Regulating electric double-layer dynamics for robust solid-electrolyte interface layer in fast-charging graphite anodes

Abstract

Graphite is used as an anode material in commercial lithium-ion batteries (LIBs) because of its stable cycling characteristics and high reversibility. However, during fast charging, the deposition of Li metal on the graphite electrode surface becomes problematic because the potential at which Li metal deposition occurs would be close to 0 V (vs. Li/Li⁺). In this study, we demonstrated an improvement in the fast-charging performance through the effective suppression of Li deposition on the anode surface during fast charging. This was achieved by introducing a metal phosphide nanodot coating layer onto artificial graphite particles. Through various analyses, including density functional theory (DFT) calculations, it was found that the cobalt phosphide(CoP) coating layer increased the concentration of PF₆⁻ anions in the inner Helmholtz layer (IHL), which in turn induced the formation of an anion-derived solid-electrolyte interface (SEI) layer. A CoP-artificial graphite (AG)/NCM 811 full cell exhibited a high capacity retention (88%) after 300 cycles, without any Li deposition. We also verified the impact of other types of metal phosphides on the fast-charging performance of LIBs. Our findings suggest that the rational design of the SEI layer is feasible through simple surface modifications that induce changes in the properties of the electric double layer. This provides a novel perspective for the design of materials suitable for the rapid charging of LIBs.