Ultrasonic Spray Deposition of Hydroxypropyl Methylcellulose Polymer Electrolyte Films

Gel polymer electrolytes were prepared using an ultrasonic spray deposition (USD) method in this study. The polymer matrix consisted of polyethylene glycol (PEG), with deionized water as the solvent and hydroxypropyl methylcellulose (HPMC) as the thickener. Lithium perchlorate (LiClO₄) and lithium chloride (LiCl) were used as lithium salts. The effects of spray flow rate, number of coating layers, and lithium salt concentration on transmittance and ionic conductivity were investigated. Surface and cross-sectional morphology were observed using optical microscopy (OM) and scanning electron microscopy (SEM). The results showed that increasing the number of coating layers decreased transmittance due to light absorption by the electrolyte film and lithium salt. The higher lithium salt concentrations, the lower transmittance. Under controllable conditions, the transmittance of the electrolyte with LiClO₄ exceeded 95%, higher than the typical value reported in the literature (approximately 80%). Ionic conductivity increased with increasing spray flow rate, number of coating layers, and lithium ion concentration. The highest ionic conductivity of the electrolyte with LiClO₄ reached 3.56 × 10^-5 S cm^-1, which compares favorably with previously reported gel polymer electrolytes. These findings indicate that the gel polymer electrolytes developed in this work are qualified for applications in electrochromic device fields.