Mechanistic Insights into the Interactions between a New Type of Hard Carbon Anode and Organic Electrolytes in Sodium-Ion Batteries

Hard carbons (HCs) are a promising class of anode material for sodium-ion batteries (SIBs) since they exhibit excellent initial Coulombic efficiency (ICE) and rate performance with glyme-based electrolytes. However, the mechanisms behind such an excellent cell performance remain ambiguous. Herein, we report on a combined experimental and theoretical study to understand how and why a new type of biomass-derived hard carbon has good compatibility with the glyme-based electrolyte vs ester-based counterpart. We first show with theoretical molecular dynamics (MD) simulations that the solvation structure of glyme-Na+ has high binding energy and large volume, thus repelling the anion (from the Na salt) away from attacking the HC anode and promoting the formation of a stable solid electrolyte interface (SEI) layer. We then show X-ray photoelectron spectroscopy (XPS) analysis, revealing a higher concentration of inorganic materials in the SEI formed with glyme-based electrolyte. We also employ micromechanical testing in combination with MD simulations to demonstrate that the glyme-derived SEI possesses a higher Young’s modulus and uniform interfacial stress distribution than the ester-based counterpart. Because of these unique properties, the new HC anode with the glyme-based electrolyte exhibits excellent electrochemical performance, e.g., 332.8 mAh g–1 at 0.05 C in the initial cycle with 93.4% initial Coulombic efficiency (ICE), 81% capacity retention rate after 1000 cycles at 1.0 C, and 245 mAh g–1 rate performance at 1.0 C.