Lithium metal batteries are what QuantumScape chose to make its solid-state cell due to their high energy density potential. Liquid electrolytes were not considered very promising: no lithium metal cell with them had high Coulombic efficiency – which means they had low cycling lives. MIT researchers used an innovative electrolyte and discovered it could help to create stable lithium metal batteries with high-nickel cathodes.
Theoretically, such a battery could deliver more than 400 Wh/kg. The MIT researchers tested a lithium metal battery that achieved more than 230 mAh/g with 4.7V. The high voltage – considering the size of the cell – was another component that made it difficult for them to live longer.
Despite that, the cell coped with more than 100 cycles with a Coulombic efficiency above 99.65 percent. Under more demanding tests, it kept an efficiency of more than 88 percent for 90 cycles.
The MIT researchers owe that to a sulfonamide-based electrolyte they had created for other applications. The tests showed it suited lithium metal batteries really well. One of the researchers, Jeremiah Johnson, told Green Car Congress he and his associates developed it for lithium-air batteries. Common liquid electrolytes use carbonates.
Amazingly, the liquid electrolyte suppressed side reactions such as stress-corrosion cracking, impedance growth on the cathode side, and metal dissolution, all of which lowered the Coulombic efficiency of previous lithium metal battery attempts.
Another benefit the sulfonamide-based electrolyte offered was the “highly reversible lithium metal stripping and plating.” According to the researchers, that makes the lithium metal anode achieve a “desirable deposition morphology” and avoid pulverizing the element in the cell. Again, that increases its performance and stability.
Considering it is a liquid electrolyte, it could be applied to regular cells and also to new ones with lithium metal for a higher energy density. According to Jun Li, one of the researchers, it does not have many expensive components, “just carbon and fluorine.” The production process is what will require more attention if further tests prove it is as good as it seems.
Image source: MIT/Nature Energy