Kinetic analysis on LiFePO 4 thin films by CV, GITT, and EIS

Abstract

LiFePO 4 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 cm 2 s –1. The D ˜ Li values obtained by GITT, and EIS techniques were in the range of 10 –14–10 –18 cm 2 s –1, 10 –14–10 –18 cm 2 s –1, respectively. The D ˜ Li values obtained by the two methods show a minimum point at x ∼ 0.5 for Li 1− x FePO 4. However, the overpotential values of the LiFePO 4 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 Li 1– x FePO 4. This contradict could be caused by the improper use of GITT and EIS techniques for measuring the chemical diffusion coefficient of Li in Li 1– x FePO 4 which constitutes two phase, i.e., LiFePO 4 and FePO 4 in this region.