Polymeric electrolytes for rechargeable lithium-based electrochemical cells and batteries would be made by blending and complexing cyanoresins with lithium salts, according to a proposal. In particular, polymeric electrolytes for separators, carbon-composite anodes, and cathodes would be formulated from appropriate blends of different polymers that are mutually insoluble and do not chemically react with each other. As a result, each polymeric component would retain its specific desired characteristics in high-energy-density batteries that would be capable of long cycle lives and high charge/discharge rates. For example, one polymeric component could provide high ionic conductivity and charge-carrier concentration while another polymeric component would provide structural integrity. Conceivably, a lithium battery made with such materials could exhibit an energy density of 80 W×h/lb for more than 1,000 charge/discharge cycles. Batteries like this could be used in applications ranging from geosynchronous satellites to electric vehicles to small consumer electronic equipment.
Heretofore, lithium anodes normally have not been stable in polymeric-electrolyte batteries because most polymeric electrolytes now in use react unfavorably with freshly plated Li. These interactions may increase the electrical resistances of anode/electrolyte interfaces and, thus, the electrical resistances of the affected cells to unacceptable levels during charge/discharge cycling, and may reduce charge/discharge capacities.
Lithium-ion conductivities in currently available candidate solid polymeric electrolyte materials range between 10–6 and 10–5 S/cm; these values are too low for effective operation at room temperature. The polymers that have been used to synthesize polymeric electrolytes include polyacrylonitrile, polyvinyl pyrrolidone, and polyethylene oxide, all of which have low dielectric constants (between 4 and 5). Low dielectric constants lead to high degrees of ion association in polymeric electrolytes; this, in turn, results in unacceptably low concentrations of charge carriers and low ionic conductivities at room temperature.
To increase ionic conductivities by the two orders of magnitude as needed for practical cells, the base polymers must be amorphous (with respect to crystalline structure), have low glass-transition temperatures (Tg's), and have large dielectric constants (preferably as large as 20). To prevent failure of cells by maintaining structural integrity of separators (preventing punch-through by lithium dendrites and thereby also preventing electronic conduction between anodes and cathodes), polymeric electrolytes to be incorporated into separators must have Tg's greater than those of polymeric electrolytes to be incorporated into compositea-material electrodes. The Tg's of the composite materials in the electrodes must also exceed those of the polymeric electrolytes.
Three commercially available amorphous cyanoresins are under consideration as candidate polymeric electrolytes. These are cyanoethyl polyvinyl alcohol (CRV), cyanoethyl pullulan (CRS), and cyanoethyl sucrose (CRU)(see figure). CRV is a rubbery solid with a relatively low Tg of 30 °C. CRS, which has a Tg of 105 °C and a high molecular weight (760,000) has been studied for use in capacitors and can be processed to form excellent thin films. CRU is a viscous liquid at room temperature, with a molecular weight of 782. These cyanoresins are ideal candidates to be blended and complexed with appropriate Li salts to form polymeric electrolytes of various Tg's for use with composite electrodes and as cell separators with high ionic conductivities. Conventional candidate Li salts include LiClO4, LiPF4, and LiCF3SO3.
Measurements of the dielectric constants and dielectric-loss characteristics of these three cyanoresins and of CRS/CRV and CRS/CRU blends indicate that the room-temperature dielectric constants are approximately 20 — large enough to achieve high ionic conductivities at nominal temperatures when these cyanoresins complexed with Li ions to form solvent-separated ion pairs. Li salts of CRU and CRV are expected to exhibit restricted anion mobilities, with concomitant high Li+ mobilities.
This work was done by Shiao-Ping S. Yen, Andre H. Yavrouian, James B. Stephens, and John D. Ingham of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Materials category.