Electrochromic Behavior and Phase Transformation in Li4+xTi5O12 upon Lithium-Ion Deintercalation/Intercalation.

The impact of phase transformation from spinel-structured Li4Ti5O12 to rocksalt-type Li7Ti5O12 on the electrochromic properties of the material is studied. Thin films of Li4Ti5O12 are deposited on platinum-coated substrates using radio-frequency-ion beam sputtering. In situ and ex situ optical spectroscopy (in reflectance geometry) is performed along with electrochemical characterization. In situ measurements demonstrate the reversible electrochromic behavior of the deposited thin films and the effect of the change of lithium content on the reflectance spectrum. Ex situ measurements quantify the optical constants of thin films for different charge states by modeling the reflectance spectrum with a Clausius-Mossotti relation. The model reveals the presence of one or two dominant resonant frequencies in the case of Li4Ti5O12 or Li7Ti5O12, respectively, in the UV/visible/NIR region of light. The single strong resonance in the case of Li4Ti5O12 is assigned to transition from O 2p to Ti t2g, that is, across the band gap, whereas for the Li7Ti5O12 phase, the two resonances correspond to the electronic transitions from O 2p to empty Ti t2g and from filled Ti t2g to empty Ti eg. The concentration dependence of the derived dielectric constants points out a fast lithium ion transport through the grain boundaries, thereby segregating a conductive lithium-rich phase at the grain boundaries. This increases the electronic conductivity of the thin films in the initial stages of intercalation and explains the debated mechanism of the fast discharge/charge capability of Li4Ti5O12 electrodes.