Clear route to anode-free sodium-ion battery

By eliminating a once-necessary feature, the lab of Peng Bai, assistant professor in the Department of Energy, Environmental & Chemical Engineering at Washington University in St. Louis, has developed a stable, highly efficient sodium-ion battery that is less expensive to make and significantly smaller than a traditional lithium-ion battery,

"We've found that the minimal is maximum," said Bai. "No anode is the best anode." Bai and his team report their work in a paper in 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 separated from each other; and an electrolyte – the liquid through which the ions move. When lithium ions flow from the anode to the cathode, free electrons leave through the current collector to the device being powered while the lithium ions pass through the separator to the cathode.

To charge, the process is reversed. The lithium ions pass from the cathode, through the separator, to the anode.

The concept of replacing lithium ions with sodium ions and doing away with the anode isn't new.

"We used old chemistry," Bai said. "But the problem has been, with this well-known chemistry, no one ever showed this anode-free battery can have a reasonable lifetime. They always fail very quickly or have a very low capacity or require special processing of the current collector."

Anode-free batteries tend to be unstable due to the growth of dendrites – finger-like growths that can cause a battery to short circuit or simply degrade quickly. Conventionally, this has been attributed to the reactivity of the alkali metals involved; in the case of an anode-free battery, the metal is sodium.

In this newly designed battery, only a thin layer of copper foil was used on the anode side as the current collector, i.e. the battery has no active anode material. Instead of flowing to an anode where they sit until time to move back to the cathode, the sodium ions are transformed into a metal. First, they plate themselves onto copper foil, then they dissolve away 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 of sodium ions is smooth, with a metal luster: "This kind of growth mode has never been observed for this kind of alkali metal."

'Observing' is key. Bai has developed a unique transparent capillary cell that offers a new way to look at batteries. Traditionally, in order to determine what went wrong when a battery fails, a researcher had to open it up and take a look. But that after-the-fact kind of observation has limited usefulness.

"All of the battery's instabilities accumulate during the working process," Bai explained. "What really matters is instability during the dynamic process, and there's no method to characterize that." Observing Ma's transparent, anode-free capillary cell, "we could clearly see that if you don't have good quality control of your electrolyte, you'll see various instabilities", including the formation of dendrites.

Essentially, it comes down to how much water is in the electrolyte.

Alkali metals react with water, so the research team brought the water content down. "We were hoping just to see a good performance," Bai said. Watching the battery in action, the researchers soon saw shiny, smooth deposits of sodium. It's the smoothness of the material that eliminates the morphological irregularities that can lead to the growth of dendrites.

"We went back to check the capillary cells and realized there was a longer drying process of the electrolyte," Bai said. Everyone talks about the water content in batteries, but in previous research, the amount of water had often been relegated to a statistic that merely needed to be noted. Bai and Ma realized that it was, in fact, the key.

"Water content must be lower than 10 parts-per-million," Bai said. With that realization, Ma was able to build not just a capillary cell, but a working battery. This is similar in performance to a standard lithium-ion battery, but takes up much less space because of the lack of an anode.

"Check your cell phone. Your electric car. One quarter of the cost of such items comes from the battery," Bai said. Sodium-ion batteries have the same energy density as lithium-ion batteries but use a metal that is more common than lithium, while this new version is smaller and cheaper than current lithium-ion batteries, thanks to the elimination of the anode.

"We proved you can use the simplest setup to enable the best battery," Bai said.

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