Growth of V2O5 films for electrochromic and battery applications

Vanadium pentoxide (V2O5) is an electrochromic material. Research into electrochromic devices has become more popular with the renewed interest in environmentally-friendly ‘green’ technologies. Particular interest is in the production of electrochromic coated windows for energy saving purposes. Over the past year’s scientists have struggled to develop electrochromic coated windows which would be of low cost, acceptable optical characteristics, long-term durability and at the same time uniformly coated without pinholes or gaps. Vanadium pentoxide has also shown potential as a cathode material in electrochemical power sources such as lithium ion batteries, which becomes more significant with the ever-growing demand for portable electronic devices. A large amount of work is being done to develop high performance, reliable and ecologically friendly lithium ion batteries. This research aims to address these requirements. The first part of the thesis describes the growth of V2O5 thin films grown by Aerosol Assisted Chemical Vapour Deposition (AACVD) on fluorine doped tin oxide substrates using vanadyl acetylacetonate (VO(acac)2) as the precursor. It was found that the optimal growth conditions for the films to be used as a cathode material for lithium ion batteries was 400°C for the duration of 1 hour with a post annealing step for 3 hours at 600°C to promote crystallization of the films. Even though the films showed promising electrochemical properties, they suffered from non-uniformity at lower growth temperatures, electrochemical stability issues and poor optical transmittance hence a step to improve these properties was made in the form of doping. Silver was chosen as a metal to dope the V2O5 films using silver trifluoroacetate (AgO2CCF3) as the doping precursor. It was found that 450°C was the new optimal temperature for the growth and that an increase in crystallinity and a change in preferred orientation and morphology was observed. Higher percentages of silver doping improved the electrochemical properties such as the specific capacity, stability and reversibility of the material over 500 cycles. Even though silver doping improved the electrical properties of the films there were still concerns associated with the uniformity and optical transparency. 1 | Page In order to solve these shortcomings, it was decided to change both the growth method and the substrate. Atomic Layer Deposition (ALD) was employed with Vanadium(IV) Tetrakis(DimethylAmide) (V(NMe2)4) as the vanadium source and indium doped tin oxide glass (ITO) as the new substrate. Films were grown using both thermal and plasma ALD for 400 cycles, growth temperature was found to be optimal outside of the ALD window hence the process included a significant chemical vapour deposition (CVD) component. Improvements in the electrical, optical and morphological film quality were observed with ALD when compared to the AACVD grown films. Even though the electrochemical properties of the film were drastically improved the cycling stability still remained an issue. In an effort to solve this and possibly further improve the electrochemical properties silver doping was performed in the ALD mode. The silver precursor, 2,2,6,6 tetramethyl-3,5-heptadionato silver (I) (AgC11H19O2) was added in a super-cycle ALD mode so as to give nominally 5% and 10% silver doped V2O5 films. The electrochemical properties of the films were significantly improved in terms of performance and cycling stability. A detailed comparison between the two growth techniques, the effects of doping and the subsequent properties of the films are presented in this thesis