This Materials World Network (MWN) project examines the universally observed Mixed Glass Former Effect (MGFE) where independent of the mobile cation, independent of the two glass formers used, independent of whether the system is all-oxide, all-sulfide, or even mixed oxy-sulfide, and independent of the over-all mobile cation concentration, the ionic conductivities of Mixed Glass Former (MGF) glasses are always higher than that of the two parent binary glasses at the same level of mobile cation concentration. With the recent and well known problems of liquid polymer electrolytes in millions of lithium batteries, there is a renewed interest in solid electrolytes for lithium batteries. Such MGF glasses make ideal candidates due to their anomalously high ionic conductivities and other advantageous properties brought about by the mixing of the glass formers. However, before wide spread application of these electrolytes can be implemented, a detailed understanding of the MGFE must be developed first. For this reason, the intellectual merit of this project is that an international research team has been assembled to form a Materials World Network of research capability from among three universities in the US and three universities in Europe. Researchers in the US are preparing the glasses and measuring the composition dependence of the conductivity and examining short range structures through vibrational spectroscopy (Martin-Iowa State University). Both short and intermediate range structures of the glasses are being examined using high resolution x-ray diffraction (Petkov-Central Michigan University) and tracer diffusion coefficients are being measured (Dieckmann-Cornell). The European collaborators are providing complimentary neutron diffraction (Brjesson-Chalmers Institute of Technology, Sweden) and nuclear magnetic resonance (Eckert- Westfalische Wilhems-Mnster University, Germany) data to extend the detail of structural studies. European collaborators are also providing theoretical modeling and simulation of both the ionic conductivity and structure of the glasses (Maass- Technical University Ilmenau, Germany) and for theoretical modeling of the ion dynamic processes that are apparently greatly magnified in MGFE glasses (Funke-Mnster U., Germany). The project synergistically combines both structural and dynamical studies of the MGFE in oxide, sulfide, and oxy-sulfide glasses to determine the nature, extent, and role of the favorable structural features of MGFE glasses.
The broader impacts of this project are to use new modalities of international collaboration to provide young researchers new education experiences to broaden and deepen their research abilities, and to expand and develop their professional and international awareness to enhance their global citizenry. This is achieved by providing students with unique extended collaborative research and education experiences in the European collaborators' laboratories, and by having them host European students in the US to create professional linkages and experiences throughout this program. Deep collaborations among other MWN projects in similar countries are developed to leverage learning among the MWNs and speed up the development of best practices for such international collaborations. Collaboration is also established with local 2- and 4- year colleges to draw undergraduates, especially women and minorities, to the program to foster new 4 year graduates and new graduate students. The framework of the MWN is also used to develop new modalities of distance utilization of research equipment through common operating systems and high-speed internet connections, and to develop sustainable collaborations among the partners of the program that foster long term progress on research.
This award is co-funded with the Office of International Science and Engineering.
Intellectual Merit: The sun, a very large source of 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, in this project we are examining new ternary glasses based upon two network formers (e.g., silicon, boron, phosphorous or germanium) that form a highly cross-linked arrangement of chemical bonds. In one ternary glass system Na2O + B2O3 + P2O5, the sodium ion (Na+) conductivity, the rate at which Na+ move through the glass is nearly 100 times larger than either of the binary Na2O + B2O3 or Na2O + P2O5 systems. This increased speed at which the Na+ moves was studied in terms of the atomic level chemical bonding in these ternary glasses. It was found that the chemical bonding in sites in which the Na+ ions reside become significantly weaker in the ternary glasses compared to either of the binary glasses. These weaker chemical bonds were found to be associated with the unique way in which boron (B) is bonded in the glass to form special tetrahedral species which have weaker ionic bonds to the Na+ ions. In another ternary system that was studied in the project, Na2S + GeS2 + PS5/2, the Na+ ion conductivity becomes much slower, more than 10 times slower in the ternary glasses compared to either of the binary glasses. This decrease in the Na+ conductivity, while a negative trend for use in new sodium batteries, none-the-less holds significant information about the factors of the glass that control the conductivity. For these reasons, the chemical structures of these glasses was also studied at the atomic level using a variety of spectroscopic technique. It was found that the phosphorous (P) structures in the glass act like Na+ ion traps that increase the strength of the chemical bonds and thereby these traps significantly slow down the rate at which the Na+ ions can move through the network. Broader Impacts: There were two significant Broader Impact outcomes of this project. First, the project increased the number of domestic US students who earned the Ph.D. and B.S. degrees in Materials Science and Engineering. Second, the project increased the diversity of US students who earned the Ph.D. and B.S. degrees in Materials Science and Engineering. In the first Broader Impact, the project graduated two Ph.D. US students and graduated 11 B.S. US students. Of these, one Ph.D. was a women, 50%, and four of the B.S. students graduated were women, 36%. Both of these graduation percentages are significantly higher than the average graduation rate of US women in Material Science and Engineering.
