A self fire-extinguishing and high rate lithium-fluorinated carbon battery realized by ethoxy (pentafluoro) cyclotriphosphazene electrolyte additive design

Abstract

The lithium/carbon fluoride (Li/CF_x) battery has attracted significant attention due to its highest energy density among all commercially available lithium primary batteries. However, its high energy density also poses a significant risk during thermal runaway events, and its poor electrochemical performance at high discharge current densities limits its application in high-power devices. In this study, we propose a flame-retardant and interface-affinity enhancing electrolyte additive, ethoxy (pentafluoro) cyclotriphosphazene (PFPN), to regulate the electrolyte in Li/CF_x batteries. The PFPN molecules participate in the lithium solvation shell, and its fluorine groups shows better affinity with the carbon fluoride cathode compared to traditional carbonate electrolytes. Furthermore, as a bulky molecule, PFPN reduces the desolvation energy of Li⁺ ions after participating in lithium ion coordination, making it easier for Li⁺ ions to be utilized at the carbon fluoride cathode, significantly enhancing the battery's rate performance. In a carbonate electrolyte containing 16 % PFPN additive (1 M LiBF₄/EC+DMC, volume ratio 1:1), the CF_x cathode delivers 277 % higher capacity at a current density of 1 A g⁻¹ compared to a CF_x cathode without the additive. Additionally, due to the ability of PFPN to produce fluorine and phosphorus radicals at high temperatures that interrupt the combustion chain reaction, the electrolyte can achieve up to 10 instances of zero-second self-extinguishing, reducing the battery self-extinguishing time by 90 %. This study is the first to investigate the safety and flame-retardant electrolyte design of carbon fluoride batteries, providing a method to improve the power performance and safety of Li/CF_x batteries at the same time.