AbstractThis paper presents the effect of salt concentration on the electrochemical, thermal, and structural properties of the solid polymer electrolyte (SPE) prepared with the polymer sodium carboxymethylcellulose (NaCMC) and the salt sodium triflate (NaCF3SO3). The electrolyte was prepared in the form of membranes using solution cast technique. Deionized water was used as a common solvent for both the precursor materials. The membranes were characterized using electrochemical impedance spectroscopy (EIS), Fourier‐transform infrared spectroscopy (FTIR), X‐ray diffraction (XRD), and differential scanning calorimetry (DSC). XRD results show reduction in crystallinity of the electrolyte with an increase in the salt concentration. The FTIR result confirms polymer‐salt interaction. The ionic conductivity of the electrolyte membrane was found to be dependent on the concentration of NaCF3SO3. The maximum ionic conductivity of the SPE membranes was observed to be 1 × 10−4 Scm−1 at room temperature (36°C). DSC results show an increase in glass transition temperature (Tg) with increasing salt concentration. The total ionic transference number of the highest conducting sample was found to be ~1, which shows that the conductivity of SPE membranes is predominantly due to the transport of ions.