Thermodynamic investigations of ternary lithium-transition metal-oxygen cathode materials

The reaction of lithium with three transition metal oxides (MnO, LiFeO2, and LiCoO2) has been investigated by an equilibrium electrochemical technique in cells of the type: (−) Al, Li0.9Al/LiClKcl(l)/[Li xMOy] (+) Each system exhibits long constant voltage plateaus characterized by three-phase equilibria. The compositional range of reaction of lithium with MnO is 2.0 equivalents, whereas 3.0 equivalents may be reacted with the compounds LiFeO2 and LiCoO2. The ternary iron and cobalt oxide systems have been found to be kinetically fast (10–15 mA/cm2) and reversible at 400°C. The free energies of formation ΔGfo of Li5FeO4, LiFeO2, and LiCoO2 were calculated and found to be −399.88, −154.18, and −131.62 kcal/mole, respectively. Replacement of sulfide with oxide cathode materials might reduce the high temperature lithium battery corrosion problems currently associated with sulfur-containing cells.