N-Doped Carbon Layer Construction and Targeted Defect Repair Enables Direct Regeneration of Spent LiFePO₄ Cathodes

The growing number of spent LiFePO₄ (LFP) batteries presents a major challenge. Traditional recycling methods are economically inefficient and environmentally harmful, and there is an urgent need for an innovative and eco-friendly solution. This study constructed a novel direct regeneration approach for LFP batteries using melamine and phytate lithium through a one-step solid-state sintering process. Phytate lithium served as an essential lithium supplement, whereas melamine acted as an electron donor and nitrogen source. The reducing environment created by melamine pyrolysis is conducive to eliminating Fe_Li defects and reconstructing Li⁺ diffusion channels. Additionally, the N-doped carbon layer derived from N atoms in melamine can form more active sites that improve the electrical conduction properties of the regenerated LFP (RLFP) material. The RLFP exhibited excellent electrochemical performance. Compared with spent LFP, it exhibited a significantly higher initial capacity of 150 mAh g⁻¹ at 0.2 C. After 300 cycles at 1 C, it retained 82% of its initial capacity. At 5 C, its cycling stability, with a retention rate of 77% after 300 cycles, is comparable to that of commercial products. Overall, a cost-effective and environmentally sustainable recycling strategy for retired LFP batteries is determined, contributing to the advancement of sustainable energy storage technologies.