A fundamental study of the interplay between ordered morphology, grain structure, and ionic conductivity in mixtures of block copolymers and lithium salts is proposed. The results of the work have the potential to impact the development of high specific energy batteries with solid electrolytes. The work will focus on mixtures of polystyrene-polyethyleneoxide block copolymers, synthesized by anionic polymerization, and bis-trifluoromethylsulfonimide (LiTFSI), a salt that is often used in batteries containing polymer electrolytes. Small-angle X-ray scattering and depolarized light scattering will be used to determine the nature of the order-disorder and order-order phase transitions in these materials. Conductivity measurements will be made on the same samples as those used in the X-ray and light scattering studies using AC impedance spectroscopy. Measurements will be made over a wide range of block copolymer compositions, salt concentration, temperature, and annealing conditions.
NON-TECHNICAL SUMMARY:
The fundamental work proposed in this project is motivated by the need to design safe rechargeable batteries for clean energy-related applications such as electric vehicles and stationary energy storage. The proposed nanostructured ion-conducting solid will be used as the electrolyte in a lithium battery. By replacing the flammable liquid electrolyte that is used in current lithium batteries by a solid, one can use higher energy electrodes that, in turn increase the energy density of the battery. The absence of a flammable component makes the battery safe. The co-PIs will use solid-state batteries for demonstrations in their outreach activities involving high school students. Garetz works with the David Packard Center for Technology and Educational Alliances in New York City to mentor local high school students in fields related to technology and research. Balsara works with Math and Science Summer Academy program at Berkeley, and enrichment program which brings students from the Oakland School District to the University of California. The PIs will continue an ongoing collaboration with Professor M. Banaszak at A. Mickiewicz University, Poland on the subject of ion-containing polymers that arose from an NSF-sponsored workshop in Poland.
Technical accomplishments This is a collaborative project aimed at obtaining a fundamental understanding of a new class of electrolytes for lithium batteries. One of the limitations of current lithium batteries is safety. About one in ten million lithium-ion batteries exhibits catastrophic failure that is often initiated by combustion of the electrolyte. The electrolytes that we are studying are inherently nonflammable. In addition, they are mechanically rigid and tough, and this prevents deformation of the electrodes during battery cycling. The electrolytes are nanostructured, comprising ordered co-continuous domains of a hard polymer that gives the electrolyte the desired mechanical properties and soft salt-containing domains necessary for ion transport during charging and discharging the battery. The domains are formed spontaneously when the electrolyte is prepared by casting or melt-processing. We report on three fundamental discoveries related to the relationship between structure and properties of this class of electrolytes. When the ordered domains form from a homogeneous disordered phase, one obtains coexistence of order and disorder. This has important consequences on the manner in which the ordered phase forms. In nanostructured polymers without salt, the ordered phase grows until the entire sample is ordered. In nanostructured electrolytes, the initial growth of the ordered phase is followed by decay due to partitioning of the salt into the ordered domains. For practical applications, it is important to have a completely ordered electrolyte, and our work outlines regimes where this is possible. An important property of our electrolytes is the fact that the orientation of the domains is random; coherent order is restricted to regions that we call grains. We have, for the first time, established the relationship between grain structure and ion transport. The work described above was the fruit of collaboration between two research groups with complementary skills. The group at Polytechnic Institute at NYU specializes is characterization of grain structure while the group at UC Berkeley specializes in synthesis and characterization of ion transport. Student development and broader impacts The NSF funds were used to support two graduate students Jacob Thelen and Mahati Chintapali at UC Berkeley. They have received multidisciplinary scientific training in the crucial area of materials necessary for producing the next generation of batteries for transportation and other applications. The importance of this field is recognized by academia, industry, and national laboratories. The specific background in characterization of charge transport in nanostructured block copolymers is relevant for numerous other technologies. The work done by these students encompasses synthesis and characterization of polymers, optics, scattering, electrochemistry, and thermodynamics. Our collaborations with scientists at the national facilities (SSRL and APS NIST) has exposed the students to senior scientists with complementary backgrounds and research interests. Leveraging the nation's investment in research infrastructure for education of graduate students is a long-standing tradition in our groups. Throughout the course of this grant, PIs and graduate students have engaged with the Professional Development Program at UC Berkeley. Balsara served as the chair of the UC Berkeley Senate Committee that oversees the program in 2011. This committee advises and reviews pre-college to graduate level programs aimed at helping outstanding students who have encountered restricted educational opportunities due to racial, ethnic, gender preferences, or other socio-economic conditions achieve their academic promise. The Professional Development Program organizes several outreach programs and the Da Vinci program is one that is particularly well-suited for interactions with the NSF team. Here students are taught about polymers, electrolytes and batteries and then given first hand experience on what it is like to work on these subjects. The activities were tailored to educate and excite students, who are primarily young girls from local middle schools. Significance and Societal Impact Lithium-ion batteries, used traditionally to power personal electronics, are being considered for powering electric vehicles, storing energy from the sun and wind, and for providing electrical power on airplanes. The electrolyte used in current lithium batteries is a flammable organic solvent. Safety concerns have limited the use of large format lithium ion batteries. Our research is related to the fundamental thermodynamic and transport characteristics of solid polymer electrolytes for lithium batteries. The electrolytes are nonflammable and thus have the potential to deliver energy more safely than current systems.
