SYNTHESIS AND CHARACTERISATION OF INTERGROWTH TUNGSTEN BRONZES AND EVALUATION OF THEIR ELECTROCHEMICAL PERFORMANCE

Abstract

The UK is becoming increasingly reliant on renewable energy sources as a replacement for fossil fuels. There is increasing pressure to reduce the consumption of fossil fuels and, as a result, there is increasing pressure on the energy industry to meet demands for renewable energy. However, this demand cannot be met without the means to store energy during off-peak hours, for use during peak energy consumption hours. Lithium ion batteries provide the solution to this demand, though much work needs to be done to bring battery technology up to the required capacity.
Since their commercialisation in 1991, lithium-ion batteries have become safer and more efficient, partly as a result of the adoption of graphitic anodes, at the expense of electrochemical capacity. In recent years, new energy materials have come forward promising greater capacity to store and provide energy whilst retaining the all-important safety features of intercalation materials. In particular, the work of Griffith et al. has hinted towards safe, reliable and cost-efficient metal oxide anodes with greater electrochemical capacity than the graphitic anodes currently in use across the globe (K. J. Griffith et al., 2018). By selecting the right structural motifs, materials previously thought unsuitable for use as electrodes might provide the solution to the world’s demand for safe and storable energy. This thesis focuses upon the unique interlocking (or ‘intergrowth’) tungsten
bronze (ITB) phase, first reported by Hussain and Kihlborg, for which testing under battery conditions has not yet been conducted (A. Hussain and L. Kihlborg, 1976). K0.13WO3, which is predicted to exhibit an ITB phase, was synthesised and attempts to elucidate its structure via PXRD were made. Galvanostatic discharge-charge data up to an including the third discharge demonstrated that K0.13WO3 has a capacity of 1.60 Li+/TM on the third discharge, exceeding the capacities of Nb16W5O47 (~1.5 Li+/TM) and Nb18W16O93 (~1.4 Li+/TM) reported by Griffith et al. at similar rates of discharge. Furthermore, solid-state 7Li and 6Li MAS NMR experiments were conducted, suggesting that Li+ reversibly intercalated into WO3-like environments in the ITB phase as opposed to the hexagonal sites which were also present.