Abstract: The present invention refers to a method for preparing a solid polymer electrolyte based on modified cellulose, the method comprising the steps of: a) lithiation or sodiation of at least a hydroxyl group of each repeating unit of a “cellulose” to obtain Li(cellulose) or Na(cellulose); b) functionalization of the Li(cellulose) or Na(cellulose) obtained in step a), in the presence of an aprotic solvent by reacting it with an organic linker, wherein the organic linker serves to covalently attach at least one organic salt to the “cellulose”, and the use of the solid polymer electrolyte in lithium or sodium secondary batteries.