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

Li_4/3Ti_2/3O₂–LiFeO₂ solid solution, Li_{(4 − x)/3}Ti_{(2 − 2x)/3}Fe_xO₂ (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 Li_1.2Ti_0.4Fe_0.4O₂ 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.