Br-doped Li4Ti5O12 and composite TiO2 anodes for Li-ion batteries: Synchrotron X-ray and in situ neutron diffraction studies
Synchrotron X-ray diffraction data were used to determine the phase purity and re-evaluate the crystal-structure of Li4Ti5O12-xBrx electrode materials (where the synthetic chemical inputs are x = 0.05, 0.10 0.20, 0.30). A maximum of x′ = 0.12 Br, where x′ is the Rietveld-refined value, can be substituted into the crystal structure with at least 2% rutile TiO2 forming as a second phase. Higher Br concentrations induced the formation of a third, presumably Br-rich, phase. These materials function as composite anodes that contain mixtures of TiO2, Li4Ti5O12-xBrx, and a Br-rich third, unknown, phase. The minor quantities of the secondary phases in combination with Li4Ti5O12-xBrx where x′ ∼ 0.1 were found to correspond to the optimum in electrochemical properties, while larger quantities of the secondary phases contributed to the degradation of the performance. In situ neutron diffraction of a composite anatase TiO2/Li4Ti5O12 anode within a custom-built battery was used to determine the electrochemical function of the TiO2 component. The Li4Ti5O12 component was found to be electrochemically active at lower voltages (1.5 V) relative to TiO2 (1.7 V). This enabled Li insertion/extraction to be tuned through the choice of voltage range in both components of this composite or in the anatase TiO2 phase only. The use of composite materials may facilitate the development of multi-component electrodes where different active materials can be cycled in order to tune power output.