Modulating interphasial chemistry through PEI/PI separator coating for thermally robust high-voltage batteries

Operating lithium-ion batteries at high voltages is essential for achieving the higher energy densities required in emerging applications ranging from electric vehicles to compact consumer electronics. However, pushing conventional LCO cathodes beyond 4.5 V, especially under elevated temperatures, often leads to accelerated electrolyte oxidation, unstable CEI, and rapid performance decay. These challenges have limited the widespread implementation of high-voltage cells despite their clear advantages in energy storage capability.

A research team led by Feng Pan, from Peking University Shenzhen Graduate School, School of Advanced Materials, has developed a new separator-based strategy to address these issues. Instead of solely relying on electrolyte additives or cathode surface engineering, the team designed a PEI/PI-coated functional separator capable of actively modulating Li⁺ solvation structures and directing the formation of a more robust CEI. Their study outlines how this gradient-functional separator enhances thermal stability, improves interfacial chemistry, and enables long-term cycling of lithium-ion cells at 4.6 V and 60 °C—conditions under which conventional separators fail rapidly.

The team published their work in Energy Materials and Devices on 24 Oct, 2025.

The work demonstrates that PEI/PI’s strong polar groups—particularly C=O and C-N moieties—bind Li⁺ more strongly than conventional separators, reducing its desolvation barrier and accelerating interfacial ion transport. As a result, electrolyte decomposition becomes more controlled, and harmful organic by-products are suppressed. The researchers also showed that the separator significantly enhances practical properties such as electrolyte wettability, tensile strength, and dimensional stability at high temperature, helping maintain integrity under harsh operating conditions.

With PAP separator, LCO cells can reliably cycle at 4.6 V and 60 °C without a rapid degradation. The study reports substantially improved capacity retention, higher coulombic efficiency, and a thinner, more stable CEI layer, confirmed by cryo-TEM, XPS and TOF-SIMS analyses. Full pouch cells with LCO||graphite chemistry also demonstrated long-term stability at elevated temperature, underscoring the practical feasibility of the approach.

Looking ahead, the researchers expect separator-based interphase regulation to become an important direction for advancing high-voltage lithium-ion batteries. They note that further work is still needed to investigate the long-term chemical stability of PEI under cycling and its resistance to interfacial by-products. Incorporating functional additives or optimizing polymer chemistry may provide additional pathways to extend lifetime and enable even higher-voltage or high-temperature operation. By shifting the separator from a passive component to an active participant in interfacial chemistry, this strategy opens a new route toward safer, more durable, and higher-energy battery systems.

Contributors include Shuofeng Jian, Jiahui Zeng, Zhaohuang Zhan, Yumeng Lan, Hai Lin, Luyi Yang and Feng Pan from School of Advanced Materials, Peking University Shenzhen Graduate School, Shenzhen, China; Chen Guo from Shenzhen Senior Technology Material Co., LTD, Shenzhen, China; and Zu-Wei Yin from College of Energy, Xiamen University, Xiamen, China.

This work was supported by the Shenzhen Science and Technology Planning Project (JSGG20220831095604008), the National Natural Science Foundation of China (51902296), the National Center for International Research of Electric Vehicle Power Batteries and Materials (2015B01015), the Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development (2017B030301013), and the Shenzhen Key Laboratory Project of Advanced Functional Carbon Materials for High-Energy Storage (ZDSYS201707281026184).