Introduction: Although a few types of SIBs have already been commercialized, especially with liquid electrolytes for stationary energy storage applications, synthesis of suitable electrolytes with sufficient ambient temperature ionic conductivities is still an unsolved problem.

Methods and Results: In this work, an attempt has been made to synthesize solid state biopolymer electrolytes by using Azadirachta indica natural resin (AINR) as the biodegradable biopolymer host and NaBF₄ as the ionic salt. These electrolytes were synthesized by using the usual solvent-casting method with DI water as the solvent. Among several compositions investigated in this work, the best conducting composition; 70 AINR - 30 NaBF₄ (wt. %); showed an ambient temperature ionic conductivity almost close to the order of 10^-6 S cm^-1. As expected, σ vs 1/T variation exhibited Arrhenius behavior with almost constant activation energies (~ 1.0 eV) for all the compositions. As per DC polarization analysis, the best conducting biopolymer electrolyte is dominantly an ionic conductor with an ionic transference number of 0.99 and the electronic contribution to conductivity is almost negligible; which is highly desirable for an ideal electrolyte to avoid short circuits within the cell. Cyclic voltammetric studies confirm that the electrochemical stability window of the best conducting electrolyte composition is from -3 to +3 V. Drop in intensity of the FTIR optical vibrational band at 750-1250 cm^-1 in solid electrolytes compared to the pure NaBF₄ indicates the dissociation of ionic salt due to formation of AINR/Na⁺ complex.

Keywords: Sodium-ion batteries, Biopolymer electrolytes, Natural resins, Electrochemical impedance spectroscopy and FTIR

 This research was supported by the Accelerating Higher Education Expansion and Development (AHEAD) Operation of the Ministry of Higher Education funded by the World Bank (DOR Project # 62).

* Corresponding author: K. Vignarooban (kvignarooban@gmail.com)