| Li-ion rechargeable battery has emerged as one of the most important portable energy carriers in the last decade. While LiCoO2 has been used as the cathode for a decade because of the good capacity and cycle retentions, tremendous efforts have been devoted to search other low cost and environmentally viable materials. Some of the promising materials such as LiFePO4, Li3V2(PO4)3, and LiNi1/3 Mn1/3Co1/3O2 were studied in this thesis. New lithium metal fluorophosphates were also discovered as potential cathode materials.; The use of an aqueous solution synthesis route employing nanosized oxidized carbon black particles to inhibit LiFePO4 crystal growth was demonstrated. The resultant particle size of about 100 nm is reduced by 20 times compared to the solution synthesis method alone. Electron diffraction patterns and high resolution images from TEM experiments confirmed the single olivine phase nature of the material and the very small crystallite sizes. The 100 nm crystallites of LiFePO4-OCB showed vastly improved capacity (0.7 Li, 125mAh/g) compared to the 2mum particle. This improvement is due to contributions of decreased Li diffusion paths and improved contact with conductive carbon particles.; Electrochemical PITT experiments coupled with ex-situ X-ray diffraction studies revealed the structural similarities of the delithiated monoclinic single phase compositions of LixV2(PO 4)3. (x = 2, 1, 0) From Le Bail refinements of XRD patterns, monoclinic Li3V2(PO4)3 shows only 7% volume variation upon delithiation which demonstrates its excellent intercalation characteristics. Structures of delithiated single phase compositions were further studied by both powder neutron diffraction and 7Li solid state NMR. From the structure solutions, each plateau in the V vs x curve corresponds to a two-phase transition involving the reorganization of electrons and Li ions within the lattice. The existence of charge ordering in Li2V2(PO4)3 demonstrates the electrons are pinned on both long and short time scales. The structures spanning the phase transition are energetically different, governed by the site potential of the Li+ ion, and interaction with the V"+ ions in the lattice.; The solid solution regime on re-insertion (from x = 0 to x = 2.0) appears to be a consequence of the mixed V4+/V5+ state in V2(PO4)3 formed on complete emptying of the lattice, which does not display charge ordering. Therefore lithium insertion results in disorder until sufficient Li+ re-population and vanadium reduction occur. These findings explain the complex steps displayed in the voltage vs composition curves of LixV 2(PO4)3, and are also directly relevant to other materials. (Abstract shortened by UMI.)... |
