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–LiCo_0.85Mn_0.15O₂ 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 Li_1 − 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–LiCo_0.85Mn_0.15O₂–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.