Analysis of Rate and Temperature Dependence of Inhomogeneous Cathode Reaction By Means of in-Situ XAFS Imaging

Abstract

Inhomogeneous reaction in secondary batteries promotes local degradation of the electrode. Composite electrodes have inhomogeneity of conductivity at a certain scale. The inhomogeneity depends on the conductivity of each materials and the ratio of conductive additive. The study on the inhomogeneity of reaction in the composite electrode is essential as well as a development for manufacturing homogeneous composite electrodes. In the present study, we analyzed the temperature dependence of reaction distribution of LiFePO4 lithium-ion battery by using in-situ XAFS imaging technique. In LiFePO4 cathodes, remarkable inhomogeneity has been reported because of low conductivity of the active material. A cathode sheet was prepared using composite mixture of LiFePO4, acetylene black, and polyvinylidene difluoride binder. A lithium foil was used as an anode. An Al-laminated pouch cell was used for analyzing the reaction distribution in the discharging process at four conditions of temperatures (23 °C and 60 °C) and discharging rates (0.2 C and 2.0 C). XAFS imaging experiments were performed at BL-4 of Ritsumeikan Synchrotron Radiation Center in Japan. The cathode of 3 × 10 mm2 was fully covered by the X-ray beam. Obtained X-ray images were converted to X-ray absorption spectra at each detector pixel. The chemical state of active material was estimated from the energy of Fe K absorption edge, E 0. Variance in the histogram of E 0 for the whole electrode was used as a measure of the inhomogeneity. The 2-dimentonal chemical state maps at the depth of discharge of 20% showed inhomogeneous distribution at all discharging conditions. The values of variance of the histograms are 0.6 eV2, 1.2 eV2 at 23 °C for the discharging rate of 0.2 C and 2.0 C, respectively. The values at 60 °C are 1.0 eV2 for 0.2 C and 1.8 eV2 for 2.0 C. The variance becomes large at higher temperature and higher discharging rate. At high rate processes, electric current in the cathode concentrates to the limited path with low resistance in compared with low rate processes. Inhomogeneous distribution caused by conductivity in the cathode will be emphasized. The temperature dependence indicates that ion diffusion in electrolyte solution affects to inhomogeneity of the cathode reaction. The inhomogeneous reaction is promoted at higher temperature by the following mechanism. The concentration of lithium ion in the solution decreases at reaction preceding area in a cathode during discharging process. The further discharge reaction at the area restrains by the concentration gradient in the solution. When the fast diffusion in the solution supplies lithium ions continuously, the reaction proceeds without the effect of concentration gradient. In conclusions, higher discharging rate and higher temperature promote inhomogeneous cathode reaction. These results show the reaction distributions are considerably affected by the environment and the condition of use.