A team of engineers at Rensselaer Polytechnic Institute in the US have used aqueous electrolytes to assemble a fast charging, cost-efficient lithium aqueous battery that still performs well.
Aqueous electrolytes have been of interest for years because of their non-flammable nature and because they are not sensitive to moisture in the manufacturing process, making them easier to work with and less expensive. The biggest challenge with this material has been maintaining performance.
“If you apply too much voltage to water it electrolyzes, meaning the water breaks up into hydrogen and oxygen,” said Nikhil Koratkar, professor of mechanical, aerospace, and nuclear engineering at Rensselaer. “This is a problem because then you get outgassing, and the electrolyte is consumed. So usually, this material has a very limited voltage window.”
Koratkar and his team used a water-in-salt electrolyte, which is less likely to electrolyze. For the cathode, the researchers used lithium manganese oxide, and for the anode, they used niobium tungsten oxide, a complex oxide that has not been explored in an aqueous battery before.
“It turns out that niobium tungsten oxide is outstanding in terms of energy stored per unit of volume,” said Koratkar. “Volumetrically, this was by far the best result that we have seen in an aqueous lithium-ion battery.” The cell has a volumetric capacity of 200 Ah/l at a 1C charging rate, which is much higher than a state-of-art graphite anode at 50 to 110 Ah/l.
The niobium tungsten oxide is relatively heavy and dense, giving a good energy storage. The crystal structure also has well-defined channels — or tunnels — that allow lithium ions to diffuse quickly, meaning it can charge quickly. This combination of fast-charging capability and the ability to store a large amount of charge per unit volume is rare in an aqueous battery. Koratkar sees the technology being used for portable electronics, electric vehicles and grid storage.