Scientists at the U.S. Army Combat Capabilities Development Command's Army Research Laboratory worked alongside researchers from the U.S. Department of Energy's Pacific Northwest National Laboratory in this study, which looked at the chemical reactions that take place when two key components of battery interface, forming a critical component known as SEI, or solid-electrolyte-interphase.
Being able to understand the formation mechanism of this SEI is, the researchers say, crucial to unlocking future ‘better’ batteries. Using a technique that serves as a sort-of ‘molecular eye’, this research presented a dynamic picture of the chemistry and structure of SEI.
A diagram of the solid-electrolyte-interphase studied by U.S. army researchers for a better understanding of battery chemical and internal structures. Image Credit: U.S. Army.
What is SEI?
According to Army scientist Dr Oleg Borodin, a researcher working on the study, these properties are known to influence a battery’s charge-discharge rate, especially at lower temperatures.
“SEIs are critical for battery properties but elusive to characterize,” said Dr Kang Xu, a principal investigator on this research project. “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.”
The SEI is a protective layer formed on a Li-ion battery’s negative electrode. It forms as a result of electrolyte decomposition primarily during the battery’s very first cycle. Many of a battery’s properties – performance, charge ‘loss’, rate capability, safety, and cyclability – are all highly dependent on the quality of this SEI layer, and therefore understanding the nature and composition of SEI is of prime interest to researchers.
If SEI’s formation and how it has an effect on battery performance can be understood, SEI could be engineered to dramatically improve Li-ion battery performance.
U.S. army researcher Dr. Kang Xu, a member of the team involved with researching Li-ion batteries with the goal of providing identical energy density to current batteries. Image Credit: Conrad Johnson.
The Joint Army-Department of Energy Study
The research used a newly developed method known as in-situ liquid secondary ion mass spectrometry. To apply this method to their research, the team at Pacific Northwest collaborated with Army researchers to discover how the chemicals work at the interface between the electrolyte and electrode on a molecular level when a new battery is charged for its first hour.
The researchers tracked the formation and chemistry of the SEI, allowing them to map the reactions as they took place. They discovered that during its initial charge, the battery created an electric double layer at the electrolyte-electrode interface, the formation of which results in fine chemical and structural differences of SEI. Ultimately, these differences influence the battery’s performance and could act as a guide to the development of improved batteries.
The study was funded by the U.S. Department of Energy Office of Vehicle Technologies Advanced Battery Materials Research Program and others.