Gary Zhou 
A potentially crucial reason behind lithium-ion batteries’ tendency to self-discharge has seemingly been discovered by researchers at the Tesla-funded battery research center at Dalhousie University. What’s quite remarkable was that the group’s findings were amazingly simple yet potentially profound.
Electronic products such as smartphones and laptops tend to self-discharge over time. It’s an all-too-familiar scenario, where a device loses battery charge despite not being used for a period of time. But while this is considered normal today, researchers from Dalhousie University may have discovered the culprit for self-discharging lithium-ion batteries.
Dr. Michael Metzger, an assistant professor and the Herzberg-Dahn chair and in the Department of Physics and Atmospheric Science at Dalhousie University, noted that a commercial tape that holds electrodes together in lithium-ion batteries could be a key contributor to the self-discharging process.
“In commercial battery cells, there is tape — like Scotch tape — that holds the electrodes together, and there is a chemical decomposition of this tape, which creates a molecule that leads to the self-discharge. In our laboratory, we do many highly complex experiments to improve batteries, but this time we discovered a very simple thing. It’s a very simple thing — it is in every plastic bottle, and no one would have thought that this has such a huge impact on how the lithium-ion cells degrade,” Dr. Metzger said.
To understand lithium-ion battery cells and their self-discharging behaviors, Dr. Metzger and his team opened up several cells and exposed them to various temperatures. To their surprise, the team found that the electrolyte solution in the cell was bright red. Exploring further, the team placed cells with common electrolyte solution into ovens at four different temperatures. Four different oven temperatures were used, ranging from 25°C to 70°C. The cell sample at 25°C remained clear, while the sample at 55°C turned light brown, and the one at 70°C became blood red. The team then performed a chemical analysis to examine the composition of the electrolyte.
Following are the team’s observations.
“That’s when (the researchers) found that the polyethylene terephthalate, or PET, in the tape decomposes and creates the molecule that leads to the self-discharge. The molecule is called a redox shuttle because it can travel to the positive side of the electrode, then to the negative side and then back to the positive side. So, it shuttles between the electrodes and that creates the self-discharge, just like lithium is supposed to do. The problem is that the shuttle molecule is doing it all the time in the background, even when no lithium is supposed to move when the battery just sitting there.”
“It’s something we never expected because no one looks at these inactive components, these tapes and plastic foils in the battery cell but it really needs to be considered if you want to limit side reactions in the battery cell,” Dr. Metzger said.
Dr. Metzger and his team’s findings can be found below.
