Sodium-Ion-Conducting Alginate-Based Electrolyte Material for Energy Storage Applications

A green pseudosolid polymer electrolyte is prepared using sodium alginate and sodium polyphosphate via a sustainable solution-cast method with water as the medium. The amorphous anionic polymer backbone enables easy cationic movement, enhancing ionic conductivity. This water-in-salt electrolyte exhibits an electrochemical stability window of 3.2 V and a cationic transport number of 0.90%. Thermal analysis confirms stability up to 150 °C, making it suitable for high-temperature applications. X-ray diffraction analysis verifies its amorphous nature, facilitating smooth ion transport, while scanning electron microscopy reveals a smooth morphology with well-defined pores, improving electrode interface stability. At room temperature, the electrolyte displays electrical conductivity around 10⁻⁵ S cm⁻¹, increasing to 10⁻⁴ S cm⁻¹ above 40 °C. The drift ionic velocity is ≈10⁻⁵m s⁻¹, with ionic mobility of 10⁻⁷ mV s⁻¹. Cage-type hopping dominates ionic movement, requiring a low activation energy of 0.158 eV. Incorporating an ionic liquid as a plasticizer further enhances conductivity to 10⁻³S cm⁻¹. Additionally, the material exhibits dielectric relaxation due to polar group orientation. Its high capacitance with minimal electrode contribution makes it a promising candidate for energy storage applications, offering excellent electrochemical and thermal stability, along with superior electrode–electrolyte interface properties.