Because the lithium-ion batteries powering today’s laptops, phones, and electric vehicles are filled with a flammable electrolyte, engineers have considered a safer alternative: an aqueous one.
In an aqueous lithium-ion battery, the anode and cathode are immersed in a water-based saline solution that conducts ions as the battery charges and discharges. While the aqueous battery will not catch fire, its low energy output has kept the option sidelined for now.
A drawback of the aqueous option: Too much voltage breaks the water into its oxygen and hydrogen parts, consuming the electrolyte and limiting the voltage window.
Rensselaer Polytechnic Institute researchers used a special type of aqueous electrolyte – a water-in-salt electrolyte – to prevent this electrolyzing of water.
In addition to using lithium manganese oxide as a cathode, RPI professor Nikhil Koratkar and a team of engineers tried out a complex oxide for the anode, one that had not been explored before in an aqueous battery: niobium tungsten oxide.
It turns out that niobium tungsten oxide is outstanding, says the mechanical, aerospace, and nuclear engineering professor at Rensselaer, particularly in terms of energy stored per unit of volume. The aqueous battery packs a maximum amount of energy in a limited volume.
“Volumetrically, this was by far the best result that we have seen in an aqueous lithium-ion battery,” said Koratkar.
The dense-packing of niobium tungsten oxide particles in the electrode lead to a higher energy storage based on volume. The material’s crystal structure also has well-defined channels — or tunnels — that allow lithium ions to diffuse quickly, which leads to a faster charge.
The combination of fast-charging capability and the storage of a large amount of charge per unit volume, Koratkar said, is rare in aqueous batteries.
"Our work is showing that aqueous battery chemistries might displace the incumbent non-aqueous Li-ion technology, especially in applications where volumetric energy and power density, cost. and safety is of paramount importance," Koratkar told Tech Briefs.
In an edited interview with Tech Briefs below, Koratkar explains how an aqueous battery has practical implications for emerging applications like portable electronics, electric vehicles, and grid storage.
Tech Briefs: How does an aqueous electrolyte differ from the typical electrolyte present in a lithium-ion battery?
Nikhil A. Koratkar: An electrolyte conducts ions within a battery. Lithium-ion batteries use organic electrolytes; these are typically lithium salts dissolved in an organic solvent. An aqueous electrolyte simply uses "water" as the solvent into which the lithium salts are dissolved.
Tech Briefs: How does an aqueous electrolyte make a safer, more cost-efficient battery?
Nikhil A. Koratkar: Organic solvents are flammable, while water is not. Hence, an aqueous battery can never catch fire. Organic solvents are expensive, while water is cheap.
Further, organic solvents are often toxic, require careful disposal and/or recycling, and most importantly they are moisture sensitive and rapidly degrade when exposed to the ambient conditions. This means that battery manufacturing requires expensive dry-room conditions to keep the moisture out. If water is used as the electrolyte, then exposure to moisture is a non-issue, which makes battery manufacturing and assembly much cheaper and simpler.
Tech Briefs: Why haven’t aqueous electrolytes caught on?
Nikhil A. Koratkar: They can electrolyze (i.e., dissociate into oxygen and hydrogen) if the cell voltage exceeds 1.23 V at room temperature. This reduced voltage window lowers the energy density of the aqueous batteries.
Tech Briefs: How is your electrolyte less likely to electrolyze?
Nikhil A. Koratkar: The idea of the electrolyte came from Professor Chunsheng Wang's work at the University of Maryland . We have not pioneered this idea. The concept that Professor Wang came up with was to saturate the water with a highly soluble lithium salt. This was found to suppress the breakdown (electrolysis) of water to higher voltages. We used his concept in our battery.
Our innovation is the anode material: niobium tungsten oxide. This is the first time this material has been explored in an aqueous battery. We found that this material excels in an aqueous battery and delivers the highest volumetric energy and volumetric power density achieved to date in an aqueous lithium-ion battery.
Volumetric means energy stored per unit volume. The reason why this oxide works so well is that it is comprised of densely packed, micron-size particles that can store a lot of energy per unit volume. The particles also contain channels that allow fast lithium diffusion, enabling fast charging of the battery.
Tech Briefs: What inspired you to do this work?
Nikhil A. Koratkar: We want to make cheaper and safer batteries, without compromising on performance. Aqueous batteries will definitely make batteries cheaper and safer, but they invariably hurt performance. We are therefore on the lookout for new materials such as niobium tungsten oxide, that allow us to maintain (volumetric) energy density on par with conventional batteries that use organic electrolytes.
Tech Briefs: What's next for your research?
Nikhil A. Koratkar: In the future, we want to look for new complex oxides that might outperform niobium tungsten oxide. Since niobium and tungsten are heavy elements, they do not offer stellar gravimetric energy and power density (gravimetric means energy stored per unit mass). We want to search for other oxide chemistries that are lighter and could deliver high gravimetric performance in conjunction with high volummetric performance.
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