Li-Ion Diffusivity Mismatch in Commercial Level High-Ni Single-Crystalline NCM Cathode and Graphite-SiO Composite Anode: Degradation Mechanism and Controlled Charging Protocol

Abstract

High-nickel (Ni > 80%) single-crystalline [Ni_1–x–yCo_xMn_y]O₂ (NCM) cathodes and graphite-SiO composite anodes are a commercial level combination to enable the development of high-energy-density Li-ion batteries. However, full cells with this combination exhibit a significant increase in resistance and cycle degradation during cycling at room temperature. Paradoxically, this phenomenon is alleviated at an elevated temperature (45 °C). In this study, we elucidate the temperature-dependent cycle degradation mechanism in these full cells. The larger primary particle size of single-crystalline NCM cathodes compared with polycrystalline cathodes results in slower Li-ion diffusion due to an extended Li-ion pathway, but SiO particles in the composite anode display the high rate capability of Li ions. This dissymmetry in Li-ion diffusion between the cathode and anode leads to not only structural defects of the cathode but also a rapid electrolyte reduction. These factors contribute to rapid performance degradation in the cell. To solve this issue, we propose a controlled charging protocol which is based on Li-ion diffusion coefficients as a function of the state of charge.