From Structural Mechanisms to Functional Design in Hard Carbon Anodes for Sodium-Ion Batteries

Hard carbon offers reversible sodium storage at low potentials, making it a promising anode for high-energy sodium-ion batteries.Its capacity depends on both the storage sites formed during carbonization and their reversible use during cycling.This review connects these processes through electrochemically accessible storage space, defined as the sites sodium can reach, occupy and vacate reversibly within a given electrolyte, potential window and cycling time.State-resolved measurements and calculations link pore filling to low-potential capacity, while interlayer participation and transport limitations vary with material and storage conditions.