Enhanced LiMn2O4 Thin-Film Electrode Stability in Ionic Liquid Electrolyte: A Pathway to Suppress Mn Dissolution

Spinel-type lithium manganese oxide (LiMn2O4) cathodes suffer from severe manganese dissolutionin the electrolyte,compromising the cyclic stability of LMO-based Li-ion batteries (LIBs).In addition to causing structural and morphological deteriorationto the cathode, dissolved Mn ions can migrate through the electrolyteto deposit on the anode, accelerating capacity fade. Here, we examinesingle-crystal epitaxial LiMn2O4 (111) thin-filmsusing synchrotron in situ X-ray diffraction and reflectivityto study the structural and interfacial evolution during cycling.Cyclic voltammetry is performed in a wide range (2.5-4.3 V vs Li/Li+) to promote Mn3+ formation,which enhances dissolution, for two different electrolyte systems:an imidazolium ionic liquid containing lithium bis-(trifluoromethylsulfonyl)imide(LiTFSI) and a conventional carbonate liquid electrolyte containinglithium hexafluorophosphate (LiPF6). We find exceptionalstability in this voltage range for the ionic liquid electrolyte comparedto the conventional electrolyte, which is attributed to the absenceof Mn dissolution in the ionic liquid. X-ray reflectivity shows anegligible loss of cathode material for the films cycled in the ionicliquid electrolyte, further confirmed by inductively coupled plasmamass spectrometry and transmission electron microscopy. Conversely,a substantial loss of Mn is found when the film is cycled in the conventionalelectrolyte. These findings show the significant advantages of ionicliquids in suppressing Mn dissolution in LiMn2O4 LIB cathodes.