Diametric Extremes in the Ionic Conductivity of Mixed Glass Former Solid Electrolytes

Iowa State University, Ames IA

Investigators

Abstract

NON-TECHNICAL DESCRIPTION The sun, a very large source of untapped energy, has a cyclic pattern of night and day. Therefore, energy storage systems are required and batteries are promising technologies being considered if problems of safety, high cost, and low efficiency can be resolved. Given the size of energy storage systems required, very low cost and earth abundant sodium batteries are being actively researched. Sodium batteries today, however, must operate at 300 C because the battery separator, a solid electrolyte, has a low Na+ ion conductivity and this causes such batteries to be inefficient, unsafe, and expensive. For these reasons, it is important to better understand Na+ ion motion in the solid state so that newer better conducting solid electrolytes can be developed. In this project, new glassy solid electrolytes are being studied that can be made very inexpensively with high Na+ ion conductivities. In particular, Martin's group is examining new ternary glasses based upon two network formers (e.g., silicon, boron, or germanium) that form a highly cross-linked arrangement of chemical bonds. These particular glasses are being studied because it has been found that in some combinations of two glass formers the Na+ ion conduction increases dramatically whereas for other combinations the Na+ ion motion decreases significantly. By studying these two different systems and their opposite behaviors, it will be possible to learn more about how Na+ ions conduct through solids and help develop new sodium batteries based upon better solid electrolytes. TECHNICAL DETAILS Na+ ions doped into a glass former can have very high conductivities when the chemistry is optimized. A rare coincidence of high ionic conductivity and improved physical and electrochemical properties of glassy solid electrolytes can be obtained by mixing two glass formers, B and P, for example, at a constant fraction of Na+. Such optimized mixed glass former glassy solid electrolytes based upon earth abundant Na may be candidates from which next generation cheaper, safer, and more efficient large grid-scale batteries can be made. While the Na B P O mixed glass former system exhibits a positive effect, new research has also discovered systems that show never before seen negative effects. This dual behavior of the effects is also observed in sulfide glasses. Here, B additions to a Na P S glass exponentially increase the ionic conductivity, whereas Ge additions to the same glass decrease its ionic conductivity. While progress has been made in understanding ionic conduction in solid electrolytes, significant knowledge gaps still exist. In undertaking this research, two doctoral graduate students and one undergraduate student will prepare and characterize new mixed glass former glasses that exhibit these opposite mixed glass former effects. Through this research, they will develop advanced laboratory-based research skills in materials science, glass research, and solid state electrochemistry. They will also develop computational skills and knowledge through an international collaboration with a leading glass theory and simulation research group in Germany where they will spend a 10-week summer research experience learning molecular dynamics and reverse Monte Carlo simulation techniques.