The effect of choline chloride/ethylene glycol deep eutectic solvent on the properties of polyurethane-based electrolyte films

Polyurethane (P-Ure) have been widely used due to their mechanical properties that provide soft and hard segment in the polymer matrix. However, formation of hydrogen bond between P-Ure chains causes the polymer film to be brittle, exhibits poor electrode-electrolyte contact, thus lowering the ionic conductivity and may reduce the performance of polymer electrolytes (PE). The addition of plasticizers such as ionic liquid has successfully addressed these issues which are to improve its ionic conductivity along with the flexibility of the film while preserving its mechanical properties. However, the cost to produce free-standing film by incorporating ionic liquid (IL) is high and less environmentally friendly. DES was selected due to its lower cost and non-toxic nature, while still offering similar properties to ILs. Therefore, to produce flexible, free standing, and highly conducting films, varying weight percentages (10, 20, 30, and 40 wt. %) of choline chloride (ChCl)/ethylene glycol-based DES were incorporated into the P-Ure-based electrolyte using the solvent casting technique. Fixed amount of lithium bis(trifuloromethanesulfonyl)imide (LiTFSI) salt was added to all samples to provide charge carrier. The effect of different wt.% of DES on the structural, morphological, mechanical, and electrical properties of the plasticized P-Ure-based electrolyte films was analysed using Fourier Transform Infrared Spectroscopy (FTIR), Optical microscope (OM), Universal Tensile Machine (UTM), and Electrochemical Impedance Spectroscopy (EIS), respectively. The FTIR analysis confirms the interaction between P-Ure and DES. When DES was added, the ionic conductivity improved with the highest conductivity of 1.46 x 10-3 S cm-1 (40 wt. % DES) if compared to 4.99 x 10-4 S cm-1 in control sample (P-Ure0). The addition of DES into the P-Ure system effectively enhanced its performance. This improvement is attributed to DES filling the spaces between the polymer chain, which reduced hydrogen bonding and increased the film flexibility. As a result, the movement of lithium ions became easier, leading to better ionic conductivity. The increased flexibility was confirmed by the highest tensile strain value of 328.93% at 40 wt. % DES content (P-Ure40). Additionally, the improved conductivity is supported by the observed reduction in particle aggregation in the morphology of P-Ure40, which may be due to the dissociation of previously formed ion clusters. In conclusion, the incorporation of DES significantly enhances the flexibility and ionic conductivity of P-Ure-based electrolyte films, demonstrating their potential applicability in flexible energy storage device.