Electrochromic Materials Based on Ions Insertion and Extraction

Abstract

Reducing energy consumption is one of the major concerns in modern building design and construction because buildings account for ≈40% of the total energy consumption in modern society. Among various technologies for saving energy, electrochromic smart windows are considered as a highly promising one. Except for the application in saving energy, electrochromism has also attracted extensive attention in many other fields, such as, anti‐glare rearview mirrors, displays, military camouflage, flexible wearable fabric, etc. However, there are still many challenges needed to be solved, such as, the response rate and durability of electrochromic materials (ECMs). The transmission rate of ions in ECMs determines the response rate of the whole electrochromic device. Besides, the embedding depth and total amount of ions directly affect the contrast, durability, and coloration efficiency of ECMs. It is crucial to select suitable electrolyte ions to obtain high‐efficiency ECMs. Here, the challenges in ECMs based on ions insertion/extraction are presented, which are focused on advanced nanostructured ECMs’ design and electrochromic devices’ practical application. Recent advances in ECMs are presented to discuss the important questions, which are especially focused on the connection between suitable ions and advanced nanostructural ECMs. Finally, a perspective for next‐generation electrochromic devices is presented.

Extraordinarily Stable Aqueous Electrochromic Battery Based on Li4Ti5O12 and Hybrid Al3+/Zn2+ Electrolyte

“…We believe that the reduction of Ti 4+ to Ti 3+ and even Ti 2+ is predominantly caused by the insertion of Al 3+ ions, which possess smaller radii and higher valence for stronger electrostatic interactions with the Ti 4+ ions in the spinel structure. 25,42 The efficient reduction of colorless Ti 4+ into colored Ti 3+ and Ti 2+ facilitated by the incorporation of Al 3+ and Zn 2+ ions can be considered as the origin for the opaque colored state and high optical contrast of the LTO film in the hybrid electrolyte.…”

“…The CV curve after 1500 cycles of operations in a 1 M Al 3+ /Zn 2+ hybrid electrolyte (Figure S17) can also confirm that the reactivity of Zn 2+ ions becomes higher as the cycling progresses. The long activation period of Zn 2+ ions is mainly due to the larger ion radius, 25 and the saturated optical contrast is mainly due to its poor kinetics, suggesting that Al 3+ ions are still the major contributor to the coloring process. By comparing the SEM images of the films in the initial state and after 12 500 cycles (Figure S18), it can be found that the surface morphology still maintains a high consistency.…”

“…Traditional electrochromic materials including NiO, TiO 2 , Prussian Blue, and WO 3 , have been widely studied for realizing ECBs. Great emphasis has been placed on aqueous electrolytes containing earth-abundant multivalent ions (e.g., Zn 2+ , Ca 2+ , Al 3+ ) for their outstanding advantages over organic electrolytes: ,,,− (1) aqueous electrolytes can offer enhanced safety prospects due to their nonflammability; (2) smaller ion radius and higher charge densities of multivalent ions and ionic conductivity of aqueous-based media can facilitate faster responses and elevated battery energy density; (3) there is lower cost for mass production. Although notably fast response and high capacity have been demonstrated, aqueous ECBs can only maintain stable operations over ∼10 3 cycles due to the mismatch between the EC material and the aqueous electrolyte, e.g., the corrosion of WO 3 in acidic electrolyte .…”

3D Pine‐Needle‐Like W18O49/TiO2 Heterostructures as Dual‐Band Electrochromic Materials with Ultrafast Response and Excellent Stability

“…[47][48][49] A comprehensive comparison of the electrochemical performance between PNL-W 18 O 49 /TiO 2 heterostructures and other EC materials reported in the literature are presented in Figure 4i and Table S2, Supporting Information, demonstrating the obvious superiorities of PNL-W 18 O 49 /TiO 2 heterostructures, especially in response time and cycling stability in a narrow potential window of −0.8 to 0.6 V. [14,22,[41][42][43][44][45] We think the excellent EC performance of PNL-W 18 O 49 / TiO 2 heterostructures is attributed to the following aspects. [50] First, the 3D PNL-W 18 O 49 /TiO 2 heterostructures had a large specific surface area, which increased the contact area between EC materials and electrolytes and further reduced the dif-fusion path of H + , promoting the electrochemical reaction (Figure S5, Supporting Information). Second, the growth of W 18 O 49 nanothorns on the surface of TiO 2 nanofibers resulted in a bandgap shift (Figure S14, Supporting Information), which caused the conduction band to shift to a negative position.…”

Li+/Al3+ Composite Solid Electrolyte Based Electro‐Thermal Dual Responsive Devices with Optimized Optical Contrast and Cycle Durability

“…Besides, electric-induced phase modulation on VO 2 phase transition driven by electric field was verified again (Figure 5d), which was mainly due to partial phase transition induced by insertion and extraction of Li + /Al 3+ mixed dual-ions. [22,24]…”