Rutile-TiO2 based materials for lithium ion batteries

Although widely used, the most promising Li-based energy storage systems still suffer from a lack of suitable electrodes. There is therefore a need to seek new materials to satisfy the increasing demands for energy storage worldwide. TiO2 is a very promising anode material for lithium rechargeable batteries. It has a low insertion voltage of Li and high theoretical specific capacity. However, Li insertion into bulk rutile is negligible at room temperature due to the dense close packing of the rutile structure; also it suffers from a poor electronic conductivity. The electrochemical performance of pure rutile reveals that only 0.11 mol of Li can be inserted into rutile structure with a specific capacity of 26 mAh/ g. The main objective of this thesis has been to seek ways to improve the performance and charge storage capacity of rutile by compositional modification Improvement of the electronic conductivity of rutile by quenching oxygen-deficient samples and its influence on electrochemical performance have been studied and compared with that of fully oxidized rutile. An improvement in charge-discharge capacity was achieved; 0.21 Li per mol of TiO2 corresponding to 49 mAh/ g in the first cycle, but for subsequent cycles, both became similar which indicates that increasing the electronic conductivity by quenching did not give a long term improvement and suggests that lattice dimensions rather than electronic conductivity may be the reason for the poor perfomance of rutile anode. Substitution of Ti4+ with metal ions of either similar or different valence to increase the lattice dimensions and/or to increase the electronic conductivity is an option to improve the electrochemical performance of rutile TiO2. In this study, the effect of doping with large Sn4+ and co-doping with Cu-M (M= Nb, Ta) on the electrical and electrochemical performance is presented. The objective was first, to increase the unit cell dimensions of rutile by doping. This is based on the hypothesis that insertion of Li into TiO2 rutile would be easier with an expanded unit cell. Solid solutions have been prepared via solid state reaction where Ti4+ is partially replaced by either Sn4+ or a combination of divalent (Cu2+) and pentavalent ions (Nb5+, Ta5+). Single-phase solid solutions of the doped systems have been characterised by XRD and indexed on a tetragonal rutile structure; lattice parameter refinement confirms the expansion in the unit cell dimensions. Lithium test cells were fabricated using the rutile solid soultions as anodes. The first discharge step reveals that up to one mole of Li ion can intercalate into codoped Cu-Nb or Cu-Ta at room temperature with a discharge capacity up to 78 mAh/g while a specific capacity of 154 mAh/ g was delivered by Sn-doped rutile. These examples of lattice expanded doped rutile show a much higher electroactivity towards Li insertion than undoped rutile with excellent retention of capacity during cycling. Ex-situ XRD indicates excellent structural stability during cycling with no evidence of major changes in the rutile crystal structure. However, a major drawback in their electrochemical behaviour was a significant loss of capacity on cycling. The variation in the electrical properties of doped systems with the nature and composition of metal electrode and atmosphere was studied for Cu-Nb and Cu-Ta co-doped rutile. The formation of a potential barrier, due to the presence of residual phase at the grain boundary, was indicated by impedance spectroscopy (IS) in codoped system, the data showing a Schottky-like nature. The SnxTi1-xO2 system exhibits resistive behaviour, with high activation energy for all compositions. The effect of rutile TiO2 as starting material on the electrochemical performance of Li4Ti5O12 (LTO) was examined and compared with that of anatase TiO2. High purity LTO was obtained using rutile starting material but the specific capacity was slightly higher for LTO prepared using anatase than rutile.