Fine Li(4 ? x)/3Ti(2 ? 2x)/3FexO2 (0.18 ? x ? 0.67) powder with cubic rock-salt structure as a positive electrode material for rechargeable lithium batteries

Abstract

Li4/3Ti2/3O2–LiFeO2 solid solution, Li(4 − x)/3Ti(2 − 2x)/3FexO2 (0.18 ≤ x ≤ 0.67), which has the cubic rock-salt structure (Fmm, average particle size less than 100 nm), was synthesized from Fe-Ti co-precipitates by hydrothermal reaction with excess LiOH and KClO3 at 220 °C. Calcination of the products with lithium hydroxide in an oxidative atmosphere leads to the oxidation of trivalent iron to a 3+/4+ mixed valence state. Hydrothermally-obtained Li1.2Ti0.4Fe0.4O2 gave maximum initial charge (266 mA h g−1) and discharge capacities (153 mA h g−1 around 3 V) between 2.5 and 4.8 V. Calcination enabled us not only to improve the crystallinity, but also suppress the discharge capacity fading with cycle number. Two plateaus around 3 and 4 V were observed on discharging by decreasing the amount of Li extraction (0.4 Li per chemical formula). Although the cubic rock-salt structure was retained during both charge and discharge processes, a partial 3d-cation displacement from octahedral 4a to tetrahedral 8c sites and some oxygen loss were observed after electrochemical delithiation. In-situ57Fe Mössbauer spectroscopy showed evidence of the Fe3+/Fe4+ redox only around the 4 V region.