Nonflammable Fluorinated Cyclic Phosphorus-Based Compounds as Co-Solvents for Advanced Lithium Ion Battery Electrolytes

摘要

The utilization of portable energy sources like lithium-ion batteries (LIBs) is of essential worth nowadays. However, there are still certain limitations and bottlenecks addressed to nonaqueous aprotic electrolytes, as inevitable components for the operation of the battery. With this in line, state-of-the-art (SOTA) nonaqueous aprotic electrolytes reveal different challenges such as high flammability. With the advance demand of energy in the last decades an improvement in safety concerns with an improved cycling performance are highly required for the advanced batteries, tailored for targeted application. To cope with this challenge, one effective and cost favorable approach refers to the introduction of functional additives or co-solvents to the SOTA electrolyte. Phosphorus (Ⅲ and Ⅴ) fluorinated or non-fluorinated compounds have been the subject of an investigation and can be featured as effective flame retardants. However, a trade-off between flammability and cycling performance is always present , due to the high content of the flame retardant negatively influencing the operation of the battery. In this work, several fluorinated cyclic phosphorus-based containing compounds were designed, synthesized and studied as flame retardant co-solvent as electrolyte components for advanced LIB application. The selected molecules were correlated in respect to their structure-reactivity relationship and characterized by means of carefully selected physicochemical, electrochemical, analytical and structural methods. The influence of the fluorinated cyclic phosphorus-based co-solvents on the overall performance of the graphite/NCMlll cells was elucidated comprising the flammability of considered electrolyte formulations. The results obtained from self-extinguishing time (SET) and flash point determination, as well as TGA and DSC measurements were correlated to performed theoretical calculations. Furthermore, the additive/co-solvent concentration dependency on the flammability was studied by means of GC-MS, SEM and XPS analysis and correlated to theoretical calculations to understand and elucidate the mechanism of the combustion. This systematical approach allows to understand the structure-reactivity relationship of the selected compounds, their impact on the overall battery chemistry, performance, safety and mechanism and helps, to further tailor the properties of functional electrolyte co-solvents for targeted application.