Strong Ion Pairing at the Origin of Modified Li-Cation Solvation Environment and Improved Performances of Dual-Salt Electrolytes

Abstract

Organic phosphates have been widely used as fire-retardant additives or co-solvents to improve the safety of Li-ion electrolytes. However, these solvents show poor compatibility at low potentials and cannot work efficiently as sole solvent and at low salt concentration. The utilization of high concentration electrolytes was shown to improve the interfacial properties by altering the solvation structure but the drawbacks are low ionic conductivity, high viscosity, and elevated costs. Herein, a dual-salt phosphate-based electrolyte consisting of medium concentration LiNO3 and LiTFSI salts in triethyl phosphate is analyzed and found to address the above issues. Benefiting from the strong affinity between NO3− and Li+, the use of LiNO3 is found to perturb the solvation structure, with mixed anion (NO3− and TFSI−) pairing with Li+. The detailed surrounding of Li+ in dual-salt phosphate-based electrolyte is probed through nuclear magnetic resonance and Raman analysis, corroborated by density functional theory calculations. This is found to lead to the formation of a LiF-Li3N-LiNxOy-rich solid-electrolyte interphase on Lithium metal electrode surface. These results and analyses not only allow the assembly of LiNi0.5Co0.2Mn0.3O2||Li cells with significantly improved cycling and low temperature performances but also shine light on taming strong polarity phosphate electrolytes with fine solvation interplay.

Keywords: Li-ion batteries, dual-salt electrolyte, solvation structure, ion pairing, solid-electrolyte interphase