Ion–Solvent Chemistry in Alkali Metal Batteries
Authors: Chen Xiang , Zhang Xueqiang , Shen Xin , Zhang Qiang 
Published: May 2020

Alkali metal batteries are considered as promising next-generation energy storage devices due to their ultrahigh energy densities. However, the high reactivity of alkali metal toward organic solvents and salts renders inevitable side reactions, which further leads to undesirable electrolyte depletion, cell failure, and evolution of flammable gas. Ion–solvent chemistry was proposed to prove the intrinsic instability of normal organic electrolytes towards alkali metal anodes through multiscale calculations (density functional theory (DFT) calculations, ab initio molecular dynamics (AIMD) simulations), and in situ optical microscopic observations. Once complexed with cations in electrolytes, solvent molecules exhibit a significantly reduced the lowest unoccupied molecular orbital (LUMO) energy level, facilitating the electrolyte decomposition and gas evolution. The origin of such phenomenon can be explained by two mechanisms in ether and ester electrolytes, respectively. The LUMO energy levels of ion–ester complexes exhibit a linear relationship with the binding energy, regulated by the ratio of carbon atomic orbital in the LUMO, while LUMOs of ion–ether complexes are composed by the metal atomic orbitals. AIMD simulations further prove the electrolyte decomposition kinetics on alkali metal anodes. The broken of C–O bond in electrolyte solvents is always accompanied by the formation of Li/Na–O bond, validating the role of coordinated cations in promoting electrolyte decomposition. In situ optical microscopic observations validated the theoretical predictions that electrolyte gassing on Li/Na metal surface is obviously promoted after introducing cations into electrolytes.

Based on the proved ion–solvent chemistry, several strategies were proposed to build stable electrolytes for alkali metal anodes, including additive design and regulating anions in electrolyte solvation shell. Both electrolyte additives and anions from salts can participate in electrolyte solvation of cations and regulate the composition and structure of solid electrolyte interphase (SEI). More importantly, a fancy strategy of cation additive was proposed as different cations have different impacts on solvent molecules. Three principles, including the LUMO energy level decrease of solvents after coordinating with cations, the electrode potential of introduced cations in electrolytes, the binding energy between cations and solvents, were proposed to select proper cation additives for sodium metal batteries, which is further proved by electrochemical tests. Besides, the role of electrolyte solvation in regulating ion–solvent interactions was also systematically investigated. The cation additive strategy affords emerging chances for rational electrolyte design for stable and safe Na metal batteries and also affords a fresh research paradigm for other batteries.

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