Structure, morphology and reaction mechanisms of novel electrode materials for lithium-ion batteries

• Main Author: Hua, Xiao
• Published: University of Cambridge 2015
• Subjects: 540
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.675909

For the Li1+xV1-xO2 materials, the amount of excess Li does not change the average crystal structure; however, it alters the cycling performance greatly. This suggests that the local structure governs the electrochemical behaviour. This motivated us to employ the pair distribution function (PDF) technique to probe the local structure. The analyses show the displacements of V3+ form trimers. A structure model for the stoichiometric LiVO2 incorporating the V3+ distortion was constructed and refined, to assist in the assignment of the Li sites revealed by the 6Li nuclear magnetic resonance (NMR) results. For the TiO2-B materials, previous investigations of nanostructured phases suggested the Li insertion mechanism of these materials has a morphological dependence. However, the morphology of the nanoparticles, which to date shows the best cycling performance among all the TiO2 phases with various morphologies, has not been well studied due to the technical challenges which arise from the nature of the diffraction technique and the limited particle sizes. We therefore employed techniques including small-angle X-ray scattering (SAXS) and PDF analyses, to provide an accurate description of the morphology for these nanoparticles. Combining advanced structure modelling, we demonstrated that the nanoparticles adopt an oblate shape with the minor-axis along the b-axis. For the CuF2 conversion material, which theoretically has an exceptionally high specific energy density, behaves differently in practice when compared with other fluorides, such as FeF2 and NiF2. The mechanism underlying its unique electrochemical performance and poor reversibility were investigated via a variety of characterization methods, including cyclic voltammetry (CV), X-ray absorption near edge structure (XANES), PDF and NMR spectroscopy. We demonstrated that Cu dissolution takes place upon charge, associated with the consumption of the LiF phase via the formation of a Cu1+ intermediate. Such side reaction prevents Cu from transforming back to CuF2, leading to negligible capacities in subsequent cycles and making this material challenging to use in a rechargeable battery.