Anode free Sodium batteries, could they be better than lithium ion batteris?

Lithium-ion batteries are the simplest we have as far as energy density and convenience go.

The Washington University in St. Louis lab of Peng Bai, professor within the Department of Energy, Environmental & Chemical Engineering within the McKelvey School of Engineering, has developed a stable sodium-ion battery that's highly efficient, are going to be less costly to form, and is significantly smaller than a standard lithium-ion battery thanks to the elimination of a once-necessary feature. The research was published May 3, 2021, within the journal Advanced Science.

A traditional lithium-ion battery consists of a cathode and anode, both of which store lithium ions; a separator to stay the electrodes separated on either side; and an electrolyte—the liquid through which the ions move. When lithium flows from the anode to the cathode, free electrons leave through the present collector to the device being powered while the lithium passes the separator to the cathode.

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

The smooth growth of sodium metal in action is that the ultimate quality check of the electrolyte for Bai and Ma's anode-free batteries. Credit: Peng Bai Lab

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

"We used old chemistry," Bai said. "But the matter has been, with this well-known chemistry, nobody ever showed this anode-free battery can have an inexpensive lifetime. They always fail very quickly or have a really low capacity or require special processing of the present collector."

Anode-free batteries tend to be unstable, growing dendrites—finger-like growths which will cause A battery to short or just to degrade quickly. This conventionally has been attributed to the reactivity of the alkali metals involved, during this case, sodium.

In this newly designed battery, only a skinny layer of copper foil was used on the anode side because the current collector, i.e., the battery has no active anode material. Rather than flowing to an anode where they sit until time to maneuver back to the cathode, within the anode-free battery the ions are transformed into a metal. First, the plate themselves onto copper foil, then they dissolve away when it is time to return to the cathode. "In our discovery, there are not any 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 luster. "This quite growth mode has never been observed for this type of alkaline metal."

"Observing" is vital. Bai has developed a singular, transparent capillary cell that gives a replacement thanks to check-out batteries. Traditionally, when A battery fails, to work out what went wrong, a researcher can open it up and take a glance. But that after-the-fact quiet observation has limited usefulness.

"All the battery's instabilities accumulate during the working process," Bai said. "What really matters is instability during the dynamic process, and there is no method to characterize that." Watching Ma's anode-free capillary cell, "We could clearly see that if you do not have good internal control of your electrolyte, you will see various instabilities," including the formation of dendrites, Bai said.

The spiky growth and picoliter bubble are the basis explanation for early failures in previous anode-free batteries. Credit: Peng Bai Lab

Essentially, it comes right down to what proportion water is within the electrolyte.

Alkali metals react with water, therefore the research team brought the water content down. "We were hoping just to ascertain an honest performance," Bai said. Watching the battery in action, the researchers shortly saw shiny, smooth deposits of sodium. It is the smoothness of the fabric that eliminates morphological irregularities which will cause the expansion of dendrites.

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

"Water content must be less than 10 parts-per-million," Bai said. Thereupon realization, Ma was ready to build not just a capillary cell, but a working battery that's similar in performance to a typical lithium-ion battery, but takes up much less space due to the shortage of an anode.

"Check your telephone. One-quarter of the value of such items comes from the battery," Bai said. Sodium batteries use a more common metal than lithium batteries; they need an equivalent energy density as lithium batteries; and that they are smaller and cheaper than lithium batteries, because of the elimination of the anode. "We proved you'll use the only set up to enable the simplest battery," Bai said.

Paper: DOI: 10.1002/advs.202005006