In 2019, the Nobel Prize in chemistry rewarded the American John B. Goodenough, the Briton Stanley Whittingham and the Japanese Akira Yoshino for their work on lithium-ion batteries . It is an understatement to say that the invention of this technology in the 1970s was a real revolution. Present in smartphones, laptops, connected watches, game consoles, two wheels or even electric cars, the lithium battery has offered our devices an autonomy almost as upsetting from a societal point of view as the invention of electricity.
Sodium, promises and limits
However, this small energy receptacle still poses several problems, starting with the materials it requires for its design: lithium and cobalt, rare and expensive raw materials, and which should become even more so in the face of the increase. constant demand (especially in the electric car industry). For these reasons, researchers are constantly busy in the lab to try to develop a viable alternative to the battery lithium ion . One of them is based on sodium salts, much cheaper and above all, present in abundance in the oceans and the earth’s crust.
If we consider it promising for many years, the sodium-ion battery still has limits, and not least: not only can it not store as much energy as a lithium battery but it also still supports too badly repeated charges. Over the cycles, small inactive sodium crystals end up accumulating at the cathode, and come to kill the device. A team of researchers from Washington State University (WSU) and the Pacific Northwest National Laboratory (PNNL, a public laboratory also located in Washington State), managed to get around these two obstacles, making for the first time with a sodium-ion battery, a device as efficient as a lithium battery.
Cathode and electrolyte, a crucial link
Yuehe Lin, professor of mechanical and materials engineering, and Xiaolin Li, researcher in materials electrochemistry, had the idea of creating a liquid electrolyte (the conductive substance present in batteries and which contains mobile ions) boosted in ions sodium. More salty, the mixture seemed to coexist better with the cathode, itself reworked with metallic oxides in a thin layer. The new formula proved to be a winner: with sodium ions able to move smoothly and continuously, more crystal crust formed at the cathode. After 450 charge cycles, their battery also demonstrated that it retained an 82% energy retention capacity.
"Our research has revealed the essential correlation between the evolution of the structure of the cathode and the interaction of its surface with the electrolyte," said Yuehe Lin. "These are the best results ever reported for a sodium-ion battery with a thin-film cathode. It is also proof that it is a viable technology, comparable to lithium-ion batteries."
Being now aware of this determining link between the cathode and the electrolyte, Yuehe Lin and Xiaolin Li will try to find even more harmonious combinations of materials. One of their goals is also to do without the famous cobalt. Rare and expensive, as we have seen, it is also mainly extracted in the Democratic Republic of Congo (DRC), a state which cultivates very little transparency in the mining economy and where the exploitation of children at work is legion.