Electrochemical investigation of electrochromic devices based on NiO and WO3 films using different lithium salts electrolytes

Research Progress in Ionic Liquid-Based Electrolytes for Electrochromic Devices

Electrochromic (EC) technology has become one of the smart technologies with the most potential for development and application at this stage. Based on electrochromic devices (ECDs), this technology has shown extraordinary potential in the fields of smart windows, display devices, and sensing systems. With the optimization and iteration of various core components in ECDs, the electrolyte layer, a key component, evolved from its initial liquid state to a quasi-solid state and solid state. As driven by increasing application demands, the development trend indicates that all-solid-state, transparent electrolytes will likely become the future form of the electrolyte layer. Recently, the application of ionic liquid (IL)-based electrolytes in the field of electrochromism attracted a lot of attention due to their ability to bring outstanding EC cycling stability, thermal stability, and a wider operating voltage range to ECDs, and they are regarded as the new generation of electrolyte materials with the most potential for application. Although compared with conventional electrolytes, IL-based electrolytes have the characteristics of high price, high viscosity, and low conductivity, they are still considered the most promising electrolyte materials for applications. However, so far, there has been a lack of comprehensive analysis reports on "Research progress in ionic liquid-based electrolytes for electrochromic devices" within the EC field. In this article, the research progress of IL-based electrolytes in ECDs will be summarized from three perspectives: liquid, quasi-solid, and solid state. The future development directions of IL-based electrolytes for ECDs are discussed.

Effect of In Situ Heating on the Growth and Electrochromic Properties of Tungsten Trioxide Thin Films

Electrochromism has emerged as a pivotal technology in the pursuit of energy efficiency and environmental sustainability, spurring significant research efforts aimed at the creation of advanced electrochromic devices. Most electrochromic materials are used for smart window applications. However, current electrochromic materials have been applied to new energy vehicles, cell phone back covers, AR glasses, and so on. More application scenarios put forward more requirements for the color of the colored states. Choosing the right color change in the application will be the trend in the future. In this work, tungsten trioxide (WO3) thin films were prepared by adjusting the in situ heating temperature. WO3 with a crystalline structure showed excellent cyclic stability (5000 cycles), electrochromic performance (ΔT = 77.7% at 633 nm, CE = 37.1 cm2/C), relatively fast bleaching/coloring speed (20.0 s/19.4 s), and the darkest coloring effect (L* = 29.32, a* = 7.41, b* = -22.12 for the colored state). These findings offer valuable insights into the manipulation of smart materials and devices, contributing to the advancement of electrochromic technology.

All solid state electrochromic devices based on the LiF electrolyte

The unsafe deposition process and slow deposition rate of the electrolyte layers are the main obstacles for electrochromic devices (ECDs) toward commercial application. In this work, an ECD with a structure of glass/ITO/WO₃/LiF/NiO/ITO has been prepared by electron beam and resistance evaporation methods. The LiF electrolyte is deposited by resistance evaporation with the LiF particles and shows promising potential as the Li⁺ based electrolyte in ECDs owing to its high transparency and good ionic conductivity. The ECD shows a fast response (4.0 s for bleaching and 9.6 s for coloring), large optical transmittance modulations (∼58.9% at 625 nm, 100 s for coloring), good stability and high coloration efficiency (88.5 cm² C^-1). This work not only indicates that LiF can be used as a Li⁺ based electrolyte in an ECD, but also paves a new way to fast and safe preparation of ECDs with high performance.

Electrochromism of Nanocrystal-in-Glass Tungsten Oxide Thin Films under Various Conduction Cations

The nanocrystal-in-glass (nanocrystals embedded amorphous matrix) tungsten oxide (WO₃) thin films with a nanoporous characteristic were prepared via an electron beam evaporation technique. The e-beam evaporated WO₃ thin films present a fast colored/bleached time of 16/11, 16/14, and 12/12 s, a large optical modulation of 92, 91, and 87% at 633 nm, and a high coloration efficiency of 61.78, 62.04, and 67.59 cm² C^-1 in Li⁺, Na⁺, and Al³⁺ electrolytes, respectively. On one hand, the improved electrochromic performance is mainly attributed to the short diffusion distance and buffering effect in the host matrix, which facilitates the ion insertion/extraction and alleviates the structural collapse of the framework. On the other, owing to the strong electrostatic interactions between the trivalent cations and the host, the WO₃ thin films in Al³⁺ possess a shallow diffusion depth and long cycle life. The individual contribution from the capacitance- or diffusion-controlled process is comprehensively demonstrated. Pseudocapacitive behavior in the nanocrystal-in-glass WO₃ thin films is in favor of fast kinetics response and sound cycling stability. Our work offers an in-depth insight of the electrochromic mechanism for nanocrystal-in-glass WO₃ thin films in various electrolytes and sheds light on the fundamental principle in the electrochromic devices.