Preparation And Properties Study Of All Solid Polyurethane Based Electrolyte For Electrochromism

Due to the advantages of excellent controllability,rich color variety,and low driving voltage,electrochromic materials and devices have received extensive attention and are used in electronic displays,energy-saving buildings and military camouflage.For example,electrochromic smart windows can control the entry of external light,adjust indoor lighting and temperature,and maintain indoor comfort at low energy consumption;Flexible electrochromic devices can be integrated on clothing in the structural form of flat or fiber to realize intelligent changes in clothing color for wearable display and visual detection.As the core component of electrochromic devices,the selection and design of electrolyte materials play a crucial role in the performance of electrochromic devices.At present,the electrolytes commonly used in electrochromic devices are liquid,quasi-solid and all-solid.Due to the easy leakage of liquid state,there are certain safety hazards,and the mechanical properties are poor,which is not conducive to commercial application;The quasi-solid electrolyte still has a certain solvent,volatile,and there are still safety hazards in harsh environments such as high/low temperature.In contrast,all-solid electrolytes have excellent mechanical stability,thermal stability,electrochemical stability and safety in use,and have gradually become a research hotspot in the field of electrochromism,but their poor conductivity and interface transport characteristics hinder their rapid development.Thermoplastic polyurethane（TPU）contains soft and hard segment structure and rich polar groups,has good mechanical flexibility,and shows good dissociation and transport ability for metal cations,so TPU-based all-solid polymer electrolyte has great application potential in the field of electrochromic and is considered to be one of the most ideal all-solid electrolyte substrates.However,during the processing process,the TPU polymerization chain will undergo oriented crystallization,and the light scattering is severe when passing through the grain boundaries and grains,resulting in a decrease in transmittance,on the other hand,the more crystalline regions of the polymer,the more difficult the ion transport of the electrolyte,resulting in lower conductivity.In view of the above problems,this paper uses TPU as a matrix to analyze the effects of different lithium salts and inorganic fillers on the crystallization characteristics of TPU,improve the ionic conductivity of TPU-based polymer electrolytes,and form all-solid electrochromic devices based on organic or inorganic electrochromic materials,and explore the performance of electrochromic devices based on all-solid electrolyte thin films.The specific contents are as follows:Using LiBF₄,LiClO₄,LiTFSI and LiOTF as lithium sources,and TPU to form an all-solid polyurethane electrolyte film,the electrochemical,optical and thermodynamic properties of different polyurethane electrolyte films were comprehensively compared,and the effects of different metal salts on the crystallinity of TPU were analyzed.When LiBF₄ is added to 20 wt.%,the all-solid electrolyte has the best comprehensive performance,which has a good inhibitory effect on the crystallization of polyurethane,and its optical transmittance is only attenuated from 83%to 78%（at630 nm）within 12 h,and the conductivity can reach 1.02×10^-5 S cm^-1 at 35°C.Poly（3,4-ethylenedioxythiophene）-polystyrene sulfonic acid（PEDOT:PSS）color-changing electrode and polyaniline（PANI）counterelectrode were prepared,and a transmissive electrochromic device was assembled with an all-solid polyurethane electrolyte film,which has an optical contrast ratio of 32.9%（at 630 nm）,a coloring time of 17.5 s,a fading time of 11.2 s,and can still maintain an optical contrast of 25.2%after 3000 cycles.SiO₂ and Li_6.75La₃Zr_1.75Nb_0.25O₁₂（LLZNO）were used as inorganic fillers to improve the dissociation ability of metal salts,further inhibit the crystallinity of the all-solid electrolyte film,and the ionic conductivity of the prepared all-solid electrolyte was increased to 1.8×10^-5 S cm^-1 and2.6×10^-5 S cm^-1 at 35°C,respectively.Transmission and reflective electrochromic devices were assembled by adding 10 wt.%SiO₂ and 15 wt.%LLZNO all-solid composite electrolytes,respectively,and the transmissive devices initially had an optical contrast ratio of 31.8%,which reduced the fading time by 4.7 s and the coloring time by 3.0 s compared with the electrochromic devices assembled with all-solid polyurethane electrolyte.The reflective device requires a fading time of 9.6 s,a tinting time of only 6.2 s,an optical contrast（reflection）of 13.5%,and still maintains95.6%of the initial optical contrast after 1000 cycles.In order to verify the suitability of all-solid polyurethane-based electrolyte and inorganic electrochromic material,Mo-doped WO₃-based inorganic electrochromic layer was prepared,and the all-solid polyurethane electrolyte and LLZNO-based all-solid polyurethane composite electrolyte were respectively assembled to assemble transmissive and reflective electrochromic devices.Transmissive electrochromic devices have an optical contrast of 33.7%,a tinting time of13.7 s,a fading time of 18.5 s,and an optical contrast of 25.2%after 2000 cycles.The optical contrast（reflection）of the reflective electrochromic device is 19.6%,which is 6.1%higher than that of the device composed of PEDOT:PSS as the electrochromic layer,and can still maintain an optical contrast of 17.2%after 1000 cycles,indicating that the all-solid polyurethane-based electrolyte is suitable for inorganic electrochromic materials.