Thermochemistry of Li_1+xMn_{2- x}O₄ (0≤x≤1/3) spinel

Lithium substituted Li_1+xMn_{2- x}O₄ spinel samples in the entire solid solution range (0≤x≤1/3) were synthesized by solid-state reaction. The samples with x<0.25 are stoichiometric and those with x≥0.25 are oxygen deficient. High-temperature oxide melt solution calorimetry in molten 3Na ₂O·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 x follow similar trends. The enthalpy of formation from oxides becomes more exothermic with x for stoichiometric compounds (x<0.25) and deviates endothermically from this trend for oxygen-deficient samples (x≥0.25). This energetic trend is related to two competing substitution mechanisms of lithium for manganese (oxidation of Mn³⁺ to Mn ⁴⁺ 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.