Electrochromic (EC) materials can be integrated in thin-film devices and used for modulating optical transmittance. The technology has recently been implemented in large-area glazing (windows and glass facades) in order to create buildings which combine energy efficiency with good indoor comfort. This critical review describes the basics of EC technology, provides a case study related to EC foils for glass lamination, and discusses a number of future aspects. Ample literature references are given with the object of providing an easy entrance to the burgeoning research field of electrochromics.

Anodic electrochromic (EC) oxides are of major interest as counter electrodes for smart window applications owing to their unique optical properties upon charge insertion and extraction. However, performance optimization of such oxides has been hampered by limited understanding of their EC mechanism, particularly in Li + -conducting electrolytes. This paper reports on NiO x films with 1.16 ≤ x ≤ 1.32, prepared by sputter deposition. These films were immersed in an electrolyte of lithium perchlorate in propylene carbonate, and EC properties were studied by cyclic voltammetry and in situ optical transmittance measurements. The electrochromism was significantly enhanced at large values of x. We find that charge exchange in Ni oxide is mainly due to surface processes and involves both cations and anions from the electrolyte, which is different from the case of cathodic EC materials such as WO 3 .Whereas previous studies of Ni oxide have focused on cation intercalation, the cation/anionbased mechanism presented here offers a new paradigm for designing and developing EC devices such as smart windows for energy efficient buildings.2

Several technologies for energy saving and storage rely on ion exchange between electrodes and electrolytes. In amorphous electrode materials, detailed knowledge of Li-ion intercalation is hampered by limited information on the structure and transport properties of the materials.Amorphous tungsten oxide is the most studied electrochromic material and suffers from iontrapping-induced degradation of charge capacity and optical modulation span upon extensive electrochemical cycling. In this paper, we investigate trapping and de-trapping processes in connection with performance degradation and specifically use real-time electro-optical monitoring to identify different trap energy ranges pertinent to the ion-intercalated system. Evidence is presented for three kinds of traps which degrade electrochromic tungsten oxide during ion intercalation: (i) shallow traps which erode the colored state, (ii) deep traps which lower the bleached-state transmittance, and (iii) irreversible traps. Importantly, Li-ion detrapping from shallow and deep traps takes place by different processes: continuous Li-ion extraction is possible from shallow traps, whereas a certain release time must be exceeded for de-trapping from deep traps. Our notions for ion trapping and de-trapping, presented here, may serve as a starting point for discussing ion intercalation in various amorphous materials of interest for energy-related applications.

Ion trapping under charge insertion−extraction is wellknown to degrade the electrochemical performance of oxides. Galvanostatic treatment was recently shown capable to rejuvenate the oxide, but the detailed mechanism remained uncertain. Here we report on amorphous electrochromic (EC) WO 3 thin films prepared by sputtering and electrochemically cycled in a lithium-containing electrolyte under conditions leading to severe loss of charge exchange capacity and optical modulation span. Time-of-flight elastic recoil detection analysis (ToF-ERDA) documented pronounced Li + trapping associated with the degradation of the EC properties and, importantly, that Li + detrapping, caused by a weak constant current drawn through the film for some time, could recover the original EC performance. Thus, ToF-ERDA provided direct and unambiguous evidence for Li + detrapping.

Devices relying on ion transport normally suffer from a decline of their long-term performance due to irreversible ion accumulation in the host material, and this effect may severely curtail the operational lifetime of the device. In this work, we demonstrate that degraded electrochromic WO 3 films can sustainably regain their initial performance through galvanostatic de-trapping of Li + ions. The rejuvenated films displayed degradation features similar to those of the as-prepared films, thus indicating that the de-trapping process is effectively reversible so that long-term performance degradation can be successfully avoided.De-trapping did not occur in the absence of an electric current. 2Energy conservation is widely recognized as an essential part of a sustainable global energy system. 1 This realization brings attention to the buildings sector, which is responsible for a large and growing part of the global use of energy. 2,3 Energy-efficient fenestration is of considerable interest in this context, 4 and electrochromic (EC) "smart windows" are of particular importance. 5-7 These windows are able to vary their throughput of visible light and solar radiation by the application of a low electrical voltage and can provide energy efficiency along with indoor comfort in buildings. [8][9][10] Amorphous WO 3 is the most widely studied EC material, and its optical absorption can be varied through Li + ion intercalation 11 and accompanying insertion of charge balancing electrons. This mechanism can be expressed, schematically, as 11where e -denotes electrons. Thin films of amorphous WO 3 change from optically transparent to dark blue when Li + ions are inserted, and the films return to their transparent state if these ions are extracted.High optical modulation and long-term durability are needed for EC-based fenestration.These requirements are not easily compatible, and repeated insertion and extraction of large amounts of Li + ions lead to a gradual accumulation of Li + -ions in the host material 12-15 (often referred to as "ion-trapping") with ensuing loss of EC performance. Removal of the trapped ions to re-gain the initial EC properties is essential for maintaining good device performance.It has been proposed that the host structure contains different types of intercalation sites: 13, 14 a network of connected sites with low inter-site barriers, which allows fast diffusion of the intercalated ions throughout the film, and other sites with high energy barriers which are able to trap the diffusing ions. It has been suggested by Bisquert 14, 15 that the high-energy barrier trapping sites can be filled by Li + ions having sufficient energy or by waiting for a long enough time.