Li ion diffusivity and rate performance of the LiFePO₄ modified by Cr doping

This study reports the root cause of the improved rate performance of LiFePO₄ after Cr doping. By measuring the chemical diffusion coefficient of lithium (D_Li) using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), the correlation between the electrochemical performance of LiFePO₄ and Li diffusion is acquired. The diffusion constants for LiFePO₄/C and LiFe_0.97Cr _0.03PO₄/C measured from CV are 2.48 × 10 ^-15 and 4.02 × 10^-15 cm² s^-1, respectively, indicating significant increases in diffusivity after the modification. The difference in diffusivity is also confirmed by EIS and the D_Li values obtained as a function of the lithium content in the cathode. These results suggest that Cr doping facilitates Li ion diffusion during the charge-discharge cycles. The low diffusivity of the LiFePO ₄/C leads to the considerable capacity decline at high discharge rates, while high diffusivity of the LiFe_0.97Cr_0.03PO ₄/C maintains the initial capacity, even at high C-rates.