A solid polymer electrolyte for a battery is disclosed. The solid polymer electrolyte includes a polymer capable of solvating a lithium salt, a lithium salt, and nanocellulose in the form of nanofibers or nanocrystals onto which are grafted anions of lithium salt, the nanofibers or nanocrystals cellulose providing increased mechanical strength to the solid polymer electrolyte to resist growth of dendrites on the surface of the metallic lithium anode.