High Temperature Co-doped LiMn₂O₄-Based Spinels. Structural, Electrical, and Electrochemical Characterization

High-temperature cubic spinels were obtained by thermal treatment at 1100 °C of the corresponding LiCo y Mn 2 - y O 4 (0 ≤ y ≤ 1) spinels. The samples were characterized by X-ray powder diffraction, thermogravimetric analysis, electron paramagnetic resonance (EPR) spectroscopy, and electrical and electrochemical measurements. The lattice parameter of the new phases in the 0.3 < y ≤ 1 range is larger compared to the starting ones. The EPR spectra of the new compounds are also different from the starting ones. The electrical conductivity of the new phases depends on the Co content. For y > 0.65 the sharp increase in conductivity observed is associated with a change in electron hopping from Mn 3+ /Mn 4+ ions to Co 2+ /Co 3+ ones. At high temperatures, the conductivity is explained in terms of phonon-assisted polaron hopping among either Mn 3+ /Mn 4+ or Co 2+ /Co 3+ nearest neighbors. At low temperatures electron hopping beyond the nearest neighbors accounts for the conductivity. The electrochemical behavior of the new compounds as positive electrodes was analyzed. The discharge curves show both the 4 and 5 V plateau due to the Mn 4+ /Mn 3+ and Co 4+ /Co 3+ reduction, respectively. Differences in electrochemical characteristics compared to the starting samples are found. From electrochemical and thermogravimetric measurements, the oxidation states of the transition metal ions and the chemical composition of the high-temperature samples are estimated.