Direct upcycling of spent lifepo₄ cathodes into high-performance limnfepo₄ material

Abstract

A process for direct upcycling of spent LiFePO₄ (sLFP) cathode materials includes milling a mixture of particles of sLFP cathode material with particles of each of a manganese source, a lithium source, and a phosphate source. The resulting milled particles are pre-sintered to de-gas the mixture and form isomorphous intermediates then subjected to secondary milling. A carbon coating is applied to the de-gassed milled particles and the carbon-coated particles are processed by upcycle sintering to produce uLMFP / C particles that may be used to form cathodes for assembly of a LiMnFePO₄ (LMFP) battery cell.

Description

[0001] DIRECT UPCYCLING OF SPENT LiFePO4CATHODES INTO HIGH-PERFORMANCE LiMnFePO4 MATERIAL

[0002] RELATED APPLICATIONS

[0003] This application claims the benefit of the priority of U. S. Provisional Application No. 63 / 735,176, filed December 17, 2024, which is incorporated herein by reference in its entirety.

[0004] GOVERNMENT RIGHTS

[0005] This invention was made with government support under DE-EE0010399 awarded by U. S. Department of Energy and under 2213895 awarded by the National Science Foundation. The government has certain rights in the invention.

[0006] FIELD OF THE INVENTION

[0007] The present invention relates to a direct upcycling scheme (sLFP-uLMFP) for spent LFP cathodes to produce carbon-coated LMFP material with evenly distributed elements.

[0008] BACKGROUND

[0009] Lithium-ion batteries (LIBs) utilizing lithium iron phosphate (LiFePO4 or LFP) as a cathode material have been widely adopted for use in electric vehicles (EVs) and energy storage systems. Their stability, safety, and affordability have secured them near half of the market share. However, as the production of LFP-based LIBs expands, so does the volume of degraded batteries reaching the end of their lifespan. Recycling of these batteries, especially the high-value cathode active materials, becomes crucial. Recently, a direct regeneration strategy has emerged as a preferred recycling method for degraded LFP cathodes. This approach seeks to recover the electrochemical performance of spent cathode materials by restoring their compositions and structures while avoiding the need for complete disintegration of spent LFP into elemental constituents and re-synthesis. It is favored due to its simplicity, efficiency, environmental friendliness, and affordability, and stands as the sole economically viable approach for recycling LFP LIBs. Direct regeneration holds significant potential in addressing the impending “retirement wave” of LFP-based LIBs and rejuvenating spent LFP to its original performance. However, the energy density of LFP remains limited due to its low operating voltage (-3.4V) and nominal specific capacity. Recently, manganese-rich olivine cathode material, LiM Fei-xPCU (LMFP), has emerged as the next generation of phosphate-based cathodes and is projected to gradually replace LFP cathodes by 2030. LMFP achieves a higher operating voltage (-3.8V) compared to its counterpart, LFP, and offers cost-effectiveness and safety advantages over layered oxide cathodes including NCM and LCO. Consequently, rather than merely recovering the performance of spent LFP cathodes, there is a growing need to upcycle them into state-of-the-art phosphate-based cathodes — the LiM Fei- PCU cathodes.

[0010] Some researchers have synthesized LiM Fei-xPCU using the chemically decomposed salt or element level products of spent LFP as partial raw materials. For example, Zhou et al., reported in “Direct Upcycling of Leached FePCU from Spent Lithium-Ion Batteries toward Gradient-Doped LiM Fei-xPCU Cathode Material”, Advanced Energy Materials, 2024. 14(7): p. 230276, demonstrated an LMFP synthesis scheme using leached iron phosphate (FePCL) as feedstock, a residue from lithium extraction in spent LFP. A carbon-coated, Mn-gradi ent-doped LiMno.25Feo.7sPO4 material was produced, however, the 25% Mn gradient doping in this material has limitations in improving energy density. As reported by Ji, G., et al. in “Sustainable upcycling of mixed spent cathodes to a high-voltage polyanionic cathode material,” Nature Communications, 2024. 15(1): p. 4086, a deep eutectic solvent was developed for leaching mixed spent LFP and Mn-rich cathodes, followed by sintering the decomposed salts into LiMno.5Feo.5PO4 at a gram-grade level. This “sLFP-Li / Fe / PO43‘ salts-uLMFP” strategy has redundant steps, a complex process, and consumes a significant amount of energy. In view of the absence of practical, economically reasonable solutions, direct upcycling of spent LFP cathodes to LiMmFei-xP04 cathodes (sLFP-uLMFP) remains an increasingly urgent challenge for achieving upcycling without the need to break down the chemical structure of spent LFP.

[0011] SUMMARY

[0012] The inventive approach employs a straightforward, robust, and scalable mechanochemical process that directly upgrades spent LFP cathodes to LiM Fei-xPCU (LMFP) cathode material without requiring prior decomposition of spent LFP. The resulting LMFP product demonstrates a uniform distribution of manganese (Mn) and iron (Fe) on an elemental scale within the particle interior, along with a consistent carbon coating on the particle surface. Remarkably, its electrochemical performance rivals that of commercially available LMFP, exhibiting an energy density approximately 12% higher than pristine LFP. This robust upcycling method effectively transforms various spent LFP cathodes, sourced from a variety of types of end-of-life LFP batteries, into high-performance LMFP cathodes at ~100g scale, underscoring its practical applicability and resilience.

[0013] The innovative direct upcycling scheme (sLFP-uLMFP) for spent LFP cathodes produces LMFP material with an evenly distributed carbon coating. The inventive protocol directly utilizes spent LFP as feedstock to synthesize LMFP, eliminating the prior sLFP decomposition process. The approach involves a two-step process: milling (e.g., ball, jet, or sand) for elemental mixing and mechanochemical activation, followed by solid-state sintering to modify the structure and achieve carbon coating. Robustly demonstrated, this direct upcycling method effectively transforms various spent LFP cathodes, collected from different types of end-of-life (EoL) batteries, into high-performance uLMFP cathodes with different Mn: Fe ratios at ~100g scale. Remarkably, the upcycled LiMno.6Feo.4PO4 material exhibits excellent electrochemical performance, comparable to pristine materials. This environmentally friendly and economical approach holds significant potential in reshaping the recycling system for spent LFP LIBs.

[0014] The inventive scheme employs a scalable, isomorphous intermediate-assisted upcycling strategy that directly converts sLFP cathodes into high-performance LiM Fei- PO4 with tunable Mn / Fe ratios (i.e., 0<x<l) via solid-state synthesis. This approach leverages the formation of isomorphous intermediates (LiFePCU-LiMnPCU, LFP-LMP) from an inhomogeneous mixture of sLFP and Li / Mn / P feedstocks via pre-sintering, which replenishes Li deficiencies in sLFP and, more importantly, enables facile mechanochemical activation of the intermediate precursor to induce amorphization and reduce particle size. These features enhance intimate mixing of previously incompatible precursors, thereby lowering the thermodynamic and kinetic barriers to LMFP formation. Such an activated intermediate can be readily transformed into a solid solution via Mn-Fe interdiffusion driven by ion concentration gradients between adjacent LFP-rich and LMP-rich regions. By contrast, precursors of the same chemical composition but that do not form an isomorphous intermediate form non-uniform phase domains with varying Mn / Fe ratios, leading to poor electrochemical performance. Importantly, this isomorphous intermediate-assisted synthesis method has been successfully applied to diverse sLFP feedstocks, demonstrating kilogramscale production of upcycled LMFP with electrochemical performance comparable to their commercial counterparts.

[0015] In one aspect, the inventive process for direct upcycling of spent LiFePCL (sLFP) cathode materials includes milling a particle mixture comprising sLFP cathode material particles with particles comprising each of a manganese source, a lithium source, and a phosphate source to form milled particles; pre-sintering the milled particles to form isomorphous intermediate particles; secondary milling the isomorphous intermediate particles to produce milled intermediate particles; applying a carbon coating to the milled intermediate particles to form carbon-coated particles; and upcycle sintering the carbon-coated particles to form uLMFP / C particles. The phosphate source may be a phosphate salt selected from (NFUjFFPC, H3PO4, and LiFbPCh. The lithium source may be a lithium salt selected from Li2COs, LiCFFCCh, Li2C2C>4, LiOH, and LiFFPC. The manganese source may be a manganese salt selected from MnCCh, MnO, MnC2C>4, and Mn(CH3COO)2. In some embodiments, the particle mixture comprises LiMmFei-xPC, wherein 0<x<l and a ratio of Li: P is within a range of 1.0- 1.1.

[0016] In some embodiments, milling comprises ball milling which may include disposing the particle mixture in a ball mill jar; adding a solvent to the particle mixture; and operating a ball mill at a speed within a range of 300 r / min to 700 r / min for from 2-10 hours. Presintering may include heating the milled particles in a furnace at a range of about 300°C to 500°C for about 1 hour to 5 hours in an inert atmosphere. Secondary milling may include disposing the isomorphous intermediate particles in a ball mill jar; adding a solvent to the isomorphous intermediate particles; and operating a ball mill at a speed of about 300 r / min to700 r / min for 2-10 hours. Applying a carbon coating may include adding 1 wt%-20 wt% sugar to the milled intermediate particles. Upcycle sintering comprises heating the form carbon-coated particles in a furnace at a range of about 600°C to 800°C for about 8 hours to 20 hours in an inert atmosphere.

[0017] In another aspect, a method for direct upcy cling of spent LiFePC>4 (sLFP) cathode materials includes premixing particles of sLFP cathode material with particles comprising each of a manganese source, a lithium source, and a phosphate source to form LFP / Li / Mn / PC -mix particles; first milling the LFP / Li / Mn / PO4-mix particles to form Li / Mn / Fe / PO4-milled particles; pre-sintering the Li / Mn / Fe / PC -milled particles to form LMFP_pre-sintered particles comprising isomorphous intermediates; second milling the LMFP_pre-sintered particles to further mix the elemental components and form milled LMFP_pre-sintered particles; applying a carbon coating to the milled LMFP_pre-sintered particles to form carbon-coated LMFP_ps particles; and upcycle sintering the carbon coated LMFP_ps particles to form uLMFP / C particles having a crystalline structure. The phosphate source may be a phosphate salt selected from (NF jFFPC, H3PO4, and LiFbPCh. The lithium source may be a lithium salt selected from Li2COs, LiCHsCCh, Li2C2C>4, LiOH, and LiffcPC. The manganese source may be a manganese salt selected from MnCCh, MnO, MnC2C>4, and Mn(CH3COO)2. In some embodiments, the particle mixture comprises LiMmFei-xPC, wherein 0<x<l and a ratio of Li: P is within a range of 1.0-1.1.

[0018] In some embodiments, milling comprises ball milling. First milling may include ball milling which includes disposing the LFP / Li / Mn / PCL-mix particles in a ball mill jar; adding a solvent to the LFP / Li / Mn / PCL-mix particles; and operating a ball mill at a speed within a range of 300 r / min to 700 r / min for 2-10 hours. Pre-sintering may include heating the Li / Mn / Fe / PCL-milled particles in a furnace at a range of about 300°C to 500°C for about 1 hour to 5 hours in an inert atmosphere. Second milling may include disposing the LMFP_pre-sintered particles in a ball mill jar; adding a solvent to the LMFP_pre-sintered particles; and operating a ball mill at a speed of about 300 r / min to700 r / min for from 2-10 hours. Applying a carbon coating may include adding 1 wt% to 20 wt% sucrose to the milled LMFP_pre-sintered particles. Upcycle sintering may include heating the carbon-coated LMFP_ps particles in a furnace at a range of about 600°C to 800°C for about 8 hours to 20 hours in an inert atmosphere.

[0019] BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic of the direct upcycling process from LFP to LMFP diagrammatically showing the crystalline structure and process evolution of LFP cathode recycling using different method, with comparison of electrochemical performance of the LFP cathode at different stages, where a pristine LFP cathode exhibits an initial capacity of 160 mAh / g at 3.4V, which diminishes after cycling due to degradation but is restored to 160 mAh / g at 3.8 V following the upcycling process; FIG. IB is a schematic illustration of the upcycling strategy involving pre-sintering to transform inhomogeneous crystalline feedstocks into olivine-type LFP / LMP intermediate, followed by intermediate-activation and intermediate-driven homogenization to yield the LMFP solid solution; FIG. 1C plots thermal behaviors of the LFP / Li / Mn / P feedstock under an argon atmosphere; and FIG. ID shows DSC curves of LFP / LMP intermediate before and after activation.

[0020] FIGs.2A-2C illustrate direct upcycling of LFP to LMFP, where FIG.2A illustrates selection principles for Mn, Li, and phosphate sources; FIG. 2B plots thermochemical properties of the LFP / Li / Mn / PCL mixture after grinding with a mortar; and FIG. 2C plots thermochemical properties of the LFP / Li / Mn / PCh mixture after milling treatment. FIGs. 3A and 3B illustrate structural evolution of uLMFP during upcycling, where FIG. 3A shows the processing sequence and the chemical reaction equations for the inventive upcycling process from LFP to LMFP and FIG.3B provides photographs of batch material as it progresses through the steps of an exemplary embodiment of the upcycling sequence; FIG. 3C provides XRD profiles indicating the crystalline structure evolution of LMFP precursors throughout the upcycling process; FIG.3D provides XRD profiles reveal the crystalline structural evolution of the LFP / Li / Mn / PCk precursors throughout the upcycling process; FIG. 3E shows soft XAS highlighting the evolution of iron valence during upcycling, with reference Fe L2,3-edge XANES of FeCL (Fe2+) and Fe2Os (Fe3+); and FIG. 3F shows soft XAS showing manganese valence remains at 2+ during upcycling, with the reference XAS of manganese.

[0021] FIGs.4A-4B illustrate the microstructure and element distribution of uLMFP, where FIG. 4A is a STEM image of uLMFP; and FIG. 4B is a TEM-EDS of a uLMFP particle, revealing the uniform distribution of Mn, Fe, and PCL ions; FIG.4C provides EELS profiles of uLMFP / C at points pl to pl6 on a scan line from the surface to the interior on a random particle; FIG. 4D plots the intensity ratio of Mn and Fe L-edges (leso / bis) along the scan line; and FIG. 4E provides DSC curves showing the melting temperature of cLFP, uLMFP-wp, uLMFP, and cLMFP.

[0022] FIGs. 5A-5D illustrate electrical performance characterization and comparison of uLMFP, where FIG. 5A provides initial charge-discharge curves; FIG. 5B plots dQ / dV curve analysis; FIG. 5C provides a rate and cycling performance comparison; and FIG. 5D is an energy density and energy efficiency comparison.

[0023] FIG. 6A-6J illustrates upcycling of spent LFP to LMFP, where FIGs. 6A-6D are scanning electron microscopy (SEM) images showing the morphological of spent LFP electrode, spent LFP powder, upcycled uLMFP-S powder, and the pristine LMFP powder, respectively; FIG.6E is a STEM of uLMFP-S particle, showing a uniform -3.16 nm carbon coating layer; FIG. 6F provides XPS Cis to elucidate the Carbon chemical state changes during the upcycling process; FIG.6G provides XPS FIs to elucidate the Fluorine chemical state changes during the upcycling process; FIG. 6H provides initial charge-discharge curves; FIG. 61 plots cycling performance of uLMFP-S samples with different feedstock compositions at 100g scale; and FIG. 6 J plots electrochemical performance of uLMFP-S based half and full coin cells with high loadings. DETAILED DESCRIPTION OF EMBODIMENTS

[0024] FIG. 1A provides a schematic illustration of an embodiment of the inventive upcycling process from spent LFP to uLMFP. The spent LFP, sourced from an EoL LFP battery, serves as the Li source, Fe source, and partial phosphate source. To upcycle the spent LFP cathode into LMFP materials, additional Mn sources, Li sources, and phosphate sources are required.

[0025] Referring to FIG. 2A, three criteria are proposed for selecting Li / Mn / PCU salts. Lithium salts may include Li2COs, lithium acetate (LiCELCCE), lithium oxalate (Li2C2C>4), lithium hydroxide (LiOH), and LiELPCh. Candidates for Mn salts include MnCCh, MnO, Mn(II) oxalate (MnC2C>4), Mn(II) acetate (Mn(CH3COO)2). Phosphate salts may include NH4H2PO4, H3PO4, and LiELPCh. Firstly, post-annealing, the counter ions of the Li / Mn / PCL salts should decompose into gas, leaving no residuals, and the mass ratio of Li / Mn / PCL should be maximized. Secondly, chemical compatibility with the olivine structure to preserve crystal structure and avoid corrosion, i.e., the acidity of Li / Mn / PCU salts should be neutral (pKa between 4-8) to prevent the breakdown of the LFP / LMFP crystalline structure. Thirdly, the melting point of Li / Mn / PCL salts should be below 900°C, close to the decomposition temperature of LFP. Based on these criteria, Li2CC>3, MnCCh, and NH4H2PO4 were selected as the Li, Mn, and phosphate sources, respectively. These salts are also commonly employed in the industrial production of pristine LMFP via solid-state sintering.

[0026] The overall upcycling process from LiFePCU to LiM Fei xPCU (exemplified here with x=0.5) is expressed as Reaction (1):

[0027] LiFePO4+NH4H2PO4+0.5Li2CO3+MnCO3^2LiMn0.5Fe0.5PO4+H20 (g)+1.5CO2(g)+NH3(g)

[0028]

[0029] (1) LMFP can be considered a substitutional solid solution, in which Mn2+partially replaces Fe2+within the LFP structure, forming an olivine framework. According to the Hume-Rothery rules for solid solution formation, favorable substitution is expected when the dopant and host cations share similar crystal structures, ionic radii (within 15%), valence states, and electronegativities. However, these selected Li / Mn / P feedstocks (monoclinic, trigonal, or tetragonal crystal system) are structurally incompatible with the orthorhombic olivine lattice of LFP, hindering direct one-step conversion.

[0030] To address this incompatibility, the inventive approach employs an isomorphous intermediate-driven conversion strategy depicted in FIG. IB): (i) the strategic formation of LMP as a viable intermediate via pre-sintering of feedstocks (Reaction 2), leveraging its isomorphism with LFP, followed by (ii) the reconstruction between LMP and LFP to yield a homogeneous LMFP solid solution (Reaction 3).

[0031] A NH4H2P04+0.5Li2C03+MnC03-+ LiMnPO4+H2O (g)+1.5CO2(g)+NH3(g)

[0032] (2) A

[0033] x

[0034]

[0035] LiMnPO4+(l-x)LiFePO4-> LiMnxFe(1-x)PO4(3)

[0036] Reaction 2 mimics the LMP synthesis route and is thermodynamically favorable (Gibbs free energy (AG) < 0) and kinetically viable, facilitated by the thermal decomposition of Li / Mn / P feedstocks. Thermogravimetric and differential scanning calorimetry (TG-DSC) analysis reveals that thermal decomposition of the LFP / Li / Mn / P feedstocks initiates above -200 °C, coinciding with the onset of LMP formation (FIG. 1C).

[0037] In-situ X-ray diffraction (XRD) tracks the progressive transformation of MnCCh into LMP over this temperature range, while ex-situ XRD detects coherent olivine-type intermediates comprising the coexistence of LFP and LMP phases. Complementary scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy (STEM-EDS) mapping reveals Mn and P convergence within the LFP / LMP intermediate matrix, directly evidencing LMP phase evolution.

[0038] Despite the fact that Mn2+and Fe2+in the LFP / LMP intermediate satisfy the Hume-Rothery criteria for substitutional solid solution formation, Reaction 3 proceeds sluggishly due to its negligible thermodynamic driving force (AG - 0) and the intrinsically slow ion diffusion within the olivine lattice.

[0039] The formation of homogenized LMFP solid solution (Reaction 3) requires atomic-scale Mn-Fe interdiffusion driven by local concentration gradients. However, the inherently slow mobility of Mn and Fe ions within the olivine lattice presents a significant kinetic barrier to the reaction. To overcome this kinetic barrier, mechanochemical activation is employed to facilitate the solid-state upcy cling process by: (i) reducing particle size and enabling atomic-scale mixing to shorten ion diffusion pathways; (ii) introducing structural disorder and increasing surface area to create ion migration shortcuts; and (iii) lowering thermodynamic barriers through the stored energy in amorphous phases. The mechanistic basis by which mechanochemical activation promotes the upcycling reaction is elucidated in this note. The characteristic diffusion time (f) required to reach ion concentration equilibrium is determined by:

[0040] L2

[0041]

[0042] (4) where L represents the diffusion path length, and D denotes the diffusion coefficient. The ion diffusion coefficient within a solid matrix is influenced by total free volume (vf), ionic radius ( / '), and thermal activation energy (a), and is expressed as:

[0043] (_K£d) Ea D

[0044]

[0045] = Doexpvf exptRT) where Do represents the diffusion constant, y is a parameter of order unity, and a serves as a numerical correction factor. R denotes the universal gas constant, while T corresponds to the reaction temperature. The total free volume (vf) - associated with amorphization, disordering, defects, and matrix surface area - plays a crucial role in diffusion behavior. Furthermore, the constituent ions exhibit varying ionic diffusivities that depend on their ionic radii ( / ')•

[0046] According to Eqs. (4) and (5), improving solid-state reaction kinetics and reaching ion concentration equilibrium require reducing the ion diffusion path length and increasing ion mobility. During milling, mechanical energy from oscillating balls (ball milling), sand (sand milling), or pressurized gas (jet milling) induces bond breakage, decomposes the crystalline structure, increases specific surface area, reduces particle size, and improves mixing. This structural disorder enhances ion diffusion by increasing the total free volume (vf) within the matrix. The concomitant reduction in particle size further shortens the diffusion path, enabling faster attainment of concentration equilibrium.

[0047] In a ball milling system, materials are processed through impact and attrition as grinding media collide with them inside a rotating vessel. Zirconia (ZrCh), a common medium with a Mohs hardness of 8-9, is often employed. Milling efficiency is governed by material properties like hardness, density, brittleness, and ductility, with hardness being the most critical factor. For single-material systems, activation can be optimized by adjusting oscillation frequency, media hardness, and the quantity and size of the media. In mixed-material systems, however, component interactions complicate the process. Softer materials fracture more easily and can act as a buffer, protecting harder materials from size reduction and leading to a broader post-milling particle size distribution.

[0048] The mechanical and thermal properties of the feedstock materials are summarized in Table 1. TABLE 1

[0049] Material Thermal Density Mohs Shear Modulus stability (g / cm3) hardness (GPa) LFP Tm~942°C -3.60 5-6 -50-60 Li2CO3Td~720°C 2.00-2.11 3 26-32 MnCO3Td~350°C -3.42 3.5-4 52.1 NH4H2PO Td~190°C -1.80 -2

[0050] 4

[0051] Sucrose Td~186°C 1.59 1.5 33-38 ZrO25.7-6.0 8-9 53.4-86.4

[0052] Within the LFP / Li / Mn / P mixture, the Mohs hardness values of LFP (~5-6), Li2CO3(~3), MnC03(~3.5-4), and NH4H2PO4 (-2) differ significantly. During milling, the softer Li / Mn / P salts absorb most of the impact energy, breaking down and coating the harder LFP particles. This coating acts as a buffer, dissipating energy and shielding the LFP from sufficient mechanical activation. Consequently, the LFP remains mainly crystalline, leading to ineffective mixing and, after sintering, an inhomogeneous LFP / LMP mixture rather than a uniform LMFP solid solution.

[0053] To overcome the kinetic challenges of converting sLFP / Li / Mn / P feedstock into a homogenized LMFP solid solution, a pre-sintering step (450°C, 3 h) is employed. This transforms the Li / Mn / P salts into an LMP phase, creating an LFP / LMP precursor with similar mechanical properties and an identical olivine structure. According to the Hume-Rothery rules, this property similarity promotes homogenization. Subsequent milling of this compatible precursor shortens diffusion pathways and increases defect density, thereby accelerating ion diffusion and the kinetics of solid-solution formation.

[0054] Mechanochemical activation simultaneously addresses limitations by reducing particle size and mixing LFP-rich and LMP -rich domains at the atomic scale, introducing disorder and high-surface-area features that create fast diffusion pathways, and generating amorphous phases with elevated free energy to lower thermodynamic barriers. STEM-EDS mapping confirms finer particles and enhanced nanoscale mixing, while XRD peak suppression indicates disorder and amorphization. These changes yield a 112% increase in crystallization enthalpy (9522 vs. 4494 J g FIG. ID) and a 150 °C reduction in crystallization temperature (491.6 °C vs. 641.9 °C), demonstrating that mechanochemical activation transforms the intermediate into a metastable, high-energy state with reduced thermodynamic and kinetic barriers to Reaction 3.

[0055] During the final homogenization sintering, atomic-scale Mn-Fe interdiffusion, driven by ion concentration gradients between adjacent LFP-rich and LMP-rich domains, transforms the metastable LFP / LMP intermediate into a single-phase LMFP solid solution (Reaction 3). In addition, as a common practice to facilitate the reaction and enhance electronic conductivity, a carbon source is added before sintering as both a reducing agent and a carbon source. Sucrose is widely used for this purpose, however, other possible carbon sources include polyvinyl alcohol (PVA), starch, graphene oxide, and glucose. In-situ high-temperature XRD of the activated intermediate reveals the progressive formation of olivine-structured LiM Fei- PCU at 650 °C, accompanied by peak sharpening, indicative of improved crystallinity. The resulting upcycled LMFP (uLMFP) exhibits an XRD pattern in excellent agreement with both a commercial LMFP (LiMno.6Feo.4PO4, cLMFP) benchmark and the standard reference pattern (JCPDS: 13-0336), confirming the successful synthesis of a phase-pure, well-crystallized olivine material.

[0056] In the industrial production of LMFP via solid-state sintering, ball milling is often employed for elemental mixing and mechanochemical activation, followed by solid-state annealing to form the crystalline structure and achieve carbon coating. As will be recognized by those in the art, while ball milling was used in the embodiments described herein, alternative milling techniques may be used, for example, sand milling or jet milling. Accordingly, references to “milling”, or “ball milling”, include known alternative milling techniques. The inventive direct upcycling of LFP to LMFP emulates the industrial synthesis process of commercial LMFP. The thermochemical properties of the ground LFP / Li / Mn / PCL mixture (LFP / Li / Mn / PCL mix) were characterized by TGA-DSC, revealing individual decomposition peaks for all the LFP / Li / Mn / PCL3' source components (see FIG.

[0057] 2B). A weight loss of approximately 30% was observed as the temperature increased from room temperature to 700°C. In contrast, after ball milling treatment, the melting peaks for MnCCh and Li2COs in the LFP / Li / Mn / PCL-mill disappeared (see FIG. 2C), indicating that MnCCh and Li2COs had reacted with other components. This phenomenon is consistent with gas generation during the milling process. Additionally, the weight loss of the LFP / Li / Mn / PCh-mill from room temperature to 450°C was approximately 14%, maintaining a stable mass between 450°C and 900°C. To mitigate gas generation and mass changes that could impact element mixing and crystallization during the LMFP upcycling process, a pre-sintering treatment at 450°C is used for the LFP / Li / Mn / PCri-mill specimen prior to the upcycling sintering step at 650°C.

[0058] A flow diagram of the steps of the direct upcycling process from spent LFP to LMFP is provided in FIG.3A. The process involves several key steps after mixing (101): grinding to premix the raw material particles, milling (102) to mix, size-reduce, and chemically activate the particles, pre-sintering (103) to de-gas, secondary milling to further mix the elemental components and apply a carbon coating (104), and upcycling sintering (105) to form the crystalline structure. The intermediate product after each step is named as LFP / Li / Mn / PCU-mix, Li / Mn / Fe / PCh-mill, LMFP_pre-sintering, LMFP_ps / sucrose, and uLMFP / C, respectively. Specifically, the pre-sintering step (103) at 450°C serves to de-gas and establishes a stable microenvironment via the formation of isomorphous intermediates for subsequent crystalline structure forming during the upcycling sintering step (105) at 650°C. Additionally, sucrose (or other suitable carbon source) is coated onto the surface of the LMFP_pre-sintering, acting as both a reducing agent and a carbon coating source (105). This sugar coating ensures a favorable carbothermal environment during the sintering process, and the residual conductive carbon coating enhances the electrical conductivity of the LMFP materials. Alternative sugars for forming this coating include PVA, starch, graphene oxide, and glucose. Based on the TG analysis, the mass loss of the LMFP-sp / sucrose with 10 wt% sucrose is approximately 8.3% as the temperature surpasses 650°C, primarily due to the carbonization of sucrose. For purposes of this evaluation, pristine LFP was used as the feedstock to eliminate variations that could be caused by different spent LFP feedstocks. Subsequently, various spent LFP cathodes were examined based on the optimized process. The right side of FIG. 3A illustrates the chemical reaction that occurs during processing, identified as two reaction steps, the first reaction corresponding to steps ( 101 )-(l 03) and the second reaction corresponding to steps (104)-(105).

[0059] FIG. 3B employs an alternative approach to illustrating the inventive process for upcycling of spent LFP cathode to uLMFP / C at a 50g scale batch, providing photographic images of the materials at each step within the process. Panel (a) depicts collected LFP-EV powder from spent LIBs. Panel (b) shows a mixture of LFP / Li / Mn / PO4 sources. Panel (c) shows the mixture of LFP / Li / Mn / PCh sources distributed into ball mill jars. Panel (d) shows a mixture of LFP / Li / Mn / PCL sources after the first ball milling treatment. Panel (e) shows separation of balls and milled LFP / Li / Mn / PCk sources using a sieve. Panel (f) depicts milled LFP / Li / Mn / PCh sources. In Panel (g), LFP / Li / Mn / PCk precursor is shown after pre- sintering at 450°C. Panel (h) shows the addition of 10 wt% sucrose. In Panel (i), the obtained LMFP precursor is depicted after the second ball milling treatment. Panel (j) shows the obtained uLMFP / C after sintering at 650°C for 10 hours in an Ar atmosphere. In Panel (k), the resulting 42g uLMFP / C is depicted.

[0060] Transmission Electron Microscopy and Energy Dispersive X-ray Spectroscopy (TEM-EDS) were employed to elucidate the elemental level merging of LFP / Li / Mn / PCL during the upcycling process. In the LFP / Li / Mn / P04-mix specimen, LFP / Li / Mn / PCL particles were loosely and unevenly distributed, indicating that grinding alone is insufficient to prepare the LMFP precursor. In the Li / Mn / Fe / phosphate-ball mill specimen, the size of LFP / Mn / phosphate-related particles was reduced to less than 500 nm, and they were closely aggregated and evenly distributed at the particle level. However, most Mn- and Fe-related particles still existed individually and had not merged. In the LMFP_pre-sintering specimen, Mn was predominantly coated on the surface of Fe, forming a Mn@Fe structure. This phenomenon is consistent with literature reports of Mn doping into FePCL particles in a gradient manner. In the LMFP_ps / sucrose specimen, the sizes of LFP / Li / Mn / phosphate-related particles were further reduced, well-mixed, and even partially merged. However, within each particle, the distribution of Mn, Fe, and P elements remained non-uniform. In the uLMFP / C specimen after sintering at 650°C, the Mn / Fe / PCL elements were evenly distributed at the elemental level. The uLMFP / C demonstrated a granular structure, uniformly dispersed, with particle sizes ranging from 100 nm to 400 nm.

[0061] Referring to FIGs. 3C and 3D, X-ray Diffraction (XRD) profiles were employed to reveal the crystalline structure evolution of LMFP precursors during the upcycling process (The LFP / Li / Mn / PCh-mix sample exhibits characteristic peaks corresponding to LFP, Li2COs, MnCCh, and NH4H2PO4 at 29.76°, 21.21°, 31.37°, and 16.69°, respectively. After milling, the peaks associated with Li2COs and NH4H2PO4 vanish, and a new peak emerges at 18.46°, indicating that MnCCh and Li2COs had reacted with other components. Subsequent presintering treatment at 450°C eliminates all Li / Mn / PCL salt-related peaks, leaving only the characteristic peaks of residual LFP and newly formed LMFP. This observation aligns with the weight loss observed in the LFP / Li / Mn / PCL-ball mill specimen, which is 14% and lower than the 30% weight loss observed in the LFP / Li / Mn / PCL-mix specimen (FIGs. 2B and 2C). Furthermore, the secondary milling enhances the conversion of LFP to LMFP, demonstrating the effectiveness of the milling operation in achieving element mixing. Upon sintering at 650°C, the LMFP peaks sharpen, with a narrower full width at half maximum (FWHM), confirming the good crystalline structure of the upcycled carbon-coated uLMFP / C. Remarkably, the XRD profile of the uLMFP / C closely matches that of the commercial reference LiMno.6Feo.4PO4 cathode (LMFP-pri stine) and standard LMFP (JCPDS: 13-0336), providing additional evidence of the favorable crystalline structure achieved in the upcycled material.

[0062] The evolution of the chemical valence states of iron and manganese ions during the upcycling process was elucidated using Soft X-ray Absorption Spectroscopy (XAS). For the Fe L-edge (see FIG. 3E), the X-ray absorption spectra exhibit strong absorption features due to the spin-orbit splitting of the Fe 2p core hole around 705-710 eV, corresponding to the transition from the Fe 2p63d6to the Fe 2p63d7orbital. This result indicates that the Fe [^-absorption edge shifts to higher energy with an increasing oxidation state of Fe. The Fe Ls-absorption peaks of Fe (III) and Fe (II) appear at approximately 709.1 eV and 707.6 eV, respectively. In the LFP / Li / Mn / PCL-mix specimen, most Fe ions are present as Fe (II). After ball milling treatment, in the Li / Mn / Fe / PCL-ball mill specimen and LMFP_ps / sucrose specimen, partial Fe (II) has been oxidized to Fe (III), indicating the effectiveness of milling for elemental mixing and chemical activation. After upcycling sintering, the Fe (III) has been reduced to Fe (II) in the uLMFP / C specimen, indicating the effectiveness of upcycling sintering for crystalline structure formation. For Mn, the L-edge X-ray absorption spectra indicate that Mn ions maintain a 2+ valence state throughout the upcycling process (see FIG. 3F).

[0063] To verify the carbon coating and element distribution of uLMFP / C, high-resolution TEM characterization and elemental mapping analysis were performed. FIG. 4A provides a high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of uLMFP / C. As seen in the image, the particle edge clearly displayed an amorphous carbon layer distributed around the periphery of LMFP, approximately 2.75 nm thick, confirming successful encapsulation of LMFP particles within the carbon layer. According to the carbon element analysis, the uLMFP precursors synthesized using 10 wt% sucrose result in 3.22 wt% carbon in uLMFP / C. In HAADF-STEM, a well-defined olivine crystal structure was observed. The interplanar spacing, determined as 0.4305 nm using inverse fast Fourier transform (iFFT), corresponds to the (110) crystal plane of the Pmnb-type structure. According to the inductively coupled plasma mass spectrometry (ICP-MS) analysis, the resulting uLMFP / C has an elemental composition of Li1.o49Mno.59Feo.41Po.9s4O3.936, which closely matches the expected values of the synthesis. Referring to FIG.4B, the elemental mapping revealed a uniform distribution of carbon, iron (Fe), manganese (Mn), phosphorus (P), and oxygen (O) elements on individual uLMFP / C particles, indicating the structural uniformity of uLMFP / C.

[0064] To further investigate the valence states and concentration profiles of Fe / Mn in the uLMFP / C, a series of electron energy loss spectroscopy (EELS) analyses were conducted along a line from the surface to the interior (points 1 to 16) on a random uLMFP / C particle (see FIG. 4C, where the image at the top left indicates the 16 points). The fixed positions of the Fe L-edge (710.4 eV) and Mn L-edge (640.6 eV) confirm the homogeneous oxidation states (+2) for Fe and Mn in both surface and bulk phases. Notably, the relative intensity of the Fe L-edge and Mn L-edge remains stable from the outermost surface to the interior of the particle, confirming the uniform distribution of Fe and Mn concentrations in uLMFP / C particles. FIG. 4D plots the intensity ratio of Mn and Fe L-edges (leso / bis) along the scan line. This observation differs from that of Zhou et al. (supra), who produced Mn-gradient-doped LiMno.25Feo.7sPO4 material based on a one-step sintering using leached iron phosphate (FePCL) as feedstock. Thus, the pre-sintering and secondary milling support the uniform distribution of Fe / Mn within the entire particle. Pre-sintering secures a mass-stable environment for element mixing and crystallization during the LMFP upcycling process.

[0065] FIG. 4E provides DSC curves showing the melting temperature of commercial LFP (cLFP), uLMFP-wp, uLMFP, and commercial LMFP (cLMFP).

[0066] The above structural analysis demonstrates that uLMFP / C material has significant potential to exhibit excellent electrochemical performance. To gain a deeper understanding and comparison of the electrochemical performance, CR2032-type coin cells were assembled using different active materials. The first charge-discharge profiles for LFP-p, uLMFP / C, and LMFP-p materials at 0.1 C (1 C = 170 mAh / g) in a voltage range of 2.5-4.1 / 4.5 V (vs Li / Li) are presented in FIG. 5A. In contrast with LFP-p (which exhibits a single voltage plateau at 3.4 V), uLMFP / C shows two voltage plateaus at around 3.5 V and 4.0 V, corresponding to the oxidation reactions of Fe2+ / Fe3+and Mn2+ / Mn3+, respectively. At a discharge rate of 0.1 C, the specific discharge capacities for LMFP-p, LFP-p, and uLMFP / C are 151.9, 155.4, and 154.1 mAh / g, respectively. The discharge capacity of uLMFP / C is approaching that of LFP-p and is slightly higher than LMFP-p, indicating the successful upcycling of LFP to LMFP. The dQ / dV profiles of the three materials are shown in FIG. 5B. The profiles of LMFP-p and uLMFP / C are similar. Compared to LFP-p, uLMFP / C shows minimal voltage polarization (0.092 V), revealing the good crystalline structure of the upcycled materials. This observation confirms that uLMFP / C exhibits preferable carbon coating, with 3.6 wt% carbon, resulting in lower polarization and better electrochemical reversibility. In comparison, the carbon content for LFP-p and LMFP-p is 1.44 wt% and 1.8 wt%, respectively.

[0067] The rate capability of the three materials at different current densities ranging from 0.1 to 3 C is presented in FIG. 5C. While LMFP-p and LFP-p exhibit similar or higher capacity when the current density is below C / 3, uLMFP / C demonstrates significantly higher capacity at rates above C / 2, primarily due to the higher carbon content. At 3 C, uLMFP / C achieves a specific discharge capacity of approximately 120 mAh / g, which is -14% and -41% higher than that of LMFP-p and LFP-p, respectively. Additionally, when the current density is restored to C / 3 after charge-discharge at 3 C, the specific discharge capacity of uLMFP / C recovers from 120 mAh / g (at 3 C) to 142.1 mAh / g (at C / 3). After cycling at C / 3 for 200 cycles, uLMFP / C still demonstrates a specific discharge capacity of 140.87 mAh / g, with capacity retention rates exceeding 99%, significantly higher than those of LMFP-p and LFP-p. The excellent capacity retention of uLMFP / C confirms its outstanding structural stability, attributed to the uniform distribution of Mn and Fe and the sufficient carbon coating.

[0068] Considering the higher voltage plateaus of LMFP, the energy density of uLMFP / C (576.5 Wh / kg) increases by approximately 9% after doping with Mn (LFP-p: 527.9 Wh / kg), which is comparable to that of LMFP-p (577.2 Wh / kg), as shown in FIG. 5D. uLMFP / C demonstrates significantly higher energy density than LMFP-p and LFP-p at rates above C / 2, primarily due to the higher carbon content. At 3 C, uLMFP / C achieves a specific energy density of approximately 420 Wh / kg, which is -13.5% and -56% higher than the 370 Wh / kg and 270 Wh / kg obtained by LMFP-p and LFP-p at 3 C, respectively. Additionally, when the current density is restored to C / 3 after charge-discharge at 3 C, the specific energy density of uLMFP / C recovers from 420 Wh / kg (at 3 C) to 530 Wh / kg (at C / 3), with energy retention rates exceeding 99% after 100 cycles, significantly higher than those of LMFP-p and LFP-p. Furthermore, the energy efficiency of uLMFP / C during C / 3 cycling is approximately 94.5%, which is also much higher than the -90% and -91.5% for LMFP-p and LFP-p, respectively. This indicates that uLMFP / C exhibits better recovery of capacity, which can be attributed to the enhanced structural stability of uLMFP / C due to the carbon coating.

[0069] After parameter optimization using LFP-p, spent LFP cathode black mass from different types of end-of-life LFP cells was used to confirm the upcy cling process at a 100g scale. For example, the end-of-life LFP prismatic cells (300 Ah) were obtained from EVE Energy (Guangdong, CN) (referred to as “LFP -EVE”). The process of spent LFP cathode separation, extraction, and black mass collection involves the steps of: (a) remove outer package to expose LFP-EVE jelly roll; (b) strip LFP-EVE cathode electrode from jelly roll; (c) stir cathode electrode strip in DI water; (d) delaminate cathode layer after 3 minutes of stirring, with water pH ~7.3; (e) separate Al foil and LFP-EVE cathode strips, achieving purity and yield close to 100%; (f) drying the LFP-EVE cathode strip at 80°C; (g) dry blend and (h) grind LFP-EVE cathode strip at 20k rpm to obtain LFP-EVE powder; (i) collect LFP-EVE powder used as feedstock for LMFP upcycling (187g collected); (j) collect Al foil. The collected LFP-EVE powder was used as feedstock for Li / Fe / PCL sources. The lithium deficiency and composition of the spent LFP were determined via ICP-MS, revealing a composition of Lio.ssFePCL. With the addition of designed Li / Mn / PCL sources, three uLMFP samples using spent LFP (uLMFP-S) were prepared using the upcycling process described above to produce powder. Specifically, uLMFP-S64 was prepared using spent LFP-EVE black mass with a Mn: Fe ratio of 6:4. uLMFP-S73 was prepared using spent LFP-EVE black mass with a Mn: Fe ratio of 7:3. uLMFP-S-mix was prepared using a mix of two types of spent LFP black mass, sLFP-EVE and sLFP-A123 (from cylindrical cells obtained from A123 Systems, LLC, China), with a Mn: Fe ratio of 6:4. The tap density of uLMFP-Sl, ULMFP-S2, and uLMFP-S3 is 1.143, 1.138, and 1.176 g / cm3, respectively, slightly higher than the 1.053 g / cm3obtained for LMFP-p, indicating the good processability of uLMFP-S in electrode fabrication.

[0070] SEM-TEM-EDS was employed to elucidate the morphological evolution from sLFP-EVE to uLMFP-Sl. The sLFP-EVE cathode composite was collected from the cathode electrode by wet blending in DI water to delaminate from the Al current collector and remove the electrolyte. The SEM image in FIG. 6A provides an example of the sLFP-EVE cathode composite morphology. After drying, blending, and sieving, the collected sLFP-EVE powder (FIG.6B) was composed of sLFP particles (200 nm to 1 pm) aggregated with PVDF binder and conductive carbon (~50 nm). After the upcycling process, the obtained uLMFP-S demonstrated individual particles, with sizes ranging from 100 nm to 500 nm (FIG. 6C), like LMFP-p (FIG. 6D). Due to the presence of PVDF binder and conductive carbon, with 10 wt% sucrose addition, the carbon content in uLMFP-S 1 is 4.04 wt%, which is higher than 3.22 wt% for uLMFP / C with the same sucrose addition. The HAADF-STEM of uLMFP-Sl indicated that the particle edge clearly displayed an amorphous carbon layer distributed around the periphery of LMFP, approximately 3.16 nm thick, thicker than 2.75 nm for uLMFP / C, confirming successful encapsulation of LMFP particles within the carbon layer, as shown in FIG. 6E. In HAADF-STEM, a well-defined olivine crystalline structure was observed. The interplanar spacing, determined as 0.239 nm using inverse fast Fourier transform (iFFT), corresponds to the (041) crystal plane of the Pmnb-type structure. The elemental mapping revealed a uniform distribution of Fe, Mn, and P, indicating the structural uniformity of uLMFP-S. However, the elemental mapping of uLMFP-S revealed an uneven distribution of carbon and fluorine elements on individual uLMFP-S particles, mainly due to the presence of PVDF binder and conductive carbon in the spent LFP cathode.

[0071] To understand the surface elemental chemical states of uLMFP-S, XPS was employed to investigate the elemental chemical states of the uLMFP-S material, with a focus on Cis and FIs, which are primarily related to the evolution of the PVDF binder during the upcycling process. In the Cis spectrum of uLMFP-S-mix (FIG. 6F), the peak at 288.7 eV represents O-C=O oxygen-containing functional groups, indicating the presence of MnCCh and Li2CC>3. This peak significantly decreased for the uLMFP-S-bml, indicating that milling effectively activates these precursors. In uLMFP-S-bm2, a new peak at approximately 286 eV represents C-O-C from the addition of sucrose. This peak disappeared in the uLMFP-S-s2 specimen, indicating successful carbonization during the upcycling sintering. In the FIs spectrum shown in FIG. 6G, the peaks at 688-689 eV and 684-685.5 eV represent organic fluorine and metal fluorides, respectively. In the uLMFP-S-mix and uLMFP-S-bml specimens, most fluorine is present as organic fluorine, indicating the existence of PVDF. After sintering, most fluorine has been converted into metal fluorides, indicating the formation of LiF. Based on the Fluoride Ion Selective Electrode (ISE) test, the F content in the uLMFP-S is 0.1 wt%, indicating that most of the fluorine originally from the PVDF has been removed via carbonization.

[0072] The structural analysis reveals that the uLMFP-S material holds substantial promise for superior electrochemical performance. The initial charge-discharge profiles of spent LFP-EVE and three uLMFP-S materials at 0.1 C (1 C = 170 mAh / g), within a voltage range of 2.5-4.1 / 4.5 V (vs Li Li), are depicted in FIG. 6H. Due to lithium deficiency and structural defects, S-LFP exhibits a specific discharge capacity of 145.8 mAh / g, which is lower than the 155.4 mAh / g demonstrated by LFP-p. The specific discharge capacity of S-LFP declines rapidly, retaining merely 80% capacity after 100 cycles at C / 3 (see FIG. 61). Post-upcycling, the uLMFP-S64 achieves a specific discharge capacity of 152.3 mAh / g, closely matching those of LMFP-p and uLMFP / C. After 100 cycles at C / 3, uLMFP / C maintains a specific discharge capacity of 136.2 mAh / g, with capacity retention rates surpassing 97.8%. With a high loading electrode (CAM: PVDF: Super P = 0.94:0.03:0.03, 2 mAh / cm2), uLMFP-S64 delivers a specific discharge capacity of 128.7 mAh / g at C / 3 (see FIG. 6J). In a full cell configuration using Gr as the anode, uLMFP-S64 achieves a specific discharge capacity of 126.6 mAh / g at C / 3 and retains a specific discharge capacity of 116.3 mAh / g with 92% capacity retention after 100 cycles. These findings highlight the exceptional structural stability and validate the efficacy of the upcycling method for spent LFP cathodes.

[0073] To further demonstrate the robustness and versatility of the inventive approach, additional cathodes were synthesized from different feedstocks: “uLMFP-sE64” from sLFP-E with a Mn: Fe ratio of 6:4; “uLMFP-sE73” from sLFP-E with a Mn: Fe ratio of 7:3; and “uLMFP-sEA64” from a 1:1 blend of sLFP-E and sLFP-A with a Mn: Fe ratio of 6:4. For a 1 kg batch of uLMFP-sE64, ball milling was conducted in four 2 L jars for 5 h (MITR, YXQM-8L), followed by pre-sintering and final homogenization sintering in a tube furnace under nitrogen flow (Thermo Scientific, Lindberg / Blue M).

[0074] The uLMFP-sE73 variant, with a higher Mn: Fe ratio of 7:3 achieved a higher average voltage than uLMFP-sE64 (3.85 vs. 3.78 V) due to its increased Mn content. Although its capacity was slightly lower (148.8 mAh g1at 0.1C), attributable to the inherently lower kinetics of the Mn-rich phase, it exhibited good cyclability with 98.7% retention after 100 cycles. To test the tolerance for varied feedstock, we synthesized uLMFP-sEA64 from a blend of sLFP-E and sLFP-A (spent cylindrical cell from another manufacturer) while maintaining a Mn: Fe ratio of 6:4. This cathode delivered 147.7 mAh g1at 0.1C and 138.7 mAh g1at C / 3, and near 100% retention of its capacity after 300 cycles at C / 3. Despite minor capacity variations between uLMFP-s samples attributed to differing feedstocks, the maintained high performance across varying origins and Mn ratios underscores the exceptional scalability and versatility of our synthesis strategy. This maintained electrochemical performance across upcycled LMFP cathodes, derived from various spent LFP cathodes collected from diverse types of end-of-life batteries, demonstrates the robustness of the inventive upcycling method.

[0075]

[0076] 1: Materials and

[0077] Lithium carbonate (Li2COs, Fisher Chemical, AR> 99.99%), manganese carbonate (MnCCri, Sigma, AR > 99%), ammonium dihydrogen phosphate (NH4H2PO4, Sigma, AR > 99%), sucrose (CeH Oe, Fisher Chemical, AR > 99%), polyvinylidene fluoride (PVDF, Arkema, Mw = ~110- 130k), N-methyl-2-pyrrolidone (NMP, Fisher Chemical, AR > 99%), and Super-P were used as received. The commercial lithium iron phosphate cathode (pLFP-C, Canrd Ltd, China) served as the starting material for the upcycling study. Pristine LMFP (MSE Ltd, US) or pLMFP-C was used as a control for comparison.

[0078] Example 2: Direct upcycling of LFP to LMFP

[0079] The upcycling process involved primarily two steps: milling and sintering. Referring again to FIG. 3A, in steps (101) and (102), LFP (lithium iron phosphate) was combined in a ZrCE ball milling jar with manganese carbonate, lithium carbonate, and ammonium dihydrogen phosphate (with a molar ratio of Li: Mn: Fe: P as 1.05:0.6:0.4:1). After adding an appropriate amount of a solvent to cover the mixture, the mixture was milled at a speed within a range of 300 r / min to 700 r / min for 2-10 hours. For testing, ball milling was performed at 600 r / min for 6 hours. Appropriate solvents that may be used include ethanol, water, acetone, and methanol. Subsequently, the precursor was dried at 80°C in a convection oven and then subjected to the first sintering (step (103)) in an inert (e.g., argon, nitrogen) atmosphere within a tube furnace. A temperature range of 300°C to 500°C may be used for about 1 to 5 hours. For testing, the temperature and time were 450°C and 4 hours, respectively. Next, in step (104), the obtained LMFP (lithium manganese iron phosphate) precursor was mixed with a certain amount of sucrose, ranging from 1 wt% to 20 wt%. For testing, 10 wt% was used. The LMFP precursor was then ball milled at 300-700 r / min for an additional 2 to 10 hours, with the duration depending on the rotational speed. For testing, ball milling was performed at 600 r / min for 6 hours. Finally, the second sintering step (105) was carried out within a temperature range of 500°C - 800°C for a period of about 8 to 20 hours. For testing, the upcycle conditions were at 650°C for 10 hours in an inert (Ar, N2) atmosphere, resulting in the production of upcycled LMFP (uLMFP).

[0080] Example 3: Preparation of spent LFP powder and its direct upgrading to LMFP After parameter optimization, two spent LFP cathodes from end-of-life LFP cells were used to confirm the upcycling process. These end-of-life LFP prismatic cells (300 Ah), named sLFP-EVE, were provided by EVE company. The prismatic cells were manually disassembled in a fume hood, and the long cathode strip was carefully segregated and stored in the fume hood for 2 days to allow solvent evaporation. After disassembly, the cathode strips were washed with DI water to remove residual electrolyte salt and then dried at 50°C in a convection oven. Next, the cathode strips were dissected into approximately 4x12 mm pieces using a paper shredder, followed by grinding in a coffee blender and sieving through a sieve with 125 pm holes to separate them from the aluminum foil. The sieved sLFP-EVE powder served as the feedstock material for upcy cling, following the same process described previously. End-of-life spent LFP cylindrical cells (10 Ah), named sLFP-A123, were provided by A123 company. The cathode powder was obtained using a process similar to that used for the sLFP-EVE CAM.

[0081] Example 4: Materials Characterization

[0082] The chemical composition of LFP and LMFP powders was evaluated by inductively coupled plasma mass spectrometry (ICP-MS, Thermo Scientific, iCAP RQ model). Their crystal structures were examined by X-ray powder diffraction (XRD) employing a Bruker D2 Phaser (Cu Ka radiation, 1=1.5406 A). The morphology of the cathode powders was observed by FEI Apreo scanning electron microscope (SEM) with X-Max 80 EDS detector. The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) curves of the upcycling process were collected in alumina pans by An Instruments™ Discovery SDT 650™ simultaneous DSC / TGA. X-ray photoelectron spectroscopic (XPS) measurement was conducted with an AXIS Supra by Kratos Analytical with an Al Ka anode source working at 15 kV and 10-8 Torr chamber pressure. The spectra data were processed by Avantage software. All spectra were calibrated with the hydrocarbon C is peak at 284.6 eV. To measure the metallic fluoride content, the samples were dissolved in HC1 solution, and the concentrations of free F ions were measured using a Fluoride Ion Selective Electrode (ISE, Orion™, Thermo Scientific).

[0083] Example 5: Cell Assembly and Electrochemical Measurement

[0084] Pristine LMFP-MSE served as the control for comparison with regenerated samples. Typically, cathode powder was mixed with PVDF (KYNAR 2800) and carbon black (Super P) in NMP (Sigma-Aldrich, anhydrous 99.5%) at a mass ratio of 8:1:1 to form a homogeneous slurry. The slurries were cast using a doctor blade and then dried under vacuum at 120°C for lOh. Cathode discs of 12 mm were cut, calendared, and directly used for assembling CR2032 coin cells in an Argon environment glovebox. Li metal was used as the anode, and a polypropylene membrane (Celgard 2500) served as the separator. The electrolyte used was battery-grade lithium hexafluorophosphate (LiFPe) solution in ethylene carbonate (EC) and ethyl methyl carbonate (EMC), with a composition of 1.0 M LiPFe in ECZEMC = 30 / 70 (v / v) (Gen2, Gotion Ltd, USA). The mass loading was approximately 4 mg / cm2The cells were rested for 8 hours, followed by galvanostatic charge-discharge cycles tested using a Neware battery cycler within a voltage window of 2.5 to 4.5 V. The initial activation included 4 cycles at C / 10 (1 C = 170 mAh / g), followed by long cycling at different rates. The graphite anode (92% graphite, 2% C45 carbon, 6% Kureha 9300 PVDF binder) for full cells was made by ANL CAMP Facility, with an areal capacity of 1.93 mAh / cm2. The cathode electrode for the full cell was fabricated with 94% uLMFP-S, 3% super-p, and 3% PVDF. The graphite] |uLMFP-S full cells were assembled with a Negative / Positive (N / P) capacity ratio of 1.05 based on the specific capacities of LMFP (150 mAh / g) and graphite (340 mAh / g).

[0085] The inventive approach described herein provides an innovative direct upcycling scheme (sLFP-uLMFP) for spent LFP cathodes, producing carbon-coated LMFP material with evenly distributed elements. The protocol uses spent LFP as the feedstock to synthesize LMFP, bypassing the conventional sLFP decomposition step. Demonstrating robust efficacy, this upcycling approach successfully converts various spent LFP cathodes, collected from different types of EoL batteries, into high-performance uLMFP cathodes with varying Mn: Fe ratios on a 100g scale. The upcycled LiMno.eFeo.4PO4 material exhibits electrochemical performance comparable to pristine materials. This environmentally friendly and economical method exhibits significant potential for revolutionizing the recycling and upcycling framework for spent LFP LIBs.

Claims

CLAIMS:

1. A method for direct upcycling of spent LiFePC>4 (sLFP) cathode materials comprising:milling a particle mixture comprising sLFP cathode material particles with particles comprising each of a manganese source, a lithium source, and a phosphate source to form milled particles;pre-sintering the milled particles to form isomorphous intermediate particles; secondary milling the isomorphous intermediate particles to produce milled intermediate particles;applying a carbon coating to the milled intermediate particles to form carbon-coated particles; andupcycle sintering the carbon-coated particles to form uLMFP / C particles.

2. The method of claim 1, wherein the phosphate source is a phosphate salt selected from NH4H2PO4, (NH4)2HPO4, H3PO4, and LiH2PO4.

3. The method of claim 1, wherein the lithium source is a lithium salt selected from Li2CO3, LiCH3CO2, Li2C2O4, LiOH, and LiH2PO4.

4. The method of claim 1, wherein the manganese source is a manganese salt selected from MnCCh, MnO, M11C2O4, and Mn(CH3COO)2.

5. The method of claim 1, wherein the particle mixture comprises LiM Fei-xPC, wherein 0<x<l and a ratio of Li: P is within a range of 1.0-1.1.

6. The method of claim 1, wherein milling comprises ball milling comprising: disposing the particle mixture in a ball mill jar;adding a solvent to the particle mixture; andoperating a ball mill at a speed within a range of 300 r / min to 700 r / min for from 2-10 hours.

7. The method of claim 1, wherein pre-sintering comprises heating the milled particles in a furnace at a range of about 300°C to 500°C for about 1 hour to 5 hours in an inert atmosphere.

8. The method of claim 1, wherein secondary milling comprises ball milling comprising:disposing the milled intermediate particles in a ball mill jar;adding a solvent to the milled intermediate particles; andoperating a ball mill at a speed of about 300 r / min to 700 r / min for from 2 hours to 10 hours.

9. The method of claim 1, wherein applying a carbon coating comprises adding 1 wt%-20 wt% sugar to the milled intermediate particles.

10. The method of claim 1, wherein upcycle sintering comprises heating the carbon-coated particles in a furnace at a range of about 600°C to 800°C for about 8 hours to 20 hours in an inert atmosphere.

11. A method for direct upcycling of spent LiFePC>4 (sLFP) cathode materials comprising:premixing particles of sLFP cathode material with particles comprising each of a manganese source, a lithium source, and a phosphate source to form LFP / Li / Mn / PC -mix particles;first milling the LFP / Li / Mn / PC -mix particles to form Li / Mn / Fe / PC -milled particles;pre-sintering the Li / Mn / Fe / PC -milled particles to form LMFP_pre-sintered particles comprising isomorphous intermediates;second milling the LMFP_pre-sintered particles to further mix the elemental components and form milled LMFP_pre-sintered particles;applying a carbon coating to the milled LMFP_pre-sintered particles to form carbon-coated LMFP_ps particles; andupcycle sintering the carbon-coated LMFP_ps particles to form uLMFP / C particles having a crystalline structure.

12. The method of claim 10, wherein the phosphate source is a phosphate salt selected from NH4H2PO4, (NF ^HPC, H3PO4, and LiFbPCh.

13. The method of claim 10, wherein the lithium source is a lithium salt selected from Li2CO3, LiCT CC, Li2C2O4, LiOH, and LiH2PO4.

14. The method of claim 10, wherein the manganese source is a manganese salt selected from MnCCh, MnO, MnC2C>4, and Mn(CH3COO)2.

15. The method of claim 10, wherein the LFP / Li / Mn / PO4-mix particles comprise LiMmFei-xP04, wherein 0<x<l and a ratio of Li: P is within a range of 1.0-1.1.

16. The method of claim 10, wherein milling comprises ball milling comprising: disposing the LFP / Li / Mn / PC -mix particles in a ball mill jar;adding a solvent to the LFP / Li / Mn / PC -mix particles; andoperating a ball mill at a speed within a range of 300 r / min to 700 r / min for from 2-10 hours.

17. The method of claim 10, wherein pre-sintering comprises heating the Li / Mn / Fe / PC -milled particles in a furnace at a range of about 300°C to 500°C for about 1 hour to 5 hours in an inert atmosphere.

18. The method of claim 10, wherein second milling comprises ball milling comprising:disposing the LMFP_pre-sintered particles in a ball mill jar;adding a solvent to the LMFP_pre-sintered particles; andoperating a ball mill at a speed of about 300 r / min to700 r / min for from 2-10 hours.

19. The method of claim 10, wherein applying a carbon coating comprises adding 10 wt% sucrose to the milled LMFP_pre-sintered particles.

20. The method of claim 10, wherein upcycle sintering comprises heating the carbon-coated LMFP_ps particles in a furnace at a range of about 600°C to 800°C for about 8 hours to 20 hours in an inert atmosphere.

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