Targeted Defect Repair and Multi-functional Interface Construction for the Direct Regeneration of Spent LiFePO4 Cathodes.

Due to the low economic benefits and environmental pollution of traditional recycling methods, the disposal of spent LiFePO₄ (SLFP) presents a significant challenge. The capacity fade of SLFP cathode is primarily caused by lithium loss and formation of a Fe (III) phase. Herein, a synergistic repair effect is proposed to achieve defect repair and multi-functional interface construction for the direct regeneration of SLFP. Tannic acid (TA) forms a compact coating precursor for a carbon layer on SLFP with abundant functional groups and creates a mildly acidic environment to enhance the reducibility of thiourea (TU). Therefore, TU reduces Fe (III) to Fe (II) and repairs Li-Fe anti-site defects of SLFP, while at the same time acting as a source of N/S-doping elements for the carbon layer at a lower temperature (140 °C). The multi-functional carbon layer improves the properties of the regenerated LiFePO₄ (RLFP) due to the enhanced conductivity, structure maintenance and protection, and the improved kinetics of Li⁺ transport. Furthermore, the Fe─O and P─O bonds are strengthened, further enhancing the structural stability of the RLFP. Consequently, the RLFP demonstrates outstanding performance with a discharge capacity of 141.3 mAh g^-1 and capacity retention of 72% after 1000 cycles at 1 C.