Direct Regeneration of Spent LiFePO₄ Using Methionine for Defect Repair and N/S-Doped Interface Construction

The coming wave of end-of-life electric vehicles presents a formidable challenge for the recycling of spent Li-ion batteries, particularly those with LiFePO₄ cathodes. The degradation of the LiFePO₄ cathode is primarily attributed to Li loss-induced lattice defects and the formation of Fe(III) phase. Traditional recycling methods are often plagued by high energy consumption, environmental pollution, and low economic value. Herein, we propose a targeted direct regeneration strategy using methionine as a multifunctional reducing agent to simultaneously repair bulk crystal defects and construct a N/S-doped interface. Methionine acts as an efficient reductant, facilitating the reduction of Fe(III) to Fe(II) and the elimination of Fe-Li anti-site defects, thereby reconstructing rapid the Li⁺ diffusion pathways. Simultaneously, it can serve as a single-source precursor for in situ formation of a N/S-doped carbon coating during subsequent annealing. This multifunctional interface not only significantly enhances electronic conductivity but also stabilizes the crystal structure. As a result, the regenerated LiFePO₄ (R-LFP) exhibits a high discharge capacity of 150.76 mAh g^-1 at 1.0 C and outstanding cycling stability with >87% capacity retention after 800 cycles, a performance comparable to commercial LFP. This work provides an efficient and sustainable solution for the upcycling of spent Li-ion battery cathodes.