Analysis of the chemical diffusion coefficient of lithium ions in Li₃V₂(PO₄)₃ cathode material

The chemical diffusion coefficients of lithium ions (D_{L i+}) in Li₃V₂(PO₄)₃ between 3.0 and 4.8 V are systematically determined by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT). The D_{L i+} values are found to be dependent on the voltage state of charge and discharge. Based on the results from all the three techniques, the true diffusion coefficients (D_{L i+}^true) measured in single-phase region are in the range of 10^-9 to 10^-10 cm² s^-1. Its apparent diffusion coefficients (D_{L i+}^app) measured in two-phase regions by CV and GITT range from 10^-10 to 10^-11 cm² s^-1 and 10^-8 to 10^-13 cm² s^-1, respectively, depending on the potentials. By the GITT, the D_{L i+} varies non-linearly in a "W" shape with the charge-discharge voltage, which is ascribed to the strong interactions of Li⁺ with surrounding ions. Finally, the chemical diffusion coefficients of lithium ions measured by CV, EIS and GITT are compared to each other.