Scientists find out why lithium-ion batteries self-discharge over time. The cause is surprisingly simple and the problem is easy to fix. The importance of this discovery for the energy transition, e-mobility and all devices that use batteries as energy carriers is enormous.
Researchers at Dalhousie University in Halifax, Canada, have succeeded in finding the cause of self-discharge in lithium-ion batteries. The trigger is a simple component of the batteries that no one previously suspected. It’s probably easy to replace, and it could soon solve a problem the importance of which cannot be overestimated.
Lithium-ion batteries not only power electric cars, smartphones, notebooks and many other devices. They are also an important part of the energy transition, as emphasized by the Federal Institute for Materials Research and Testing (BAM). According to the BAM, batteries are also already being used as decentralized buffers in private homes and industrial companies to store the energy from wind or solar systems. In order to stabilize the power grid, there will be an increased need for such large stationary storage systems in the future.
To achieve this, not only must the safety and longevity of the batteries be improved, but the energy loss through self-discharge must also be reduced. Thanks to the Canadian researchers’ discovery, this problem can be considered practically solved. “It’s a small thing, but it can definitely help make batteries better,” Scinexx quoted Michael Metzger, senior author of the study.
Scientists have long suspected that so-called redox shuttles are to blame for the self-discharge of lithium-ion batteries, writes the science magazine. Redox stands for reduction-oxidation, which means that an oxidation is reversed. Instead of giving up electrons, electrons are gained. According to the battery forum Germany, redox shuttles actually serve to prevent dangerous chemical reactions caused by overcharging in lithium-ion batteries.
The redox shuttle responsible for the self-discharge is reduced at the cathode of the battery, absorbing an electron, writes “Scinexx”. Then it travels to the anode, where it is oxidized again by giving up the electron. “For every electron that is transported in this way from the negative to the positive electrode, a lithium ion also moves there and thus causes self-discharge,” explains study leader Sebastian Buechele.
The key questions the scientists asked themselves were which electrochemically active molecule it is and what the source of it is. In order to track down the culprit, they examined various rechargeable batteries that are also used in electric cars, smartphones and laptops, among other things.
It turned out that as the batteries heated up, their normally colorless electrolyte became increasingly discolored. According to “Scinexx”, at more than 25 degrees it turned slightly yellowish, then orange and finally at 70 degrees an intense dark red. At the same time, the researchers’ measurements indicated that a redox process was taking place. Using chemical analysis, Buechele and his team were finally able to identify the responsible molecule.
“According to our experiments, only dimethyl terephthalate (DMT) can be considered as a potential redox shuttle in the electrolytes,” say the scientists. Surprisingly, this is the organic compound based on polyethylene terephthalate (PET). The only question is how a plastic, from which plastic bottles and many other everyday objects are made, gets into the batteries.
It is not part of the active battery components, writes “Scinexx”. But in common battery cells, adhesive tape made of PET is used to hold the tightly rolled electrode layers together. “We never expected that because nobody pays attention to these inactive components,” says Metzger. “But it’s the chemical breakdown of this tape that creates the redox shuttle molecule.”
Thanks to this knowledge, it is now possible to solve the problem of self-discharge, according to the scientist. And the solution is very simple: “If you replace the PET adhesive tape with another, less susceptible material, there is no discharging DMT.” The researchers are already in contact with manufacturers who are understandably interested in the discovery.
