What leads to self-discharge of batteries?

Low battery? We all know it – a battery-powered device is fully charged, switched off and put aside for a few days. But the next time it is used, the battery is no longer at 100 percent, but perhaps only at 98 percent. This leads to the question: what happened to that two percent? The answer is now given by the discovery of researchers at #Dalhousie #University in Halifax, Canada.

Sebastian Büchele , a research associate at the Institute for Applied Materials at the Karlsruher Institut für Technologie (KIT) , spent three months conducting research at Dalhousie University in Halifax, Canada, where the discovery was made. Also involved was Tom B?tticher , a master's student at WWU Münster and also a visiting scientist at Dalhousie University. Both were doing research in the research group of Professor Michael Metzger.

The big topic is #self-#discharge – but what do we actually mean by self-discharge of batteries? This can be explained quite simply – self-discharge of batteries is a self-processing process that causes batteries to discharge even though no electrical consumption is connected. An example that is familiar to many is the mobile phone: the charged device lies switched off in the drawer: the assumption would be that the battery remains at the same charge level, but it discharges itself even when switched off and thus loses energy. What is the reason for this? Exactly this mystery has now been solved. A so-called #redox #shuttle is responsible for the self-discharge. But what is a redox shuttle?

A redox shuttle is a normal chemical compound. However, it can migrate through the #electrolyte of cells and pick up an electron from an electrode, in this case the negative electrode. It is the first negatively charged, migrates through the electrolyte to the positive electrode and then donates the electron there. So basically it transports an electron from the negative electrode to the positive electrode, but not through the external circuit, but within the actual cell. The consequence of this is that a so-called charge balance must always happen. This means that if a negatively charged particle must follow from the negative electrode to the positive electrode, a positive particle must also follow, and in this case this is the lithium ion. When lithium ions go from the negative electrode, also called the anode, to the positive electrode, it means nothing else than that the battery cell is discharging.

The redox shuttle molecules were already known and used in science in lithium-ion cells, but mainly for overcharge protection. However, the fact that the redox shuttle molecules are generated in-sito was not known until now. It was precisely this redox shuttle molecule that was identified by Sebastian Büchele shortly before he returned to Germany. Mr. Büchele's successors Anu Adamson was finally able to find out through further experiments in the laboratory that this generated molecule comes from the adhesive tape in the battery cells. Classically, battery cells, such as the cylindrical cells, are rolled up like a sushi roll and taped to prevent them from unrolling. Until now, no one has considered that this adhesive strip could pose any problem in the cell, let alone that it participates in any way in the electrochemical reactions in battery cells. Through further research, the researchers found that this redox shuttle is DMT (dimethyl terephthalate), a monomer of PET, meaning it is a small component of this huge polymer chain that makes up PET. Infrared studies revealed that the tape used in the cells does indeed have characteristic patterns of PET. The tape connects the electrode materials and the separator so that they are close together, and these electrode materials and also the separator are soaked in the electrolyte. Basically, the compounds in the electrolyte are responsible for allowing the tape to decompose. So in short, the compounds that you find in the electrolyte react with the PET plastic tape, so it decomposes at high temperatures, mind you, to this DMT, which ends up being the identified redox shuttle.

Initially, the research group had two observations. First, the high self-discharge in LFP graphite cells, and later also in the NMC graphite cells, and at the same time it was observed that especially at high temperatures, i.e. when the cells were charged or discharged at high temperatures, the cells then also self-discharged particularly quickly. During further investigations, it was found that the extracted electrolyte had turned red, in some cases dark red.

Now the question arose - is there a connection between this self-discharge and the color of the electrolyte? To test this, cells were charged and discharged normally at different temperatures, and then the liquid electrolyte was removed from the cells. An experimental setup in which small coin cells were built with this extracted electrolyte provided the insight that there is indeed a correlation between colored electrolyte and self-discharge. The electrolyte was then analyzed in what is called a GC-MS (gas chromatography with mass spectrometry), and that ended up being the key analytical tool, because the gas chromatography mass spectrometry identified the DMT molecule responsible for self-discharge.

"We already know from battery manufacturers and large automotive manufacturers that they are addressing the issue and in some cases are already adapting their production to eliminate this problem," says Sebastian Büchele.

So an inconspicuous adhesive tape is to blame for the self-discharge of many batteries.

If you would like to listen to an interview (in German) with Sebastian Büchele and Tom B?tticher click below and watch the latest episode of GELADEN - der Batteriepodcast .