Manipulating Protein Configuration for High-Performance Solid Polymer Electrolytes

Abstract

Solid polymer electrolytes (SPEs) are well known for their excellent mechanical properties and high safety. It will be significant to develop advanced SPEs for substituting currently used organic liquid electrolytes. However, determined by their ion-transport coupled with mobility of polymer segments, SPEs usually exhibit low ionic conductivity (10-8 ~ 10-6 S/cm) at room temperature, which severely limits their practical applications. Thus far, efforts on enhancing the ionic conductivity of SPEs have been primarily focusing on reducing crystallinity degree of the polymer hosts via introducing plasticizers, advanced nanofillers, other polymers, etc. while there have not existed effective ways to the control of the ion-transport pathways inside the SPEs. In our work, we discovered that by manipulating the configuration of denatured soy protein, the transportation of Li+ can be significantly promoted, which is of great interest for fabricating high-performance SPEs. Via carefully controlling critical factors, including denaturation of soy protein, loading level of lithium salt and temperature for the evaporation process of the electrolyte preparation, a protein-based solid electrolyte with high ionic conductivity (ca. 10-5 S/cm at room temperature) and modulus (ca. 1 GPa) was achieved, which has never been realized in any conventional SPEs. Meanwhile, the soy-protein-based SPE presents high lithium ion transference number of ca. 0.94. Molecular dynamic simulation results indicate that evaporation temperature for electrolyte preparation plays a critical role in controlling the configuration of protein. As a result, a more flexible protein configuration allows strong interactions between anions of lithium salts with the backbone oxygen and negative charge side groups of protein to form anion clusters that can facilitate the transportation of Li+. It is therefore speculated that this unique ion-transport mechanism is ceramic-like and is decoupled with mobility of protein chains. In addition, by introducing ceramic nanoparticles into denaturation of soy protein, the configuration of the protein was dramatically changed to result in a type of highly efficient nanofiller that enables fast ion-transport. A composite polymer electrolyte based on poly(ethylene oxide) (PEO) loaded with such hybrid nanofillers shows one order of magnitude improvement in ionic conductivity at room temperature, as compared with the one with untreated nanofillers. Moreover, the composite polymer electrolyte was demonstrated improved mechanical properties, and importantly improved adhesion properties that greatly benefit interfacial properties with the electrodes. The studies may open a new avenue for developing high-performance SPEs with promoted ion-transport by manipulating the configuration of the protein.