Strong ion pairing at the origin of modified Li-cation solvation and improved performances of dual-salt electrolytes

Abstract

Organic phosphates have been widely used as fire-retardant additives/co-solvents to improve the Li-ion electrolyte safety. However, these solvents show poor compatibility with low potential electrodes and cannot work efficiently as sole solvent 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 lithium nitrate (LiNO3) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salts in triethyl phosphate (TEP) 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 environment of Li+ in dual-salt phosphate-based electrolyte is probed through nuclear magnetic resonance and Raman analysis, corroborated by theoretical calculations. The dual-salt formulation is found to lead to the formation of a LiF–Li3N-LiNxOy-rich solid-electrolyte interphase on Li metal surface. The 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 a light on taming strong polarity phosphate electrolytes with fine solvation interplay.