Hard carbon anode materials for sodium-ion batteries

Recent results have shown that sodium-ion batteries complement lithium-ion batteries well because of the low cost and abundance of sodium resources. Hard carbon is believed to be the most promising anode material for sodium-ion batteries due to the expanded graphene interlayers, suitable working voltage and relatively low cost. However, the low initial coulombic efficiency and rate performance still remains challenging. The focus of this review is to give a summary of the recent progresses on hard carbon for sodium-ion batteries including the impact of the uniqueness of carbon precursors and strategies to improve the performance of hard carbon, and highlight the advantages and performances of the hard carbon. Additionally, the current problems of hard carbon for sodium-ion batteries and some challenges and perspectives on designing better hard-carbon anode materials are also provided.