March 29, 2023

Why do batteries discharge themselves? The teacher finds out by accident

Why do batteries discharge themselves?  The teacher finds out by accident

Image: Bram Van Oost/Unsplash/Reproduction

Another one from the “It’s rare, but happens often” series: you leave your notebook fully charged in standby mode, and when you come back, you see that it has lost some of its battery. Why does it happen? A professor from Canada claims to have discovered the causes of self-discharge in lithium-ion batteries – and, best of all, by accident.

The answer to this question was an old curiosity of Professor Michael Metzger, from the Department of Physics and Atmospheric Sciences at Dalhousie University, Canada.

The first idea was to open several batteries and place them at different temperatures. He believed that external heat could affect the durability of loads. But in the process, he noticed something unexpected: he saw that, on the inside, all batteries retain a red electrolyte solution.

This solution is the ‘transport medium’ for the ions circulating through the battery – in practice, it is what allows electricity to be carried. As the most common solution is colorless, scientists began to explore the reasons for the appearance of the red hue.

The team decided to place the batteries with electrolyte solution in ovens at different temperatures. When at 25°C, the liquid remained clear. At 55ºC, it turned light brown. At the highest temperature, 70ºC, it became red blood.

In chemically analyzing the electrolyte, Metzger discovered that PET (polyethylene terephthalate) films—yes, the plastic used in soda bottles—were responsible for the coloring. In a battery, the cell is in the adhesive strips that are responsible for holding the electrodes together.

It is this element that breaks down in the chemical solution strip and thus creates a molecule that causes the batteries to discharge “on their own”.

The PET molecule results from the redox process: it travels from the positive to the negative side of the electrode without stopping, which makes the batteries “keep on working” even when there is no activity.

“It’s something we never expected because nobody looks at these inactive components, these plastic strips and foils in the battery cell,” Metzger said in a statement. “But they really have to be considered if you want to limit side reactions to the load.”

In addition to curiosity, this discovery is also commercially important: a fix could create batteries that do not discharge and thus become more durable.

“It’s a very simple thing, it’s in every plastic bottle and nobody would have thought it would have such a big impact as lithium-ion cells degrade,” the professor pointed out.

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