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Solid-Electrolyte Interphase can Help Develop Better Lithium-Ion Batteries

Thanks to a molecular eye of sorts, researchers are one step closer to understanding the kind of mechanisms that occur inside the batteries and make them susceptible to fire.

Army scientist Dr Kang Xu specializes in electrochemistry and materials science to develops innovative solutions for tomorrow’s Soldiers at the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory at Adelphi, Maryland. Image Credit: U.S. Army photo by Jhi Scott.

Researchers at the Army Research Laboratory of the U.S. Army Combat Capabilities Development Command collaborated with scientists from the Pacific Northwest National Laboratory of the U.S. Department of Energy to analyze chemical reactions that take place when two major parts of a battery interface. This phenomenon creates a crucial component in the battery, often called the solid-electrolyte-interphase, or SEI for short.

Interpreting the SEI’s chemistry and formation mechanism offers a solution to develop better batteries in the future, stated the scientists.

The latest study, which was facilitated by a method that behaves as a molecular eye, provides a dynamic picture of the structure and chemistry of SEI.

It is known that these properties have an impact on the batteries’ safety, cycle life, and charge-discharge rate, specifically at low temperature, stated Dr Oleg Borodin, Army scientist and a researcher with a team that focuses on electrochemistry.

SEIs are critical for battery properties but elusive to characterize. They dictate how fast a battery could be charged for Warfighters in order to improve operational capabilities as well as preventing slow and abrupt battery failure during mission. But like dark matters, everyone knows they exist but no one knows how they work.

Dr Kang Xu, Study Principal Investigator and Scientist, Army Research Laboratory, U.S. Army Combat Capabilities Development Command

Almost four years ago, ARL researchers started working with experts “who are not battery people,” added Xu, but have special expertise in sophisticated characterization methods. Xu stated that Army scientists outlined the fundamental challenges that are faced in interpreting SEIs, and requested for help.

This association led to a series of pioneering work in SEI, and a few of the outcomes have already been published in the Nature Chemistry journal in 2018 and the Nature Nanotechnology journal in 2019.

The new study performed by the researchers has appeared in the article titled “Real-time mass spectrometric characterization of the solid-electrolyte interphase of a lithium-ion battery,” published in Nature Nanotechnology, a peer-reviewed scientific journal, on January 27th, 2020.

This method, an in-situ liquid secondary ion mass spectrometry, was developed by researchers from the Pacific Northwest National Laboratory and the Environmental Molecular Sciences Laboratory.

To apply this method, the research team collaborated with Army researchers to find out how the chemicals work at the interface between the electrolyte and the electrode on a molecular level upon charging the battery in its first hour.

The researchers tracked the formation and chemistry variation of the SEI. The technique enabled the team to map the chemical reactions as they took place. When the researchers’ work was integrated with molecular dynamics simulations, it demonstrated something that had only been contemplated before.

During the initial charging of the battery, the battery creates an electric double layer at the electrolyte-electrode interface. The formation of this electric double layer results in fine chemical and structural differences of SEI.

These differences will ultimately govern the performance of the battery itself. The kind of molecular-level interpretation of the interface could act as an insightful guide to the efforts of Army researchers in developing more improved batteries.

This team noted that before any interphasial chemistry takes place (at the time of initial charging), the self-assembly of solvent molecules results in the formation of an electric double layer at the electrode-electrolyte interface.

The formation of the electric double layer is guided by the electrode surface potential and Li+. The ultimate interphasial chemistry is predicted by the structure of the electric double layer; specifically, the surface of the negatively charged electrode repels salt anions from the inner layer and leads to an inner SEI. This SEI is dense, thin, and naturally inorganic. This dense layer is responsible for insulating electrons—the principle functions of the SEI—and conducting Li+.

An organic-rich, electrolyte-permeable external layer emerges following the formation of the internal layer. The inner SEI layer, in the presence of a highly concentrated, fluoride-rich electrolyte, has an increased concentration of LiF. This is caused by the presence of anions in the electric double layer. Real-time nanoscale observations like these will help in designing more improved interphases for upcoming batteries.

For many years, researchers had attempted to examine the SEIs present in lithium-ion batteries, with reduced success, because a method that would enable them to visualize battery operations on a molecular level was not available. Such nanoscale visualizations on a molecular level are required to interpret the chemistry that occurs at the interface.

Earlier in 2017, Army scientists teamed up with the University of Maryland and, for the first time, created a lithium-ion battery. This battery utilizes a solution of salt and water as its electrolyte and reaches the 4.0 V mark. This voltage is required for household electronics such as laptop computers, but without the dangers of fires and explosives linked with certain non-aqueous lithium-ion batteries available in the market.

Most of the commercially-available batteries are different from the aqueous lithium-ion batteries developed by this research team. By understanding the SEI present in lithium-ion batteries, existing technology can be incrementally improved as an instant solution for a majority of Army applications.

The study was funded by the U.S. Department of Energy Office of Vehicle Technologies Advanced Battery Materials Research Program; laboratory-directed research and development programs at Pacific Northwest National Laboratory; the DOE Office of Science Joint Center for Energy Storage Research; the U.S. German Cooperation on Energy Storage; and the Environmental Molecular Sciences Laboratory—a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research.


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