UCLA Chemical Engineers Develop Method to Quantify Overlooked Battery Component’s Performance

Published in the Proceedings of the National Academy of Sciences, the study was led by Yuzhang Li, an associate professor of chemical and biomolecular engineering at UCLA Samueli.

The new method measures how ions and electrons travel through the solid electrolyte interphase, which until now has been described simply as conductive or stable without examining how it affects battery performance. Using a custom setup that isolated the SEI, the researchers established a quantitative figure of merit for SEI performance.

“This changes how scientists understand and design batteries, showing that the solid electrolyte interphase isn’t just a passive layer but is actually a crucial, active component,” said Li, who is also a member of the California NanoSystems Institute (CNSI) at UCLA. “Our work is the first to treat the SEI as a functional solid-state electrolyte and to quantify its key properties: how well it conducts ions, blocks electrons and selects lithium ions. This knowledge could lead to safer, longer-lasting batteries.”

In a typical rechargeable battery, the electrolyte sits between the positive electrode, or cathode, and the negative electrode, or anode. The electrolyte conducts positive ions between the electrodes while blocking electrons, which flow through the external circuit. As the battery operates, the SEI forms on the anode where it contacts the reactive electrolyte. This thin film allows lithium ions to pass through but blocks electrons from entering the anode. SEI structures vary from battery to battery and can have major impacts on battery lifespan and charging capabilities.

Despite the important role the SEI plays, the film’s dynamic and chemically complex nature has made it difficult for scientists to quantify its effects on battery performance. To overcome this challenge, the team developed a highly specialized battery, with the SEI itself acting as the solid-state electrolyte. This allowed the researchers to apply existing measurement techniques directly to the SEI. Designing a separator-free cell that could stably and reproducibly measure SEI transport properties was a major technical hurdle that took nearly four years of trial, error and redesign. The result was a novel quantitative figure of merit, called the SEI cT number, that measures the SEI’s key properties. The new metric captures how efficiently the SEI conducts ions and blocks electrons, enabling highly accurate predictions of battery performance.

While the findings are currently at a fundamental laboratory research stage and have been demonstrated only in highly controlled cell experiments rather than in commercial or regulatory settings, the ability to characterize SEIs offers scientists a roadmap to develop better batteries with more effective interphases.

“This project started from a simple question, ‘What if we treated the SEI like a real material, not just a byproduct?’ It evolved into a multi-year collaboration and serves as a reminder that even the most familiar concepts in science can still surprise us when we look at them in a new way,” Li said.

The paper’s co-lead authors are Bo Liu and Dingyi Zhao, both doctoral students in Li’s research group at UCLA. The paper’s other authors include Katelyn Lyle, Xintong Yuan, Po-Hung Chen, Xinyue Zhang, Jin Koo Kim, Tian-Yu Wang, Haoyang Wu, Chongzhen Wang, Jiayi Yu, Keyue Liang, Jung Tae Kim and Kaiyan Liang — all current or former members of Li’s lab. The research was funded by the U.S. Department of Energy’s Office of Basic Energy Sciences through its Electron and Scanning Probe Microscopies Program. Some experiments were performed at the Electron Imaging Center for Nanosystems at CNSI.