In this article, the complexation of methylcellulose (MC) with tri-potassium phosphate (K3PO4) is investigated using an ultrasonication-assisted solution cast process. X-ray diffraction (XRD) analysis indicates that varying concentrations of K3PO4 salt (7�35 wt.) disrupt the crystalline segment of the MC host matrix. The considerable shift in the intensity and orientation of the Fourier transforms infrared spectroscopy (FTIR) transmittance bands indicate the establishment of a charge-transfer entanglement between the polymer host and the dispersed salt. The optimum sample (28 wt.) exhibits a high ionic conductivity of 1.74 � 10�5 S cm�1, potential stability of 3.48 V, and a cation transference number (ti) of 0.945. These findings show the relevance of the developed SPEs for possible application in electrochemical devices. © 2022 Taylor & Francis Group, LLC.