Hard Carbon in Alkali Metal Ion Batteries

Hard carbons (HCs) are nongraphitizable and consist of numerous short‐range carbon layers stacked together. The carbon layers are twisted around each other and form large layer spacing as well as diverse pore types, making it ideal for extensive application in alkali metal ion batteries (AMIBs). However, the impact of complex microstructure on alkali metal ion storage has not been fully recognized, which is not conducive to the improvement of electrochemical properties of HCs. Therefore, this article not only systematically analyzes the ion storage mechanism of HCs in AMIBs but also focuses on the relationship between the microstructure (graphite microcrystals, defects, and nanopores) and the electrochemical performance of HCs. Subsequently, it provides countermeasures to the problems of the electrochemical properties. Finally, the key challenges and perspectives for high‐performance HC anodes are also discussed.