Electrochromic Behavior and Phase Transformation in Li₄₊Ti₅O₁₂ 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 t_2g, 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 t_2g and from filled Ti t_2g to empty Ti e_g. 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.