Batteries that can effectively, affordably and safely compete with the internal combustion engine require new materials development and design strategies. Energy storage in portable consumer rechargeable lithium ion batteries has reached ~ 3.0 Ah, insufficient for powering electric vehicles, but with reasonable, > 30,000, charge/discharge cycles. The use of metallic lithium as the anode (to increase the cell voltage) and flow-through cathodes or cathodes in which oxygen is reduced, can increase energy density. Replacement of lithium with less expensive, more available sodium will reduce costs. However, current high energy and power density Li battery technology suffers from safety concerns and poor performance at low temperatures. Replacement of liquid electrolytes with solid electrolytes will improve safety, and development of low-barrier conducting materials will improve wintertime behavior. Next generation lithium batteries such as lithium air and flow-through cathode batteries have already been designed with solid electrolytes (alone or in combination with liquid electrolytes). Here, one aspect of this multidisciplinary problem will be addressed, namely the formation of soft solid crystal electrolytes with low-affinity channels for lithium or sodium ion conduction. All solid-state lithium ion organic conductors have the benefits of increased safety, but the limitation of poor ionic conductivity, while ceramic/glass conductors have higher ionic conductivities but are brittle and can have poor adhesion to the electrodes. Engineering of solid-state organic materials with specific ion conduction pathways that can enhance ion migration offers promise as a means to achieve higher solid-state ionic conductivities, while soft, more malleable organics will afford better adhesion to the electrodes. There is only limited progress in this area, making the development of new synthetic routes for the formation of specific architectures with ion channels an important avenue of research.
Proposed is a project on design and fabrication of a novel class of solid electrolytes made from lithium salt cocrystals. The proposed materials posess ion channels with weak interactions between the ions and channel walls. These weak interactions arise from the deliberate use of polarizable (soft) functionality on the walls, which interact poorly with the non-polarizable (hard) lithium ions according to the Pearson Hard Soft Acid Base Concept. The resulting materials will be soft solids with good conductivity, decreased flamability, and improved low-temperature conduction. Two preliminary materials show lithium ion conduction with negligible activation barrier, and equal conductivity at room temperature and -78 C. A major goal of the proposed work is to increase the thermal stability of prototype materials at high temperatures. This will be achieved by developing systems with greater intermolecular interactions through pi-stacking or covalent linkage. The resulting materials would be the first solid electrolytes to have favorable conductivities over the entire range of global temperatures. Variation of anion size and matrix affinity will be used to optimize the selective conduction of cations in the matrix. The use of sodium ions in place of lithium ions will also be explored in an effort to design electrolytes for sodium batteries as well. These materials will be fabricated into films for device testing. Preliminary results on the use of Polyhedral oligomeric silsesquioxane polyethylene glycol (POSS-PEG) as a binder are promising, and provide junctions between the cocrystals for DC conductivity without affecting the temperature independent behavior.
Intellectual Merit : The proposed class of materials represent a new class of material for solid electrolytes. They exhibit behavior slightly superior to pure polymer electrolytes at room temperature, and exceeding superiority at low temperature. Such materials have the potential to lead to the design of solid state batteries that work across all ranges of global temperature, and posess increased safety due to the absence of volatile flammable electrolytes.
Broader Impacts : Energy renewables is an increasingly important sector of the United States Economy. Batteries will continue to play a major role in energy storage for some time. The proposed work may lead to new materials for the improvement of safety and functioning of batteries for the betterment of US energy independence. More importantly, the project will train young scientists in order to supply the market's increasing demand in the field of ion conduction, which is relevant to numerous applications in this growing economic sector.
