Reuse and regeneration of lithium-ion battery cathodes

Abstract

A method for regenerating spent cathode material in lithium-ion batteries involves lithiating the cathode material in a relithiation solution containing a reducing agent at a temperature ranging from 60°C to 180°C for a time sufficient to repair compositional defects in the cathode material. The lithiated material is then sintered until properties are fully restored. The relithiation solution can be a source of Li ions combined with a naturally occurring organic reducing agent such as citric acid, ascorbic acid, tartaric acid, or the like.

Description

[Technical Field]

[0001] Related Applications This application claims the benefit of priority to U.S. Provisional Patent Application No. 63 / 090,136, filed October 9, 2020, the entire contents of which are incorporated herein by reference.

[0002] Government Rights This invention was made with government support under Grant No. CBET-1805570 awarded by the National Science Foundation. The government has certain rights in this invention.

[0003] The present invention relates to a method for directly regenerating spent lithium-ion batteries. [Background technology]

[0004] Olivine-type lithium iron phosphate (LiFePO4 or LFP) is one of the most widely used cathode materials for lithium-ion batteries (LIBs) due to its high thermal stability, long cycle life, and low cost. Due to these advantages, LFP batteries have accounted for more than one-third of the total LIB market and currently dominate applications in power tools, electric buses, and grid energy storage. Global demand for LIBs is projected to reach 440 GWh by 2025, meaning millions of tons of used LIBs will soon be produced after the end of their useful life (3–10 years). Effective reuse and remanufacturing of used LIBs can help reclaim valuable materials, reduce the energy used to extract natural resources, and mitigate environmental pollution from end-of-life disposal of discarded batteries, making LIBs more affordable and sustainable.

[0005] Current efforts to recycle LIBs have focused on recovering valuable materials. For example, pyrometallurgical and hydrometallurgical processes have been commercially used to recycle LIBs containing cobalt (Co) and nickel (Ni). These processes generally involve dismantling the battery, smelting, and / or acid leaching, followed by multi-step chemical precipitation and separation, ultimately decomposing the LIB cells into simple compounds (e.g., CoSO4, NiSO4, Li2CO3) that can be used to resynthesize new cathode materials. Due to the high value of transition metals (e.g., approximately $30 / kg for Co), such recycling processes can achieve reasonable economic returns despite high operating costs. Unfortunately, their high energy demands and reliance on corrosive chemicals (acids, oxidizers) generate significant greenhouse gas (GHG) emissions and secondary waste, thereby raising further environmental concerns—a common criticism from those opposed to the transition to electric energy sources. Furthermore, much of the cathode's value, represented by its tailored composition and structure, is completely lost in these destructive recycling processes. Therefore, more efficient approaches with significantly reduced energy costs and waste generation are needed, especially for LIBs made without expensive metals, such as LFP, because the economic value of these recycled base products is not sufficient to offset the high costs of the pyrometallurgical and hydrometallurgical processing processes. This is especially true when considering that global battery manufacturers have collectively produced approximately 100,000 tons / year of LFP cathodes since 2015. The large number of these batteries nearing the end of their lifespan increases the urgency for battery recycling solutions.

[0006] Decades of research have shown that the degradation of LFP cathode performance is primarily due to Li vacancy defects (Li v ) and Fe occupancy at Li sites (Fe Li ) has become clear that the cause is Li v The defect is Fe 2+ Fe 3+Not only does it cause oxidation to Fe 2+ It also induces partial migration of Li to the lithium site. + "This degradation mechanism may offer an opportunity to directly recover degraded LFP particles and form new LFP particles that can be immediately used to fabricate new battery cells," said Dr.

[0007] The reuse of spent lithium-ion batteries (LIBs) is urgently needed to address its environmental and global sustainability challenges. The method of the present invention relates to a solution. Summary of the Invention

[0008] The present invention describes a method for the direct regeneration of spent LiFePO4 cathode material in lithium-ion batteries by low-temperature solution lithiation followed by short-time sintering. This relatively low-energy and mild chemical process allows for profitable processing even for recycling LiFePO4 that does not contain valuable elements (Ni or Co). Greenhouse gas emissions have been shown to be very low. This economical and environmentally friendly recycling method shows great potential for industrial applications.

[0009] Our method, an efficient and environmentally friendly LIB regeneration method based on defect-targeted repair, represents a paradigm shift in LIB reuse strategies. Specifically, by combining low-temperature aqueous relithiation and rapid post-sintering, we demonstrate successful direct regeneration of spent LiFePO4 (LFP) cathodes, one of the most important materials for EV and grid storage applications. The composition, structure, and electrochemical performance of LFP cathodes can be restored to the same level as virgin LFP, even in a wide range of degradation states. Lifecycle analysis shows that this defect-targeted direct regeneration approach can significantly reduce energy use and greenhouse gas (GHG) emissions, leading to significant economic and environmental benefits over current hydrometallurgical and pyrometallurgical processing methods.

[0010] In one aspect of the present invention, a method for regenerating spent cathode material in a lithium-ion battery includes lithiating the cathode material in a relithiation solution containing at least one reducing agent at a temperature ranging from 60°C to 180°C for a time sufficient to repair compositional defects in the cathode material, and sintering the lithiated material. The relithiation solution may include a lithium salt and at least one reducing agent, and the at least one reducing agent may be one or a combination of naturally occurring organic reducing agents. The naturally occurring organic reducing agent may be selected from the group consisting of citric acid, ascorbic acid, tartaric acid, oxalic acid, sugar, or combinations thereof. In some embodiments, the relithiation solution may be a mixture of 0.01-4 M LiOH solution and 0.01-2 M citric acid. In some embodiments, the lithium salt is selected from the group consisting of LiOH, LiSO, LiCl, LiCHO, and LiNO. The cathode material may be LiFePO. Prior to the relithiation step, the cathode material can be obtained by disassembling the lithium-ion battery to remove the cathode strip, placing the cathode strip in a solvent to separate the lithium-containing powder from other components in the cathode strip, and washing and drying the separated lithium-containing powder. In some embodiments, the sufficient time is in the range of 1 hour to 18 hours. The temperature can be in the range of 60 to 120°C, and the sufficient time can be at least 5 hours. The sintering step can be carried out in a furnace under an inert atmosphere at a sintering temperature in the range of 400°C to 800°C for a sintering time in the range of 50 to 300 minutes. The sintering time can include a temperature ramp to gradually heat the lithiated material at a controlled rate. The relithiation solution can be recycled and reused for subsequent relithiation processes.

[0011] In another aspect of the present invention, a method for regenerating LiFePO4 cathode material from a used lithium-ion battery includes disassembling the lithium-ion battery to remove the cathode strip, immersing the cathode strip in a solvent to separate the lithium-containing powder from other components in the cathode strip, washing and drying the separated lithium-containing powder, placing the lithium-containing powder in a container containing a relithiation solution including a reducing agent, heating the container and solution to a temperature in the range of 60°C to 180°C for a time sufficient to repair compositional defects in the cathode material, and sintering the lithiated material at a sintering temperature in an inert atmosphere. The relithiation solution may include a lithium salt and at least one reducing agent, which may be one or a combination of naturally occurring organic reducing agents. The naturally occurring organic reducing agent may be selected from the group consisting of citric acid, ascorbic acid, tartaric acid, oxalic acid, sugar, or a combination thereof. In some embodiments, the relithiation solution may be a mixture of 0.01-4 M LiOH solution and 0.01-2 M citric acid. In some embodiments, the lithium salt is selected from the group consisting of LiOH, Li2SO4, LiCl, LiC2H3O, and LiNO3.

[0012] The sufficient time can be in the range of 1 hour to 18 hours. In some embodiments, the temperature can be in the range of 60 to 120°C, and the sufficient time is at least 5 hours. The sintering temperature can be in the range of 400 to 800°C, and sintering occurs for a sintering time in the range of 50 to 300 minutes. The sintering time can include a temperature ramp to gradually heat the lithiated material at a controlled rate. The relithiation solution can be recycled and reused for subsequent relithiation processes.

[0013] The method of the present invention does not alter any other properties of the LFP particles. vWe employ a green and efficient direct LIB recycling strategy based on defect-targeted repair to precisely eliminate site and antisite defects. We demonstrate successful direct regeneration of used LFP cathodes in various degradation states, restoring their composition, structure, and electrochemical performance to the same levels as virgin LFP cathodes. Unlike recycling via pyrometallurgical and hydrometallurgical treatments, such a defect-targeted direct recycling process requires only low-concentration lithium salts, a green and low-cost reducing agent, nitrogen, and water. With appropriate modifications, this method can also be extended to recycling other "low-cost" LIB cathodes, such as LiMn2O4 (LMO) batteries. A lifecycle analysis of LFP direct recycling demonstrates that our approach can significantly reduce energy use (approximately 80-90%) and GHG emissions (approximately 75%), leading to greater economic and environmental benefits than current approaches. [Brief explanation of the drawings]

[0014] [Figure 1] 1A-1E show the relithiation kinetics of C-LFP according to the method of the present invention: FIG. 1A is a schematic diagram of the solution relithiation process; FIG. 1B is a plot of the LFP composition evolution during relithiation at different temperatures; FIG. 1C is a plot of the saturated vapor pressure of water at various temperatures with a selection of apparatus (schematic diagram) for high pressure (>1 bar) and low pressure (<1 bar); FIG. 1D shows the XRD patterns of C-LFP and R-LFP with different relithiation times; and FIG. 1E shows the temperature dependence of the Li apparent diffusion coefficient and the required diffusion time. [Figure 2]Figures 2A-2H show microstructural characterization of different LFP particles according to an embodiment of the method of the present invention, where Figures 2A and 2E are STEM images of C-LFP and R-LFP, respectively, Figures 2B and 2F are STEM images of C-LFP and R-LFP particles, respectively, Figures 2C and 2G are Fe L-edge EELS spectra of C-LFP and R-LFP particles, respectively, and Figures 2D and 2H show Rietveld refinement patterns of neutron diffraction data of C-LFP and R-LFP, respectively. [Figure 3] 3A-3E show various aspects of the electrochemical performance of LFP electrodes. FIG. 3A is a contour plot of the time dependence of diffraction peak intensity during heating, holding, and cooling stages. FIG. 3B shows the revolution of antisite defects during heating, holding, and cooling. FIG. 3C is a plot of the cycling stability of C-LFP, R-LFP, RS-LFP, and P-LFP. FIG. 3D is a comparison of the rate performance of C-LFP, R-LFP, RS-LFP, and P-LFP. FIG. 3E is a plot of the long-term cycling stability of RS-LFP cycled at 2 C, 5 C, and 10 C for 300 cycles. [Figure 4] Figures 4A-4C show the XRD patterns and cycling stability of RS-LFP regenerated from C-LFP using different SOHs. 15%, 50%, and 60% depleted C-LFP were regenerated using the same process. The XRD patterns in Figure 4A show that a pure LFP phase was obtained in all samples after solution relithiation and sintering. As shown in Figures 4B and 4C, the capacity and stability of 15% and 60% depleted LFP can be restored to the same levels as P-LFP. This process is schematically illustrated in Figure 4D, demonstrating that complete relithiation of C-LPF with different SOHs is possible from the same reaction batch. [Figure 5]5A to 5D show the electrochemical performance results, where FIG. 5A is a plot of the results of the charge-discharge process in the first cycle at a rate of 0.1C, FIG. 5B is a plot of the cycle stability at a rate of 0.5C, FIG. 5C shows the discharge capacity of the full cell in the first cycle at a rate of 0.1C, and FIG. 5D is a plot of the cycle stability at a rate of 0.5C. [Figure 6] 6A-6E show an economic and environmental analysis comparing the approach of the present invention with other recycling methods, where FIG. 6A shows a simplified schematic of cathode production by pyrometallurgical processing ("Pyro"), hydrometallurgical processing ("Hydro"), and direct recycling ("Direct") methods, as well as from virgin mined material; FIGS. 6B and 6C show the total energy consumption and GHG emissions per kg of recycled cells from pyrometallurgical processing, hydrometallurgical processing, and direct recycling, respectively; and FIGS. 6D and 6E are a comparison of the total energy consumption and GHG emissions per kg of cathode production from used batteries using virgin material and direct recycling processes. [Figure 7] 7A and 7B are the XRD patterns and cycling performances, respectively, of C-LFP and RS-LFP regenerated by fresh and recycled relithiation solutions of LiOH and CA. DETAILED DESCRIPTION OF THE INVENTION

[0015] To demonstrate the defect-targeting direct recycling method of the present invention, a commercially available LFP cell was cycled for up to 6,500 cycles in the voltage range of 2.5–3.8 V, reaching a capacity fade of up to 50%. The cell was disassembled, and LFP powder was recovered from the cathode according to the procedure described by Y. Shi, et al. (2018), “Effective regeneration of LiCoO from spent lithium-ion batteries: A direct approach towards high-performance active particles. Green Chem. 20, 851–862). The recovered cycled LFP particles (referred to as “C-LFP”) were subjected to a relithiation treatment in a Li-containing aqueous solution at a controlled temperature and time. The relithiated LFP powder (R-LFP) was thoroughly washed with deionized (DI) water, dried, and then subjected to post-sintering to complete the entire regeneration process.

[0016] LiFePO4 "LFP" cells were cycled over 6500 cycles using an Arbin battery tester over a voltage range of 2.5 to 3.8 V, followed by C / 10 (1 C = 170 mA g -1 ) to 2 V and then disassembled. The cathode strips were thoroughly rinsed with dimethyl carbonate (DMC) to remove residual electrolyte. After drying, the cathode strips were immersed in NMP (N-methylpyrrolidone) or other suitable solvent for 30 minutes, followed by ultrasonic treatment for 20 minutes to remove the LFP powder, binder, and carbon black from the aluminum substrate. The resulting suspension was centrifuged at 3500 rpm for 5 minutes to precipitate the cycled LFP (C-LFP) powder, which was then separated and dried for regeneration.

[0017] The new cells were directly discharged to 2 V at C / 10 without any cycling, then disassembled, and the recovered LFP material was used as a reference material for comparison.

[0018] C-LFP was regenerated by solution relithiation followed by a short annealing process. For the solution relithiation process, LFP powder recovered from a cycled cell was loaded into a 100-mL reactor containing 80 mL of a solution of 0.2 M LiOH and 0.08 M CA. An exemplary range of solution composition is 0.01–4 M Li solution and 0.01–2 M reducing agent. The reactor was maintained at a wide range of temperatures for various run times for relithiation. The relithiated LFP (R-LFP) powder was thoroughly washed with deionized water and dried. The R-LFP was then mixed with excess 4% Li2CO3 and subjected to thermal sintering at different temperatures for 2 h using a heating rate of 5 °C / min in an inert (nitrogen) atmosphere. The final recycled LFP is referred to as RS-LFP. It should be noted that although the experiments described herein use LiOH as the lithium ion source, other lithium ion sources may be used, including lithium salts such as Li2SO4, LiCl, LiC2H3O2, LiNO3, among others, as would be apparent to one skilled in the art.

[0019] The crystalline structure of the powders was investigated by X-ray powder diffraction (XRD) using Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos AXIS Ultra DLD using Al Kα radiation. The composition of unused, depleted, and regenerated LFP cathodes was measured by inductively coupled plasma mass spectrometry (ICP-MS). HRTEM was recorded on a JEOL-2800 at 200 kV equipped with a Gatan OneView Camera. STEM-EDS was performed on primary particles using the same instrument in annular dark-field (ADF) mode. STEM-EELS was performed on a JEOL JEM-ARM300CF at 300 kV equipped with double correctors. Ex-situ neutron diffraction patterns were collected in high-resolution mode (Δd / d ∼0.25%) over a 2-hour period with the SNS operating at a nominal 1.1 MW and subsequently processed using VDRIVE software, a suite of neutron diffraction data reduction and analysis software available from Oak Ridge National Laboratory. Operando neutron diffraction data were collected in high-intensity mode (Δd / d ∼0.45%) while the powder was heated and cooled in a furnace under nitrogen.

[0020] To evaluate the electrochemical performance using electrodes with moderate material loading, different LFP powder samples were mixed with polyvinylidene fluoride (PVDF) and Super P65 in NMP at a mass ratio of 8:1:1. The resulting slurry was cast onto aluminum foil, followed by vacuum drying at 120 °C for 6 h. Circular electrodes were cut and compressed to approximately 3–5 mg / cm. 2 To fabricate electrodes for high-material-loading half-cell and pouch-full-cell tests, cathode casting solutions were prepared with a commercially available ratio (RS-LFP:Super P:PVDF = 95:2:3) to achieve an active material loading of approximately 19 mg / cm. 2The charge-discharge cycle was controlled at 1000 kJ / s. Constant current charging and discharging was performed on the assembled cell in the potential range of 2.5 to 3.8 V. The electrolyte was LP40 (1 M LiPF6 in ethylene carbonate / diethyl carbonate = 50:50 (volume / volume)). The cell was activated by three cycles at 0.1 C, followed by further cycling at a higher rate. Electrochemical impedance spectroscopy (EIS) tests were performed in the discharged state using a Metrohm Autolab potentiostat with a signal amplitude of 10 mV for 10 min. 6 Hz~10 -3 The frequency range was 100 Hz.

[0021] The key to reproducing C-LPF is Li v The goal is to precisely eliminate the antisite defects. In order to move the Fe ion back to its original position (M2), the high-valence state Fe 3+ Due to the strong electrostatic repulsion of Li and Fe, a high activation energy (1.4 eV) is required. Referring to Figure 1A, the locations of Li and Fe in the perfect olivine structure are defined as the M1 and M2 sites, respectively. The top panel of this figure shows the Li vacancy (Li v ) and Fe occupancy at Li sites (Fe Li ) is shown, while the bottom panel shows C-LFP with all Fe removed by treatment with CA (citric acid, middle panel) in LiOH solution. 3+ Fe 2+ Park et al. ("Anti-site reordering in LiFePO4: Defect annihilation on charge carrier injection", Chem. Mater. 26, 5345-5351 (2014)) demonstrated that R-LFP was reduced to 1.5 V (vs. Li / Li) at an extremely low rate of C / 100 ("1C" represents charge or discharge in 1 hour). +We have demonstrated successful reordering of the antisites via deep discharge up to 1000 . Theoretical studies have also revealed that a reducing environment can lower the activation barrier, which in turn can facilitate the migration of Fe. Therefore, the most important step in the effective direct regeneration of C-LFP is the transfer of Fe. 3+ is reduced to lithium ions (Li + ) to C-LFP again.

[0022] The half electrode potential of the LFP electrode is 0.40 V (vs. the standard hydrogen electrode or SHE) (Equation 1). TIFF0007803562000001.tif45170

[0023] Various reducing agents react with Fe 3+ Inorganic reducing agents such as NaBH4, Na2S2O3, and hydrogen peroxide (H2O2) are well known for their effectiveness as reducing agents in various combinations. Naturally occurring organic reducing agents are of particular interest in the process of the present invention because they are safe and environmentally friendly. Examples of suitable reducing agents include citric acid (C6H8O7), oxalic acid (C2H2O4), ascorbic acid (C6H8O6), and tartaric acid (C4H6O6), which may be used alone or in combination. Glucose (C6H 12 O6) and other sugars are also candidates for naturally occurring organic reducing agents that can be used. For example, citric acid (CA), which is found in high concentrations in citrus fruits, has a redox potential of approximately −0.34 V (vs. SHE) (Eq. 2), which may be an ideal candidate to assist the reduction of C-LFP. The Gibbs free energy of the overall reaction (Eq. 3), which combines Eqs. 1 and 2, was calculated to be −56.35 kJ / mol (see Supporting Information for details), indicating that the relithiation reaction of degraded LFP is thermodynamically favorable. In our experimental design, CA in a Li-containing aqueous solution donated electrons to convert Fe 3+ , reducing the electrostatic repulsion and subsequently lowering the migration barrier to Fe 2+from the M1 site back to the M2 site, which transfers solution Li to the Li-deficient C-LFP particles. + Promote diffusion.

[0024] The evolution of LFP composition during solution relithiation was monitored by inductively coupled plasma mass spectrometry (ICP-MS). We first synthesized LiCoO and LiNi 1-x-y Co x Mn y Relithiation was tested at 180 °C, the minimum temperature required for the relithiation of depleted layered oxides such as O2 cathodes. As shown in Figure 1B, the Li composition of C-LFP particles increased from 0.5 to 1.0 when the relithiation time was extended to 5 h. Note that an autoclave reactor capable of maintaining pressures above 11 bar (the saturation pressure of water) is preferably used for this operation (Figure 1C). Effective relithiation at temperatures below the boiling point allows the pressurized reactor to be replaced with a low-cost vessel without extra safety precautions. Compositional analysis of the relithiation solution before and after the reaction indicated that 1.9 mol% Fe was leached from the initial LFP. This can be attributed to trace amounts of Fe2O3 generated in the depleted LFP after extended cycling (Figure 1D). From a quality control perspective, leaching of the residual Fe2O3 phase may be desirable, as it results in a higher-purity LFP phase in the regenerated product. The gradually diminishing FePO4 peak (shown by the grey vertical dashed line) indicates the transformation of the FePO4 phase into the LFP phase.

[0025] With the goal of minimizing the energy consumption of the process, lower temperatures were investigated. Surprisingly, lowering the solution temperature to 80 °C resulted in only a slight change in the relithiation kinetics, as shown in Figure 1B. The solution temperature for relithiation could be continuously lowered by further extending the treatment time. For example, 100% composition recovery could be achieved after 10 and 17 hours of relithiation at temperatures of 70 and 60 °C, respectively.

[0026] Li at different temperatures+ The apparent diffusion coefficient and time were calculated. The details of the calculation are shown below. TIFF0007803562000002.tif16170In the formula, R is the gas constant, T is the absolute temperature, A is the interface between the cathode and the electrolyte (A=1.6 cm2), n is the number of electrons involved in the reaction, F is the Faraday constant, and C is the Li concentration in the electrode based on the molecular weight (M) and density (ρ) of LFP. + is the concentration (=ρ / M), and σ is the Warburg factor. The Warburg factor is the ratio of Z' to ω in the Warburg domain. -1 / 2 It can be obtained from the slope of the plot (ω is the angular frequency). TIFF0007803562000003.tif12170

[0027] Based on the obtained slope, the Li in the LFP sample + The apparent diffusion coefficient is 1.05 x 10 -15 cm 2 / s was calculated.

[0028] The apparent diffusion coefficient in solids at different temperatures can be predicted by the Arrhenius equation, TIFF0007803562000004.tif10170 Formula, D Li+ app is the lithium apparent diffusion coefficient, E a is the activation energy (3.1 eV), and k is the Boltzmann constant (8.617 × 10 -5 eV / K) and D0 is the frequency factor.

[0029] Li + The mean diffusion time and the diffusion coefficient D Li+ app The relationship between can be estimated using the following formula: TIFF0007803562000005.tif18170In the formula, t is Li + is the diffusion time, and D Li+ appare the diffusion coefficients at different temperatures, and R is the diffusion length (approximately 100 nm). The calculated diffusion times as a function of temperature are plotted in Figure 1E.

[0030] Electrochemical impedance spectroscopy (EIS) measurements revealed that Li 0.5 Li to FePO4 + Apparent diffusion coefficient (D Li+ app ) is 1.05 × 10 -15 cm 2 / s, which is consistent with previous reports. Assuming that the average particle size of LFP is 100 nm, the above D Li+ Using the calculated Li + The diffusion times shown in Fig. 1 are in good agreement with the experimental relithiation times, indicating that the solution relithiation kinetics roughly follows a semi-infinite solid-state diffusion mechanism. Effective relithiation at temperatures below the boiling point of water allows the process to be carried out at ambient pressure. This allows the pressurized reactor to be replaced with a low-cost vessel that does not require extra safety precautions, making the process even more practical for large-scale operation.

[0031] To further confirm the crucial role of citric acid (CA), the same C-LFP was treated with a LiOH solution without CA. As expected, sequential oxidation of (LiFePO4) to Fe2O3 and Fe3O4 was observed. This result also confirms that the defect-targeted repair effect is enabled by CA. In addition, CA is a low-cost additive (approximately 0.55 $ / kg) widely used in the food industry, and the relithiation process only generates CO2, HO, and acetonedicarboxylic acid (C5H6O5, approximately 10 $ / kg). It is also worth noting that C5H6O5, a key intermediate in drug synthesis, is conventionally produced by decarbonylation of CA in oleum. This suggests that our direct LFP recycling process may involve suitable precursors, providing an alternative route for the green synthesis of valuable organic molecules. Ascorbic acid ("AA") (E = -0.55 V) 23 Other reducing agents, such as tartaric acid (“TA”) (E=−0.23 V), have shown similar functionality for regenerating C-LFP, providing a variety of options for low-cost reducing agents.

[0032] The X-ray diffraction (XRD) patterns of C-LFP and samples after solution relithiation for different times (referred to as "R-LFP") further demonstrate the phase transition of degraded LFP during the solution relithiation process. For example, referring to FIG. 1D, C-LFP exhibits strong peaks at 2θ = 18° and 32° (highlighted by gray dashed lines), which are attributed to the presence of FePO4 due to the loss of lithium. As the relithiation time is extended from 1 hour to 5 hours at a temperature of 80°C, the intensity of these peaks gradually decreases and then disappears, suggesting the transformation of the FePO4 phase into the LFP phase.

[0033] To further understand the relithiation mechanism at the atomic level, high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) images were obtained. For LFP cathodes after over 6,500 cycles, the particles still exhibited clear crystallinity, with a conformal carbon coating (2–3 nm) retained on the surface (Figure 2A). Electron energy loss spectroscopy (EELS) experiments were performed to investigate the valence states of O and Fe from the surface to the interior of the particles (Figure 2B). From one representative particle, the O K edge and Fe L edge spectra from the surface (position 1) to the interior (position 6) of a C-LFP particle were compared. In the case of C-LFP, the O pre-peak gradually appeared from the surface to the bulk, and Fe was present in the interior of the C-LFP particle. 3+ The Fe L-edge gradually shifts from 707.93 eV to 709.65 eV, as shown in Figure 2C and Table 1 below, suggesting the presence of Fe 3+ This suggests that the nuclei are predominantly present in the body. Position 1 indicates the surface of the particle, and position 6 indicates the interior side of the particle. TIFF0007803562000006.tif85170

[0034] In the EELS spectrum taken from another representative particle, a clear O K edge pre-peak appears in the spectrum obtained from the particle surface, and Fe 3+ The EELS results above indicate that FePO4 and LiFePO4 phases coexist and are randomly distributed in different particles. Several two-phase models, including the shrinking core model, the mosaic model, and the domino cascade model, have been proposed to understand the local structure of delithiated LFP, but they have been established mainly for the first charge-discharge cycle. Our results suggest high heterogeneity in the phase distribution in LFP particles after long-term charge-discharge cycles.

[0035] FIG. 2D shows the Rietveld refinement pattern of the neutron diffraction data of C-LFP, and the detailed structural information is listed in Table 2 below, where: Phase 1 LiFePO4: Space group: Pnma, R wp=2.56%, a=10.2926(10)Å, b=5.9905(6)Å, c=4.6989(4)Å, α=β=γ=90°, fraction: 52.9%, phase 2 FePO4: space group: Pnma, R wp = 2.56%, a = 9.8284(9) Å, b = 5.7955(5) Å, c = 4.7831(4) Å, α = β = γ = 90°, fraction: 47.1%. TIFF0007803562000007.tif106170

[0036] Overall, C-LFP exhibits 47.1% Li deficiency (loss) and 4.81% Fe / Li antisite defects. A computational study by Malik et al. ("Particle size dependence of the ionic diffusivity", Nano Lett. (2010), 10, 4123-4127) showed that in a 100 nm LiFePO4 particle, 0.1% of the antisites lead to approximately 5% of the Li + It has been shown that Li can be trapped. It is generally believed that the loss of Li inventory is the main cause of capacity degradation in LFP batteries, and the effect of antisite defects has often been overlooked. Olivine-type LFP has the Pnma space group and can trap Li + is confined within channels supported by interconnected FeO6 octahedra and PO4 tetrahedra. + Since this is the only route for diffusion, such Li sites are used by Fe 2+ The fact that Li occupies a large part of + This can block diffusion, leading to loss of capacity and rate performance.

[0037] For the R-LFP samples, Li, as revealed by HAADF-STEM images of representative R-LFP particles, + Peripheral Fe 2+ All of them show ordered structures along the <0010> direction (Fig. 2E). +Diffusion channels are shown. The carbon shell was also retained after solution relithiation. The disappearance of the O pre-peak and the unchanged Fe L-edge at 707 eV (Figure 2F) confirmed that the oxidation state of Fe remained 2+ from the surface to the bulk. Neutron diffraction data (Figure 2G) further confirmed that a pure orthorhombic LFP phase was obtained after the relithiation process, with the proportion of antisite defects reduced to a low of 2.2%, which is even lower than that of virgin LFP (referred to as "P-LFP") (2.5%).

[0038] Figure 2H shows the Rietveld refinement pattern of the neutron diffraction data of R-LFP. Table 3 below lists the structural parameters obtained from the Rietveld refinement, where LiFePO4: space group: Pnma, R wp =4.09%, a=10.3146(9)Å, b=6.0000(6)Å, c=4.6909(4)Å, α=β=γ=90°. TIFF0007803562000008.tif92170

[0039] After a short sintering treatment of R-LFP, a highly crystalline LFP (referred to as "RS-LFP") with further reduced antisite defects was obtained. The XRD patterns of each sample were examined to identify possible structural changes. As the sintering temperature increased from 400 to 800 °C, the average grain size grew from 72 to 96 nm, but no phase change was observed. A uniform carbon coating was also maintained on the particle surface, as indicated by the uniform distribution of C, P, and Fe elements in elemental mapping, further suggesting targeted repair of compositional and microstructural defects in C-LFP.

[0040] Fe in the sintering process LiOperando neutron diffraction was performed to quantify the development of antisite defects. The contour plot of the time dependence of peak intensity shown in Figure 3A confirmed that the pure LFP phase was maintained during heating and cooling, demonstrating the good stability of R-LFP particles. After cooling, the proportion of antisite defects decreased to 1.5%, further reduced compared to that of P-LFP (2.5%). In Figure 3B, dots with error bars correspond to the proportion of antisite defects. In contrast, after a solid-state reaction-based regeneration process (SS), in which Li-containing precursors (e.g., Li2CO3, LiOH) were mixed with depleted LFP particles for sintering, significant phase impurities (e.g., Li3PO4, Fe2P) were always present. Ensuring uniform Li distribution within the LFP particles and acting to eliminate phase impurities after sintering is an additional advantage of the solution-based relithiation process.

[0041] The electrochemical performance of the LFP sample was first evaluated using a half cell. A cycle test was performed at 0.1 C (1 C = 170 mA g -1 ) for two cycles of activation, followed by an additional 100 cycles at 0.5 C (Figure 3C). P-LFP was activated at 161 mAh g at 0.5 C. -1 The capacity of C-LFP recovered from the used cell was only 103 mAh g at 0.5 C. -1 and after 100 cycles, this is an additional 98 mAh g -1 The initial capacity of R-LFP was 159 mAh g -1 The capacity dramatically improved to 100.7%, indicating that the electrochemical activity was restored after aqueous relithiation. However, after 100 cycles, only 93.7% of the initial capacity was maintained. Such degradation is probably due to the degradation of Li during the aqueous relithiation process. +This is thought to be related to proton exchange, which has little effect on the crystal structure but induces side reactions due to the presence of protons. A subsequent short sintering step helps to create more stable particles that can provide the same capacity and stability as P-LFP. It was found that a temperature that is too low can exacerbate structural defects, as reported in earlier studies of LFP synthesis, while a temperature that is too high can increase the crystal grain size. In both cases, the capacity of RS-LFP was inferior to that of P-LFP. Increasing the sintering temperature from 400 to 600 °C increased the first-cycle discharge capacity (at 0.5 C) to 148 mAh g -1 from 159mAh·g -1 When the temperature was further increased from 600 to 800°C, the capacity increased to 141 mAh g -1 The capacity retention was 99%, 99%, 99%, 97%, and 94% for the samples sintered at 400°C, 500°C, 600°C, 700°C, and 800°C, respectively.

[0042] After thermal sintering at 600°C for 2 hours, RS-LFP achieved 159 mAh g at 0.5 C. -1 With a capacity loss of less than 1% after 100 cycles, RS-LFP can deliver a capacity of 150 mAh / g even when cycling at 0.5 C is extended to 1000 cycles.

[0043] The rate capability of C-LFP can also be restored after complete regeneration, as shown in Figure 3D. Specifically, P-LFP exhibited 163 mAh g at 0.2 C, 2 C, and 10 C, respectively. -1 , 141mAh·g -1 , and 99mAh·g -1 The RS-LFP has a capacity of 162mAh g -1 , 144mAh·g -1 , and 102 mAh·g -1In contrast, C-LFP has a capacity of 115 mAh g due to Li loss and structural defects. -1 , 82mAh·g -1 , and 66 mAh·g -1 In addition, the regenerated LFP also exhibited excellent long-term cycling stability. No obvious capacity loss was observed after 300 cycles at rates of 2C, 5C, and 10C (Figure 3E). The significantly improved rate capability and high stability of RS-LFP suggest that both the composition and structure of C-LFP were fully restored after relithiation and short-time sintering.

[0044] The high-load half-cell showed an initial capacity of 156 mAh / g, which remained at 157 mAh / g after 50 cycles at 0.5 C. The pouch cell (3 cm x 3 cm) showed a capacity of 28.6 mAh (3.17 mAh / cm) at a rate of 0.1 C. 2 ) capacity with no capacity degradation after 30 cycles. These results further suggest great potential for directly using recycled LFP in the fabrication of new cells without sacrificing cell-level performance.

[0045] Generally, a 20% capacity loss is considered the end of life for electric vehicle (EV) batteries. Considering secondary use, a 50% capacity decline can be assumed to be the lower limit of LIB life for any application. In reality, LIB waste streams may consist of cells at various states of health. Therefore, we tested our method on a mixture of cycled cathode materials with states of health (SOH) of 40%, 50%, and 85% to fully explore the effectiveness of our process. When subjected to the same regeneration process as previously described, the cycled LFP mixture showed complete recovery of composition, structure, and electrochemical performance to the same levels as P-LFP. Using the same process, we regenerated RS-LFP from C-LFP with different SOHs of 15%, 50%, and 60% degradation. The XRD patterns in Figure 4A reveal that a pure LFP phase was obtained after solution relithiation. Figures 4B-4C show the capacity and stability of 60% depleted LFP, recovering to the same level as P-LFP. This process is shown schematically in Figure 4D, demonstrating the complete relithiation of C-LPF, a different SOH, from the same reaction batch.

[0046] These results suggest significant advantages of using low-temperature solution relithiation for the treatment of spent batteries in various states of health, since all cathodes reached stoichiometric composition through self-saturation.

[0047] To further explore the practical applications of regenerated LFP (RS-LFP), we investigated the concentration of approximately 19 mg / cm 2 We fabricated electrodes with thicknesses corresponding to commercially available products with material loadings of 1000 uF and used them to assemble both half cells (with Li metal as the counter electrode) and pouch cells (with graphite as the anode).

[0048] The corresponding electrochemical performance was evaluated by constructing a cathode casting solution with a commercially available ratio (RS-LFP:Super P:PVDF=95:2:3), and the active material loading was approximately 19 mg / cm. 2The electrolyte was LP40 (1 M LiPF6 in EC / DEC) and the cell was activated with three cycles at 0.1 C followed by a further cycle at 0.5 C.

[0049] Figure 5A shows the results of the charge-discharge process at a 0.1C rate for the first cycle, yielding a reasonable first-cycle coulombic efficiency (97.6%) and a capacity of 166 mAh / g. The half-cells yielded capacities of 169 and 165 mAh / g. Next, cycling stability was evaluated at a 0.5C rate, and the results are plotted in Figure 5B. For such a high-material-loading cathode, RS-LFP delivered an initial capacity of 156 mAh / g and maintained a capacity of 157 mAh / g after 50 cycles, suggesting good stability. Figure 5C shows that the discharge capacity of the full cell at a 0.1C first cycle was 166 mAh / g, which nearly achieved the theoretical capacity of LFP (172 mAh / g). The assembled pouch cell delivered a discharge capacity of 28.6 mAh (3.17 mAh / cm). 2 Note that a total capacity of 157 mAh / g can be achieved. Figure 5D is a plot of the cycling stability at a rate of 0.5 C. Notably, these results are comparable to those of several major commercial LFP suppliers worldwide: A123 Systems, LLC (154 mAh / g), Phostech Lithium Inc. (156 mAh / g), Likai (158 mAh / g), Valence Technology, Inc. (149 mAh / g), and Sitelan (156 mAh / g). The capacity can be maintained at 157 mAh / g even after 30 cycles. These results further confirm that the recycled LFP exhibited excellent electrochemical performance even with electrodes of thickness comparable to commercially available products, indicating great potential for practical applications.

[0050] Figure 6A shows a simplified flowchart for direct recycling of cathodes from spent LIB materials, as well as cathode regeneration from pyrometallurgical recycling ("Pyro"), hydrometallurgical recycling ("Hydro"), and new cathode material production. Note that currently, pyrometallurgical and hydrometallurgical recycling routes are not used commercially to recover cathode materials from spent LFP batteries due to economic losses. These are included here as possible end-of-life treatment options for LFP batteries, assuming that battery recycling will become mandatory before novel recycling technologies become available. Compared to other processes, the distinct advantages of developing a direct recycling process for LFP batteries are: 1) simplified operating equipment and processes; 2) reduced operating temperatures and shorter operating times; and 3) elimination of the use of strong acids and bases. These features are associated with potential economic and environmental benefits that can be analyzed using the EverBatt model developed by Argonne National.

[0051] These three different recycling methods are modeled assuming an annual plant processing capacity of 10,000 tonnes of used batteries (Figure 6B). The total lifecycle energy usage for the pyrometallurgical and hydrometallurgical treatment processes is 18.4 and 30.6 MJ kg, respectively. -1 LFP cells. In pyrometallurgical processes, 55% of the energy use is attributable to high-temperature smelting. In hydrometallurgical processes, 87.8% of the energy use comes from the upstream production of chemicals consumed in the process. The total energy use for direct reuse is only 3.5 MJ kg -1LFP cells, which are significantly lower than other processes. GHG emissions are also an important factor to consider when evaluating recycling approaches. As shown in Figure 6C, the total GHG emissions from the direct recycling process are only 26.6% and 27.7% of those from the pyrometallurgical and hydrometallurgical processes, respectively. Furthermore, the total energy usage per kg of cathodes made from direct recycling of used batteries is only 22.3% of that from cathodes made from virgin materials (Figure 6D). GHG emissions from cathode production via direct regeneration of used batteries are 46.2% lower than from virgin materials (Figure 6E).

[0052] The total costs of pyrometallurgical processing, hydrometallurgical processing, and direct recycling are $3.4, $2.4, and $2.1 per kg of used battery cells processed, respectively. It is assumed that any recycled Al, Cu, and graphite are sold to recover a portion of the costs. However, it is important to note that the high costs of the pyrometallurgical and hydrometallurgical recycling processes cannot be offset by net revenues due to the use of expensive equipment, large amounts of materials, and high energy consumption. This is the primary reason why the industry does not currently recycle LFP cells. In contrast, direct recycling, as described herein, allows cell manufacturers to use the regenerated cathode material without further recomposition, potentially generating a profit of $1.04 per kg of used battery recycled.

[0053] The significant reductions in total energy usage, GHG emissions, and costs achieved by the direct regeneration method based on low-temperature aqueous relithiation of the present invention provide an important option for the reuse of spent LIBs. Existing methods for LFP reuse are still based on hydrometallurgical or other destructive processes. Ideally, solid-state sintering by adding a desired amount of lithium to spent LFP cathode powder could also restore its original composition. However, determining the exact lithium addition amount for a large number of spent cells with significantly different SOHs can be difficult in practice. More importantly, targeted repair of defects cannot be achieved, as evidenced by the relatively low capacity of recycled LFP from solid-state sintering. Chemical lithiation in aprotic solvents (e.g., acetonitrile) using strong reducing agents can also be used to refunctionalize spent LFP, but the highly corrosive nature of such systems may limit its practical application.

[0054] Another advantage of using the ambient solution process of the present invention is that the relithiation solution itself can be reused. For example, a used solution containing LiOH and CA was used to relithiate a second batch of used LFP under the same conditions. The XRD patterns and cycling stability of RS-LFP regenerated with fresh and recycled solutions are compared in Figures 7A and 7B. As can be seen in Figure 7B, the traces for the fresh and recycled solutions completely overlap. Therefore, a pure LFP phase can be obtained even with a recycled solution of LiOH and CA. The capacity and stability of RS-LFP reached the same levels as that regenerated with a fresh solution of LiOH and CA. The successful demonstration of recycling and reuse of the LiOH and CA relithiation solution adds further efficiency to reduce the overall operating costs of the direct recycling method of the present invention.

[0055] The methods and procedures described herein demonstrate a defect-targeted remediation method for more efficient and sustainable reuse of spent LIB materials. Such improvements represent a paradigm shift toward green LIB reuse, potentially generating revenues simply not possible with existing reuse processes. Fully restoring the electrochemical performance of spent LFP cathodes to the level of virgin material could improve market acceptance of recycled battery materials. Furthermore, under the Everbatt model, and assuming an annual plant processing capacity of 10,000 tons of spent batteries, our direct regeneration route can achieve a regeneration efficiency of 3.5 MJ kg -1 These results suggest that LFP cells have lower energy consumption (equivalent to only 19% and 11% of the pyrometallurgical and hydrometallurgical processes, respectively) and lower GHG emissions of 0.7 kg / kg LFP cells (26.6% and 27.7% of the pyrometallurgical and hydrometallurgical processes, respectively). Importantly, the cost of direct recycling per kg of spent LFP cells can be reduced to $2.1 compared to $3.4 and $2.4 for the pyrometallurgical and hydrometallurgical processes, respectively. Note that while some uncertainty remains regarding the costs of battery collection and transportation, these costs can be assumed to be the same regardless of the recycling process used, since the batteries will likely be collected from the same sources. Therefore, the efficiency gained from direct recycling is attributable to significantly improved operational design and reduced chemical usage.

Claims

1. 1. A method for regenerating a used olivine-structured polyanionic cathode material of a lithium ion battery, comprising: lithiating the cathode material in an aqueous relithiation solution comprising at least one organic reducing agent, a cathode material, and a lithium salt by heating the aqueous relithiation solution at a temperature in the range of 60°C to 180°C to replenish the lithium in the cathode material, producing a replenished lithiated cathode material; and heating the replenished lithiated cathode material to repair defects in the replenished lithiated cathode material; A method comprising:

2. The method of claim 1, wherein after lithiation of the cathode material, the aqueous relithiation solution contains 1.9 mol% Fe.

3. 10. The method of claim 1, wherein the at least one organic reducing agent is selected from the group consisting of citric acid, ascorbic acid, tartaric acid, oxalic acid, a sugar, or a combination thereof.

4. 10. The method of claim 1, wherein the at least one organic reducing agent comprises citric acid and the aqueous relithiation solution comprises 0.01M to 4M LiOH and 0.01M to 2M citric acid.

5. The lithium salt is LiOH, Li ２ SO ４ , LiC ２ H ３ O ２ , and LiNO ３ 2. The method of claim 1, wherein the compound is selected from the group consisting of:

6. The cathode material is LiFePO ４ The method of claim 1, wherein

7. 10. The method of claim 1, wherein the lithiation of the cathode material in the aqueous relithiation solution is carried out for at least 1 hour.

8. 10. The method of claim 1, wherein lithiating the cathode material in the aqueous relithiation solution is carried out at a temperature in the range of 60° C. to 120° C. for at least 5 hours.

9. 10. The method of claim 1, wherein the heating is carried out in a furnace at a temperature in the range of 400° C. to 800° C. for 50 to 300 minutes.

10. 10. The method of claim 9, wherein heating comprises a temperature gradient to gradually heat the replenished lithiated cathode material at a controlled rate.

11. 10. The method of claim 1, wherein the aqueous relithiation solution is recyclable and reusable for subsequent relithiation processes.

12. LiFePO from used lithium-ion batteries ４ 1. A method for regenerating a cathode material, comprising: disassembling the lithium ion battery to remove the cathode strip; recovering cathode powder from said cathode strip; placing the cathode powder in a vessel containing an aqueous relithiation solution comprising at least one organic reducing agent and a lithium salt; heating the container and the aqueous relithiation solution at a temperature in the range of 60°C to 180°C for at least 1 hour to form a replenished lithiated cathode material; and heating the replenished lithiated cathode material in an inert atmosphere at 400°C to 800°C; A method comprising:

13. The method of claim 12, wherein after heating the container and the aqueous relithiation solution to form the replenished lithiated cathode material, the aqueous relithiation solution contains 1.9 mol% Fe.

14. 13. The method of claim 12, wherein the at least one organic reducing agent is selected from the group consisting of citric acid, ascorbic acid, tartaric acid, oxalic acid, a sugar, or a combination thereof.

15. 13. The method of claim 12, wherein the at least one organic reducing agent comprises citric acid and the aqueous relithiation solution comprises 0.01M to 4M LiOH and 0.01M to 2M citric acid.

16. The lithium salt is Li 2 CO 3 , LiOH, Li ２ SO ４ , LiC ２ H ３ O ２ , and LiNO ３ 13. The method of claim 12, selected from the group consisting of:

17. 13. The method of claim 12, wherein heating the vessel and the aqueous relithiation solution occurs for a period of time ranging from 1 hour to 18 hours.

18. 13. The method of claim 12, wherein heating the vessel and the aqueous relithiation solution is carried out at a temperature in the range of 60°C to 120°C for at least 5 hours.

19. 13. The method of claim 12, wherein heating the replenished lithiated cathode material is carried out in a furnace at a temperature in the range of 400° C. to 800° C. for 50 minutes to 300 minutes.

20. 20. The method of claim 19, wherein heating the replenished lithiated cathode material comprises a temperature gradient to gradually heat the replenished lithiated cathode material at a controlled rate.

21. 13. The method of claim 12, wherein the aqueous relithiation solution is recyclable and reusable for subsequent relithiation processes.

22. The method of claim 3 , wherein the at least one organic reducing agent comprises citric acid.

23. 15. The method of claim 14, wherein the at least one organic reducing agent comprises citric acid.

24. 10. The method of claim 1, wherein the aqueous relithiation solution comprises 0.01M to 4M Li and 0.01M to 2M organic reducing agent.

25. The lithium salt is Li ２ CO ３ The method of claim 1, wherein

Images