One-of-a-kind tool helped solve anode puzzle that thwarted previous attempts

When it comes to batteries, lithium ion is the best we have in terms of energy density and convenience.

For the moment.

The University of Washington, St. Louis laboratory of Peng Bai, assistant professor in the Department of Energy, Environmental and Chemical Engineering at the McKelvey School of Engineering, has developed a stable sodium ion battery that is highly efficient, will be less expensive to manufacture and is significantly smaller than a traditional lithium-ion battery due to the elimination of a function once necessary.

“We have found that the minimum is the maximum,” Bai said. “No anode is the best anode.”

The research was published on May 3, 2021 in the journal Advanced science.

A traditional lithium-ion battery consists of a cathode and an anode, both of which store lithium ions; a separator to keep the electrodes separate on each side; and an electrolyte – the liquid through which ions move. As lithium flows from the anode to the cathode, free electrons flow through the current collector to the powered device while the lithium passes the separator at the cathode.

To charge, the process is reversed and lithium passes from the cathode, through the separator, to the anode.

The concept of replacing lithium with sodium and removing the anode is not new.

“We used old chemistry,” Bai said. “But the problem was, with this well-known chemistry, no one has ever shown that this anode-less battery can have a reasonable lifespan. They always break down very quickly or have a very low capacity or require special treatment. of the current collector. “

Batteries without an anode tend to be unstable, growing dendrites – finger-like growths that can cause a battery to short or simply degrade rapidly. This has conventionally been attributed to the reactivity of the alkali metals involved, in this case sodium.

In this newly designed battery, only a thin layer of copper foil was used on the anode side as a current collector, that is, the battery has no active anode material. Instead of flowing to an anode where they rest until they come back to the cathode, in the anode-less battery the ions are transformed into metal. First, they stick to a sheet of copper, and then they dissolve when it’s time to return to the cathode.

“In our discovery, there are no dendrites, no finger-like structures,” said Bingyuan Ma, the paper’s first author and a doctoral student in Bai’s lab. The deposit is smooth, with a metallic sheen. “This type of growth pattern has never been observed for this type of alkali metal.”

“Observe” is the key. Bai has developed a unique transparent hair cell that offers a new way of looking at batteries. Traditionally, when a battery fails, in order to determine what is wrong, a researcher can open it up and take a look. But this type of after-the-fact observation has limited utility.

“All battery instabilities accumulate during the working process,” Bai said. “What really matters is instability during the dynamic process, and there is no way to characterize that.” Looking at Ma’s anode-less hair cell, “We could clearly see that if you don’t have good quality control over your electrolyte, you will see various instabilities,” including the formation of dendrites, Bai said.

Basically, it depends on how much water is in the electrolyte.

Alkali metals react with water, so the research team reduced the water content. “We were just hoping to see a good performance,” Bai said. Watching the battery in action, the researchers quickly saw smooth, shiny sodium deposits. It is the softness of the material that eliminates morphological irregularities that can lead to the growth of dendrites.

“We went back to check the hair cells and realized there was a longer drying process for the electrolyte,” Bai said. Everyone talks about the water content of batteries, but in previous research the amount of water was often relegated to a simple statistic that should be noted.

Bai and Ma realized that was, in fact, the key.

“The water content should be less than 10 parts per million,” Bai said. With this realization, Ma was able to build not only a hair cell, but a functional battery that performs similarly to a standard lithium-ion battery, but which takes up much less space due to the lack of ‘anode.

“Check your cell phone. Your electric car. A quarter of the cost of these items comes from the battery,” Bai said. Sodium batteries use a more common metal than lithium batteries; they have the same energy density as lithium batteries; and they are smaller and cheaper than lithium batteries, thanks to the elimination of the anode.

“We’ve proven that you can use the simplest setup to activate the best battery,” Bai said.

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