Charge carrier release in lithium ion batteries - DMT

In February of this year, a report from an American research institute shook the LION battery world. The reason for the slow, unintentional self-discharge of lithium-ion batteries had been found: a polyester adhesive tape that is not actually involved in the chemical reaction and is used, for example, to seal the film wraps or film stacks inside the battery. The study concludes that DMT (dimethyl terephthalate, a precursor or basic material for the production of polyesters) is the charge carrier that causes the self-discharge.
2 June, 2023 By
Charge carrier release in lithium ion batteries - DMT
CMC Klebetechnik GmbH, Herr Gerald Friederici

In addition to other processes such as thermal ageing, mechanical stress or environmental influences such as UV, organic insulation materials can of course also be chemically damaged. Polyester film is known to be a very chemically resistant product and is therefore used in countless applications in electrical engineering. It has good temperature resistance, high tensile strength and is resistant to numerous solvents, bases and alkalis. However, due to the manufacturing process (polymerization), polyester plastic is not particularly resistant to hydrolysis.

During hydrolysis, the long polymer chains of the polyester (PET; polyethylene terephthalate) are broken down by the polar water (H2O). Slowly but surely, this causes a polyester film to lose its mechanical strength, for example (clearly noticeable in the hydrolysis test at 85°C/85% rH after just 1,000 hours).

Application technology

Gerald Friederici

If you have any questions, please contact:

Gerald Friederici

Phone: +49 6233 872 356

friederici (at)

Polyester resins are produced using two common methods: the most common is via pure terephthalic acid as a terephthalate monomer. The less common way today is via DMT (dimethyl terephthalate) - the chemical substance discovered by the American research institute as the cause of self-discharge. However, DMT is not contained in the finished polymerized polyester. DMT can only be produced as a degradation product through chemical decomposition, regardless of the manufacturing process. 

In addition to hydrolysis (transesterification and depolymerization by water), there is also methanolysis. This process is used in plastics recycling: in methanolysis, the polyester structure is broken down at 180 to 280°C and further processed into fuel substitute products, for example.

It stands to reason that the electrolyte solutions used in most lithium-ion batteries, which have a high methanol content, will cause such methanolysis to occur over time, even at lower temperatures.

The solution: the CMC 61100 PEN adhesive tape from CMC Klebetechnik

The CMC 61100 special adhesive tape for battery applications offered by CMC Klebetechnik therefore does not use the polyester films commonly used for thermal class B as a film carrier. Instead, a related, higher-quality PET plastic is used: polyethylene naphthalate (PEN). PEN is produced via the methyl ester route and is considerably more resistant to hydrolysis. PEN also resists methanolysis much better than PET. This is why CMC Klebetechnik only uses PEN films for applications in lithium-ion batteries. These are mechanically and electrically identical to comparable polyester films and allow a 1:1 replacement without the need for design or process changes. After qualification of the adhesive tape, rolls of CMC 61100 PEN film adhesive tape can seamlessly replace the previously used polyester adhesive tape.

In addition, the PEN adhesive tape CMC 61100 is also a perfect alternative to adhesive tapes with more hydrolysis/methanolysis-stable plastics in terms of price.

Photo: Lithium-ion polymer rechargeable batteries (abbreviated as LiPo, LIP, Li-poly). isolated on white. iStock. Teodor Costachioiu