Chemical state of Iron of LiFePO₄during charge-discharge cycles studied by in-situ X-ray absorption spectroscopy

In-situ X-ray absorption spectroscopy (XAS) at the Fe K-edge X-ray absorption near edge was used to investigate the chemical change of LiFePO ₄, which is a candidate cathode material for lithium ion batteries, during charge-discharge cycles. The relative amount of FePO₄ formed in the LiFePO₄ by the charge-discharge cycles was estimated from the XAS spectra. The results show that the amount Of FePO₄ in the LiFePO₄ increased with charging and decreased with discharging. While a linear relationship between the relative amount of FePO₄ and electrical capacity was observed during the initial charging, it deviated from linearity during the charge-discharge cycles. This may be attributed to the irreversible diffusion paths of lithium ions and/or the partial formation of inactive FePO₄ in the LiFePO4 during the charge-discharge cycles. It is shown that in-situ XAS has sufficient potential to nondestructively characterize heteroseneous electrochemical reactions in electrode materials during charge-discharge cycles.