as a high pH is necessary to prevent protonation and hydroxide anions are consumed in the redox relithiation equation. The decrease in pH at lower concentrations after the hydrothermal step, shown in Error! Reference source not found., suggests that there is some reduction in total OH' after relithiation, and therefore it is involved in the relithiation even when KOH is used to increase the pH instead of LiOH. There is no bulk change in pH after hydrothermal treatment in the 4 M KOH solution as the amount of hydroxide consumed does not produce any measurable change in pH. Table 4

[0113] pH without additives pH after

[0114] Solution Composition Hydrothermal Treatment 0.1 M KOH + DI 13.00 10.17 Water

[0115] 2 M KOH + DI Water 14.04 13.98

[0116] 4 M KOH + DI Water 14.31 14.32

[0117] FIG. 7C diagrammatically illustrates the full process by which lithium is liberated and made available for relithiation. Essentially, the spent cathode material is subjected to hydrothermal regenerating at elevated temperature and pressure in a solution the includes KOH. Overall, these results indicate that relithiation from sparingly soluble lithium salts is enabled through sink-driven dissolution, where Li consumption by the delithiated lattice continuously shifts the local solubility. The role of hydroxide concentration is central. At 0.1 M KOH, protonation dominates preventing bulk Li reinsertion, while at 2 M KOH, thermally activated LiPF6 and LixPOyFx are not sufficiently neutralized. At 4 M KOH, the large OH- reservoir neutralizes acidic byproducts, prevents protonation, and supports continuous Li dissolution. In view of the foregoing results, the minimum concentration for effective relithiation using the inventive scheme appears to be about 2 M KOH and that higher concentrations can be employed. While no upper limit was established during testing, the success obtained with 4 M KOH suggests that concentrations exceeding this level would likely be a waste of materials, with any excess KOH needing to be washed away prior to annealing.

[0118] Example 4: Demonstration on Alternative Cathode Materials

[0119] The principles of hydrothermal reactions and prior demonstrations of hydrothermal relithiation on various transition metal oxide materials support the feasibility of using residual lithium in 4 M KOH for relithiating other cathode materials. To test this, the method was applied to LiCoO2(LCO), LiNio.6Coo.2Mno.2O2 (NCM622), and LiNi0.87Co0.05Mn0.08Al0.03O2 (NCMA), with results summarized in FIGs. 8A-8C, respectively. All samples were prepared using the same procedure used for NCM 111. Given the variability in high-Ni layered cathodes in morphology and composition, which can influence electrochemical performance, regeneration was performed on spent cathodes from cycled, filled cells, and a mimicked "scrap" batch derived from unfilled electrodes. The scrap sample (Scrap-NCM622-FullRec2) underwent the same hydrothermal treatment and was reassembled into batteries for direct comparison with spent-derived samples, which experienced bulk lithium loss through cycling.

[0120] In Figs. 8A-8C, the upper left panels are backscattering SEM images from spent materials of LCO, NCMA, and NCM622, respectively. The upper right panel in each figure shows backscattering SEM images following regeneration using the inventive process for LCO, NCMA, andNCM622. Backscattered SEM imaging of all cathodes before and after hydrothermal treatment confirmed effective impurity removal, with darker PVDF and lighter conductive carbon residues no longer visible. Importantly, no particle cracking was observed in the recovered samples. The plots in the lower panel of each figure show initial discharge capacities of LCO, NCMA, and NCM622, respectively. The morphological retention and full stoichiometric recovery for each cathode material resulted in discharge capacities near or within pristine expectations, as indicated by the vertical dashed lines in each plot. For LCO and NCMA, pristine reference materials were unavailable, so vertical lines indicate expected initial discharge capacities based on values from literature. Posttreatment performance for these materials fell within these expected ranges, suggesting reasonable recovery. For the NCM622 sample, despite aging to 90% of their original discharge capacity, both Scrap-NCM622-FullRec. and NCM622-FullRec. showed similar electrochemical performance consistent with pristine NCM622 expectations, indicating that prior aging did not impair discharge capacity. Cracking observed in the spent NCM622 confirmed cycling-induced damage and lithium depletion compared to the scrap sample. ICP-MS results before and after hydrothermal treatment, provided in Tables 5-7 below, for NCM622, LCO, and NCMA, respectively, confirm successful relithiation. TGA data (not shown) further validated the impurity removal.

[0121] Table 5

[0122] Sample Ni Mn Co Li / [TM] Scrap-NCM622 0.60 0.20 0.20 1.01 Scrap-NCM622-FullRec 0.60 0.20 0.20 1.04 S-NCM622 0.60 0.20 0.20 0.97 NCM622-FullRec 0.60 0.20 0.20 1.00 Table 6

[0123] Sample Co Li / [TM]

[0124] S-LCO 1.00 0.95

[0125] LCO-FullRec 1.00 1.00

[0126] Table 7

[0127] Sample Ni Mn Co Al Li / [TM] S-NCMA 0.87 0.08 0.05 0.03 0.97 NCMA-FullRec 0.87 0.08 0.05 0.03 1.01

[0128] The inventive approach provides a modified process for hydrothermal relithiation of NCM cathodes, which process utilizes remaining lithium on the surface of the cathode materials post-disassembly as a Li source, without the need for excess additive Li to drive relithiation. Testing demonstrates that successful relithiation is achieved using a highly alkaline KOH aqueous solution of 2 M KOH or higher concentration. The KOH solution was more effective in removing conductive carbon and binder impurities compared to traditional LiOH treatments, leading to improved cathode performance. Experiments with delithiated NCM (D-NCM) confirmed that relithiation could occur using surface lithium compounds present after battery discharge and disassembly. The inventive approach proved effective in restoring the structural integrity and electrochemical performance of the spent NCM. This method was further validated in other cathode materials: LCO, NCMA, and NCM622.

[0129] The process and results disclosed herein demonstrate hydrothermal relithiation with existing surface lithium for efficient and cost-effective recycling of NCM cathodes. This approach not only minimizes the need for additional lithium salts but also addresses the critical challenge of impurity removal, contributing to more sustainable battery recycling practices. Not typically limited by bulk Li loss, this process is particularly attractive for scrap CBM. For spent battery materials, the presence of the CEI is essential, which advocates for the complete discharge of cells prior to disassembly. This ensures a maximum amount of lithium returns to the cathode, either within the bulk crystal or as a part of the CEI. The scalability and economic viability of the inventive approach underscore its potential for broader application in the LIB recycling industry. By optimizing the hydrothermal conditions and further understanding the role of surface lithium, this novel technique could be pivotal in addressing the growing challenge of LIB waste management.

Claims

WHAT IS CLAIMED IS:

1. A method for relithiating a spent Li-ion cell, comprising:discharging the cell;disassembling the cell to separate a cathode material;hydrothermally treating the separated cathode material in a KOH-based aqueous solution for a predetermined time to provide a regenerated cathode material; and annealing the regenerated cathode material;wherein residual lithium on surfaces of the separated cathode material provide sufficient lithium for relithiation.

2. The method of claim 1, wherein the KOH-based aqueous solution has a concentration of 2 M KOH or higher.

3. The method of claim 1, wherein the KOH-based aqueous solution has a concentration of 4 M KOH.

4. The method of claim 1, further comprising, after hydrothermally treating and prior to annealing:separating the regenerated cathode material from the solution;washing the regenerated cathode material; anddrying the regenerated cathode material to provide a cathode precipitate.

5. The method of claim 4, wherein, prior to annealing, a lithium salt is added to the cathode precipitate in an amount sufficient to compensate for lithium loss during annealing.

6. The method of claim 5, wherein the lithium salt is selected from Li₂CO₃, LiCO3, LiOH, and Li2O2.

7. The method of claim 1, wherein hydrothermally treating comprises adding the separated cathode material to a reactor at an elevated temperature and pressure, wherein the elevated temperature is within a range of 120 °C to 220 °C and the elevated pressure is within a range of 2 psi to 200 psi.

8. The method of claim 1, wherein the cathode material is selected from LiNio.33Coo.33Mno.33O2 (NCM 111), LiCoO2(LCO), LiNio.6Coo.2Mno.2O2 (NCM622), and LiNi0.87Co0.05Mn0.08Al0.03O2 (NCMA).

9. The method of claim 1, wherein the cathode material is LiNio.33Coo.33Mno.33O2 (NCM 111) or LiCoO2(LCO) and hydrothermally treating comprises heating the separated cathode material and the aqueous solution to 220 °C for 4 hours.

10. The method of claim 1, wherein the cathode material is LiNio.6Coo.2Mno.2O2 (NCM622) and hydrothermally treating comprises heating the separated cathode material and the aqueous solution to 160 °C for 4 hours.

11. The method of claim 1, wherein the cathode material is LiNi0.87Co0.05Mn0.08Al0.03O2 (NCMA) and hydrothermally treating comprises heating the separated cathode material and the aqueous solution to 120 °C for 6 hours.

12. A method for relithiating cathode material from a spent Li-ion cell, comprising:hydrothermally treating a separated cathode material from the spent Li-ion cell in a KOH-based aqueous solution for a predetermined time to provide a regenerated cathode material;separating the regenerated cathode material from the solution;washing the regenerated cathode material;drying the regenerated cathode material to provide a cathode precipitate; and annealing the regenerated cathode material;wherein residual lithium on surfaces of the separated cathode material provide sufficient lithium for relithiation.

13. The method of claim 12, wherein the KOH-based aqueous solution has a concentration of 2 M KOH or higher.

14. The method of claim 12, wherein the KOH-based aqueous solution has a concentration of 4 M KOH.

15. The method of claim 12, wherein, prior to annealing, a lithium salt is added to the cathode precipitate in an amount sufficient to compensate for lithium loss during annealing.

16. The method of claim 15, wherein the lithium salt is selected from Li2CO3, LiCO3, LiOH, and Li2O2.

17. The method of claim 12, wherein hydrothermally treating comprises adding the separated cathode material to a reactor at an elevated temperature and pressure,wherein the elevated temperature is within a range of 120 °C to 220 °C and the elevated pressure is within a range of 2 psi to 200 psi.

18. The method of claim 12, wherein the cathode material is selected from LiNio.33Coo.33Mno.33O2 (NCM 111), LiCoO2(LCO), LiNio.6Coo.2Mno.2O2 (NCM622), and LiNi0.87Co0.05Mn0.08Al0.03O2 (NCMA).

19. The method of claim 12, wherein the cathode material is LiNio.33Coo.33Mno.33O2 (NCM 111) or LiCoO2(LCO) and hydrothermally treating comprises heating the separated cathode material and the aqueous solution to 220 °C for 4 hours.

20. The method of claim 12, wherein the cathode material is LiNio.6Coo.2Mno.2O2 (NCM622) and hydrothermally treating comprises heating the separated cathode material and the aqueous solution to 160 °C for 4 hours.

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