Microscopy images of the niobium tungsten oxide particles that constitute the battery anode. Image: Rensselaer Polytechnic Institute.
Microscopy images of the niobium tungsten oxide particles that constitute the battery anode. Image: Rensselaer Polytechnic Institute.

As the lithium-ion batteries that power most phones, laptops and electric vehicles become increasingly fast-charging and high-performing, they also grow increasingly expensive and flammable. In a paper in Energy Storage Materials, a team of engineers at Rensselaer Polytechnic Institute show how they can – by using aqueous electrolytes instead of the typical organic electrolytes – assemble a substantially safer, cost-efficient battery that still performs well.

A conventional battery comprises two electrodes – an anode and a cathode – immersed in a liquid electrolyte that conducts ions as the battery charges and discharges. Aqueous, water-based electrolytes have been eyed for this role because of their non-flammable nature and because, unlike non-aqueous, organic electrolytes, they aren't sensitive to moisture in the manufacturing process, making them easier to work with and less expensive. The biggest challenge with aqueous electrolytes, however, has been maintaining their performance.

"If you apply too much voltage to water it electrolyzes, meaning the water breaks up into hydrogen and oxygen," explained Nikhil Koratkar, a 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 this study, Koratkar and his team 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 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," Koratkar said. "Volumetrically, this was by far the best result that we have seen in an aqueous lithium-ion battery."

Niobium tungsten oxide is relatively heavy and dense, making 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 faster.

The combination of a fast-charging capability and the ability to store a large amount of charge per unit volume, Koratkar said, is rare in aqueous batteries. Achieving that kind of performance, together 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.

This story is adapted from material from Rensselaer Polytechnic Institute, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.