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

Yug Joshi; Aparna Saksena; Efi Hadjixenophontos; Jochen M Schneider; Guido Schmitz

doi:10.1021/acsami.9b19683

Electrochromic Behavior and Phase Transformation in Li_{4+ x}Ti₅O₁₂ upon Lithium-Ion Deintercalation/Intercalation

ACS Appl Mater Interfaces. 2020 Mar 4;12(9):10616-10625. doi: 10.1021/acsami.9b19683. Epub 2020 Feb 20.

Authors

Yug Joshi¹, Aparna Saksena², Efi Hadjixenophontos¹, Jochen M Schneider², Guido Schmitz¹

Affiliations

¹ Institute of Materials Science, University of Stuttgart, 70569 Stuttgart, Germany.
² Materials Chemistry, RWTH Aachen University, 52074 Aachen, Germany.

PMID: 32041397
DOI: 10.1021/acsami.9b19683

Abstract

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.

Keywords: Clausius−Mossotti; Li4Ti5O12; complex refractive index; electrochromic; lithium titanate; phase transformation; reflectance spectroscopy; thin film.