Stable phases of the isothermal (800^oC) cross-section of the pseudo-ternary Li-Mn–Co–O system

Chem. Met. Alloys 1 (2008) 283-287

https://doi.org/10.30970/cma1.0066

Alexander A. ANDRIIKO, Arseniy Ye. SHPAK, Nataliya Ye. VLASENKO, Nataliya M. STEPANENKO

The equilibrium phase composition has been studied in two (pseudo)binary cross-sections of the Li-Mn-Co-O system at 800°C in ambient air atmosphere: I – LiMn_2-xCo_xO_n (0≤x≤2) and II – LiMn_xCo_1-xO_n (0≤x≤1). Section I contains a homogeneous spinel-structured phase in the composition range 0≤x≤1, with the lattice parameter decreasing with increasing Co content. The compositions become 2-phase for 1<x≤2, consisting of a Li-deficient spinel structure Ме₃О₄ (Ме = Со, Mn) and layered LiMeO₂ with the structure of α‑NaFeO₂. The compositions in section II are single-phase only if the Mn/Co atomic ratio does not exceed ¼ (0≤x≤0.25). Further increase of the Mn content results in the decomposition of this phase into three compounds with fixed compositions according to the overall reaction LiMn_xCo_1-xO_n → a LiMn_0.25Co_0.75O₂ + b LiMn_1.1Co_0.9O₄ + c Li₂MnO₃. The phase LiMn_0.25Co_0.75O₂ disappears for 0.6≤x≤1. Electrochemical testing of single-phase samples from both cross-sections showed that the replacement of up to 20% of the Mn atoms in lithium-manganese spinel increases the reversible capacity of this compound when used as material for the positive electrode of a Li-ion cell. The electrochemical performance of LiCoO₂ deteriorates with the partial replacement of Co atoms by Mn.

Voltage profiles of galvanostatic charge and discharge curves for LiMn₂O₄ (1) and LiMn_1.6Co_0.4O₂ (2) samples.

Keywords

Lithium / Manganese / Cobalt / Oxides / Stable phases / Electrochemical properties / Li-ion cell