Weakly Solvating Electrolytes\nfor Safe and Fast-Charging\nSodium Metal Batteries

Electrolytes for high-performance sodium metal batteries\n(SMBs)\nare expected to have high electrode compatibility, low solvation energy,\nand nonflammability. However, conventional flammable carbonate ester\nelectrolytes show high Na<sup>+</sup> desolvation energy and poor\ncompatibility with sodium metal anodes, leading to slow Faradaic reactions\nand significant degradation of SMBs. Herein, we report a weakly solvating\nelectrolytes (WSEs) design developed by an ionized ether-induced solvent\nmolecule polarization strategy. The steric hindrance and electron-withdrawing\neffect of the pyrrolidine cation weaken the solvation ability of the\nionized ether and enable carbonate ester with low solvation energy\nthrough intermolecular polarization interactions. It enables WSEs\nwith fast Na<sup>+</sup> migration kinetics and electric-field-reinforced\ncationic electrode/electrolyte interface, thereby promoting the stability\nand reversibility of SMBs even under high-charge-rate conditions.\nThe Na||Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> battery\nwith ionized ether-based WSEs exhibits a capacity retention of 83.5%\nwith an average Coulombic efficiency (CE) of 99.69% after 500 cycles\nat 10C. Furthermore, the Na||Na<sub>2</sub>Fe<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub> cells maintained 92.8% capacity retention after 1000\ncycles at 5C with an average CE of 99.77% at a cutoff voltage of 4.5\nV. The ionized ether also eliminates the fire and safety risks associated\nwith WSEs. This work offers valuable insights into the design of WSEs\nfor safe and high-performance sodium metal batteries.