Hard Carbon as Sodium-Ion Battery Anodes: Progress and Challenges

Hard carbon (HC) is the state-of-the-art anode material for sodium-ion batteries due to its excellent overall performance, wide availability, and relatively low cost. Recently, tremendous effort has been invested to elucidate the sodium storage mechanism in HC, and to explore synthetic approaches that can enhance the performance and lower the cost. However, disagreements remain in the field, particularly on the fundamental questions of ion transfer and storage and the ideal HC structure for high performance. This Minireview aims to provide an analysis and summary of the theoretical limitations of HC, discrepancies in the storage mechanism, and methods to improve the performance. Finally, future research on developing ideal structured HCs, advanced electrolytes, and optimized electrolyte-electrode interphases are proposed on the basis of recent progress.