Investigating the Superior Performance of Hard Carbon Anodes in Sodium‐Ion Compared With Lithium‐ and Potassium‐Ion Batteries

Abstract Emerging sodium‐ion batteries (NIBs) and potassium‐ion batteries (KIBs) show promise in complementing lithium‐ion battery (LIB) technology and diversifying the battery market. Hard carbon is a potential anode candidate for LIBs, NIBs, and KIBs due to its high capacity, sustainability, wide availability, and stable physicochemical properties. Herein, a series of hard carbons is synthesized by hydrothermal carbonization and subsequent pyrolysis at different temperatures to finely tune their structural properties. When tested as anodes, the hard carbons exhibit differing ion‐storage trends for Li, Na, and K, with NIBs achieving the highest reversible capacity. Extensive materials and electrochemical characterizations are carried out to study the correlation of structural features with electrochemical performance and to explain the specific mechanisms of alkali‐ion storage in hard carbons. In addition, the best‐performing hard carbon is tested against a sodium cathode Na 3 V 2 (PO 4 ) 3 in a Na‐ion pouch cell, displaying a high power density of 2172 W kg −1 at an energy density of 181.5 Wh kg −1 (based on the total weight of active materials in both anode and cathode). The Na‐ion pouch cell also shows stable ultralong‐term cycling (9000 h or 5142 cycles) and demonstrates the promising potential of such materials as sustainable, scalable anodes for beyond Li‐batteries.