Single-Ion-Conducting SPP–SPA Blend Hydrogel-Based Pseudo-Solid Polymeric Electrolyte Material for Na⁺-Ion Constructed Energy Storage Devices

Abstract

A pseudo-solid blend polymeric hydrogel electrolyte material, sodium polyphosphate (SPP)–sodium polyacrylate (SPA), is successfully synthesized by using SPP and SPA in an aqueous medium. Large anionic polymer chains with small Na+ cations allow movement of cations only inside the matrix and make it a Na+-ion-based single-ion-conducting electrolyte. SPP–SPA has a gel matrix with good elasticity, flexibility, and adhesive property, which allow better transport of ions inside the matrix with a good electrode–electrolyte interface. Because water molecules present inside the matrix act like a water-in-salt electrolyte, it shows an electrochemical stability window of 2 V with good current density. This single-ion-conducting electrolyte shows a cationic transport number of ≈97%. The electrolyte shows room-temperature conductivity, matrix diffusivity constant, ionic mobility, and drift ionic velocity in the order of 10–4 S/cm, 10–8 m2/s, 10–6 m2/V s, and 10–3 m/s respectively. This electrolyte material follows a cage-type (n > 1) ionic conduction mechanism with the requirement of a low amount of activation energy, that is, 0.314 eV. A high amount of capacitance is associated with the electrolyte that has an insignificant electrode contribution. This SPP–SPA pseudo-solid polymeric electrolyte can be used as a highly promising smart material for energy storage and capacitor applications.