A chance discovery in a Canadian lab could help extend the life of laptop and phone batteries.
According to scientists from Dalhousie University in Halifax, common adhesive tape in batteries may be the reason many devices lose some of their power while off or not being used, which is a phenomenon known as self-discharge.
"In our laboratory we do many highly complex experiments to improve batteries, but this time we discovered a very simple thing," Michael Metzger, an assistant professor in Dalhousie University's physics and atmospheric science department, said in a news release. "In commercial battery cells there is tape — like Scotch tape — that holds the electrodes together and there is a chemical decomposition of this tape, which creates a molecule that leads to the self-discharge.""
The solution is simple too, Metzger says: replace the polyethylene terephthalate, or PET, plastic tape commonly used inside batteries with something more durable and stable.
"It's a commercially relevant discovery," Metzger said. "It's a small thing but it can definitely help improve battery cells."
Metzger and his team have been trying to understand why lithium-ion battery cells in inactive devices tend to lose some of their power and self-discharge; something that has long frustrated and puzzled consumers and manufacturers alike.
The electrodes that power batteries are held in an electrolyte solution that is usually a form of lithium. After exposing several battery cells to different temperatures, researchers were surprised to see that electrolyte solution had turned bright red when it normally should be clear, which was something they had never encountered.
Chemical analysis of the red electrolyte solution revealed that at higher temperatures, a new molecule had been created inside the battery through the decomposition of common PET adhesive tape, which is often used to hold components together inside batteries. Strong and lightweight, PET is also frequently used for plastic packaging, drink bottles, clothing fibres and more.
Researchers realized that the red molecule, dimethyl terephthalate, was acting as a redox shuttle – meaning that it can transport electrons between a battery's positive and negative electrodes, creating self-discharge and depleting power even when a battery is not in use. Ideally, the shuttling of electrons within a battery should only happen when a device is on.
"It's a very simple thing – it is in every plastic bottle and no one would have thought that this has such a huge impact on how the lithium-ion cells degrade," Metzger said. "It's something we never expected because no one looks at these inactive components, these tapes and plastic foils in the battery cell but it really needs to be considered if you want to limit side reactions in the battery cell."
The findings are outlined in a pair of studies published on Jan. 20 and Jan. 23 in the Journal of The Electrochemical Society. Metzger said the research has been attracting attention from companies eager to improve battery performance.
"The self-discharge is a super important metric for them," Metzger said of a U.S. company he visited. "One of the engineers said, 'I heard you guys found out something is wrong with PET tape.' So, I explained to him that it's causing this self-discharge and asked him, 'What are you using in your cells?' He said, 'PET tape.'"
