Structural analysis of lithium-excess lithium manganate cathode materials by 7Li magic-angle spinning nuclear magnetic resonance spectroscopy

The local structures of lithium-excess lithium manganese spinel oxides were studied by high-resolution solid-state 7Li magic-angle spinning (MAS) NMR spectroscopy. Two resonance lines at ∼500 and ∼555 ppm were observed for the spinels in 7Li MAS NMR spectra. Spinel stability tests in which spinel powder was stored in electrolyte solution were performed to analyze the changes in the lithium local structure after manganese dissolution. After the spinel stability test, the intensity of the resonance at ∼500 ppm decreased, whereas new resonance line at 0 ppm was observed. The lithium content of the 0 ppm peak increases with the storage time in electrolyte. SEM and chemical analysis suggested a surface coating of non-spinel lithium compounds, the presence of defects on particles surface and fluorine incorporation into the aged spinel. In addition, about 60–70% of lithium remains in the spinel framework after the storage.