Hybrid polysaccharide–NaI derived transparent and flexible solid-state electrolyte films for sustainable design of supercapacitors

Abstract

Conventional liquid electrolytes utilized in supercapacitors suffer from leakage, flammability, and poor adaptability making them unsuitable to design flexible, wearable, and portable devices. Despite the significant advancements in designing flexible electrode materials towards the fabrication of safer and integrated energy storage systems, the development of optically transparent and mechanically flexible polysaccharide-based solid-state electrolytes remains comparatively limited. Herein, we report a series of flexible solid-state electrolyte (FSSE_NaI-x; where x = 27, 43, 53, 60, 65, and 69 wt%) films, composed of konjac glucomannan (KGM), hydroxypropyl methylcellulose (HPMC), and sodium iodide (NaI). Among them, the FSSE_NaI-65 film exhibits optimal properties in terms of mechanical, optical, and ionic conductivity suitable for designing supercapacitor devices. It achieves a Young's modulus of ∼2.5 MPa with an exceptional elongation at break at 118%, along with an optical transparency of 88% at 800 nm. It delivers a high ionic conductivity of 2.77 mS cm⁻¹ at room temperature and a wide electrochemical stability window of 2.4 V. A solid-state supercapacitor assembled with the FSSE_NaI-65 film shows a specific capacitance of 159 F g⁻¹ at 1 A g⁻¹, with an excellent cycling stability retaining 87.5% of its specific capacitance over 4000 cycles at 5 A g⁻¹. It maintains a stable performance under bending conditions with a capacitance retention of 72.2% over 2000 cycles at 5 A g⁻¹. The device furnishes a high energy density of 22.1 Wh kg⁻¹ and power density of 500.6 W kg⁻¹ at 1 A g⁻¹ confirming its potential for next-generation flexible energy storage systems.