In situ X-ray diffraction and X-ray absorption studies of high-rate lithium-ion batteries

A combination of in situ synchrotron X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) was used to study a two-electrode lithium-ion cell made from electrodes from a high-rate lithium-ion battery. The cathode was LiNi_0.85Co_0.15O₂. Both the XRD and the XAS were done in the transmission mode. XAS was done while the cell was charged at the C/2 rate to 4.5 V. The near edge Co and Ni spectra (XANES) indicated that all of the charge compensation occurred on the Ni and the Co did not change its oxidation state during charge. The Co EXAFS indicated that the Co is randomly distributed in the NiO₂ layers. XRD data were collected on a beam line with a position sensitive detector. Complete high quality XRD patterns could be obtained in as little as 3 min. The patterns covered the range between the (0 0 3) and the (1 1 3) reflections of the cathode material and included the (0 0 2) reflection from graphite in the anode. Data were collected while the cell was cycled at rates between the C/4 and 2C rate. In all cases, at the beginning of charge, there is an initial expansion along the c-axis and a concomitant contraction along the a- and b-axes. Towards the end of charge there is a contraction along the c-axis and a slight expansion along the a- and b-axes. After high rate charges there are relaxation processes that last for a few minutes after termination of charge. The results indicate with proper choice of the active materials and electrode design lithium-ion cells can be charged and discharged at high rate with high utilization of the active material.