100, 99, 98, 97 percent – even when laptops and smartphones are switched off, batteries lose a small part of the stored electricity. Canadian scientists led by Michael Metzger from Dalhousie University in Halifax, Canada, have now found a cause for this undesirable self-discharge of lithium-ion batteries. They present them in the “Journal of The Electrochemical Society”: Plain adhesive tape that holds the electrodes together decomposes when heated and forms so-called redox shuttles. These molecules can accept electrons at the cathode and transport them to the anode. This slowly but steadily reduces the charge level of the batteries.
In numerous experiments, the researchers examined commercially available lithium-ion batteries based on lithium iron phosphate (LFP) and nickel manganese cobalt oxide (NMC811). They noticed continuous self-discharge as the temperature increased between 25 and 70 degrees Celsius. They were surprised at the strong red coloration of the liquid electrolyte. This liquid remained clearly transparent at room temperature, turned slightly brownish at 55 degrees and finally blood-red at 70 degrees.
View of the inactive components of a battery
Chemical analyzes revealed the cause of the coloration. It was the molecule dimethyl terephthalate (DMT), a decomposition product of the widespread, cheap and stable plastic polyethylene terephthalate (PET). However, PET is not used in the electrochemically active components of a battery. But Metzger and colleagues found what they were looking for in the adhesive tape that held the individual electrodes of the batteries tested together. “We never expected that. Because no one has ever looked at the inactive components of a battery,” says Metzger.
This discovery could be of great importance not only for consumers but also for the manufacturers of lithium-ion batteries. Metzger suspects that PET adhesive films are used in many batteries today. However, the manufacturers themselves do not disclose such details of their production processes. “Therefore, we are currently opening commercial batteries of all available types in our laboratory and analyzing the chemical composition of the adhesive films,” says Metzger. His first impression: “PET is surprisingly common.”
Simple solution against self-discharge
The scientists intend to publish detailed results of this series of studies very soon. By then, at the latest, you can be sure of the attention of the various battery manufacturers. But Metzger’s experiments also showed that suppressed the formation of the undesired redox shuttles by adding some vinylene carbonate to the liquid electrolyte becomes.
If this simple solution does not suffice, it is not unlikely that the production lines will be switched to adhesive films made from other, less critical plastics. It is not yet clear which materials these will be exactly. Responsible manufacturers will hardly do without numerous preliminary tests in order not to be surprised again by unnoticed interactions between a simple packaging material and the functional part of a battery.
(jle)
