Oxygen Nonstoichiometry and Phase Transitions in LiMn[sub 1.5]Ni[sub 0.5]O[sub 4−δ]

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Abstract
Crystal structure and oxygen stoichiometry in LiMn1.5Ni0.5O4−δLiMn1.5Ni0.5O4−δ , 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 O2O2 pressure, this cation-ordered spinel is oxygen-stoichiometric, δ=0δ=0 , space group P4332P4332 . Upon heating between 650 and 680°C680°C , increasing oxygen deficiency occurs exclusively in MnO6MnO6 octahedra and Mn–O–MnMn–O–Mn 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 Mn–OMn–O and Ni–ONi–O bond lengths, and disordering of Mn, Ni on octahedral sites. Hence, the transformation to space group Fd3¯mFd3¯m , 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, δ∼0.65δ∼0.65 , at 950°C950°C , which loses more oxygen at higher temperatures until, at 1100°C1100°C , the material is essentially a stoichiometric, single-phase cation-disordered rock salt, space group Fm3¯mFm3¯m . A second spinel phase persists in small amounts from 950 to 1100°C1100°C . Differences in electrochemical behavior depend on sample preparation and correlate with the oxygen content of LiMn1.5Ni0.5O4−δLiMn1.5Ni0.5O4−δ when used as a cathode in Li test cells.

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