Electrochromic Behavior and Phase Transformation in Li 4+ x Ti 5 O 12 upon Lithium-Ion Deintercalation/Intercalation

ACS Applied Materials & Interfaces, vol. 12, pp. 10616–10625

Abstract

The impact of phase transformation from spinel-structured Li 4 Ti 5 O 12 to rocksalt-type Li 7 Ti 5 O 12 on the electrochromic properties of the material is studied. Thin films of Li 4 Ti 5 O 12 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 Li 4 Ti 5 O 12 or Li 7 Ti 5 O 12, respectively, in the UV/visible/NIR region of light. The single strong resonance in the case of Li 4 Ti 5 O 12 is assigned to transition from O 2p to Ti t 2g, that is, across the band gap, whereas for the Li 7 Ti 5 O 12 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 Li 4 Ti 5 O 12 electrodes.

Authors 5

1. Yug Joshi corresponding

   University of Stuttgart

   Affiliation as printed

   Institute of Materials Science, University of Stuttgart, 70569 Stuttgart, Germany

2. Aparna Saksena Aachen Materials Chemistry

   RWTH Aachen University

   Affiliation as printed

   Materials Chemistry, RWTH Aachen University, 52074 Aachen, Germany

3. Efi Hadjixenophontos

   University of Stuttgart

   Affiliation as printed

   Institute of Materials Science, University of Stuttgart, 70569 Stuttgart, Germany

4. Jochen M. Schneider Aachen Materials Chemistry

   RWTH Aachen University

   Affiliation as printed

   Materials Chemistry, RWTH Aachen University, 52074 Aachen, Germany

5. Guido Schmitz

   University of Stuttgart

   Affiliation as printed

   Institute of Materials Science, University of Stuttgart, 70569 Stuttgart, Germany