Electrochromic Behavior and Phase Transformation in Li_4+xTi₅O₁₂ upon Lithium-Ion Deintercalation/Intercalation

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 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.

Mechanical Failure Mechanism of Silicon-Based Composite Anodes under Overdischarging Conditions Based on Finite Element Analysis

“…Considering the expansion and shrinkage of the anode due to lithiation and excessive delithiation during charging and overdischarging processes, − a thermodynamic method was applied here to simulate the evolution of mechanical deformation. − Thermal expansion and electrochemical expansion not only have similar physical meaning in which the change of temperature well corresponds to the change of lithium-ion concentration but also have a similar formula form, − as shown in Table S1. The internal thermal strain (ε th ) caused by thermal expansion can be given by where α is the thermal expansion coefficient, T ref is the reference temperature, and Δ T is the temperature increase used to represent the degree of expansion and shrinkage in our work.…”

Slow‐Moving Phase Boundary in Li4/3+xTi5/3O4

“…A schematic of the sputter chamber and more details on the deposition conditions are given in refs. [9,49] After the deposition, the layers were annealed at 550 °C in vacuum for 2 h. Only the annealed samples showed the proper reversible Li intercalation and were used throughout the study.…”

A fast self-charging and temperature adaptive electrochromic energy storage device

“…[1][2][3] The use of electrochromic materials as electrode materials for energy storage devices, combined with the advantages of energy storage and visual indication of the charge and discharge status, has attracted widespread attention. [4][5][6][7][8][9][10][11][12] However, in harsh environments or remote areas, the power grid is unavailable, which limits the charging and reuse of electrochromic energy storage devices. To solve this issue, various energy harvesting technologies, such as photovoltaic devices, 13,14 friction generators, 15,16 piezoelectric generators, 11 etc., were integrated with electrochromic energy storage devices into self-charging power systems.…”