Nanostructured electrodes for battery applications

Abstract

Nanostructured materials have attracted attention for application in energy storage devices, especially for those which require high charge/discharge current rates such as lithium ion batteries. Initial work in this thesis focused on finding a nanostructured electrode pair capable of performing as a high rate device. Previously, mesoporous nickel demonstrated high rate performance in alkaline electrolyte, but attempts to ion exchange H+ for Li+ and import in to aprotic lithium electrolytes proved futile; due to the corrosion of the active NiOOH by anions of the lithium salt. Both Nickel Phosphorous (NiP) and Mesoporous Nickel Phosphorous Alloys (MNiP) were electrodeposited and displayed an in aprotic electrolytes compared to pure Ni; characterised by a shift in pitting potential (Ep) from ~3.2V to 4.75 V VS. Li for Nio,78Po,22. MiNiP ion-exchanged in wann aqueous lithium hydroxide yielded significantly higher electrochemical activity than MNi when subsequently cycled in aprotic electrolytes, however, the ion exchange reaction was only 22 % complete. increased corrosion resistance The search for a nanostructured electrode pair moved toward finding a high rate aqueous negative electrode to partner the MNi electrode. Nanostructured titanium dioxide films, (formed by the sol-gel surfactant template method) unlike non templated, have been found to be reduced when cathodically polarised in aqueous lithium hydroxide. The result is attributed to accelerated lithium ion diffusion within the porous nano structure , allowing a rapidly reversible insertion of lithium ions forming Lix Ti02 with x as high as 0.25 in a 1.3 !lm thick film. An open circuit potential of 1.75 V is observed for the charged NiOOH2_x/LixTi02 device with an average cell potential of 1.6 V, well in excess of Ni/MH and Ni/Cd systems; which discharge between 1.0 and 1.3 V. A specific energy of 90 Wh kg-I whilst impressive for a supercapacitor is not comparable to batteries. Synthesising films of a thickness great enough to be utilised in a battery or ultra capacitor device was ultimately unsuccessful, due to cracking and exfoliation of the film from the underlying FTO substrate. To achieve higher material loadings internally nanostructured Ti02 powders (100-220 m2g- l ) also prepared by the sol-gel technique, were investigated in composite electrode films. An extra capacity for lithium insertion at low voltage and an impressive rate performance, characteristic of high surface area titanates was observed. A correlation between the observed rate performance and electrode thickness was discovered. Further study revealed that the diffusion of ions through the electrode matrix was the limiting factor for nanomaterials in thick composite electrodes rather than solid state diffusion, contrary to conventional wisdom. Nanostructured Ti02 as has been optimised such that particle relaxation time is of the order of a few minutes, therefore the composite or microstructure of the battery must also operate on this time scale. If characteristic electrode thickness > 100 !lm for commercial application are to facilitate high rates, significant efforts must now focus on optimisation of the microstructure of batteries, in tandem with continued efforts on the preparation of more uniform nanostructured materials.

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