Transmission Electron Microscopy Used To Get An Inside Look At Rechargeable Batteries
December 27, 2013

Researchers Develop Way To Study Battery Materials Under Realistic Conditions

redOrbit Staff & Wire Reports - Your Universe Online

By borrowing a page out of life scientists’ book, researchers have found a novel new way to analyze rechargeable batteries – by viewing the electrodes while they are bathed in wet electrolytes.

The technique, which mimics the conditions found within actual batteries, is similar to the ways in which life scientists use transmission electron microscopy to study wet environments. Their findings, which were detailed earlier this month in the journal Nano Letters, could prove helpful in the development of a less-expensive, longer lasting rechargeable battery, the study authors explained.

The project should also prove helpful for those studying battery materials under dry conditions, they added. The study demonstrated that many aspects of the energy-converting devices could also be investigated under dry conditions, which are far easier to use. However, wet conditions are vital to analyzing the hard-to-find solid electrolyte interphase coating, a layer which accumulates on the electrode's surface and has a dramatic impact on its performance.

“The liquid cell gave us global information about how the electrodes behave in a battery environment,” explained Chongmin Wang, a materials scientist with the Department of Energy's Pacific Northwest National Laboratory (PNNL) in Richland, Washington. “And it will help us find the solid electrolyte layer. It has been hard to directly visualize in sufficient detail.”

Despite the fact that electricity cannot be seen by the naked eye, charging a battery forces electrons into the negative electrode, where positively charged ions of lithium or some other metal flows towards them and attaches to them. The ions need to fit within pores inside the electrode, the researchers explained.

“Powering a device with a battery causes the electrons to stream out of the electrode. The positive ions, left behind, surge through the body of the battery and return to the positive electrode, where they await another charging,” PNNL said in a statement. “Wang and colleagues have used high-powered microscopes to watch how the ebbing and flowing of positively charged ions deform electrodes.”

As the ions enter the pores, it makes the electrodes swell, and they can become worn down after repeated use. Previous research designed to speed battery development had revealed that sodium ions leave behind bubbles that could interfere with the battery’s function.

Thus far, however, transmission electron microscopes have only been able to accommodate dry battery cells (also known as open cells). In actual batteries, liquid electrolytes bathe the electrodes, creating an environment that ions can easily move through. So Wang and his colleagues set out to develop a wet battery cell inside a transmission electron microscope at the Department of Energy’s Environmental Molecular Sciences Laboratory (EMSL).

“The team built a battery so small that several could fit on a dime. The battery had one silicon electrode and one lithium metal electrode, both contained in a bath of electrolyte,” PNNL explained. “When the team charged the battery, they saw the silicon electrode swell, as expected.”

“However, under dry conditions, the electrode is attached at one end to the lithium source – and swelling starts at just one end as the ions push their way in, creating a leading edge. In this study's liquid cell, lithium could enter the silicon anywhere along the electrode's length. The team watched as the electrode swelled all along its length at the same time,” the laboratory added.

According to Wang, the electrode swelled more and more uniformly, just like it would inside an actual battery. However, the total amount of size it gained was approximately the same whether the research team was working with a wet or dry battery cell – which suggests that either state would allow them to study some battery components.

“We have been studying battery materials with the dry, open cell for the last five years,” Wang said. “We are glad to discover that the open cell provides accurate information with respect to how electrodes behave chemically. It is much easier to do, so we will continue to use them.”