GOALI: Self-Assembled Multivalent Lithium Salts for Solid Polymer Electrolytes

Temple University, Philadelphia PA

Investigators

Abstract

TECHNICAL SUMMARY: This GOALI project is a collaboration between Temple University, Hybrid Plastics, and MaxPower, Inc., and addresses the fundamental question of whether it is possible, using an understanding of the self-assembly of nanomaterials and lithium ion diffusion in polymers, to design solid polymer electrolytes (SPEs) that have both high room-temperature conductivity (> 1 x 10-3 S/cm) and lithium ion transference numbers, tLi+, that approach 1. The SPEs so formed will have the advantages of liquid electrolytes without the accompanying safety problems of volatility, flammability and dendrite formation. Multi-ionic lithium salts will be synthesized from polyoctahedral silsesquioxane (POSS) nanoparticles and combined with polyethylene oxide (PEO), since preliminary data indicate that these SPEs exhibit improved lithium ion transport properties. The polyoctahedral silsesquioxanes are Janus-like nanoparticles, with hydrophobic phenyl groups at one end and ionic groups based on -Si-O-BF3- Li+ at the other end. In the morphology that forms, the phenyl groups cluster and the -Si-O-BF3- groups orient towards the PEO phase. The electron withdrawing POSS cage and BF3 groups delocalize the negative charge on the anion so that the dissociated Li+ can be solvated by the surrounding PEO matrix. The PEO is completely amorphous, so that the resulting solid structure is not the result of PEO crystallinity but instead it is proposed to be the result of phenyl crystallites and crosslinks formed from -Si-O-BF3 anion--- Li+---O-H2CH2 bridges that connect the phenyl clusters and PEO chains. Enhanced conductivity may be the result of this morphology, in which Li+ ions are loosely coordinated to several -Si-O-BF3 anions, and may migrate along the interfacial regions with a lower activation energy. Preliminary data show ionic conductivities of 3 x 10-4 S/cm, close to the target value of 1 x 10-3 S/cm, with tli+ = 0.6. Better designed multi-ionic lithium salts in which the Li+/phenyl group ratio is increased will be investigated. The purpose is to minimize the amount of non-conductive phase needed to maintain a solid without PEO crystallinity and to maximize the amount of low Tg conductive phase with solvated Li+ ions. Morphology, mechanical and electrochemical properties will be correlated to elucidate the factors that contribute to enhanced conductivity. This project is supported by the NSF Solid State and Materials Chemistry Program. NON-TECHNICAL SUMMARY: The potential impact of the proposed research is improved solid polymer electrolytes that would enhance the performance of lithium/lithium ion batteries used in large electrical energy storage applications such as electric vehicles/hybrid electric vehicles or matching the output of fluctuating power sources including wind and solar with fluctuating demand. Solid polymer electrolytes are intrinsically safer than the volatile liquid electrolytes currently used in small lithium ion batteries for portable devices. The research will focus on the development of new materials which have ionic conductivities comparable to those of current liquid electrolytes, but are safer and have better performance overall. Material synthesis will be performed in collaboration with Hybrid Plastics, Inc., a company with the expertise and scale-up facilities necessary for the project. Long-term battery testing for complete evaluation of the new solid polymer electrolyte materials that are developed will be accomplished through collaboration with MaxPower, Inc, a lithium battery manufacturer. Undergraduate/graduate students will participate in interdisciplinary research in polymer-, organic- and electro-chemistry both in academic and industrial environments.