A gamma fluorinated ether as an additive for enhanced oxygen activity in Li–O2 batteries

Abstract

Perfluorocarbons (PFCs) are known for their high O₂ solubility and have been investigated as additives in Li–O₂ cells to enhance the cathode performance. However, the immiscibility of PFCs with organic solvents remains the main issue to be addressed as it hinders PFC practical application in Li–O₂ cells. Furthermore, the effect of PFC additives on the O₂ mass transport properties in the catholyte and their stability has not been thoroughly investigated. In this study, we investigated the properties of 1,1,1,2,2,3,3,4,4-nonafluoro-6-propoxyhexane (TE4), a gamma fluorinated ether, and found it to be miscible with tetraglyme (TEGDME), a solvent commonly used in Li–O₂ cells. The results show that with the TE4 additive up to 4 times higher O₂ solubility and up to 2 times higher O₂ diffusibility can be achieved. With 20 vol% TE4 addition, the discharge capacity increased about 10 times at a high discharge rate of 400 mA g_C⁻¹, corresponding to about 0.4 mA cm⁻². The chemical stability of TE4 after Li–O₂ cell discharge is investigated using ¹H and ¹⁹F NMR, and the TE4 signal is retained after discharge. FTIR and XPS measurements indicate the presence of Li₂O₂ as a discharged product, together with side products from the parasitic reactions of LiTFSI salt and TEGDME.