Chromogenic materials are capable of optical change reversibly in response to physical stimuli (e.g., electric field, temperature, illumination, and atmosphere). Among them, electrochromic materials are expected to be widely used in smart windows, screen displays, multi-functional energy storage devices and other fields due to their characteristics such as large adjustment range, fast response rate, high coloring efficiency and good cycle stability. However, compared with semi-solid-state electrochromic devices that are difficult to package and organic electrochromic materials that are prone to denaturation and failure, inorganic all-solid-state electrochromic materials and devices have better comprehensive application. This paper focuses on the typical inorganic all-solid-state electrochromic materials and devices, presents a brief review on the current preparation methods of each structure layer of electrochromic devices and compares its advantages and disadvantages, introduces in detail the main alternative electrochromic materials and its key performance evaluation index, and explains the principle of several representative electrochromic devices, proposes to use transparent flexible electrodes with both high light transmittance, low surface resistance and excellent bending fold to replace the traditional rigid substrate in order to realize multi-field responsible device application development. Finally, the application prospect of inorganic all-solid-state electrochromic devices is prospected from the perspective of performance bottleneck, process difficulty and industrialization opportunity, which provides reference for the industrialization process of electrochromic devices.

A new inorganic/organic composite film PEDOT:Ce@TiO₂ was prepared through drop casting-secondary polymerization method using the mixture solution of PEDOT oligomer and cerium-containing polyoxotitanate cage [Ti₈O₇(HOEt)(OEt)₂₁Ce]. PEDOT:Ce@TiO₂ exhibits a special nanostructure with gully-like rough surface, which shows strong hydrophobicity while good wettability to acetonitrile solution. PEDOT:Ce@TiO₂ can be used as cathodically electrochromic film and supercapacitor electrode material. The mass specific capacitance of PEDOT:Ce@TiO₂ is 71.2 F/g at a current density of 1 A/g, which is 1.7 times higher than that of the PEDOT film. An all-solid-state electrochromic supercapacitor prototype device was further assembled using PEDOT:Ce@TiO₂. With charging completed the electrochromic area of this device shows dark green, and it turns bright yellow with discharging completed.

As an important transition metal oxide, manganese dioxide (MnO₂) has attracted more and more attention due to its abundant reserves, varied crystal types and excellent material properties. Nanostructured MnO₂ has smaller size and larger specific surface area, that makes it can further optimize its material properties and expand its application fields. In the introduction, this article starts with the introduction of the application of manganese dioxide, and points out that nanostructuring and variability in crystal form have an important influence on the structure and properties of manganese dioxide. The main text summarizes and reviews the research progress in recent years from two aspects: the preparation methods and the applications of nanostructured MnO₂. (1) This paper introduces the progress in the preparation methods of nanostructured MnO₂ including hydrothermal, Sol-Gel, chemical precipitation, solid-phase synthesis. Then the advantages and disadvantages of preparation methods, the morphologies and properties of nanostructured MnO₂ are summarized. (2) The applications of nanostructured MnO₂ including energy-storage electrodes, electrochromic devices, catalysts and bio-sensors are reviewed. Nanostructured MnO₂ can be used as the cathode material of batteries and the electrode material of supercapacitors. Manganese- containing composite oxides prepared by adjusting the crystal form of MnO₂ and compounding are used as the cathode material of the lithium ion batteries, which can increase the capacities and improve the cycle stability of batteries. It has been industrialized as a cathode material for lithium-ion power batteries, and has good application prospects in the field of new energy vehicles. As the electrode material of electrochromic devices, MnO₂ is usually used by combining with other materials with large optical modulations since the color of pure MnO₂ mainly changes between brown and yellow and its optical modulation is small. For example, polyaniline/MnO₂ hybrid electrochromic film has a great difference in morphology, structure and electrochromic performance compared with pure polyaniline film, showing higher optical modulation, coloration efficiency and cycle stability. Nanostructured MnO₂ plays important roles in the catalytic conversion of ethylbenzene and the catalytic elimination of air pollutants. Nanostructured MnO₂ can increase the current response, reduce the detection limit, and greatly improve the sensitivity of detection. In recent years, it has been gradually paid attention to and widely used in the field of biosensors. For example, MnO₂ nanosheets assisted fluorescence polarization biosensors can be effective in detection of Ag⁺ in environmental water samples, PtAu-MnO₂ binary nanostructures modified graphene paper show good sensing performance in non-enzymatic glucose detection. In conclusion part, current existing problems are analyzed. The development direction of nanostructured MnO₂ applied in lithium-ion battery cathode materials and electrochromic devices are pointed out. The future prospects for development of nanostructured MnO₂ are discussed.

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.

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.

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.

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 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.

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.

Vanadium dioxide (VO₂), as a transition metal oxide, has thermochromic property, which undergoes metal to insulator transition (MIT) in response to external temperature changes, and is accompanied by numerous changes in physical property. It has attracted widespread attention in the field of smart windows. In recent years, research on the preparation method of VO₂, the phase change mechanism, and the improvement of optical performance are quite rich. However, practical applications still face technical bottlenecks and challenges such as higher intrinsic transition temperature (T_c), lower luminous transmittance (T_lum), insufficient solar modulation ability (ΔT_sol), nonideal metastability and durability, and uncomfortable color for human eyes (brownish yellow). At present, there are many researches related to the improvement of the performance of VO₂ itself owing to its insufficient optical property, and general methods for improving its performance such as elements doping, multilayer film structure design, and microstructure design have been widely adopted. This review summarizes the general performance improvement strategies of VO₂ film, and highlights the latest research progress of VO₂-based smart window service performance, low-temperature flexible preparation and color modulation in practical applications. Future development trends are also discussed in terms of skin comfort and environmental friendliness.

Nanoscale tungsten oxide, as a functional semiconductor with unique physical and chemical properties, is widely used in environment, energy, life science and information technology fields. Based on the application of first-principles study in nanoscale tungsten oxide, the functions of theory calculations are reviewed in the paper. Firstly, the development and basic theory of the first principles and density functional theory are illustrated based on quantum mechanics. Then, the commonly related software in such field of semiconductors, such as MS (Materials studio) and VASP (Vienna ab initio simulation package) are introduced. Furthermore, the recent study of the first-principles on tungsten oxide in terms of electronic structure, interaction of materials, molecular thermodynamics, and so on, is clarified. Finally, the existing problems and future developments of theory calculations used in the field are summarized and prospected.