Solid polymer electrolyte (SPE) membrane is a key component in the development of all-solid-state lithium-ion batteries (ASS LIBs) that are environmentally friendly, high-energy density, and safe. SPE membranes with high ionic conductivity usually have poor mechanical and thermal stability. To overcome these problems, SPE membranes require supporting components such as plasticizers that have a wide range of physicochemical properties, including deep eutectic solvent (DES). This work studied the effect of the addition of choline chloride/urea DES (ratio: 1/2 mol/mol) on ionic conductivity, flexibility, and thermal stability of PVA/LiClO4-based SPE membrane. DES was synthesized by mixing the choline chloride (ChCl) and urea with ratio: 1/2 (mol/mol) under stirring at 80 °C for 3 h. The resulting DES (ChCl/urea) was a clear liquid with a density of 1.26 g/mL. SPE membranes were prepared by mixing PVA/LiClO4 (85/15 wt%) as the initial matrix and ChCl/urea (ratio: 1/2 mol/mol) as plasticizer with weight variation (1, 3, 5, 10 wt%). The addition of 5 % (wt%) ChCl/urea in PVA/LiClO4 (85/15 wt%) SPE membrane showed an optimum condition with ionic conductivity of 9.95 × 10−5 S cm−1 (obtained from AC conductance extrapolation) and 7.14 × 10−5 S cm−1 (obtained from Nyquist plot), transference number of 0.88, lower elastic modulus of 2.77 MPa, and better thermal stability where the decomposition temperature interval observed at 246.80-315.89 °C. This study also explained the relationship of ionic conductivity to dielectric constant, electric modulus and transport parameters of the SPE membrane where there are higher ionic conductivity, higher dielectric properties, and lower electric modulus. The DES ChCl/urea addition also showed complexation and interaction improvement, which was approved through the 7Li chemical shift and binding energy (B.E.) analysis, which in turn correlate to ionic conductivity increment. Based on the results obtained, DES ChCl/Urea successfully improved the conductivity, flexibility, and thermal stability of the PVA/LiClO4 SPE membrane. © 2025 Elsevier Ltd