Wet conditions are required to examine the hard-to-locate solid electrolyte interphase layer, a coating that collects on the electrode's surface and greatly impacts battery performance.

According to the Department of Energy’s Pacific Northwest National Laboratory, researchers have come up with a technique to microscopically look at battery electrodes while they are bathed in wet electrolytes, imitating realistic conditions inside actual batteries.

The research revealed that many aspects of battery materials can be examined under dry conditions, which are much easier to utilize. However, wet conditions are required to examine the hard-to-locate solid electrolyte interphase layer, a coating that collects on the electrode’s surface and greatly impacts battery performance.

“The liquid cell gave us global information about how the electrodes behave in a battery environment,” noted Chongmin Wang of the Department of Energy’s Pacific Northwest National Laboratory in a statement. “And it will help us find the solid electrolyte layer. It has been hard to directly visualize in sufficient detail.”

Wang and his colleagues have utilized high-powered microscopes to observe how the ebbing and flowing of positively charged ions deface electrodes. Squeezing into the electrode’s pores makes the electrodes swell, and repeated use can wear them down. For instance, recent work revealed that sodium ions leave bubbles behind, potentially meddling with battery function.

Prior to this study, the transmission electron microscopes have only been able to accommodate dry battery cells, which researchers call open cells. In a real battery, electrodes are bathed in liquid electrolytes that offer an environment ions can easily travel through.

Wang and his colleagues created a wet battery cell in a transmission electron microscope at the DOE’s Environmental Molecular Sciences Laboratory. The researchers developed a battery so tiny that several could be placed on a dime. The battery contained one silicon electrode and one lithium metal electrode, both held in a bath of electrolyte.

When the researchers charged the battery, they observed the silicon electrode swell. However, under dry conditions the electrode is attached at one end to the lithium source — and swelling begins at just one end as the ions advance their way in, forming a leading edge. In this study’s liquid cell, lithium could invade the silicon anywhere along the electrode’s length. The researchers saw the electrode swell all along its length at the same time.

“The electrode got fatter and fatter uniformly. This is how it would happen inside a battery,” posited Wang.

“We have been studying battery materials with the dry, open cell for the last five years,” added Wang. “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.”

According to Wang, the researchers couldn’t observe the elusive solid electrolyte interphase later. However, the researchers plan to decrease the thickness of the wet layer by at least 50 percent to raise the resolution. This might offer enough detail to see the solid electrolyte interphase later.

“The layer is perceived to have peculiar properties and to influence the charging and discharging performance of the battery,” explained Wang. “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’s findings are described in detail in the journal Nano Letters.