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Researchers at the UCLA Samueli School of Engineering said they have developed a new way to evaluate rechargeable lithium metal battery performance by directly measuring the properties of the solid electrolyte interphase, or SEI — a thin film that forms inside batteries during operation and plays a critical role in lifespan and reliability. The findings could help chemical engineers more accurately assess and design battery materials for longer-lasting and safer energy storage systems.
The study, published in Proceedings of the National Academy of Sciences, focuses on treating the SEI as an active material rather than a passive byproduct of battery operation. The team developed a custom experimental setup that isolated the SEI and allowed researchers to quantify how well it conducts lithium ions while blocking electrons, properties that strongly influence charging behavior and degradation.
According to the researchers, this approach challenges conventional descriptions of the SEI as simply “stable” or “conductive.” Instead, the team established a quantitative figure of merit — referred to in the paper as the SEI cT number — that captures how efficiently the interphase supports ion transport and suppresses unwanted electron flow. This metric enabled more accurate predictions of battery performance in controlled laboratory cells.
The work builds on experimental measurements combined with modeling to better understand transport behavior within the chemically complex SEI layer. To achieve this, the researchers designed a specialized, separator-free battery cell in which the SEI itself functioned as the solid-state electrolyte, allowing direct application of established electrochemical measurement techniques. Developing a stable and reproducible system required several years of iterative design, according to the authors.
While the findings remain at a fundamental research stage and have not yet been demonstrated in commercial battery systems, the authors say the method provides a framework for systematically comparing SEI quality across different battery chemistries. Future research will focus on applying the metric to guide the development of improved interphases that enhance battery durability and charging performance.