Electrochromic materials show reversible color-changeable characteristics, which are widely used in smart windows, displays, adjustable reflective mirrors, electronic paper, military camouflage and other fields. Compared with other kinds of display devices, electrochromic display devices have advantages of multi-colors, high contrast, no blind visual angles, and maintainable color after power off. As a typical cathodic colored electrochromic material, molybdenum oxide exhibits advantages of short response time and colored state closer to the sensitive wavelength band of the human eye to light, so that the electrochromic devices composed of molybdenum oxide is important in the research. This paper briefly introduces the definitions and applications of electrochromism, electrochromic materials and devices. In particular, electrochromic technology recently realized a demonstration application in smart phones, indicating the good prospects for the future development of electrochromic technology. We summarized mainly the research progress of the preparation of molybdenum oxide films, the modification of molybdenum oxide, and the molybdenum oxide based electrochromic devices. Finally, we presented current existing problems and solutions on molybdenum oxide based electrochromic films and devices with development prospects.

Optical property, such as color, transmittance, reflectance and emissivity, of electrochromic materials can be changed reversibly under low applied voltages. Electrochromic materials have a wide range of regulatable spectrum, which can realize the broadband control from the visible to mid-far-infrared. Electrochromic materials show a wide application prospect in the fields of intelligent window, display, anti-glare rearview mirror, intelligent thermal control, and camouflage. At present, most of researches on inorganic electrochromic materials focus mainly on transmission characteristics, but less on reflection characteristics. This is mainly because most inorganic electrochromic materials have single color and are not as easy to design as organic electrochromic materials. In recent years, through special preparation and structural design, the research on reflective properties of inorganic electrochromic materials has gradually attracted researchers’ attention. Based on reflection characteristics of inorganic electrochromic materials, methods and principles of regulating the reflectance spectrum in the visible near infrared to mid-far-infrared bands are introduced, and the latest research progress is summarized. Within the visible band, reflectance spectrum control is mainly achieved by vanadium pentoxide (V₂O₅) and V₂O₅ doping, microstructure of one dimensional photonic crystal, Fabry Perot nanocavity structure and localized surface plasmon resonance (LSPR). Within the mid-to-far infrared band, electrochromic devices (ECDs) based on the molecular vibration absorption of tungsten oxide (WO₃) or other electrochromic materials and related theory are designed and fabricated to regulate reflectance spectra. Finally, the practical application of inorganic electrochromic materials in future is prospected.

Electrochromic materials are capable in a dynamically modulation over their absorption, transmission, reflection and emissivity covering the visible and infrared regions, which is recognized as one of the most effective strategies for an artificial control over the optical spectrum. In practical devices, the modulations for color (visible light) and heat (near-infrared irradiation) are both in demand, and thus it is of significant importance to achieve the dual-band control within single electrochromic device. However, the present design of electrochromic materials mainly focuses on the adjustment ability of visible spectrum or color transformation, while leaving the modulation of infrared spectra often negligible. In this study, a multi-color tungsten oxide film with F-P cavity structure is selected to explore the wide-spectrum electrochromism of visible and infrared spectral regions. The tungsten oxide film material with F-P cavity structure presents diversified and bright colors in the visible region with various color patterns, and diversified color alternation can be realized under applied biases. At the same time, the infrared reflectance of colorful tungsten oxide films can also be modulated significantly, for example, infrared modulation rates higher than 25.00% can be obtained in the middle wavelength (around 3.5 μm) in the colored films. The research shows that the multi-color tungsten oxide film can realize the discrete and controllable adjustment of the visible and infrared spectral region in the wide spectrum, and has great potential in the application of smart window, thermal management, radiative cooling and other fields, which can even achieve the in-demand cooperative modulation of light and heat performance.

Electrochromic materials are attractive for applications like smart windows and thermal management which can modulate transmittance from visible to near-infrared wavelengths. Lithium titanate has been proven to be a potential electrochromic cathode material, but its application prospects in the field of smart windows still lack data support. In this work, Li₄Ti₅O₁₂ thin films with high transmittance and good crystallinity were prepared by Sol-Gel spin coating. The electrochromic properties of the films were characterized by different methods. It is found that all Li₄Ti₅O₁₂ thin films are very sensitive to the test conditions, such as scanning rate, displaying excellent dual band modulation property. Also, the thickness has a significant effect on the initial state transmittance, modulation amplitude, response time, voltage window and cycle durability of the Li₄Ti₅O₁₂ thin films. The colored and bleached response time of the 450 nm Li₄Ti₅O₁₂ is 19.1 and 8.9 s, respectively, while the transmittance modulation is 45% in the visible light region (550 nm). The performance of the films have not been significantly degraded after 20000 s continuous cycle. It is worth mentioning that the transmittance modulation in the near infrared band (1000 nm) is as high as 80%, showing excellent energy saving potential. Finally, Glass/FTO/Li₄Ti₅O₁₂/LiNbO₃/NiO_x/ITO, an all-solid-state inorganic electrochromic device with good cycle performance from gray to blue was successfully assembled. Above data demonstrate that lithium titanate is a strong competitive material for the popularization of electrochromic smart window.

The all-solid-state electrochromic devices (ECDs) have been widely developed for energy-saving windows, screen displays, multi-functional energy storage devices due to their characteristics such as large optical contrast, fast switching speed and good cycle stability. However, traditional all-solid-state ECDs based on monolayer electrolyte system are often limited by ordinary optical transmittance and inadequate sputtering efficiency. Herein, the all-solid- state ECDs integrated with LiAlO_x/Ta₂O₅/LiAlO_x (ATA) sandwich structured electrolyte were successfully fabricated by reactive DC magnetron sputtering technique. By means of introduction of ATA sandwich structured electrolyte, the prepared ECDs with seven layer (ITO/NiO/LiAlO_x/Ta₂O₅/LiAlO_x/WO₃/ITO) was endowed with superfine optical transmittance and substantial sputtering efficiency simultaneously. The ATA-based ECDs realized satisfactory coloration efficiency of 79.6 cm²/C, fast switching speed as short as 1.9 s for coloring and 1.6 s for bleaching, and excellent cycling stability over hundreds of cycles. Furthermore, ATA sandwich structured electrolyte makes full use of the excellent ionic conductivity and stability of Ta₂O₅, as well as enough lithium ions to meet the demand for fast color switching. Hence, ATA-based all-solid-state ECDs by continuous DC sputtering is expected to provide effective guidance for the mass production and practical application of the high-performance ECDs.

Electrochromism has been developed for more than 50 years since it was discovered in the 1960s. The optical properties, such as color, transmittance, reflectance and emissivity, of electrochromic materials can be changed reversibly under low applied voltages. Electrochromic materials have a wide range of regulatable spectrum, which can realize the broadband control from visible to mid far infrared. Electrochromic materials show a wide application prospect in the fields of intelligent window, display, anti-glare rearview mirror, intelligent thermal control and camouflage. In the past decade, electrochromism research has shown a trend of vigorous development, especially in China. Electrochromism is attracting researchers in the fields of nanomaterials, vacuum coating, lithium-ion batteries, supercapacitors, sensing and display to get involved in this interdisciplinary field, gradually becoming one of hot research topics.

In the last special issue: Electrochromic Materials and Devices (2021, 36(2)), we published 5 papers. “Multi-functional Electrochromic Devices: Integration Strategies Based on Multiple and Single Devices”, “Electrochromic Devices Based on Tungsten Oxide and Nickel Oxide: a Review”, “Progress in Flexible Electrochromic Devices”, “Dynamic Process of Ions Transport and Cyclic Stability of WO₃ Electrochromic Film”, and “High-conductivity Hydrophobic Fumed-SiO₂ Composite Gel Electrolyte for High Performance Electrochromic Devices”. In this special issue, experts working in the field of electrochromism from Harbin Institute of Technology, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Institute of Nanotechnology and Nano-Bionics, Chinese Academy of Sciences, Beihang University and University of Shanghai for Science and Technology contributed 5 papers. LI Yao’s group introduces the methods and principles of regulating the reflectance spectrum in the visible near infrared to mid far infrared bands and the latest research progress, and prospects the practical application of inorganic electrochromic materials in future. WANG Jinmin’s group introduces electrochromic materials and devices, preparation of molybdenum oxide films, modification of molybdenum oxide, and the research progress of molybdenum oxide based electrochromic devices. The paper also presents current existing problems and solutions of molybdenum oxide based electrochromic films and devices, and looks forward to their development prospects. DIAO Xungang’s group reports the preparation and electrochromic properties of lithium titanate thin films by spin coating method, and assembly of an all-solid-state inorganic electrochromic device. JIN Pingshi and CAO Xun’s group reports all-solid-state electrochromic devices based on LiAlO_x/Ta₂O₅/LiAlO_x (ATA) sandwich structured electrolyte fabricated by reactive DC magnetron sputtering technique. ZHAO Zhigang’s group reports the preparation of a multi-color tungsten oxide films with F-P cavity structure and their wide-spectrum electrochromic properties of visible and infrared spectral regions.

The special issue of Electrochromic Materials and Devices focuses on the latest research progress and future development trends in the field of electrochromic, and hopes that colleagues will benefit from it. It is expected that the electrochromism research in China will develop more rapidly in the future, gradually leading the new trend of international electrochromism research.

Electrochromism is a typical multidisciplinary crossing research field, involving organic and inorganic materials, physics and chemistry, electronics and optics, testing and controlling and packaging technologies, solar energy and radiation spectral modulation and energy conservation, color and thermal controlling, and many other basic and applied researches. In recent years, electrochromism researches exhibits a colorful and exciting revival trend, especially after many Chinese scientists and companies came into this field. It brings vitality into the international electrochromism research community, which has been a little tranquil for many years. This conclusion can be detected from comparison of the participants number in international and domestic professional academic electrochromic conferences. The bi-annually International Meeting on Electrochomism-IME has been held the 13th session until now, with the number of participants remaining around 150 in recent several times. But for the two-yearly Chinese Meeting on Electrochromism-CME, the number of participants has been quickly increasing from 26 at the 1st, 150 at the 2nd, 178 at the 3rd, to 352 at the 4th CME held in August 2019. Both Chinese academics and industries are very interested in electrochromism. Each CME participants are composed of a third from academic researchers, a third from graduate students, and a third from industries.

In March 2020, commissioned by editorial board of Journal of Inorganic Materials, Prof. WANG Jinmin and I organized the special issue (Electrochromic Materials and Devices) for the Journal. It was the exact and tough period when COVID-19 epidemic ravaged in Wuhan, the whole China and then in Europe. Our scientists and graduate students were working even harder in order to fight against the epidemic with great solidarity and simultaneously to contribute and devote to our beloved motherland. Our electrochromism colleagues strongly support and cooperate, actively contributing manuscripts. As a result, series of the work such as manuscripts invitation and submission and reviews etc., had been progressing very smoothly. We had early planned to publish just one issue, but received much more manuscripts with quite high quality, and finally decided to publish two special issues with permission of the journal’s editorial.

We have invited the most prestigious research groups in China to contribute to our special issue, and they are all leaders in the field of inorganic electrochromism researches. It is expected that these professional academic works can further promote the vigorous development of electrochromic research and industrialization technologies in China. On behalf of my colleagues in the field of electrochromism, I would like to express my sincere thanks and respect to the authors and editors who have made outstanding contributions to the special issue’s publication.

Gel electrolytes are chemically stable, nonflammable and easy to encapsulate, however, their moderate ionic conductivity (10^-4-10^-5S?cm^-1) hinders their further development for the usage in electrochromic devices (ECDs). Here, we developed a highly conductive hydrophobic SiO₂/PMMA/PC/LiClO₄ gel polymer electrolyte (H-SiO₂ GPE). Electrochemical behaviors of ECDs were characterized by electrochemical impedance spectroscope (EIS), cyclic voltammetry (CV) and chronoamperometry (CA). Systematic analyses show that the ionic conductivity of the H-SiO₂ GPE can reach 5.14 mS?cm^-1 (at 25 ℃) by introducing only 0.5wt% hydrophobic fumed SiO₂. This increase is due to the good compatibility and hydrophobic-hydrophobic attractions between SiO₂ additive and organic electrolyte, which promotes the dissociation of lithium perchlorate. Additionally, the viscosity as a function of shear rate for GPE with various fumed silica contents shows the behavior of shear thinning, which indicates the formation of a three-dimensional network structure. This structure provides ion transport channels, leading to a clearly improved switching speed for ECDs assembled with hydrophobic SiO₂ GPEs (t_bleaching=4 vs 8 s and t_coloring=14 vs 16 s). Similarly, the investigation of hydrophobic fumed SiO₂ in liquid electrolyte is demonstrated that the ionic conductivity of LiClO₄/PC liquid electrolyte without and with hydrophobic fumed SiO₂ increases from 6.94 to 7.58 mS?cm^-1, respectively. Therefore, hydrophobic fumed SiO₂ as a filler has a positive effect on electrolytes, and the proposal of the H-SiO₂ GPE provides a new idea for offsetting the trade-off between a high ionic conductivity and easy leakage when applied in ECDs.

Electrochromism is the phenomenon of reversible color/optical change of materials induced by redox reactions under an applied electric field. Since electrochromism was first introduced by Platt in 1961, electrochromic (EC) technology continues to develope due to its advantages of multiple colors energy saving and controllability, and was applied in many fields, for example, smart windows, displays, anti-dazzling rear view mirrors, etc. Recently, with the rapid development of optoelectronic and photoelectric technologies, highly integrated electronic devices attracted extensive interests, and the EC technology is developed towards functionalization and intellectualization. For example, self-powered EC devices (ECDs) were fabricated through integrating with the green energy technology, which further reduced the building energy consumption. Because of the visualization of the EC phenomena by naked eyes, the signal reading became more convenient for the sensors integrated with ECDs. In addition, because of similar device structure, electrochemical principles, active components with other functional devices, a lot of multifunctional EC technologies were explored based on single device, facilitating applications of ECDs in EC infrared control, EC energy storage, and EC actuation. In light of the recent emerging progress of EC technology, we reviewed multi-functional EC systems based on the integration of multiple devices and single device, respectively, including self-powered ECDs, EC sensors, infrared ECDs, and EC energy storage devices, etc. The integration modes, structure design and performance optimization were also summarized for different types of the multi-functional ECDs. At last, we introduced the challenges and potential pathway of multi-functional EC integration in the future.

Electrochromic devices (ECDs) are the intelligent devices change color by applying electric potential, with the advantages of wide working temperature, high optical contrast, good reversible bistability, low driving voltage, and low energy consumption, which show great application potential in the field of dynamic smart windows, full-color electronic screens, anti-glare goggles, adaptive dual-stealth camouflage, and energy storage status visualization. Cathodically coloring material tungsten oxide and anodically coloring material nickel oxide are two widely studied inorganic electrochromic materials, and complementary electrochromic devices based on WO₃ and NiO films have high commercial values in the application of large scale smart windows. Improving the performance of the complementary ECDs such as optical modulation range, response rate, cycle life and weather fastness has attracted much attention. This review focuses on the structural composition of complementary electrochromic devices and summarizes the recent research progress of the electrochromic full devices based on WO₃ and NiO. Firstly, the electrochromic mechanism and decay mechanism of WO₃ and NiO films are clarified, the effects and latest research progress of four strategies for film performance optimization that include optimizing preparation conditions, element doping modification, designing nanostructure, and introducing composite materials are discussed in detail. Secondly, according to the composition and structure design of the device, the classification system of the complementary electrochromic full device is introduced, and the influence of selection for each component material and the device structure on device overall performance are summarized. Finally, the application of the electrochromic device prospects and development trends are forecasted.

The dynamic process of ions transport in electrochromic WO₃ film is usually studied by electrochemical impedance spectroscopy. However, the detailed features are hidden since the ions insertion into WO₃ is a very complex process including structural deformation and phase transformations. Chronopotentiometry is an electrochemical characterization method that measures the response potential of a system under an imposed current. Compared to other dynamic characterization methods (impedance spectroscopy and CV), it allows direct access to the voltage contributions in different states of the solution-electrode system and has frequently been used to investigate kinetic effects such as adsorption and transport phenomena near electrode surface. In this study, chronopotentiometry is creatively applied to study ion transport kinetics and control ions insertion behavior in electrochromic WO₃ film. The results suggest that a large ions insertion flux at the interface of WO₃/electrolyte could broaden ions transport channels due to the fierce lattice expansion during Li⁺ions insertion process, which further improves the ions transportation kinetics and gifts a fast switching speed of optical performance. However, the repeating ions insertion/extraction behaviors at the interface of WO₃/electrolyte for the long-term cycle process can reduce the size of WO₃ grains as a “ball mill effect”. Especially, a large ions transport flux can aggravate the “ball mill effect”. Consequently, the structure of the WO₃ film becomes very dense, which is unfavorable for ions transport and electrolyte permeation. This dense structure also leads to an irreversible accumulation of Li⁺ ions and Li_xWO₃ in the WO₃ host structure, resulting in a decay of optical modulation ability and electrochromic activity. This work offers an efficient method to analyze ion transport kinetics in intercalation materials and a new understanding of the relationship between ion transport behavior and cyclic stability of electrochromic materials.

Electrochromic materials with ability of changing color in response to periodically adjusted bias are an important class of optoelectric functional materials. The controllable modulation of light absorption and transmission can make a great contribution in applications such as smart windows, electrochromic displays and antiglare rear-view mirrors. In recent years, electrochromic technology has developed rapidly. However, the research so far mainly focuses on the traditional rigid electrochromic devices (ECD), mostly based on transparent conductive glass such as indium tin oxide (ITO) glass. The rigid electrochromic devices have some noticeable problems such as large thickness, poor conformability, low mechanical strength, high cost, etc., which hinder their further development of electrochromic technology and their forward commercialization. With the upsurge of developing flexible devices that can be used in wearable devices and e-skin, flexible electrochromic devices (FECD) have attracted extensive attention due to their possibility of foldability, wearability and even embeddability, and have become a research hotspot in the electrochromic field. Starting from the materials for preparing FECD, this review systematically summarizes the recent progress and trend of flexible electrochromic devices based on inorganic, organic, inorganic/organic composite and other new materials. The review also focuses on the research progress of up to date stretchable electrochromic devices. At the same time, challenges in performance improvement and practical application of flexible electrochromic devices at the present stage as well as the corresponding measures taken in the literatures are discussed. Finally, the key to the preparation and performance improvement of flexible electrochromic devices is defined, and the future development trend is prospected.