Ultralong‐Cycling‐Life Sodium Metal Capacitors Enabled by Hetero‐Salt Additive Strategy with NaF/LiF Hybrid Solid Electrolyte Interphase

ABSTRACT Replacing the battery‐type anode in conventional sodium‐ion capacitors with a sodium metal electrode can significantly enhance their energy density. However, their development faces critical constraints from uncontrolled sodium dendrite growth and unstable solid electrolyte interphase (SEI), which significantly compromises cycling stability and safety. Herein, we propose a hetero‐salt additive strategy by incorporating lithium difluoro(oxalato)borate (LiDFOB) into poly(ethylene glycol) diacrylate (PEGDA)‐based gel polymer electrolytes, constructing a uniquely stable NaF/LiF‐hybridized SEI that enhances interfacial ionic conductivity and suppresses dendrite growth. Simultaneously, Li + competitively coordinates with solvent molecules to reconfigure solvation structures, achieving accelerated ion‐transfer kinetics. This design enables sodium‐metal capacitors (SMCs) to achieve a remarkable energy density of 159 Wh kg −1 (at 300 W kg −1 ) and an extraordinary power density of 13 800 W kg −1 (at 92 Wh kg −1 ), while maintaining 95.1% capacity retention after 20 000 cycles at 2 A g −1 . Such a unique SEI design provides new insights for developing high‐performance SMCs.