Surface engineering based on in situ electro‐polymerization to boost the initial Coulombic efficiency of hard carbon anode for sodium‐ion battery

Abstract Hard carbon (HC) is considered as a commercial candidate for anode materials of sodium‐ion batteries due to its low cost and excellent capacity. However, the problem of low initial Coulombic efficiency is still urgently needed to be solved to promote the industrialization of HC. In this paper, 2,2‐dimethylvinyl boric acid (DEBA) is used to modify the surface of HC to prepare HC‐DEBA materials. During the cycling, the C = C bonds of DEBA molecules will be in situ electro‐polymerized to form a polymer network, which can act as the passive protecting layer to inhibit irreversible decomposition of electrolyte, and induce a thinner solid electrolyte interface with lower interface impedance. Therefore, HC‐DEBA has higher initial Coulombic efficiency and better cycling stability. In ester‐based electrolyte, the initial Coulombic efficiency of the optimized HC‐DEBA‐3% increases from 65.2% to 77.2%. After 2000 cycles at 1 A·g −1 , the capacity retention rate is 90.92%. Moreover, it can provide a high reversible capacity of 294.7 mAh·g −1 at 50 mA·g −1 . This simple surface modification method is ingenious and versatile, which can be extended to other energy storage materials.