Electronic and Electrochemical Properties of Li_1–<i>x</i>Mn_1.5Ni_0.5O₄ Spinel Cathodes As a Function of Lithium Content and Cation Ordering

The electronic and electrochemical properties of the high-voltage spinel LiMn_1.5Ni_0.5O₄ as a function of cation ordering and lithium content have been investigated. Conductivity and activation energy measurements confirm that charge transfer occurs by small polaron hopping, and the charge carrier conduction is easier in the Ni:3d band than in the in Mn:3d band. Seebeck coefficient data reveal that the Ni^2+/3+ and Ni^3+/4+ redox couples are combined in a single 3d band, and that maximum charge carrier concentration occurs where the average Ni oxidation state is close to 3+, corresponding to <i>x</i> = 0.5 in Li Li_1–<i>x</i>Mn_1.5Ni_0.5O₄. Accordingly, maximum electronic conductivity is found at <i>x</i> = 0.5, regardless of cation ordering. The thermodynamically stable phases formed during cycling were investigated by recording the X-ray diffraction (XRD) of chemically delithiated powders. The more ordered spinels maintained two separate two-phase regions upon lithium extraction, while the more disordered samples exhibited a solid-solubility region from LiMn_1.5Ni_0.5O₄ to Li_0.5Mn_1.5Ni_0.5O₄. The conductivity and phase-transformation data of four samples with varying degrees of cation ordering were compared to the electrochemical data collected with lithium cells. Only the most ordered spinel showed inferior rate performance, while the sample annealed for a shorter time performed comparable to the unannealed or disordered samples. The results presented here challenge the most common beliefs about high-voltage spinel: (i) low Mn³⁺ content is responsible for poor rate performance and (ii) thermodynamically stable solid-solubility is critical for fast kinetics