Electrospun Vanadium-based Oxides as Positive Battery Electrodes

Abstract

Energy storage is an important area of research as demand for reliable, efficient and intermittent energy increases. The development and research of energy storage technologies, in particular lithium ion batteries, is intimately linked to the research of electrode materials. These materials need to be abundant, capable of providing high capacities, possess long cycle life and have good rate retention. Nanosizing of energy storage materials shows promise in addressing these requirements.

This thesis focusses on the synthesis, characterisation and electrochemical properties of electrospun fibrous vanadium-based pentoxides for lithium ion batteries. Vanadium pentoxides (V2O5) are candidate electrode materials due to their high theoretical capacity and layered structure that can reversibly intercalate lithium ions. Starting solutions consisting of vanadium oxytripropoxide, poly(vinyl acetate) (PVAc) and ethanol were used for electrospinning the nanostructured hierarchical fibres. Dopant cations were introduced into the electrospinning solution using sodium acetate, barium oxide, aluminium isopropoxide or titanium (IV) isopropoxide. After electrospinning the as-spun materials were converted into fibrous metal oxides through a single step calcination process. Varying the calcination treatment from of the as-spun fibres showed that crystallite size and fibre morphology were dependant on temperature. Crystallite size and conductivity of the electrospun V2O5 increased with calcination temperature though electrochemical performances did not correlate. Fibres calcined at 500 °C were shown to provide the best compromise between crystallinity, conductivity and capacity. Fibre diameter was controllable by varying the amount of PVAc in the electrospinning starting solution while keeping all other parameters constant. Crystallite size increased with a decrease in fibre diameter, however, no trend in electrochemical performance was observed with V2O5 fibre diameter. Despite this, results indicated that fibrous hierarchical structures are necessary to maintain material integrity and promote higher conductivity during cycling. The structural stabilisation of V2O5 during cycling was investigated using redox-inactive dopants Ti4+ and Ba2+ with loadings of approximately 10 atomic percent (at%). The doped materials improved both rate retention and cycling stability though only Ti4+ doped V2O5 offered improved capacity. The dopants were shown to structurally stabilise V2O5 via phase change prevention. Further dopant investigation showed that dopant location was an important factor in the overall electrochemical performance of the V2O5 host material, and that dopant loading, and oxidation state can influence their location as rationalised by atomistic simulations. Interstitial 2 at% Na+ and 3 at% Ba2+ in electrospun V2O5 provided both improved structural and electronic effects on the V2O5 structure and resulted in improved electrochemical performance. The substitutionally located 3 at% Al3+ provided an improved structural effect but decreased electronic conductivity and lowered capacity. The simplicity of the electrospinning technique combined with the reliability of V2O5 fibre production shows promise in wider implementation of this alternate electrode material for other metal-ion energy systems. For future research of electrospun V2O5 it is recommended that a hierarchical fibrous structure be utilised along with the incorporation of an interstitially located dopant metal. Electrospun V2O5 shows promise as an electrode material by contributing towards providing an alternative abundant metal oxide for efficient energy storage technologies.