Perfluorocarbon Additive Behaviour on Lithium Oxygen Battery

Lithium oxygen batteries have drawn a lot of attention recently due to their potential for high energy density, around three times the amount of the theoretical specific energy of LiCoO₂, the commercial lithium ion battery.[1][2] Despite its high theoretical energy density, current lithium oxygen batteries suffer from poor cycle efficiency, poor rate capability, and low cycle life which impede practical application. One of the causes of the drawbacks is the low oxygen solubility and the oxygen transport limitations of the electrolyte. Tackling these challenges is important especially to realize the high rate lithium oxygen battery.

Perfluorocarbons (PFC), the main component of artificial blood, have been investigated as additives for lithium oxygen battery due to their ability to absorb significant volume of oxygen[3]. They are expected to improve the mass transport of oxygen and thus decrease the polarization during discharge. In this work, fluorocarbon compound was incorporated in the ether electrolytes and its performance is tested in lithium oxygen battery. Our results demonstrate that addition of PFC improves the discharge voltage significantly up to 200 mV for 20% PFC addition in 0.1 M LiClO₄ in tetraglyme(TEGDME) electrolyte (Figure 1a). We also discover that the effect of PFC additive is more pronounce at thicker electrode, in agreement with the hypothesis that PFC enhances the concentration of oxygen and/or diffusion of oxygen throughout the electrode thickness. However, subsequent investigation on the effect of PFC additive towards the oxygen reduction using Glassy Carbon (GC) Disk electrode in 0.1 M TBAClO4:DME reveals that the PFC additive reacts with  the superoxide radical and yields irreversible species during the cyclic  voltammetry. This indication is further confirmed by Rotating Ring Disk Electrode (RRDE) on the Au Ring/ GC disk electrode by performing the collection efficiency measurement (N_k). It is found that the collection efficiency increases with increasing rotation speed on the electrolyte with PFC additive, proving that the superoxide species is depleted by chemical reaction with PFC (Figure 1b). Result on the diffusion coefficient of oxygen and superoxide, solubility of oxygen and kinetic of superoxide reaction with PFC will also be discussed to assess the real effect of PFC toward oxygen reduction reaction.  In summary, this work has added deeper insight on the consideration of using PFC as electrolyte additive for lithium oxygen battery.

[1]       L.-L. Zhang, Z.-L. Wang, D. Xu, X.-B. Zhang, and L.-M. Wang, “The development and challenges of rechargeable non-aqueous lithium–air batteries,” Int. J. Smart Nano Mater., vol. 1, pp. 1–20, Feb. 2012.

[2]       A. Kraytsberg and Y. Ein-Eli, “Review on Li–air batteries—Opportunities, limitations and perspective,” J. Power Sources, vol. 196, no. 3, pp. 886–893, Feb. 2011.

[3]       R. Battlno, T. R. Rettich, and T. Tominaga, “The Solubility of Oxygen and Ozone in Liquids,” J. Phys. Chem. Ref. Data, vol. 12, no. 2, pp. 163–178, 1983.