Layered lithium manganese oxide cathodes
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The synthesis, characterisation and electrochemical performance of layered lithium manganese oxide materials have been investigated in terms of their application as an intercalation cathode in rechargeable lithium batteries. Non-stoichiometric LiₓMnᵧO₂, stoichiometric LiMnO₂ (α-NaFeO₂ type) with doped forms, LiₓMn₁₋ᵧMeᵧO₂ and LiMn₁₋ᵧMeᵧO₂ (where Me = Al, Mg, Li, Ni, Co), were prepared by solid-state and solution synthesis routes coupled with ion exchange from sodium precursors. These materials were investigated by X-ray and neutron powder diffraction, as well as chemical and compositional analysis, SEM, TEM, surface area and galvanostatic cycling measurements. The structure and performance of non-stoichiometric materials is highly dependant on the synthesis conditions and ion exchange process which determine the defect chemistry. LiₓMnᵧO₂ exhibits high capacities, 190mAhg⁻¹ at a rate of 25mAg⁻¹ (C/8) with good retention of this capacity (fade rate of ~0.1% per cycle). However, performance is hindered by a first cycle charge capacity which is less than the subsequent discharge. This is a problem for lithium ion cells which require a slight excess of Li on the first charge to form the SEI layer on the carbon anode. The performance of LiMnO₂ is less dependant on synthesis conditions, exhibiting high discharge capacities, ~200mAhg⁻¹at 25mAg⁻¹with minimal fade rate and essentially theoretical charge capacity on the first cycle. Doping of layered materials was found to result in a reduction in the initial dip and rise in capacity over the first few cycles, improved rate capability and first cycle efficiency for non-stoichiometric materials as well as higher overall capacity >200mAhg⁻¹. Structural transformation from a layered to spinel-like configuration on cycling has been investigated. Performance of the spinel-like material formed in situ is markedly superior to directly prepared spinels, being attributed to the formation of a nanostructure able to accommodate the lattice strain caused by the Jahn-Teller distortion. Ball milling and variation in carbon and Kynar concentration were investigated as well as the possibility of electrodes containing both stoichiometric and non-stoichiometric components, to permit an excess of charge capacity on the first cycle in order to accommodate the irreversible losses due to SEI surface layer formation when cycling against a carbon based anode.
Thesis, PhD Doctor of Philosopy
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