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New Aqueous Lithium-Ion Battery Developed
December 23, 2019

As the use of lithium-ion batteries as a power source continues to grow across a wide variety of sectors including mobile phones, laptop computers and even electric vehicles, dual concerns continue to arise — cost and safety. While lithium-ion batteries become increasingly high-performing and fast-charging, they also have become more expensive and flammable.

Now, according to information provided by Rensselaer Polytechnic Institute (Rensselaer), a team of engineers has demonstrated how they could assemble a substantially safer, cost-efficient battery that still performs well by using aqueous electrolytes instead of the typical organic electrolytes.

Batteries of all types consist of two electrodes, an anode and a cathode, which are surrounded by some kind of electrolyte. In lithium-ion batteries, the electrodes are immersed in a liquid electrolyte that conducts ions as the battery charges and discharges.

Aqueous electrolytes have been eyed for that role because of their non-flammable nature and because, unlike non-aqueous electrolytes, they aren’t 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, an endowed chair 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.”

In research published recently in Energy Storage Materials, Prof. Koratkar and his team, including Fudong Han, an endowed chair assistant professor of mechanical, aerospace, and nuclear engineering and Aniruddha Lakhnot, a doctoral student at Rensselaer, used a special type of aqueous electrolyte known as 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 Prof. Koratkar said had 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,” Prof. Koratkar said. “Volumetrically, this was by far the best result that we have seen in an aqueous lithium-ion battery.”

The niobium tungsten oxide is relatively heavy and dense, Prof. Koratkar explained. That weight makes its energy storage based on mass about average, but the dense-packing of niobium tungsten oxide particles in the electrode makes its energy storage based on volume quite good. The crystal structure of this material also has well-defined channels — or tunnels — that allow lithium ions to diffuse quickly, meaning it can charge quickly.

According to Prof. Koratkar, the combination of fast-charging capability and the ability to store a large amount of charge per unit volume is rare in aqueous batteries.

Achieving that kind of performance, with a low cost and improved safety, has practical implications. For emerging applications such as portable electronics, electric vehicles, and grid storage, the ability to pack the maximum amount of energy into a limited volume becomes critical.

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