The rapid development of potable and flexible electronic devices has accelerated the need for flexible lithium ion batteries based on solid electrolytes. Solid polymer electrolyte based batteries offer many advantages including higher stability, safety, and mechanical flexibility; however, two main issues remain including the lower bulk ion conductivity and the higher solid-to-solid interfacial impedance. This study focuses on the use of protein-based biomaterial to improve the battery performance. Most proteins are intrinsically surface-active molecules due to their large amphiphatic molecules and have the potential to improve interfacial adhesion. Results of this study demonstrate that the added protein-based biomaterial effectively improves and lowers the interfacial impedance, and increases the capacity of the battery.