Hybrid fluorinated ionic liquid electrolyte for high-voltage lithium metal batteries

Abstract High-voltage lithium metal batteries hold the promise of higher energy density; however, they pose significant challenges for electrolytes, particularly concerning high-voltage stability and compatibility with lithium metal. Ionic liquids offer potential solutions due to their unique properties, but their high viscosity limits their applications. Although diluents like 1,2-dimethoxyethane have been explored to reduce viscosity, they are unstable under high-voltage conditions. In this work, we investigated the structural design of ionic liquid cations and examined how different cations influence both solvation structure and performance with a fluorinated diluent. We show that a highly fluorinated cation boosts the electrolyte’s oxidative stability through interactions with anions and diluents, enabling stable cycling in 4.5 V Li/NMC cells—outperforming non-fluorinated counterparts. By enhancing oxidative stability and interfacial properties through fluorination, we demonstrate a viable strategy to overcome the limitations of traditional ionic liquid electrolytes, potentially extending beyond this study to inspire further electrolyte design innovations.