First-Principles Simulations of Salt-Concentrated Electrolytes for Li-Based Batteries: How Solvents Tune Solvation Structures and Li-Ion Conductivity

Salt-concentrated nonaqueous electrolytes, due to their special properties in increasing the stability of batteries by the formation of anion-derived solid electrolyte interphases (SEIs), have attracted considerable attention in recent years. Despite extensive efforts to explore the microscopic solvation structures of electrolyte solutions, a clear relationship between the microstructures and electrolyte performance, especially the Li-ion conductivity, is still in demand. In this work, we performed ab initio molecular dynamics (AIMD) simulations as well as density function theory (DFT) calculations for three as-designed electrolytes, namely lithium bis(fluorosulfonyl)imide (LiFSI) with acetonitrile (AN), 1,2-dimethoxyethane (DME), and 2,2-dimethyl-3,6,9-trioxa-2-siladecane (siloxane). We observed that for the above electrolytes at high concentrations, Li-ion conduction proceeds when the solvation structure changes from one form to another in a few tens of fs, involving the binding/ debinding of both the solvent and FSI anion with the Li-center. The dynamics of binding between the solvents and Li decrease with the increase in the strength of the solvation sheath, which is influenced by the polarity of the solvent. The steric shielding effect was clearly detected in the siloxane-LiFSI system which became almost nonconductive at a concentration of 3 mol L-1. It should be noted that despite the high concentration of each electrolyte (>= 5 mol L-1), there is still a certain amount of free solvents according to the simulation results. Our results deepen the understanding of the Li-ion conduction process in salt-concentrated electrolytes and provide guidelines for designing high-performance electrolytes.