Flow batteries can be rapidly “recharged” by replacing the electrolyte liquid
A flow battery is a rechargeable fuel cell that pumps a solution of charged metals dissolved in an electrolyte through a membrane to convert chemical energy into electricity. Flow batteries can be rapidly “recharged” by replacing the electrolyte liquid while simultaneously recovering the spent material for external recharging.
Researchers at the U.S. Department of Defense’s Sandia National Laboratories have discovered a new family of metal-based liquid salt electrolytes, for use in just such flow batteries. The electrochemically reversible Metal-based Ionic Liquids (MetILs) could lead to batteries packed with 3-10 times the energy density of other available storage technologies.
The main innovation is the use of a non-aqueous electrolyte which does not need to be dissolved in a solvent – it is its own solvent. The Sandia team has invented a method for synthesizing MetILs from low-cost materials that contain transition metal atoms. These atoms generate a voltage and an ionic current as they pass through the flow battery membrane. The MetILs show ion-mediated electrical conductivity, and act as both electrolyte and ionic charge carrier. Their low vapor pressure at operating temperature dramatically improves the operational safety of flow batteries.
Because the electrolyte is not diluted in a solvent, the concentration of the active metal can be roughly tripled while maintaining high efficiency of the flow battery cycle. By itself, this triples the energy capacity. At the same time, however, the operating voltage of the MetILs flow battery can be twice that of a standard vanadium-based flow battery, which again doubles the energy capacity. Finally, the chemistry of MetILs flow batteries allows the flow of chemical species which transfer 2-3 elemental charges for each metal ion that passes through the membrane. This provides another factor of 2-3 in energy density. If all factors were operating in the same flow battery, the energy capacity of the liquid electrolyte could be increased by 25-30 times, leading to storage of approximately 1.5 kWh/kg of electrolyte.
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