Unlocking High‐Energy, High‐Power, and Long‐Life Sodium‐Ion Batteries Through Interfacial‐Functionalization‐Induced Sodium Rebalancing

ABSTRACT Large‐scale application of hard carbon‐based sodium‐ion batteries is limited by insufficient energy density and sluggish interfacial kinetics. While interfacial engineering offers a potential solution, it often introduces unintended side effects at the system level, locking full‐cell performance into an inherent trade‐off hard to overcome synergistically. This study proposes a novel paradigm of “interfacial functionalization‐induced systematic sodium inventory rebalancing,” enabling simultaneous improvement in full‐cell performance and cost optimization via minimal modifications. By introducing Sn(OTf) 2 into an ester‐based electrolyte, a dynamically self‐healing Na–Sn composite SEI is in situ constructed, achieving ultrafast ion transport and effective dendrite growth suppression. Meanwhile, additional capacity from the interfacial Sn alloying reaction enables precise sodium inventory management, establishing an “interfacial gain‐compensated system loss” mechanism that breaks the trade‐off among energy density, power density, and cycle life at once. An Ah‐class Na 3 V 2 (PO 4 ) 3 ||hard carbon full cell (cathode loading: 32 mg cm − 2 ) fabricated using this strategy delivers a high energy density of 182.5 Wh kg − 1 , while exhibiting excellent cycling stability (87.6% capacity retention after 1100 cycles at 5C) and rate capability (90.28% capacity retention at 30C). This work provides a new theoretical framework and feasible technical pathway for reverse design of full cells through customized anode interface chemistry.