Hard carbons for sodium-ion batteries and beyond

Sodium-ion batteries (SIBs) are one of the most promising alternatives to lithium-ion batteries (LIBs), due to the much more abundant resources of Na compared with Li in the world. Developing SIB technology to satisfy the increased demand for energy storageis therefore a significant task. However, one of the biggest bottlenecks is the design of high-performance and low-cost anode materials, since the graphite anode in commercial LIBs is not suitable for SIBs due to thermal dynamic issues. Hard carbon materials have been regarded as having the greatest potential as anodes in commercial SIBs owing to their excellent cost-effectiveness, but their relatively limited performance compared to the graphite in LIBs as well as the dimness of the sodium storage mechanisms still need further investigation. In this review, we summarize the progress of recent research into hard carbons for SIB applications, including the fundamentals of SIBs, sodium storage mechanisms, structures and the electrochemical performances of different types of hard carbons in SIBs and other types of sodium-based energy storage as well as the main challenges in this field. We aim to provide a general insight into hard carbons and their applications in SIBs, opening up future perspectives and possible research directions.