In today’s society, the utilization of portable energy sources like lithium-ion batteries (LIBs) is of essential worth. However, LIBs still have certain limitations and bottlenecks addressed to nonaqueous aprotic electrolytes, as inevitable components for the operation of the battery.¹ With this in line, state-of-the-art nonaqueous aprotic electrolytes (SOTA) reveal different challenges such as electrolyte decomposition and high flammability. One effective and cost favorable approach refers to the introduction of additives or co-solvents to the SOTA electrolyte.^2-3 Hereby, the electrolyte decomposition can be minimized when increasing the cut-off potential (beyond 4.5 V vs. Li/Li⁺) that prevents the increased capacity fading of the battery. Furthermore, phosphorus (III and V) fluorinated or non-fluorinated compounds have been the subject of an investigation and can be featured as effective flame retardants.^2-3 With the advance demand of energy in the last decades, an increase in the energy density, as well as an improvement in safety concerns are highly required for the advanced batteries, tailored for targeted application.^2-5

In this work, several fluorinated cyclic phosphorus-based containing compounds were designed, synthesized and investigated as bifunctional - high-voltage and flame retardant – additives/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. Furthermore, the influence of the fluorinated cyclic phosphorus-based additives/co-solvents on the overall performance of the graphite/NCM111 cells was studied. In addition, the flammability of electrolyte formulations were investigated. The results obtained from self-extinguishing time test (SET) and flash point determination, as well as TGA and DSC measurements were correlated to theoretical calculations to understand 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 and safety and helps, to further tailor the properties of electrolyte additives/co-solvents for specific application.

1 D. Gao, RSC Adv. (2015), 5, 17566.

2 K. Xu, Chem. Rev. (2004), 104, 4303.

3 K. Xu, Chem. Rev. (2014), 114, 11503.

4 S. S. Zhang, J. Power Sources (2006), 162, 1379.

5 M. Asif, Renew. Sustainable Energy Rev. (2007), 11, 1388.