Sol–gel-derived vanadium and titanium oxides as cathode materials in high-temperature lithium polymer-electrolyte cells

Abstract

Binary and ternary vanadium- and titanium-containing xerogels have been prepared by hydrolysis of the metal isopropoxides and subsequent condensation. Powder X-ray diffraction (XRD) has been used to show that pure gel-derived titanium(IV) oxide possesses a struture resembling poorly crystalline anatase, whereas for all the vanadium-containing materials prepared in isopropyl alcohol solution the data are consistent with some two-dimensional (2D) order and a similar short-range arrangement of VO₅ moieties to that in crystalline V₂O₅. In contrast, the vanadium oxide xerogel obtained from an aqueous milieu shows evidence only of one-dimensional order, interlayer distance 14.2 Å. Thermal analysis and XRD have been used to show that all the vanadium-containing gels lose water in three stages and that no structural change occurs until the third stage of water loss, which occurs simultaneously with crystallisation. The oxides have been employed as the active component of the cathode in lithium polymer-electrolyte cells operating at 120°C and their cycling performance has been investigated. The binary oxides showed no improvement in performance over similar crystalline materials whereas the ternary materials, whether chemical or physical mixtures, showed good reversibility and gave observed energy densities which compared favourably with that of V₆O₁₃ in a similar cell. This improvement in performance has been attributed to the preferential reduction of the Ti^IV over V^IV near the low-voltage limit which prevents a reorganisation of the microstructure of the material.