Kinetic analysis on LiFePO4 thin films by CV, GITT, and EIS

LiFePO4 thin films were deposited on Ti substrates by pulsed laser deposition (PLD). The apparent chemical diffusion coefficients of lithium in the films, D˜Li, were measured by cyclic voltammetry (CV), galvanostatic intermittent titration technique (GITT), and electrochemical impedance spectroscopy (EIS). The average D˜Li values calculated from CV results were in the order of 10−14 cm2 s–1. The D˜Li values obtained by GITT, and EIS techniques were in the range of 10–14–10–18 cm2 s–1, 10–14–10–18 cm2 s–1, respectively. The D˜Li values obtained by the two methods show a minimum point at x ∼ 0.5 for Li1−xFePO4. However, the overpotential values of the LiFePO4 thin film electrodes obtained from the GITT results and the diffusion impedance deduced from the impedance spectra also show the minimum values at x ∼ 0.5 for Li1–xFePO4. This contradict could be caused by the improper use of GITT and EIS techniques for measuring the chemical diffusion coefficient of Li in Li1–xFePO4 which constitutes two phase, i.e., LiFePO4 and FePO4 in this region.