Oxygen Nonstoichiometry and Phase Transitions in LiMn1.5Ni0.5O4 − δ

Abstract

Crystal structure and oxygen stoichiometry in , a potential lithium-battery cathode, vary with temperature, as observed in samples quenched from different temperatures and by in situ diffraction and thermogravimetry techniques. When prepared in high pressure, this cation-ordered spinel is oxygen-stoichiometric, , space group . Upon heating between 650 and , increasing oxygen deficiency occurs exclusively in octahedra and bonds, which induces a volume increase of the 12d octahedra, a reduction of Mn as shown by X-ray absorption near-edge structure, equalization of and bond lengths, and disordering of Mn, Ni on octahedral sites. Hence, the transformation to space group , shown by Rietveld refinement of variable-temperature neutron diffraction data, is a direct consequence of oxygen loss from the structure. On further oxygen loss, a second phase transformation occurs to give a cation-deficient cubic rock salt phase, , at , which loses more oxygen at higher temperatures until, at , the material is essentially a stoichiometric, single-phase cation-disordered rock salt, space group . A second spinel phase persists in small amounts from 950 to . Differences in electrochemical behavior depend on sample preparation and correlate with the oxygen content of when used as a cathode in Li test cells.