Investigation of positive electrodes after cycle testing of high-power Li-ion battery cells: I. An approach to the power fading mechanism using XANES

Cylindrical lithium-ion (Li-ion) cells with a nickel-cobalt oxide (LiNi0.73Co0.17Al0.10O2) positive electrode and a non-graphitizable carbon (hard carbon) negative electrode were degraded using cycle tests. The degraded cells were disassembled and examined; most attention was paid to the positive electrodes in order to clarify the origin of the power fade of the cells. X-ray absorption near-edge structure (XANES) analysis demonstrated that the crystal structure of the electrode at the surface changed from rhombohedral to cubic symmetry. Furthermore, a film of lithium carbonate (Li2CO3) covered the surface of the positive electrode after the cycle tests. Using a combination of X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR), and glow discharge optical emission spectrometry (GD-OES) measurements, a schematic model of the changes occurring in the surface structure of the positive electrode during the cycle tests was constructed. The appearance of both an electrochemically inactive cubic phase and lithium carbonate films at the surface of the positive electrode are important factors giving rise to power fade of the positive electrode.