Researchers Find What Really Happens in a Battery

For the first time, researchers have used a novel technique to study battery electrodes in action.

Materials scientist Chongmin Wang of the Department of Energy's Pacific Northwest National Laboratory and colleagues used transmission electron microscopy to study electrodes immersed in electrolytes.

Until now, scientists based their research only on electrodes present in dry conditions. The present research can change the way rechargeable batteries are studied by providing a new way of monitoring them in their actual, wet environment.

Although many aspects of a rechargeable battery can be studied under dry conditions, some factors can only be found when the electrodes in the battery are bathed in an electrolyte, researchers said. The study could also pave way for development of long-lasting rechargeable batteries.

"The liquid cell gave us global information about how the electrodes behave in a battery environment," said Wang. "And it will help us find the solid electrolyte layer. It has been hard to directly visualize in sufficient detail."

Researchers said that the technique can be used to study solid electrolyte interphase layer, which is a 'mystery layer' that can be found only in wet conditions. This layer builds up on an electrode's surfaces and affects the battery's performance.

Stop and Go

When a battery is charged, positive lithium ions rush to meet electrons on the negative electrode. These lithium ions fit within the electrode.

As electrons are exhausted during battery use, positive ions are left behind on the electrode. These ions then get back to the positive electrode and wait for the battery to be charged once again.

Powerful microscopes used by Wang and colleagues showed that the movement of these ions damage the electrodes. The positive ions that get into the pores of the electrode fatten them up, resulting in greater wear and tear.

Previous research has shown that in Sodium-Ion Batteries, the ions leave bubbles in the electrodes, disrupting battery function.

TEM is used for studying microstructure of materials. However, TEM has been solely used in the study of dry batteries.

In the present study, Wang and colleagues built a tiny battery made of one small silicon electrode and a one lithium metal electrode. Both electrodes were kept in an electrolyte bath.

The team then charged the battery. They found that under dry conditions, the electrode swelled at just one place, which was expected. However, when the electrodes were placed in an electrolyte, the swelling occurred throughout the electrode.

"The electrode got fatter and fatter uniformly. This is how it would happen inside a battery," said Wang.

Wang added that the team plans to further study the solid electrolyte interphase.

The layer is perceived to have peculiar properties and to influence the charging and discharging performance of the battery," Wang said in a news release. "However, researchers don't have a concise understanding or knowledge of how it forms, its structure, or its chemistry. Also, how it changes with repeated charging and discharging remains unclear. It's very mysterious stuff. We expect the liquid cell will help us to uncover this mystery layer."

The study is published in the journal Nano Letters.