Thermochemistry of Li1+xMn2−xO4 (0⩽x⩽1/3) spinel

Lithium substituted Li1+xMn2−xO4 spinel samples in the entire solid solution range (image) were synthesized by solid-state reaction. The samples with image are stoichiometric and those with image are oxygen deficient. High-temperature oxide melt solution calorimetry in molten 3Na2O·4MoO3 at 974 K was performed to determine their enthalpies of formation from constituent binary oxides at 298 K. The cubic lattice parameter was determined from least-squares fitting of powder XRD data. The variations of the enthalpy of formation from oxides and the lattice parameter with image follow similar trends. The enthalpy of formation from oxides becomes more exothermic with image for stoichiometric compounds (image) and deviates endothermically from this trend for oxygen-deficient samples (image). This energetic trend is related to two competing substitution mechanisms of lithium for manganese (oxidation of Mn3+ to Mn4+ versus formation of oxygen vacancies). For stoichiometric spinels, the oxidation of Mn3+ to Mn4+ is dominant, whereas for oxygen-deficient compounds both mechanisms are operative. The endothermic deviation is ascribed to the large endothermic enthalpy of reduction.