Synthesis, Cation Distribution, and Electrochemical Properties of Fe-Substituted Li[sub 2]MnO[sub 3] as a Novel 4 V Positive Electrode Material

solid solution was synthesized using solid-state reaction and hydrothermal-postannealing methods and characterized as a positive electrode material for rechargeable lithium batteries. Although the maximum Fe content was limited up to 30% by solid-state reaction, the content can extend up to 75% by the hydrothermal-postannealing method. Neutron and X-ray Rietveld analysis reveal that the basic structure of the sample is a layered rock-salt structure isostructural with in which Fe ions exist on both Li (3a) and Co (3b) sites. Elemental analysis and Mössbauer spectra show Fe ions exist as 3+/4+ mixed-valence state after the samples were postannealed above 650°C. The initial charge capacity of Li/sample cells was above 100 mAh/g when the upper voltage limit was 4.3 V. The plateau around 4 V was observed for all Li/sample cells on first discharge. The maximum of initial discharge capacity was about 100 mAh/g down to 2.5 V for the Li/(50% Fe-substituted sample) cell, when the positive electrode was obtained by postannealing at 650°C in air. The capacity fading of the 4 V plateau could be suppressed by adjusting the Fe content to less than 50%, postannealing temperature between 600 and 700°C, and by 10% Ni substitution.