Boost sodium-ion batteries to commercialization: Strategies to enhance initial Coulombic efficiency of hard carbon anode

Sodium-ion batteries (SIBs) are regarded as one of the most promising candidates for large-scale energy storage system due to low cost and inexhaustible sodium reserves. The commercialize application of SIBs relies on the development of advanced cathode and anode materials. Among the various available anode materials, hard carbon material is considered to be the most potential anode material, which presents appropriate sodiation/ desodiation electric potential, high capacity, and simple synthesis method. However, the insufficient initial Coulombic efficiency (ICE) of hard carbon severely restricts the practical commercialization in SIBs. Hence, we review the influence elements that cause an inadequate ICE of hard carbon, such as the decomposition of electrolytes, high defects concentration, excessive surface functional groups, and irreversible sodium ions and so on. In order to obtain a moderate ICE, strategies including structure and morphology modification, defect and surface engineering, composition adjustment, electrolyte and binder optimization, material pre-treatment are confirmed to be effective. This review provides a further understanding of obtaining hard carbon electrodes with high ICE, which will contribute to the prosperity of SIBs in the near future.