Studies of Spinel-to-Layered Structural Transformations in LiMn2O4 Electrodes Charged to High Voltages

This research sheds light on the intriguing phenomena of structural transformations that take place in spinel LiMn2O4 (space group Fd3m) when charged to high anodic potentials (4.3 to 5.1 V) in lithium cells. It was established from the XRD and electron diffraction analyses that the orthorhombic o-LiMnO2 (space group Pnma) is formed due to the partial transformation of spinel starting at 4.7 V. The Raman spectroscopy measurements of the LiMn2O4 samples charged to 5.1 V also provided evidence of o-LiMnO2, although its band overlaps with the delithiated spinel species. An additional important finding is that some layered-type LiMnO2 is formed upon cycling of the spinel material to 4.5 V, even upon the first charge. LiMnO2 exists as a stable component of the solid electrolyte interphase developed on these electrodes. We also concluded the formation of the layered LiMnO2 structure via solid-state Li-7 NMR. The analysis of those results further indicated that the onset of the creation of the intermediate spinel-layered (splayered) phase clearly takes place between 4.5 and 4.7 V.