Degradation of High-Nickel-Layered Oxide Cathodes from Surface to Bulk: A Comprehensive Structural, Chemical, and Electrical Analysis

Multiple applications of lithium-ion batteries in energy storage systems and electric vehicles require highly stable electrode materials for long-term battery operation. Among the various cathode materials, high-Ni cathode materials enable a high energy density. However, cathode degradation accompanied by complex chemical and structural changes results in capacity and voltage fading in batteries. Cathode degradation remains poorly understood; the majority of studies have only explored the oxidation states of transition-metal ions in localized areas and have rarely evaluated chemical degradation in complete particles after prolonged cycling. This study systematically investigates the degradation of a high-Ni cathode by comparing the chemical, structural, and electrical changes in pristine and 500 times-cycled cathodes. Electron probe micro-analysis and X-ray energy dispersive spectroscopy reveal changes in the Ni:O ratio from 1:2 to 1:1 over a large area inside the secondary particle. Electron energy loss spectroscopy analysis related to structural changes is performed for the entire primary particle area to visualize the oxidation state of transition-metal ions in two dimensions. The results imply that the observed monotonic capacity fade without unusual changes is due to the continuous formation of the Ni(2+)phase from the surface to the bulk through chemical and structural degradation.