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, 7 Li NMR, and galvanostatic electrochemical methods. Special attention has been paid to the compositions inside the quadrangle restricted by solid solutions LiCoO 2 -LiCo 0.85 Mn 0.15 O 2 with the layered structure of α-NaFeO 2 and solid solutions LiMn 2 O 4 -LiMnCoO 4 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] 2 O 4 with a part of Li atoms in octahedral sites, cation-deficit layered compounds Li 1 − δ [Co,Mn]O 2 , and Li 2 MnO 3 . 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 LiCoO 2 -LiCo 0.85 Mn 0.15 O 2 -LiMn 2 O 4 -LiMnCoO 4 are determined by the content and average oxidation number of Mn atoms, which is higher than in the normal spinels Li[Mn, Co] 2 O 4 . Thus, the specific capacities of the polyphase compositions are lower in comparison with the binary solid solutions Li[Mn,Co] 2 O 4 or pure LiCoO 2 .