Unraveling the Solvent Effect on Solid-Electrolyte Interphase Formation for Sodium Metal Batteries

Ether-based electrolytes are considered as an ideal electrolyte system for sodium metal batteries (SMBs) due to their superior compatibility with the sodium metal anode (SMA). However, the selection principle of ether solvents and the impact on solid electrolyte interphase formation are still unclear. Herein, we systematically compare the chain ether-based electrolyte and understand the relationship between the solvation structure and the interphasial properties. The linear ether solvent molecules with different terminal group lengths demonstrate remarkably distinct solvation effects, thus leading to different electrochemical performance as well as deposition morphologies for SMBs. Computational calculations and comprehensive characterizations indicate that the terminal group length significantly regulates the electrolyte solvation structure and consequently influences the interfacial reaction mechanism of electrolytes on SMA. Cryogenic electron microscopy clearly reveals the difference in solid electrolyte interphase in various ether-based electrolytes. As a result, the 1,2-diethoxyethane-based electrolyte enables a high Coulombic efficiency of 99.9 %, which also realizes the stable cycling of Na||Na3V2(PO4)3 full cell with a mass loading of approximate to 9 mg cm-2 over 500 cycles. The 1,2-diethoxyethane (DEE) molecule promotes the moderate Na+-DEE coordination, thus leading to the uniform and inorganic-rich solid electrolyte interphase, as proved by the cryogenic electron microscopy characterizations. Consequently, the Na metal anode is highly stable in the NaPF6/DEE-based electrolyte with a high Coulombic efficiency of 99.9 % and dendrite-free deposition behavior.+image

Automated summary

Ether-based electrolytes play a crucial role in sodium metal batteries, and the solvation structure and interfacial reaction mechanism greatly affect the electrochemical performance. 1,2-diethoxyethane emerges as a promising ether-based electrolyte with high Coulombic efficiency and stable cycling.