Ion–Solvent Complexes Promote Gas Evolution from Electrolytes on a Sodium Metal Anode

Lithium and sodium metal batteries are considered as promising next‐generation energy storage devices due to their ultrahigh energy densities. The high reactivity of alkali metal toward organic solvents and salts results in side reactions, which further lead to undesirable electrolyte depletion, cell failure, and evolution of flammable gas. Herein, first‐principles calculations and in situ optical microscopy are used to study the mechanism of organic electrolyte decomposition and gas evolution on a sodium metal anode. Once complexed with sodium ions, solvent molecules show a reduced LUMO, which facilitates the electrolyte decomposition and gas evolution. Such a general mechanism is also applicable to lithium and other metal anodes. We uncover the critical role of ion–solvent complexation for the stability of alkali metal anodes, reveal the mechanism of electrolyte gassing, and provide a mechanistic guidance to electrolyte and lithium/sodium anode design for safe rechargeable batteries. Safe rechargeable batteries: Ion–solvent complexes in alkali metal batteries have been studied by first‐principles calculations and in situ optical microscopy. The ion–solvent complexes have low LUMOs and are readily reduced on an alkali metal anode. A general mechanism for organic electrolyte decomposition and gas evolution was discovered.