Unveiling Residual Charge Effects in the Direct Regeneration of Spent LiFePO₄

The emergence of safe crushing technologies for spent lithium-ion batteries treatment will significantly increase the proportion of incompletely discharged spent LiFePO₄ (IDS-LFP) in future recycling industry. The IDS-LFP and fully discharged S-LFP (FDS-LFP) with different residual charge show different microstructures and physicochemical properties. However, the current research in direct regeneration mainly focus on FDS-LFP, the regeneration of IDS-LFP and the relationship between residual charges and regeneration effect that has received less attention. Therefore, we classify the FDS-LFP and IDS-LFP based on their physicochemical properties and investigate the differences in their repair process using three kinds of regeneration methods. Compared to FDS-LFP, IDS-LFP exhibits a higher lithium intercalation energy barrier due to vacancies in the bulk. In addition, the proportion of Fe_Li anti-site defects in regenerated IDS-LFP (IDR-LFP) (1.3%) is higher than regenerated FDS-LFP (FDR-LFP) (0.6%), confirming that IDS-LFP cannot be completely regenerated. As a result, FDR-LFP delivers a specific discharge capacity of 137.1 mA h g^-1 at 1 C, retaining 81.5% capacity retention after 400 cycles. In sharp contrast, IDR-LFP only provides a specific discharge capacity of 105.1 mA h g^-1 at 1 C. This study provides foundational insights for handing S-LFP with varying residual charge.