Revitalizing sodium-ion batteries via controllable microstructures and advanced electrolytes for hard carbon

Sodium-ion batteries (SIBs) with low cost and high safety are considered as an electrochemical energy storage technology suitable for large-scale energy storage. Hard carbon, which is inexpensive and has both high capacity and low sodium storage potential, is regarded as the most promising anode for commercial SIBs. However, the commercialization of hard carbon still faces technical issues of low initial Coulombic efficiency, poor rate performance, and insufficient cycling stability, due to the intrinsically irregular microstructure of hard carbon. To address these challenges, the rational design of the hard carbon microstructure is crucial for achieving high-performance SIBs, via gaining an in-depth understanding of its structure–performance correlations. In this context, our review firstly describes the sodium storage mechanism from the perspective of the hard carbon microstructure's formation. We then summarize the state-of-art development of hard carbon, providing a critical overview of emergence of hard carbon in terms of precursor selection, microstructure design, and electrolyte regulation to optimize strategies for addressing practical problems. Finally, we highlight directions for the future development of hard carbon to achieve the commercialization of high-performance SIBs. We believe this review will serve as basic guidance for the rational design of hard carbon and stimulate more exciting research into other types of energy storage devices.