Revealing the Effect of Curvature Structure in Hard Carbon Anodes for Lithium/Sodium Ion Batteries

Heteroatom doping is the most common means to enhance the Li⁺/Na⁺ ions storage of hard carbon (HC). The explanation of the storage mechanism of heteroatom-doped HC is to increase the active site or widen the layer spacing while ignoring the effect of local bending structure induced by it. Meanwhile, the storage mechanism by the localized bending structure also lacks in-depth study. Herein, a locally curved configuration and an amorphous structure are designed by introducing different heteroatoms, respectively, and the mechanism of the two types of structures on the Li⁺/Na⁺ ions storage is explored. The density functional theory (DFT) calculation shows that the adsorption energy of Li⁺/Na⁺ ions is optimal at the appropriate curvature of 27.72 m^-1. Serving as anode for lithium/sodium ion batteries in ester electrolytes, the optimized HCs demonstrate satisfied specific capacity and high-rate capability, respectively. Furthermore, the charging capacity below 1.0 V of HC with suitable curvature microstructure reaches 84.8% and 90.1% of the total charge capacity, confirming that the curvature defects can better control the delithiation/desodiation process, and provide a higher energy density. This study enlightens new insights into the storage mechanisms of Li⁺/Na⁺ ions and provides guidance for better design of heteroatom-doped carbon anodes with superior performance.