The population of oxygen vacancies in Li1+yMn2-yO4-delta type cathode materials - The primary factor of temperature dependent structural changes

Based on the results of differential scanning calorimetry and synchrotron b ased in situ X-ray diffraction, during cooling, the structural phase transi tion in LiMn2O4 spinel materials, when cooled below 280 K, is associated to the oxygen vacancies created during synthesis or heat-treatment. At low te mperature (below 260 K), two coexisting phases, the spinel structure with c ubic symmetry and another structure with tetragonal symmetry were observed. The fraction of tetragonal phase decreases with decreasing population of o xygen vacancies, becoming undetectable when the population of oxygen vacanc ies is very low The tetragonal structure in samples quenched to room temper ature from 995 degreesC, as reported in the Literature, is also tentatively related to the oxygen vacancies created during heating and preserved durin g quenching. (C) 2001 The Electrochemical Society.