Perfluorocarbons (PFCs) are known for their high O 2 solubility and have been investigated as additives in Li-O 2 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-O 2 cells.Furthermore, the effect of PFC additives on the O 2 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-O 2 cells. The results show that with the TE4 additive up to 4 times higher O 2 solubility and up to 2 times higher O 2 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 À1, corresponding to about 0.4 mA cm À2 . The chemical stability of TE4 afterLi-O 2 cell discharge is investigated using 1 H and 19 F NMR, and the TE4 signal is retained after discharge.FTIR and XPS measurements indicate the presence of Li 2 O 2 as a discharged product, together with side products from the parasitic reactions of LiTFSI salt and TEGDME.

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Lithium-oxygen (Li-O 2 ) cells are popular due to their potentially high energy density. A characteristic fingerprint of a given cell is the voltage profile during constant-current discharge. We suggest that the typical initial dip and the following increase of the voltage result from a temporary increase and slow decrease in the concentration of dissolved superoxide, respectively, feeding into the Nernst equation. The steady-state superoxide concentration decreases as the surface area of the solid precipitation product (Li 2 O 2 ) increases. Importantly, these products bury the electrochemically active carbon surface. Assuming that the electrochemical step only occurs on bare carbon, the Tafel equation provides an expression for the increasing overpotential as a result of the shrinking effective electrode area. This boils the discharge voltage profile down to the sum of two logarithms, grasping all relevant features in recorded discharge voltage profiles.

In laboratory experiments, Li-O2 systems show "sudden death" at capacities far below the theoretical value. Identifying how discharge products limit the total capacity is crucial in Li-O2 system. We investigated the effect of Li2O2 seed layer deposited on carbon cathode under potentiostatic conditions at increasing overpotentials to the subsequent slow discharge at galvanostatic condition. The discharge capacity attainable in the second step is found to vary by more than a factor of 3 depending on the history, i.e., the seed layer. These results provide evidence that the battery history is decisive for the total discharge capacities.

Perfluorocarbon compounds (PFC) are known for their high O2 dissolution capability and have been investigated as additives/electrolyte solvents to improve Li-O2 batteries performance. Nevertheless, systematic studies that go beyond the proof of concept that fluorocarbon additives enhance the performance of Li-O2 batteries have not been carried out yet. In this work, we investigate 1-methoxyheptafluoropropane additive (1-PFC), a fluorocarbon with an ether functional group that has been considered as one of the candidates as additives in the Li-O2 battery. Using electrochemical methods and physical characterization of discharge products, we found that the enhancement of the discharge capacity of Li-O2 cells with 1-PFC additive is most likely correlated with instability of the 1-PFC additive against superoxide radicals, rather than the improvement in O2 solubility.