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

When it comes to batteries, lithium-ion are the best we have as far as energy density and convenience.

For now.

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

“We’ve found that the minimal is maximum,” Bai said. “No anode is the best anode.”

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

A traditional lithium ion battery consists of a cathode and anode, both of which store lithium ions; a separator to keep 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 current collector to the device being powered while the lithium passes the separator to the cathode.

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

The concept of replacing lithium with sodium 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, growing dendrites — finger-like growths that can cause a battery to short or simply to degrade quickly. This conventionally has 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 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, in the anode-free battery the 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 is smooth, with a metal luster. “This kind of growth mode has never been observed for this kind of alkali metal.”