Exploring the Relationship between Interfacial Chemistry and Thermal Safety of a Flame Retarding Semisolid Electrolyte
Authors: Sun Annie , Pol Vilas G. 
Published: August 2024

The safety-related performance of LIBs still remains as a major issue even after more than three decades of commercialization.1 When typical electrolytes composed of flammable organic liquid solvents vaporize and decompose exothermically during thermal runaway, they pose serious risk to human health and infrastructure.2 Conventional lithium-ion batteries (LIBs) are energy storage devices which function as electrochemical systems subject to highly dynamic heterogeneous reaction phenomena. As a result, the tolerance levels of tangible harm to users and surrounding environments may be amplified considerably as scale and pervasiveness of technology increases. Beyond consumer devices, large-scale energy storage to complement renewable power generation technologies is instrumental to the goal of decarbonization via electrification and thus global goals for carbon neutrality in the 21st century. Renewable power generation benefits substantially from decoupling the spatial and temporal points of energy generation from that of demand.
Recent efforts in academia and industry alike have endeavored to replace volatile and flammable conventional liquid electrolytes (composed of classes of compounds such as carbonates or ethers) with intrinsically nonflammable liquid electrolytes (composed of fluorinated and phosphorus-containing organic solvents).3–5 Simultaneously, solid-state electrolytes have been touted as an avenue to not only wholly replace the liquid component of the cell to improve safety, but also as a higher-energy density next-generation battery electrolyte.6 However, these strategies are often examined as materials-level design choices for improving “built-in” safety and not as changes to a dynamic and multiscale electrochemical system.
In this study, we find that the intrinsic safety of a system is highly dependent on the interfacial degradation pathways of the electrode-electrolyte configuration, which acts as a direct complement to the thermal behavior which may result in thermal runaway. Through detailed multimodal calorimetry and characterization of the interfacial composition, this work subsequently brings the question of flammability into focus as a downstream consequence of the interfacial reaction kinetics. This is accomplished through the combined efforts of thermal analyses as well as X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy accompanied with electron crystallography analyses, and nuclear magnetic resonance studies of the ionic transport behavior through the polymeric electrolyte. Through these efforts, design principles centered around holistic assessment of the lifespan of materials and their electrochemical and thermal interactions are developed towards the greater goal of improving the safety and resilience of LIBs and their devices against the wide spectrum of abuse scenarios created by their expanding application space.

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