Synthesis and characterisation of electron conducting mesoporous Ti and Ta oxide for use as potential electrode materials in lithium ion batteries

Mesoporous titanium oxide with a pore size of 20-30 Å was impregnated with conducting polythiophene nanowires in order to improve conductivity to take advantage of the ca. 1000 m2/g surface areas for possible applications in charge storage in devices requiring fast Li+ insertion kinetics. Pristine mesoporous titanium oxide produced a peak capacity of 301 mAh/g at current densities of 0.2 mA.cm-2 and 187 mAh/g at 1 mA.cm-2. The conductivity of the composite improved from 3.56 x 10-2 mS.cm-1 to 5.79 mS.cm-1 but significantly reduced capacity. This was attributed to pore blocking and a significant increase in the weight of the sample preventing Li+ diffusion into the pores of the material. An investigation of variation in polymer loading level and pore size using polypyrrole nanowires was then undertaken in order to improve performance. The best synthesis conditions were achieved using 5% polymer loading and host materials with the largest pore size. Excessive polymer loading and smaller pore sizes lead to decreased performance due to inhibition of Li+ transport. The C18 templated TiO2 composite produced the best capacity retention at 58% retention, and the C12 composite produced the highest initial capacity of 170 mAh/g using a current density of 1 mA.cm-2. To improve the interface between the polymer and host materials a synthesis was adapted using a catalyst-free UV initiated polymerisation of vapour-loaded pyrrole monomer with mesoporous Ti and Ta oxides. The best materials showed improved conductivity for both the Ti and Ta oxides as well as improved Li+ capacity (190 mAh/g) relative to the pristine material (128 mAh/g) and superior capacity retention (49% as compared to 22%) for the Ti composites. The retention in surface area was 87% compared to 49% reported previously for analogous materials synthesised by catalyst-initiated method. This yielded Li+ capacities of 170 mAh/g further highlighting the superiority of this new photochemical approach. Since polymer doping seemed to improve conductivity while inhibiting Li+ transport an alternative approach using hexamethyldisilathiane (HMDST) to exchange Ti-O units with Ti-S at the surface of the mesopore channels to reduce the band gap between the valance band and the conduction band, increasing the conductivity while retaining the porosity and thermal stability. Lower temperatures generally yielded materials with superior properties and although conductivity was improved using higher loading levels of HMDST, this produced a significant drop in initial capacity (137 - 41 mAh/g), but superior retention on cycling. The best performing material was synthesised using large amounts of regent (3.5 mL) at lower heating temperatures (100 °C) to maximise the combination of surface area, initial capacity, conductivity, and capacity retention (76%). Finally a vanadium hydride gel was prepared by thermal treatment of tetraphenylvanadium(IV) followed by hydrogenation. This V(III)-based material is redox active and the hydride ligand very light relative to oxide and phosphate supporting ligands normally used in V(III)-based battery materials. For this reason it could potentially lead to greater Li+ insertion capacities, so electrochemical evaluation was warranted. The best material demonstrated a peak capacity of 131 mAh/g, at a discharge rate of 1 mA.cm-2. After repeated charge discharge cycling for 50 cycles, the material retained 36% of its capacity.