Researchers have developed a way to microscopically view battery electrodes while they are bathed in wet electrolytes（电解质）, mimicking realistic conditions inside actual batteries. While life sciences researchers regularly use transmission electron microscopy to study wet environments, this time scientists have applied it successfully to rechargeable battery research. The results, reported in December 11's issue of Nano Letters, are good news for scientists studying battery materials under dry conditions. The work showed that many aspects can be studied under dry conditions, which are much easier to use. However, wet conditions are needed to study the hard-to-find solid electrolyte interphase（固体电解质膜） layer, a coating that accumulates on the electrode's surface and dramatically influences battery performance.

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

Ebb, Flow, Swell

Even though electricity seems invisible, storing and using it in batteries has some very physical effects. Charging a battery jams electrons into the negative electrode, where positively charged lithium ions (or another metal ion such as sodium) rush in to meet and hold onto the electrons. Those ions have to fit within pores within the electrode.

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.

Wang and colleagues have used high-powered microscopes to watch how the ebbing and flowing of positively charged ions deform electrodes. Squeezing into the electrode's pores makes the electrodes swell, and repeated use can wear them down. For example, recent work funded through the Joint Center for Energy Storage Research--a DOE Energy Innovation Hub established to speed battery development--showed that sodium ions leave bubbles behind, potentially interfering with battery function.

But up to this point, the transmission electron microscopes have only been able to accommodate dry battery cells, which researchers refer to as open cells. In a real battery, electrodes are bathed in liquid electrolytes that provide an environment ions can easily move through.

So, working with JCESR colleagues, Wang led development of a wet battery cell in a transmission electron microscope at EMSL, the DOE's Environmental Molecular Sciences Laboratory on the PNNL campus. 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.