Formation of stable phases of the Li–Mn–Co oxide system at 800 °C under ambient oxygen pressure

Abstract

The synthesis method of lithiated d-metal oxides using molten formate mixtures as precursors has been developed and the isothermal (800 °C) cross section of pseudo ternary Li–Mn–Co oxide system under ambient oxygen pressure has been investigated by XRD, 7Li NMR, and galvanostatic electrochemical methods. Special attention has been paid to the compositions inside the quadrangle restricted by solid solutions LiCoO2–LiCo0.85Mn0.15O2 with the layered structure of α-NaFeO2 and solid solutions LiMn2O4–LiMnCoO4 with the structure of spinel. It was found that, depending on the composition, three types of equilibrium phases could be formed: spinels Li[Li,Mn,Co]2O4 with a part of Li atoms in octahedral sites, cation-deficit layered compounds Li1 − δ [Co,Mn]O2, and Li2MnO3. Areas of (co)existence of these phases were plotted on the composition plane of the pseudo-ternary Li–Mn–Co system. Electrochemical properties of the compositions inside the quadrangle LiCoO2–LiCo0.85Mn0.15O2–LiMn2O4–LiMnCoO4 are determined by the content and average oxidation number of Mn atoms, which is higher than in the normal spinels Li[Mn,Co]2O4. Thus, the specific capacities of the polyphase compositions are lower in comparison with the binary solid solutions Li[Mn,Co]2O4 or pure LiCoO2.