A new method allows researchers to look at battery electrodes that are bathed in wet electrolytes under a microscope; this mimics the conditions that would exist inside a real battery.
The recent study showed wet conditions are necessary for delicate observations, such as examining the solid electrolyte interphase layer (a coating on the electrode's surface that "influences" battery performance), a Pacific Northwest National Laboratory (PNNL) news release reported.
"The liquid cell gave us global information about how the electrodes behave in a battery environment," materials scientist Chongmin Wang of the Department of Energy's PNNL, said. "And it will help us find the solid electrolyte layer. It has been hard to directly visualize in sufficient detail."
When a battery charges electrons are "jammed"into the negative electrode; lithium or other metal ions meet the electrons and cling onto them. It is essential that the ions are able to fit within the electrode "pores."
When the battery is powered electrons rush out of the electrode; the positive ions "surge through the body of the battery and return to the positive electrode, where they await another charging," the news release reported.
The research team was able to view this "ebbing and flowing" of positively charged ions deforming electrodes.
When the ions squeeze themselves through an electrode's pores they cause it to swell and deform. This can eventually "wear down" the electrode.
Until now researchers could only observe the process through a transmission electron microscopes if the battery contained dry conditions (also known as a "open cells). In "real batteries" the electrodes are bated in liquid electrolytes that help the ions move.
The researchers built a battery smaller than a dime that had electrodes bathed in liquid electrolytes.
When the researchers looked at the battery under a microscope they saw the electrode swell. In dry cell observations one end of the electrode is attached to a lithium source, which causes the swelling to begin on one side. In the "wet" battery lithium could enter from anywhere, causing most of the electrode to swell at the same time.
"The electrode got fatter and fatter uniformly. This is how it would happen inside a battery," Wang said.
The team believes both wet and dry batteries can be used in the study of different aspects of battery material.
"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."
This study did not find the solid electrolyte interphase layer; in the future they will try to thin-out the electrolyte layer in hopes of unveiling new detail.
"The layer is perceived to have peculiar properties and to influence the charging and discharging performance of the battery," Wang said. "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."
