William James Macklin

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

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 A. 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 TiIV over VIV near the low-voltage limit which prevents a reorganisation of the microstructure of the material.

Vanadium phosphate glasses. Effect of composition on their structure and performance as cathodes in high-temperature lithium polymer-electrolyte cells

Five vanadium phosphate (V2O5–P2O5) glasses, containing between 58 and 88 mol% V2O5, have been prepared using the melt-quenching technique. IR spectroscopy, in conjunction with powder X-ray diffraction (XRD) of the devitrified materials, has been used to show that the glasses containing ⩽ 70 mol% V2O5 have a microstructure which is similar to that of β-VPO5 whereas the remainder show both orthorhombic-V2O5 and β-VPO5 structural features. Molar-volume data suggest that there is a fairly abrupt change in microstructure atca. 75 mol% V2O5.The glasses have been employed as the active component of the cathode in lithium polymer-electrolyte cells operating at 120 °C and the cycling performance has been investigated as a function of glass composition. After an initial capacity decline the cells showed good reversibility, although this was not achieved at capacities as high as that of V6O13 in a similar cell. Glasses containing less than 78 mol% V2O5 cycled at considerably lower capacities than those with higher vanadium content and this has been related to the above change in microstructure from a layered V2O5-type to a network β-VPO5 type.