Modeling High Concentration\nBisalt-in-Sulfolane Electrolytes\nand the Observation of Ligand-Bridged Cation-Pair Complexes

Despite the abundance of sodium over lithium in Earth’s\ncrust and the copious amounts of expensive lithium salt required to\nmake Li-ion high-concentration electrolytes (HCEs), studies of HCEs\nmade from sodium salts remain sparse. A comparative molecular-level\nstudy of Li- and Na-ion HCEs and mixed cation or bisalt HCEs in an\norganic solvent is missing. To fill this gap, we studied model HCEs\nof pure and mixed Li and Na salts of bis(fluorosulfonyl)amide (FSI)\nin sulfolane using a confluence of classical molecular dynamics (MD),\nab initio MD (AIMD) simulations, and quantum chemical cluster calculations.\nWhile Li-ion HCEs display transport properties superior to those of\nNa-ion HCEs, the latter’s performance can be considerably improved\nby replacing even 25% of Na-ions with Li-ions. While the effects of\ndoping are largely systemic, a larger sensitivity of the identity\nof solvation shells of Li-ions to the Li-content of the HCE is observed;\nin contrast, those of Na-ions are more oblivious to it. Fascinating\nligand-bridged, short-distance cation pairs observed in the classical\nMD simulations are confirmed using density functional theory-based\nAIMD simulations. Quantum chemical calculations in the gas phase reveal\nthe thermodynamic stability of such cation pairs complexed with multiple\nanions and solvent molecules.