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 ⁷Li 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. (Graph Presented).