Ether-enhanced ionic liquid electrolyte enables robust interphase and fast kinetics for high-voltage LiNi0.5Mn1.5O4

The most important key challenge for 5 V-class LiNi 0.5 Mn 1.5 O 4 (LNMO) lithium metal batteries is to design electrolytes that are stable above 4.5 V while enabling fast Li + transport. A promising approach is ionic liquid bases electrolyte, composed of 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)imide (Pyr 14 FSI) and lithium bis(fluorosul-fonyl)imide (LiFSI), where both the Li + concentration and the amount of a fluoroether-based co-solvent (BTFE, bis(trifluoroethyl)ether) are independently adjusted to achieve a multidimensional control over the local Li + solvation environment. The Li + solvation sheath can be reshaped to a compact, anion-rich configuration, characterized by full-contact ion pairs (FCIPs) and enhanced Li + –FSI − coordination by adjusting the LiFSI and BTFE ratio. The ionic conductivity is markedly enhanced from 0.49 to 1.34 mS cm −1 , and the anodic stability window is extended up to 5.4 V vs. Li/Li + . Most crucially, a rapid formation of a dense, oxidation-resistant cathode–electrolyte interphase (CEI) during the initial charge cycle can be observed for systems with anion-rich solvation sheath, thereby suppressing further electrolyte oxidation and mitigating impedance growth. As the results, LNMO cells employing the optimized electrolytes retain 84.1 % of their initial capacity after 100 cycles, compared to only 30.68 % after 70 cycles for carbonate based electrolytes. This work provides a reference for the effect of ether co-solvents on the coordination structure of Li + in ionic liquid electrolytes and explores the use of such electrolyte in 5 V high-voltage lithium secondary battery systems with LNMO as the cathode.