The rapid proliferation of new energy vehicles (NEVs) leads to an unprecedented surge in the demand for lithium-ion batteries (LIBs). Among various LIBs, lithium iron phosphate (LFP) batteries have a significant market due to their inherent advantages, including superior thermal and chemical stability, low manufacturing cost, long cycle life (i.e., 2000-6000 cycles), and environmental friendliness. However, the service life of LFP batteries in NEVs is normally 6-10 years. These batteries are retired after decaying their capacity to below 80% of the initial value. The massive and growing volume of spent LFP batteries presents both a critical environmental challenge and a waste of valuable metal resources. Improper disposal can lead to the release of toxic substances from the electrolyte and heavy metals. Conversely, the spent cathodes contain abundant lithium (Li) and iron (Fe) that hold a significant economic value for recovery. Therefore, developing green, efficient, and economically viable recycling technologies is crucial for closing the material loop, ensuring resource security, and mitigating environmental hazards. This review represents the existing research status of two primary routes for managing spent LFP cathode materials, i.e., direct regeneration and hydrometallurgical/pyrometallurgical recycling. The underlying mechanisms, advantages, limitations, and key technological bottlenecks of each method are critically analyzed. Furthermore, future perspectives for the sustainable and efficient utilization of spent LFP materials are proposed to guide both academic research and industrial application.

This review systematically analyzes and compares the principal technologies for processing spent LFP cathode materials based on their structural integrity. For LFP materials that the olivine crystal structure remains largely intact despite Li⁺ loss, some direct regeneration methods are applicable. These motheds include (1) solid-state sintering, which involves supplementing missing Li, Fe, or P sources and calcining at high temperatures (i.e., typically 600-800 ℃) under an inert atmosphere to restore stoichiometry and repair defects; (2) hydrothermal synthesis, which uses aqueous lithium salts (e.g., LiOH, Li₂SO₄) and reducing agents (e.g., organic acids) at lower temperatures (i.e., 80-220 ℃) to re-lithiate the material and reduce Fe³⁺ back to Fe²⁺; and (3) electrochemical re-lithiation, where an external electric field drives Li⁺ insertion into the degraded cathode, potentially without dismantling the battery cell. For severely degraded LFP with an irreversibly damaged structure, full recycling methods are employed to extract valuable metals. These are primarily based on hydrometallurgy, i.e., acid leaching (using inorganic acids like H₂SO₄, H₃PO₄ or organic acids like oxalic, citric, formic acid), alkaline leaching (using NaOH for selective Li extraction), and bioleaching (using microbial metabolites). Pyrometallurgical processes that are often assisted by some additives like Na₂CO₃ or NaOH are also reviewed for their ability to convert LFP into separable metal compounds at high temperatures.

Direct regeneration is advantageous due to its short process flow, low cost, and high resource efficiency, making it a preferred industrial approach when applicable. Solid-state sintering offers high throughput and excellent recovery of electrochemical performance (discharge specific capacity up to ~147 mA·h·g^-1), but suffers from high energy consumption and potential impurity introduction. Hydrothermal synthesis operates under milder conditions (as low as 80 ℃), yielding repaired LFP with performance comparable to virgin material (>145 mA·h·g^-1), though it requires a strict control of pH value and a specialized autoclave equipment. Electrochemical methods avoid disassembly but face some challenges in scaling up due to the low repair efficiency. For full recycling, acid leaching achieves the maximum metal extraction rates (i.e., Li > 98%), but generates large volumes of acidic waste and introduces impurities. Organic acids offer a greener alternative but are costly and often less effective for Fe/P recovery. Selective Li leaching with oxalic acid or formic acid shows a promising potential via leaving Fe as a solid residue. Alkaline leaching is highly selective, but requires harsh conditions. Bioleaching is environmentally friendly, but suffers from slow kinetics and low efficiency. Pyrometallurgy is robust and feedstock-flexible, effectively converting LFP into soluble Li salts and magnetic Fe compounds, with Li recovery of exceeding 99% with Na₂CO₃ as an additive. However, it is energy-intensive and can emit toxic gases like HF and P₂O₅ from fluorinated binders and electrolytes, necessitating costly gas treatment. A key bottleneck across all the methods is a pretreatment step (i.e., discharging, disassembly, separation of cathode material from Al foil), which is often labor-intensive and inconsistent. Furthermore, the presence of impurities like Al, Cu, and residual carbon significantly affects the purity and performance of regenerated or recovered products. Most novel technologies remain in a lab-scale, with some challenges in industrial scalability, economic viability, and secondary waste management.