DOE energy innovation hub backs two key future battery technologies
The Center was set up four years ago with a five year remit to explore new battery technology for transportation and the electricity grid that, when scaled to commercial production, are capable of delivering five times the energy density at one-fifth the cost of commercial batteries available in 2011.
The Center has investigated 1,500 compounds for electrodes and 21,000 organic molecules relevant for liquid electrolytes as well as filing 52 invention disclosures and 27 patent applications, says director George Crabtree. Five techno-economic models created by JCESR for designing virtual batteries on the computer are being used to evaluate the best pathways for beyond-lithium-ion systems to reach 400 watt hours per kilogram (400 Wh/kg) and $100 per kilowatt hour ($100/kWh).
To do this, the researchers have developed an organic “redox flow” battery, which consists of two energy-dense liquids that store and release charge as they flow through the battery and undergo reduction and oxidation (“redox”) reactions. The design replaces the solid electrodes in conventional lithium-ion batteries with energy-dense organic liquids that charge and discharge as they flow through the battery.
The organic molecules in these redox flow batteries are highly versatile and can be tailored to store large amounts of energy inexpensively, a key requirement for the grid.
The first innovation in the design involves the use of inexpensive, recyclable and versatile organic molecules as energy storing active ingredients, and linking these organic molecules together in oligomers of up to 10 molecules, polymers of up to 1000 molecules and colloidal particles of a million to a billion molecules. These linked “macromolecules” are large enough to be blocked by the second innovation, a special porous polymer membrane, providing a simple solution to prevent crossover of the active materials between the anode and cathode liquids.
For transportation the Center has developed a battery with a lithium metal anode protected from degradation by a graphene oxide membrane, a polymer-composite sulfur cathode, and an electrolyte that is sparingly soluble for the polysulfides that form during charge and discharge. This battery system is attractive because of its high theoretical energy density and the low cost of sulfur.
Key to success will be achieving a very low ratio of electrolyte to sulfur content. Full cell testing of each of these concepts is now underway, and proof-of-principle prototypes will soon be evaluated.
The Center continues development of other prototypes, but the target of $100/kWh at the pack level will be achieved with these two systems by the end of next year, says Crabtree.
“For the lithium-sulfur battery, we are concentrating our efforts on three design features,” he said. “One is electrolytes that limit undesirable reactions of the polysulfides that form during charge and discharge. Another is binders in the sulfur cathode that trap polysulfides before they dissolve in the electrolyte and ensure mechanical integrity of the cathode during cycling.”
“The third is special membranes that prevent movement of polysulfides from the cathode to the anode and maintain a smooth anode surface during cycling. We believe that the final embodiment of the lithium-sulfur prototype will require a combination of these features to meet the cost and performance targets.”
JCESR is also working on Sulphur-Air batteries for low cost grid storage, as well as multivalent batteries with new materials to increase the battery energy storage capacity by a factor of two or three. Cathodes for multivalent batteries are a major challenge.
