Polymer‐Regulated Solvation and Interphase Engineering for Long‐Life and Safe Quasi‐Solid‐State Anode‐Free Sodium Batteries

Abstract Anode‐free sodium metal batteries (AFSMBs) offer a promising solution to enhance the inherently low energy of sodium‐ion batteries (SIBs) while circumventing the challenges in processing highly reactive Na metal anodes. However, their practical viability is severely hindered by short lifespan, driven by accelerated irreversible Na loss in zero‐Na‐excess cell configurations, alongside safety concerns of liquid electrolyte leakage. Here, a design of long‐life quasi‐solid‐state AFSMBs is demonstrated by leveraging polymer regulation of Na + solvation behavior and anode interphase chemistry to reduce Na loss while enhancing cell safety. The polyoxymethylene with reduced local steric hindrance and weak Na + chelation shapes a weakly solvating polymer‐stabilized anion‐rich Na + solvation structure. It facilitates Na + transport and formation of robust inorganic–organic dual‐layered solid electrolyte interphase (SEI), enabling smooth Na metal deposition in quasi‐solid‐state electrolytes. This chemistry yields quasi‐solid‐state AFSMBs with a long lifespan of 500 cycles and 79% capacity retention at a high rate of 1 C. The 1.2 Ah pouch cells retain 81% capacity over 200 cycles, delivering a volumetric energy of 340 Wh L⁻ 1 , surpassing LiFePO 4 ||graphite lithium‐ion batteries, while achieving a comparable gravimetric energy of 190 Wh kg⁻ 1 . Such cells also exhibit high reliability against nail penetration in the open air at a fully charged state.