Unveiling the Key to Spent LiFePO₄ Regeneration: Formation and Action of Carbon Dots

Introducing organic molecules to repair structure defects through hydrothermal treatment has emerged as a promising strategy for the direct regeneration of spent LiFePO₄ (SLFP). Here, we find a universal rule that abundant carbon dots (CDs) are generated from various organic molecules during the direct regeneration process, playing a key role in the regeneration of SLFP. By employing purified CDs to regenerate SLFP, we demonstrated that CDs can not only manifest a reduction effect during the hydrothermal process but also reconstruct the uneven carbon layer in the subsequent sintering stage. As expected, excellent electrochemical performances are exhibited by the regenerated LiFePO₄ (RLFP), delivering a high discharge specific capacity of 140.1 mAh g^-1 at a 1 C rate and maintaining a capacity retention rate of 91% after 1000 long-term cycles. After systematically analyzing the structure and electrochemical performance of LiFePO₄ (LFP) regenerated with CDs and their corresponding precursors, we propose the selection principle of organic molecules in the regeneration process of SLFP: organic molecules prone to form CDs under the hydrothermal condition are the most suitable for the regeneration of SLFP. This work offers important insights into the regeneration mechanisms of LFP and provides key guidelines for the selection of reducing agents.