Discharge condition dependence of in-plane inhomogeneous cathode reaction analyzed by X-ray absorption near edge structure imaging

Inhomogeneous reaction distributions have been observed in composite electrodes of active materials with poor electrical conductivity. Localized electronic pathways in the electrode are one cause of these in-plane reaction distributions. The influence of the carbon content and surface modification of the current collector on the inhomogeneous reaction was examined for the LiFePO4 cathodes of a lithium-ion battery. The temperature and rate dependences of the reaction distribution were investigated at 300, 313, and 333 K and discharge rates of 0.2 and 2.0C by in situ X-ray absorption near edge structure imaging for test cells containing electrolyte solutions with concentrations of 0.1, 0.5, and 1.0 M. The chemical state maps and histograms demonstrated that a higher discharge rate and higher temperature promoted the inhomogeneous reaction at the LiFePO4 cathode. It was revealed that the reaction spots sharpened owing to IR drop in the reaction channels at the higher discharge rate, while the limited ionic diffusion affected the in-plane inhomogeneous reaction at room temperature and lower electrolyte concentrations. [Display omitted] •Inhomogeneous reactions in LiFePO4 cathodes were investigated by XANES imaging.•Electrode modifications are effective for delocalizing the reaction distribution.•Temperature and discharge rate dependences of reaction distributions were analyzed.•Higher discharge rate and higher temperature promoted inhomogeneous reaction.