Abstract: During the direct repair and regeneration of spent lithium iron phosphate (LiFePO₄, LFP) batteries, residual impurities such as Cu and Cl are difficult to remove completely and may affect the structural integrity, ion transport, and electrochemical performance of the regenerated cathodes. In this study, density functional theory (DFT) calculations were performed to evaluate the feasibility of transforming residual Cu and Cl impurities into beneficial co-dopants, and to clarify their modulatory mechanisms in regenerated LFP in terms of structural stability, electron transport, Li-ion diffusion, delithiation behavior, and mechanical reliability. A Cu/Cl co-doped LFP model was constructed by substituting Cu for Fe and Cl for O, with a Li vacancy introduced for charge compensation, reflecting the possible defect configurations during impurity incorporation. Structural optimization results show that the co-doped system retains the olivine Pnma framework, and the variations in lattice parameters and cell volume are less than 3%, indicating that Cu/Cl co-doping does not cause severe lattice distortion or structural collapse. A calculated formation energy of −1.27 eV further demonstrates that the co-doped system is thermodynamically favorable. Electronic structure analysis reveals that Cu/Cl co-doping introduces Cu 3d states near the valence band maximum and promotes a downward shift of the conduction band, thereby narrowing the bandgap from 3.85 eV to 1.36 eV. The reduced bandgap and redistribution of electronic states near the Fermi level suggest weakened electron localization and enhanced electron transport capability in the co-doped system. Climbing image nudged elastic band (CI-NEB) calculations further reveal that the Li-ion migration energy barrier along the one-dimensional 010 channel decreases from 0.56 eV for pristine LFP to 0.31 eV for Cu/Cl co-doped LFP, indicating improved Li-ion diffusion kinetics. During delithiation, the average delithiation voltage increases from 3.42 V to 3.58 V, while the volume change is reduced to 2.09%, suggesting that Cu/Cl co-doping can simultaneously improve energy density and alleviate structural strain during charge/discharge cycling. Mechanical property calculations show that the bulk modulus, shear modulus, and Young′s modulus of the co-doped system are all increased, demonstrating enhanced resistance to compression, shear deformation, and elastic deformation. In addition, the decrease in the universal elastic anisotropy index indicates a more uniform mechanical response, which is beneficial for suppressing stress concentration and microcrack formation in regenerated LFP particles. Overall, Cu/Cl co-doping is theoretically feasible and can synergistically improve the electronic conductivity, Li-ion transport, structural stability, and mechanical reliability of LFP cathodes. These findings provide a theoretical basis for impurity regulation in the direct regeneration of spent LFP batteries and offer a practical strategy for converting unavoidable residual impurities into functional dopants for high-performance regenerated cathode materials.