Researchers in the United States have just developed a new kind of accumulator, sodium-free and anode-free. The discovery could lead to smaller, greener and cheaper batteries.
Researchers have been working on sodium-ion batteries for years as a promising alternative to lithium-ion batteries used in most electronic devices. In addition to being more abundant, and therefore cheaper, sodium (Na) has a largely reduced environmental impact compared to lithium. However, these batteries generally have a more limited capacity and service life.
In a new study published in the journal Advanced Science, researchers at the Washington University of Saint-Louis in the United States succeeded in creating a sodium battery by simply removing the anode. It is replaced by a copper sheet as a current sensor. Rather than being captured by the anode, the sodium ions are transformed into metal and then plated on the copper sheet. The charging phase of the battery dissolves this metal and the ions return to the cathode. Removing the anode will significantly reduce the size of the batteries.
A capillary cell unique in the world
A common problem in accumulators is the formation of dendrites, structures that grow on electrodes like tree branches and reduce performance, and can lead to short circuits. Prototypes of similar batteries had also suffered from gas bubbles, foams or disconnected metal particles.
To better understand this mechanism, the researchers created a unique tool, a transparent “hair cell”. Rather than making prototypes of accumulators, then destroying them to analyze the evolution of the internal structure, this capillary cell allows to follow the live operation. “All battery instabilities accumulate during operation,” said Peng Bai, who led the research. “ What really matters is instability during the dynamic process, and there is no method to characterize it.” The creation of this transparent tool made it possible to observe the formation of sodium metal in real time.
Reducing water content to reduce dendrites
Alkaline metals such as sodium react to water. By consulting the capillary cells, the researchers were able to realize that the drying of the electrolyte took longer. The secret to reducing the formation of dendrites and other irregularities has therefore been to reduce the amount of water in the electrolyte. “Water content must be less than 10 parts per million,” said Peng Bai.
This discovery may surprise with its simplicity. “We’ve proven that you can use the simplest solution to create the best battery,” says Peng Bai. The performance of this new battery is equivalent to conventional lithium-ion accumulators, with the same energy density. Without anodes, they will be smaller and cheaper and could impact future generations of devices, both smartphones and electric cars.