Oxygen evolution from olivine M n1-xM x P O4 (M=Fe,Ni,Al,Mg) delithiated cathode materials

Olivine LiMnPO4 is a promising cathode material for Li-ion batteries. One drawback of this material is the propensity of its delithiated phase, MnPO4, to evolve oxygen gas above approximately 200 °C. During thermal runaway of cells, this oxygen gas can burn the electrolyte and other cell components and thereby jeopardize safety. Partial substitution of Mn with M=Fe, Ni, Al, or Mg has been used to improve the lithium intercalation kinetics of LixMnPO4; however, the effect of these substitutions on oxygen evolution is not fully documented. In this paper, we calculate phase diagrams and oxygen evolution diagrams for these Mn1-xMxPO4 delithiated cathode materials. To generate the phase diagrams, we use subregular solid-solution models and fit the energetic parameters of these models to density functional theory calculations of special quasirandom structures. The resulting thermodynamic models describe the effect of mixing on the initial temperature of oxygen evolution and on the cumulative amount of oxygen evolution at elevated temperatures. We find that addition of Fe increases the initial temperature and decreases the cumulative amount of oxygen evolution. Mn0.5Fe0.5PO4 exhibits an initial temperature 50 °C higher than MnPO4 and releases 70% less oxygen gas at 300 °C. Al is insoluble in MnPO4, so addition of Al has no affect on the initial temperature. However, Al addition does slightly decrease the amount of oxygen evolution due to an inactive AlPO4 component. Mg and Ni both decrease the initial temperature of oxygen evolution, and therefore may worsen the safety of MnPO4.