Variation of Ti Valence with Lithium Content in Lithiated/Delithiated Li₄Ti₅O₁₂ Studied by X-Ray Absorption near the Edge Structure

Synchrotron-based x-ray absorption near-edge structure (XANES) spectroscopy is applied to investigate the variation of Ti valence in the discharge and charge processes of Li₄Ti₅O₁₂. Through analysis of the XANES data, the average valence of Ti can be calculated, which allows quantitative determination of the ratios of Ti³⁺/Ti⁴⁺ as a function of Li content in the electrodes. It is found that the ratios in the composites of Li₄Ti₅O₁₂/acetylene black (AB)-carbon are larger than those in Li₄Ti₅O₁₂/Ag/AB-carbon after the discharge to 1.1 V, indicating a larger amount of Li inserted into the former than that into the latter. This finding provides a good explanation for the fact that the Li₄Ti₅O₁₂/AB-carbon samples exhibit larger storage capacities than the Li₄Ti₅O₁₂/Ag/AB-carbon ones prepared in this work, concerning the larger Ti³⁺/Ti⁴⁺ ratio and the larger amount of Li⁺ inserted in the electrode for satisfying the charge neutralization requirement.