The population of oxygen vacancies in Li_1+yMn_2-yO_4-δ type cathode materials. The primary factor of temperature dependent structural changes

Based on the results of differential scanning calorimetry and synchrotron based in situ X-ray diffraction, during cooling, the structural phase transition in LiMn₂O₄ spinel materials, when cooled below 280 K, is associated to the oxygen vacancies created during synthesis or heat-treatment. At low temperature (below 260 K), two coexisting phases, the spinel structure with cubic symmetry and another structure with tetragonal symmetry were observed. The fraction of tetragonal phase decreases with decreasing population of oxygen vacancies, becoming undetectable when the population of oxygen vacancies is very low. The tetragonal structure in samples quenched to room temperature from 995°C, as reported in the literature, is also tentatively related to the oxygen vacancies created during heating and preserved during quenching.