High-capacity Li-excess lithium nickel manganese oxide as a Co-free positive electrode material

Abstract

For the design of high-capacity and Co-free positive electrode material, stoichiometric LiNi1/2Mn1/2O2 and Li-excess Li1+x(NiyMn1-y)1-xO2 (0<x<1/3, y = 0.4 and 0.5) samples were prepared using coprecipitation–calcination. The basic LiNi1/2Mn1/2O2 structure is hexagonal layered rock-salt structure (R3¯m), but the sample has a biphasic composite of main Li-poor Li1-xTM1+xO2 (TM, transition metal) and minor Li-rich Li1+xTM1-xO2 phases after calcination below 950 °C. The fraction of the minor Li-rich phase decreased concomitantly with increasing calcination temperature. Only at 1000 °C was single-phase LiNi1/2Mn1/2O2 obtainable. The biphasic samples exhibited better electrochemical performance than that for the single-phase sample. Based on this result, two Li-excess Li1+x(NiyMn1-y)1-xO2 (0<x<1/3, y = 0.4 and 0.5) samples were synthesized by setting an excess amount of Li2CO3 (nominal Li/(Ni + Mn) ratio = 2) and calcination in air. Both samples were usefulness as a high-capacity positive electrode material exceeding 180 mA h g−1, with good cycling properties at 2.0–4.6 V.