A Molecule Crowding Strategy to Stabilize Aqueous Sodium‐Ion Batteries

Aqueous sodium‐ion batteries (ASIBs) have recently emerged as a compelling choice for large‐scale energy storage when considering their safe operational characteristics and cost‐effectiveness. Nonetheless, the inadequate electrochemical stability window (ESW) of water (1.23 V) and severe dissolution of electrodes in aqueous electrolytes have proven bottlenecks to enabling high energy density and long lifespan of ASIBs. Here, a molecular crowding electrolyte is introduced, featuring nonflammable polyethylene glycol dimethyl ether (PED) as a crowding agent and a hydrogen bond receptor to further diminish the water activity. The 4.5 m (mol kg−1) NaClO4‐4 wt% H2O‐96 wt% PED electrolyte displayed an impressive ESW of 3.4 V, all while maintaining a moderate salt concentration of 4.5 m. Meanwhile, this electrolyte demonstrated substantial mitigation of vanadium dissolution in Na3V2(PO4)3. Consequently, in a comprehensive evaluation, the aqueous NaTi2(PO4)3||Na3V2(PO4)3 full cell exhibited an energy density of 47 Wh kg−1 at 1 C with a 74% retention after 100 cycles, and displayed negligible capacity decay (≈0.014% per cycle) at 5 C in 1000 cycles with high average coulombic efficiency of 99.8%. This work offers a universal strategy for the design of aqueous electrolytes with wide ESW and the ability to effectively suppress vanadium dissolution. Aqueous sodium‐ion batteries with polyethylene glycol dimethyl ether (PED) as a crowding agent can achieve stable cycling. The PED‐H2O interaction further diminishes the water activity, thus enhancing the electrochemical stability. Additionally, combined with increased viscosity, vanadium dissolution can be significantly suppressed. Consequently, the NaTi2(PO4)3||Na3V2(PO4)3 full cell exhibits a good cycling durability.