In the ongoing search for materials for new and improved batteries, sodium as a cheap and widely abundant option ticks some important boxes, but bringing these experimental designs up to speed is far from a straightforward undertaking. A new design out of Washington University in St Louis demonstrates a promising path forward, by having a thin layer of copper fulfill the role of one of the electrodes, resulting in a battery that is significantly smaller and less expensive to produce, without compromising on performance.
A typical lithium-ion battery features two electrodes, called the cathode and the anode, which pass ions in an electrolyte solution to one another as the device is charged and discharged. For some time, scientists have explored how sodium metal can replace lithium, with some promising advances being made, and also how these designs might be able to work without the anode component.
“We used old chemistry,” Peng Bai, who led the research. “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.”
Bai and his team believe they have come up with a solution to these shortcomings, by doing away with the anode and incorporating a thin layer of copper foil instead. This sits on the anode side of the battery's current collector, which gathers the free electrons as the battery is discharged and channels them into the device being powered.
As this experimental battery is charged, instead of the ions passing from the cathode through the separator material to the anode, they plate themselves onto the copper foil and transform into shiny, smooth metal. Then as the battery is discharged, the material dissolves away and the ions are returned to the cathode.
The scientists were able to observe their battery's performance in real time through a purpose-built, transparent capillary cell, which allowed them to spot instabilities and the formation of fatal, tree-like structures called dendrites, which cause batteries to short and fail. This enabled the team to make adjustments, such as reducing the water content in the electrolyte to limit the reactions with the sodium alkali metal, which can normally lead to dendrites.
“All of the battery’s instabilities accumulate during the working process,” Bai says. ”What really matters is instability during the dynamic process, and there’s no method to characterize that. We could clearly see that if you don’t have good quality control of your electrolyte, you’ll see various instabilities."
After ironing out these kinks in their see-through capillary cell, the researchers were able to build a proper working version of their anode-free sodium battery. Despite the lower cost owing to the use of sodium, and the smaller size owing to the elimination of the anode, they report the device had similar performance to a conventional lithium-ion battery.
“We’ve found that the minimal is maximum,” Bai said. “No anode is the best anode.”
The research was published in the journal Science Advances.
