Current lithium batteries use a nanoporous polypropylene membrane filled with lithium salts dissolved in high dielectric constant solvents as the separator between electrodes. This technology has problems with safety and regardless of the choice of anion, which moves 5-10X faster than Li+, resulting in anion build-up at electrodes that significantly reduces battery efficiency, maximum battery power and recharge time. The obvious choice for a replacement membrane is a single-ion conducting polymer (ionomer) that has all anions covalently bonded to the polymer, has no solvent to leak out of the battery and can easily be made into a thin film. Unfortunately, the best ionomer membranes have Li+ ion conductivities 100X too small for practical applications. The PI leads a five-PI DOE-funded team that is attempting to design superior ionomers for lithium battery membranes, but also is facing a very broad design space, with 30 polar groups that could be added as side chains and 20 anions that could be attached to polymers. Combined with at least five backbones that have low Tg, there are thousands of possible combinations. Furthermore, he is a co-PI on a seven-PI Army MURI aiming to synthesize ionomer membranes for actuators, which additionally have flexibility in the choice of cation.
The intellectual merits of this research are two-fold: (1) The proposed research will suggest anion, polar side group, backbone combinations worthy of synthesis and directly aid the seven synthesis students on the two teams mentioned above. (2) Ab initio calculations can be rapidly applied to a wide variety of anions and cations in different polar media, enabling a detailed understanding of ion interactions and solvation by polar groups, which should propel our modeling efforts with Sanat Kumar at Columbia.
The broader impacts of our proposed research are three-fold. (1) The understanding of how to design ionomers for improved ion-conduction will not only impact advanced lithium batteries and actuators, but is also vital for other battery membranes (such as commercial ones transporting F-) and the membrane electrode assemblies for fuel cells. All ion-conducting membranes suffer from the fact that only a tiny fraction of counter-ions participate in conduction, and the proposed research directly addresses boosting the conducting ion content. (2) Materials development in the energy field is expected to play a very important role in the future of the United States economy and way of life. (3) Graduate students trained in this "energy materials" arena will be in enormous demand in both US industry and academia for at least the next ten years. Penn State has superb undergraduates and research motivates them to attend graduate school (15 of 25 undergraduate researchers in the PI's group over the past 14 years have gone on to graduate school in science and engineering) with many current undergraduates interested in "energy materials", including current REU-funded student Daniel King, a Materials Science and Engineering junior who has won our department's Undergraduate Research Fellowship for three consecutive years.
Many emerging energy applications require materials that can conduct a particular type of ion and not conduct either electricity or other ions. Examples of uses for such single-ion conductors are 'separator membranes' for batteries and fuel cells that need to transport a particular ion (such as the lithium ion). Our research used a commercial computer program to quantify interactions that the ion of interest has with the membrane to design superior membranes. Several interesting candidate polymer membrane materials were identified in this research and in a separate project these membranes were constructed and tested for their ion conduction properties. One important thing that we learned is that there need to be non-ionic components of the membrane that interact favorably with the ion of interest. We developed new methods to study the role of those 'solvation' interactions in the presence of the stronger interactions from the oppositely charged ions that must also be part of the membrane. Only with proper solvation and weak interactions with the opposite charge can membranes acheive the ionic conductivity needed for practical application of these membranes as energy materials. Membranes with proper solvation and weak-binding anions were synthesized and tested based on the results of our research and a small start-up company PolyK Technologies is currently optimizing the performance of these membranes for lithium batteries.
