Defect-Accelerated Direct Regeneration of Spent LiFePO₄ via a Mechanochemical Route with Short-Term Annealing

The green and efficient regeneration of spent lithium iron phosphate (S-LFP) is crucial for the sustainable development of lithium-ion batteries. While the well-preserved olivine structure of spent S-LFP provides a foundation for direct regeneration, its intrinsically low lithium-ion diffusion coefficient poses a kinetic challenge for efficient relithiation. Conventional regeneration methods rely on harsh conditions to accelerate the diffusion of lithium ions into the lithium vacancies of the S-LFP crystal lattice. To address this challenge, this study proposes a “defect-accelerated regeneration” strategy, which achieves efficient repair of S-LFP by performing ball milling in a solution system containing lithium acetate (LiOAc) and citric acid (CA), followed by short-term annealing. The mechanochemical effects induced by this process enable simultaneous lithium replenishment and the controlled introduction of crystal defects, which act as preferential pathways for lithium-ion migration and synergistically enhance the reaction kinetics. Results demonstrate that the regenerated lithium iron phosphate exhibits a complete crystal structure and improved electrochemical performance: a specific discharge capacity of 154.14 mAh g^–1 at 0.1C and a capacity retention rate of 93.14% after 500 cycles at 1C, higher than those of the original S-LFP. This work demonstrates a pathway for the low-cost and low-energy consumption recycling of S-LFP.