Evaluation of All-Solid State Electrochromic-Supercapacitor with Varying Surface Roughness of WO₃ Film; Nanoparticle Deposition System(NPDS) vs Sputtering

Development of energy storage systems is essential since the need of renewable energy arises in order to reduce global carbon emissions. With the demand for lithium-ion batteries still overwhelming, safety of lithium-ion batteries is the issue since explosion of batteries often occur by overcharging or electrolyte leakage. Even though supercapacitors based on metal oxide are promising due to its higher stability compared to batteries, capacity loss of device under overcharging is inevitable. To overcome this problem, there are two-strategies; electrochromism and solid-state electrolyte. In this study, supercapacitor combined with electrochromic technology which can visually check the amount of charge/discharge are studied. From the point of energy storage perspective, WO₃ film is considered to be one of the electrochromic-capacitive bifuncitional material for a pseudo-capacitor. Moreover, solid-state electrolyte is an attractive alternative to prevent explosion. In addition, inorganic solid-state electrolyte has advantages for smaller volume, lighter weight and higher thermal stability compared to those for conventional liquid and gel electrolyte.<br/>Herein, we have investigated the effect of WO₃ deposition methods on evaluating characteristics of electrochromic supercapacitor (ECS). We have used two dry deposition methods to fabricate WO₃ thin film to build ECS along with all-solid state electrolyte. Two types of full-cell ECS were produced by depositing inorganic electrolyte and positive layer such as LiMn₂O₄ in a series on WO₃thin films which was fabricated by NPDS and sputtering method. Deposition methods change surface roughness greatly in WO₃ thin film owing to nanoparticle deposition system(NPDS) and sputtering method. NPDS is a novel dry deposition method which can produce films by accelerating powder to supersonic speed due to its pressure difference within the system. When powders were collided onto the electrode, powders broke and crushed while resulting highly rough surface. Since this method does not require any binder or solvent, it is an eco-friendly process. Moreover, low vacuum and low cost can make this system very attractive for the next generation deposition method. Second, sputtering method was used to deposit WO₃ thin film. Sputtering method is one of the commercialized thin film production method where It can fabricate various films under high vacuum with mass-production of thin films. Moreover, the advantage of high uniformity in film thickness, results in very low surface roughness.<br/>Evaluation of WO₃ films deposited via NPDS vs. sputtering were compared with respect to optical, electrochemical and energy storage properties of full devices in comparison. It was found that the surface roughness of WO₃ thin film using NPDS is almost 10 times higher than that of WO₃ thin film using sputtering method. With high surface roughness of WO₃ film, WO₃ film using NPDS showed larger electrochemical reaction area. Finally, differences in surface roughness have affected device performances such as transmittance, charge capacity and areal capacitance. For instance, initial transmittance of the device with WO₃ film using NPDS was found to be lower than that of WO₃ film using sputtering due to increased light scattering caused by rough surface of WO₃ film using NPDS. However, charge capacity and areal capacitance were improved in the device due to high roughness film owing to its larger reaction area. Therefore, advantageous deposition of thin film for target performance of ECS devices can be selected with varying surface roughness of WO₃thin film.