Yaobin Xu inserts a sample into a transmission electron microscope to examine the function of a rechargeable battery. Credit: Andrea Starr | Pacific Northwest National Laboratory
Recent research led by a team at the Department of Energy’s Pacific Northwest National Laboratory has shown that solid electrolyte interphase (SEI) is not an insulator as it was previously believed. The findings show that SEI actually behaves similarly to a semiconductor, explaining the long-standing question of how SEI functions during battery operation.
SEI is a filmy moss-like buildup that occurs inside rechargeable batteries and was previously believed to be a source of performance loss. In the study, published in Nature Energy, the researchers conducted the first direct measurement of the electrical properties at the boundary separating the solid electrode and the liquid electrolyte in a rechargeable battery.
The SEI layer at the focus of the research is thinner than a sheet of tissue paper. The layer forms during a battery's first charging cycle and ideally maintains its stability throughout the lifespan of the battery. However, when viewing the internals of an aging battery, a large buildup is often seen on the negative electrodes. This buildup was assumed to be a driver of battery performance loss.
The researchers developed a novel technique combining transmission electron microscopy with nanoscale manipulation of microfabricated metal needles within the microscope to directly analyze the electrical conduction across the SEI. The researchers used the method to analyze the SEI properties in copper or lithium metal with four different electrolytes. In all cases, the SEI layer leaked electrons as the voltage increased, signifying that the layer is semi-conductive.
The researchers later concluded that the organic compounds within the SEI layer are responsible for electron leakage and that reducing organic components within SEI would enable a longer lifespan of rechargeable batteries.