The potential dependent electrochemical impedance spectroscopy and lithium diffusion kinetics of LiFePO4

Abstract LiFePO 4 cathode material prepared by solid state carbo-thermal reduction exhibited pretty good electro-chemical performance. It showed an initial discharge capacity of 152.6 mAh·g − 1 and only 2.3% capacity fading was observed after 50 cycles. The coulomb efficiencies were between 98 and 100%. The electrochemical impedance spectroscopy measured at series of equilibrium potentials within the voltage range 2.5–4.5 V showed that they were highly voltage dependent. This indicated the different kinetics steps of charge carrier transportation through the whole battery components. The coefficients of lithium ions and polarizations of charge carrier transportation as a function of equilibrium potentials were determined from the EIS profiles. An inverted V shaped curve was observed as to the relation between the lithium diffusion coefficient and equilibrium potential. However, contrary to the previous reports, based on Einstein equation, the lithium ion diffusion in the phase transition region from LiFePO 4 to FePO 4 was calculated to be in the range 10 − 11 –10 − 12  cm·s − 1 , which was about 1–2 order of magnitude larger than that of other region.