Battery researchers find possible reason for self-discharge

Researchers at the Tesla-funded battery research centre at Dalhousie University have discovered an amazingly simple reason why lithium-ion batteries self-discharge over time, even when they are not in use: The use of adhesive tape.

In lithium-ion battery cells, coiled electrodes of these batteries are held together with simple PET adhesive tape. At higher temperatures in the battery, this plastic decomposes to form dimethyl terephthalate (DMT) – and this molecule acts like an “electrochemical shuttle” that discharges the battery. The finding could lead to a solution in which the PET tape is possibly replaced by a more stable material that does not decompose.

DMT acts as a so-called redox molecule. It reduces at the cathode of the battery (the “red” part of the name), picking up an electron in the process. At the anode, the molecule oxidizes (the “ox” part), releasing the electron.

In numerous test series with NMC batteries, researchers found that the electrolyte, which is actually colourless, became increasingly discoloured at temperatures above 25 degrees – the warmer it got, the darker the liquid became. The darker – from yellowish to orange to dark red – the electrolyte became, the more frequently the active redox reaction could be detected.

DMT is a component of the widely used plastic PET – which was present in the battery cells studied only in the form of adhesive tape. “It’s something we never expected because no one looks at these inactive components,” says senior author Michael Metzger of Dalhousie University. He calls it a “commercially relevant discovery” because now this problem can be fixed with a different material.

The only problem is that not every cell format rolls up the electrodes and then fixes them with a PET-containing adhesive tape. This is not necessarily the case with pouch cells or prismatic cells. As Metzger states, the team is already in contact with battery manufacturers to inform them about this small but effective change.

dal.ca, iopscience.iop.org, iopscience.iop.org