Lithium-rich, oxygen-deficient spinel obtained through low-temperature decomposition of heterometallic molecular precursor

A heterometallic single-source molecular precursor Li₂Mn₂(tbaoac)₆ (1, tbaoac = tert-butyl acetoacetato) has been specifically designed to achieve the lowest decomposition temperature and a clean conversion to mixed-metal oxides. The crystal structure of this tetranuclear molecule was determined by single crystal X-ray diffraction, and the retention of heterometallic structure in solution and in the gas phase was confirmed by the NMR spectroscopy and mass spectrometry, respectively. Thermal decomposition of this precursor at the temperatures as low as 310 ^oC resulted in a new metastable oxide phase formulated as Li-rich, oxygen-deficient spinel Li_1.5Mn_1.5O_3.5. This formulation was supported by a comprehensive suite of techniques including thermogravimetric/differential thermal analysis (TGA/DTA), elemental analysis, inductively coupled mass spectrometry (ICP-MS), iodometric titration, X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy studies (HR-TEM), and Rietveld refinement from powder X-ray diffraction data. Upon heating to about 400 ^oC, this new low-temperature phase disproportionates stoichiometrically, gradually converting to layered Li₂MnO₃ and spinel Li_1+xMn_2-xO₄ (x < 0.5). Further heating to 750 ^oC results in formation of thermodynamically stable Li₂MnO₃ and LiMn₂O₄ phases.