Modulation of the Optical Properties of Lithium Manganese Oxide via Li‐Ion De/Intercalation

Abstract

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...

Characterization and Quantification of Depletion and Accumulation Layers in Solid‐State Li+‐Conducting Electrolytes Using In Situ Spectroscopic Ellipsometry

“…Significant changes of the refractive index and extinction coefficient in the visible range upon Li + de‐/intercalation were quantified for lithium manganese oxide. [ 22 ]…”

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

“…In the former case, the fit was not satisfactory, and in the latter case, the fit parameters were not consistent for all the spectra, probably because of over parametrization. The methodology used for fitting has been described in our previous work . The derived model spectra are shown in Figure b by hollow circles.…”

“…The reflected beam was collected normally to the surface by another fiber and transferred to a BW-Tek Exemplar spectrometer. The schematics are described in our previous work, see ref .…”

“…The methodology used for fitting has been described in our previous work. 35 The derived model spectra are shown in Figure 4b by hollow circles. The optimized parameters are summarized in Table 1.…”

Orbital-hybridization-created optical excitations in Li2GeO3

“…Materials that combine good optical characters with high electrochemical performances are of great technological interest 1 – 3 . They can be the central ingredient in the lithium-based batteries governing modern electronic devices and are used in electro-optic applications.…”