Stable phases of the isothermal (800oC) cross-section of the pseudo-ternary Li-MnCoO system

 

Chem. Met. Alloys 1 (2008) 283-287

 

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 800C in ambient air atmosphere: I LiMn2-xCoxOn (0≤x≤2) and II LiMnxCo1-xOn (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 34 ( = , 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 LiMnxCo1-xOna LiMn0.25Co0.75O2 + b LiMn1.1Co0.9O4 + c Li2MnO3. The phase LiMn0.25Co0.75O2 disappears for 0.6x1. 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 LiMn1.6Co0.4O2 (2) samples.

 

Keywords

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