Ionic liquid are promising electrolytes (ILE) for high-voltage lithium metal batteries because of their wide electrochemical stability windows, high thermal stability, and non-flammability. [1]. However, their high viscosity and strong ion pairing often suppress Li⁺ mobility and limit ionic conductivity [2]. Although co-solvent addition can improve transport properties, the effects of lithium salt concentration and co-solvent ratio on Li⁺ solvation and ion dynamics remain insufficiently understood.
In this experiment study, combined with molecular dynamics simulations, the effects of LiFSI con-centration and bis(2,2,2-trifluoroethyl) ether (BTFE) content on the Li+ solvation sheath and self-diffusion behavior in [Pyr14][FSI]- and [Pyr12O1][FSI]-based ILEs are investigated. Particular attention was paid to the link between electrolyte composition, solvation nanostructure, and ions transport.
The results show that increasing LiFSI concentration weakens the Li+–FSI- coordination in both systems, while BTFE indirectly regulates the solvation environment and enhances ion mobility. In [Pyr12O1][FSI]-based ILEs, the ether oxygen of the cation contributes to the peripheral Li+ solvation sheath, facilitating Li+ transport such that the Li+ self-diffusion coefficient surpasses that of FSI− at high lithium salt concentration. In contrast, [Pyr14][FSI]-based ILEs exhibit optimal Li+ mobility at moderate BTFE ratio. The optimized micelle-like solvation structure further enabled stable cycling for 150 cycles in LNMO high-voltage lithium metal batteries [3].

[1] X. Ren, L. Zou, X. Cao, et al.,Enabling High-Voltage Lithium-Metal Batteries under Practical Conditions, Joule, 3 (2019) 1662-1676.
[2] Z. Yuan, A. Chen, J. Liao, et al.,Recent advances in multifunctional generalized local high-concentration electrolytes for high-efficiency alkali metal batteries, Nano Energy, 119 (2024).
[3] Y. Chen, K. Yao, W. Qin, C. Rodenbücher, J. Luo, C. Korte, Ether-enhanced ionic liquid electrolyte enables robust interphase and fast kinetics for high-voltage LiNi0.5Mn1.5O4, J. Energy Storage, 147 (2026) 120193.