In large-scale all-solid-state storage technologies, solid polymer electrolytes (SPEs) provide greater safety and longer cycle life than traditional liquid or gel polymer electrolytes. Polymer electrolytes (PEs) derived from biopolymers have been intensively explored for use in electrochemical devices due to their great flexibility, low cost, and environmental sustainability. However, biopolymer-based electrolytes cannot meet the expectations of practical applications at room temperature due to their low ionic conductivity. Over the years, improving the performance of this class of electrolytes has been the focus of intense research and development, notably polymer blending, plasticization, and structural functionalization. Here, we investigate the performance of an all-biopolymer solid electrolytes based on a methylcellulose-pectin blend doped with potassium phosphate. FESEM micrographs, as well as the shifting and changing intensity of FTIR bands in the electrolyte specimens confirm the polyblend homogeneity with no phase separation. The increased amorphous fraction of the composite polymer electrolytes (CPEs) is seen in the XRD and DSC patterns of the plasticized and unplasticized samples. Impendence studies performed on the system recorded a maximum ionic conductivity of 1 � 10�5 Scm�1 by doping with 50 wt. K3PO4. This value further increased to 5.9 � 10�4 Scm�1 upon adding 25 wt. EC to the polymer system. This sample also possesses an electrochemical stability window of 4.24 V and an ion transference number of 0.95. © The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2022. Springer Nature or its licensor holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.