Unlocking High‐Rate Sodium Storage in Hard Carbon via Interfacial Conformation Entropy Modulation in Localized High‐Concentration Electrolytes

ABSTRACT Ether‐based electrolytes are extensively employed in sodium‐ion batteries (SIBs) featuring hard carbon (HC) anodes, owing to their favorable interfacial wettability and intrinsically low solvation energy. Nevertheless, conventional NaPF 6 ‐based systems are still hindered by limited initial Coulombic efficiency, suboptimal rate capability, and inadequate low‐temperature ion transport. Here, the concept of interfacial conformation entropy ( S ICE ) is introduced as a mechanistic descriptor to capture the reconfigurability of solvation sheath and its influence on Na + desolvation and interfacial migration. To validate this concept, a locally high‐concentration electrolyte (LHCE) is formulated by incorporating 10 vol% 1,4‐dioxane (14DX), a sterically hindered and weakly coordinating cyclic ether, into 1 M NaPF 6 in diethylene glycol dimethyl ether (DEGDME). This tailored solvation microenvironment enhances S ICE effectively, enabling solvent conformational flexibility that accelerates Na + desolvation and directs the formation of a highly conductive, mechanically robust interphase. Consequently, the HC|Na cell delivers ultrafast‐charging durability and maintains a reversible capacity of 154.28 mAh g −1 after 9000 cycles at 10C with a capacity retention as high as 89.46%. Simultaneously, an Ah‐level NVP|HC pouch cell further substantiates the practical viability of this strategy. These findings highlight S ICE as a powerful paradigm for the rational design of high‐rate SIB electrolytes.