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.

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a change in its optical constants during lithiation. However, a quantification of the optical properties and their tailoring via dis/charging has not been probed yet.The present study aims at the quantification of the optical constants, that is, the complex refractive index (CRI) and its change during intercalation by varying the lithium content of Li x Mn 2 O 4 from x = 0 to x = 1. Furthermore, an attempt is made to establish a link between this change in optical constants to the band structure of the material, learned from various sources. [11,15,[18][19][20][21] Similar characteristics have been reported for other electrochromic materials, such as Nb 2 O 5 , WO 3 , V 2 O 5 , and MoO 3, [22] which are known to change their optical properties, namely the real (n) and imaginary (k, or extinction coefficient) part of the CRI during an electrochemical reaction. In these materials, intercalation of charged species results in the generation of different electronic absorption bands in the optical spectrum or insertion of additional bound charges acting as harmonic oscillators. [22] For the present study, the reflection spectrum is measured at different stages of intercalation to clarify such electrochromic phenomenon. The measured spectra are evaluated to extract the CRI of the material for different lithium contents. Furthermore, an innovative method of recording the reflectance spectrum in situ during the electrochemical reaction is demonstrated (Figure 1c) that provides an additional time resolution to the spectrometry. Results Structural CharacterizationThe X-ray diffractogram (XRD) spectra of representative samples of LMO in the as-deposited and annealed states are shown in Figure 2a. No characteristic LMO diffraction peaks are observed for the as-deposited layer (bottom curve of Figure 2a). Only a small hump, observed at 61.98° (marked with *), could be due to the LMO structure corresponding to {440} planes. This reflection is very broad and shifted in comparison to the work of Wickhamt and Croft. [23] (JCP2 database) that predicts the position at 63.78°. The shape and the shift in the reflection suggest that the LMO layer is nanocrystalline and stressed, as it is usually the case for sputter-deposited layers.The optical response of lithium manganese oxide (LiMn 2 O 4 , LMO) on intercalation with Li ions is quantitatively characterized. For this purpose, a layer of LMO and a layer of platinum, acting as current collector/reflector, are deposited on oxidized silicon wafers. The active layer is structurally characterized using X-ray diffractogram and transmission electron microscopy. Well-defined intercalation states are prepared electrochemically and investigated by optical spectrometry in reflectance geometry. The measured dispersion curves are described by the Clausius-Mossotti dispersion equation to derive the complex refractive index as a function of wavelength and intercalation state. The observed variation of the effective resonant wavelength is consistent with the change in the band structure of LMO with l...

The work demonstrates a novel route to synthesize disorder rich rock salt-type Li2MnO3−δ electrodes flaunting remarkably high capacity due to dynamic phase transformation during cycling.