Tailoring the Structure and Ionic Conductivity of Polysaccharide-based Polymer Electrolytes through Deep Eutectic Solvent Plasticization

Abstract

Natural polymer electrolytes (PE) consisting of polysaccharides is widely studied to overcome the leakage issues in commonly used liquid electrolytes (LE). However, the presence of hydroxyl group in polysaccharides will produce brittle electrolyte films leading to poor ionic conductivity. Deep eutectic solvent (DES) is known to be an alternative to conventional plasticizer due to its low toxicity, low volatility, and high thermal stability. In this study, using the solution casting technique, choline chloride/ethylene glycol DES were incorporated into tamarind seed polysaccharide (TSP) matrix to obtain flexible and free-standing electrolyte films. Lithium triflate (LiTf) was added to provide additional conducting species to the system. Adding DES at various weight percentages (0.2, 0.3, 0.4, 0.5, and 0.6 wt%) produced solid, flexible, and self-supporting TSP-polymer electrolyte films. This is supported by increased tensile strain, reduced modulus, and tensile stress, indicating enhancement in the flexibility of the films. EIS analyses show that adding up to 0.4 wt% DES significantly improves ionic conductivity in polysaccharides (1.19 x 10^-4 S cm^-1 from 1.33 x 10^-5 S cm^-1 in TSPL). This improvement is due to the successful modification of hydrogen bonding which reduce the TSP matrix crystallinity and promote a more amorphous phase. This can be further proven by FTIR analysis, which highlight the successful TSP-DES complexation via io-dipole interaction. Optical microscopy confirms these findings by showing a smoother surface, indicating structural changes associated with the higher conductivity. However, the addition of more than 0.4 wt% DES reduces conductivity and limits Li⁺ ion mobility due to surface aggregation and the presence of neutral ions as shown in the OM analysis. The development of this TSP-polymer electrolyte film has potential as electrolyte in energy storage devices after further modification.