Recent advances of tailoring defects and pores in hard carbon for sodium storage

Sodium-ion batteries are promising alternatives to Lithium-ion batteries in the field of large-scale energy storage for abundant sodium resources. Hard carbons (HCs) are the most commonly used anode materials of sodium-ion batteries for balanced electrochemical performance. The major challenges lie in low initial coulombic efficiency, insufficient reversible capacity, and the costs. Defects, pores, and graphitization degree are the main characteristics of HCs. The synergistic effects of defects and pores decide the surface adsorption distribution of electrolytes and the real electrochemical active area, which determine the solid-electrolyte interface formation process and initial coulombic efficiency values. Sodium cluster stored in closed pores contributes to low-voltage plateau capacity with high reversibility. Suitable defect distribution on the inner wall of the closed pores ensures stable cluster formation. This review focuses on the defects and pores of HC and corresponding modification strategies, which are highlighted by their synergistic effects. We expect to offer valuable guidance for constructing next-generation HC anodes.