- Lithium-ion batteries lose their charge over time , even when not in use
- Responsible for this is an adhesive tape made of PET plastic , which releases dimethyl terephthalate (DMT) when heated, thereby triggering a redox shuttle
- If you use an alternative material instead , the batteries no longer self-discharge
Lithium-ion batteries lose their charge over time, even when not in use. Researchers have now discovered the reason for this self-discharge and developed a solution to the problem.
Halifax (Canada). Lithium-ion batteries have a high energy density, a long service life and are relatively light in weight. Unfortunately, the batteries used in electric cars and smartphones, for example, experience a gradual loss of performance due to lithium deposits. Another problem with lithium-ion batteries is that they lose their charge over time, even when they are not in use.
In science , there has long been the assumption that this self-discharge is caused by a so-called redox shuttle. This is a process in which an electrochemically active molecule is reduced at the cathode and absorbs an electron. The molecule then migrates to the anode, where it oxidizes again, releasing electrons.
“For every electron that is transported from the negative to the positive electrode in this way, a lithium ion also moves there, causing the self-discharge.”
Which molecule causes the self-discharge?
Researchers led by Sebastian Buechele from Dalhousie University have now investigated which molecule causes self-discharge in lithium-ion batteries and why the molecular shuttle occurs.
According to their publication in the Journal of The Electrochemical Society , the scientists have studied in detail different types of lithium-ion batteries, including NMC batteries used in mobile devices. As part of their study, they exposed the batteries to different temperatures, analyzed the electrochemical behavior and the chemical composition of the individual components.
Electrolyte changes color when heated
During the analysis, Buechele’s team discovered that a normally colorless electrolyte became increasingly discolored as the batteries warmed up. At a temperature above 25 degrees Celsius, it turned yellowish, then orange, and at 70 degrees it was intensely dark red. At the same time, the electrochemical measurements provided evidence of an active redox process. A subsequent chemical analysis using gas chromatography-mass spectrometry showed which molecule was responsible for these processes.
PET plastic cause redox shuttle
According to Buechele, the result was surprising.
“According to our experiments, only dimethyl terephthalate (DMT) can be considered as a potential redox shuttle in the electrolytes.”
DMT is an organic compound with a benzene ring that forms the basic unit of the plastic polyethylene terephthalate (PET). The self-discharge of lithium-ion batteries is therefore caused by a component of the plastic.
However, it was still unclear how the PET plastic gets into the lithium-ion batteries because it is not used in any active battery components. Scientists eventually discovered that many battery cells use an adhesive tape to hold the electrode layers together, which is made of PET, explains Michael Metzger.
“We never expected that because nobody pays attention to these inactive components. But it is the chemical breakdown of this tape that creates the redox shuttle molecule.”
The study thus shows that a simple adhesive tape triggers self-discharge in lithium-ion batteries.
Heat degrades PET plastic tape
When the battery heats up, a chemical reaction occurs that breaks down the PET plastic of the tape. This releases DMT, which becomes the electrochemical shuttle and slowly drains the battery.
“This is a commercially significant discovery. It’s a small thing, but it can definitely help make batteries better.”
Replacing the PET tape with another material that does not release discharging DMT can fix the discharging problem. The scientists are currently in contact with various battery manufacturers.
“One of the engineers said to me, ‘I heard you guys spotted a problem with the PET tape’. I then explained to him that this is the cause of the self-discharge and asked him what they use in their battery cells. The answer was: PET strap.”
Journal of The Electrochemical Society, doi: 10.1149/1945-7111/acaf44