Solution Phase Catalysts for Lithium-Oxygen Batteries

A lithium-oxygen battery (LOB) would deliver the highest energy density of a rechargeable battery. But there is a central problem of catalytic action required for the rechargeable LOB: conventional solid state catalysts are not effective enough to catalyze the multiphase electrochemical reactions on the cathode side. The discharge product Li₂O₂ is also in solid state; it will accumulate at the catalyst surface and hence block the electrode reactions during the discharge process. Meanwhile, during the charging process, it is also difficult to make good contact between the solid catalyst and Li₂O₂. Therefore, instead of solid catalysts, some novel solution phase catalysts have been developed to solve the above problems^1-3.

Here, we present a solution-phase catalyst, iron phthalocyanine (FePc), and provide a mechanism involving iron-oxygen coordination and a redox shuttle across the electrolyte⁴. The Fe-oxygen coordination and the electron dislocation in the big conjugated structure lowers the energy of the high energy intermediates in the ORR and OER. The dissolved FePc can act as a molecular shuttle of (O₂)⁻ species and electrons between the surface of electronically conducting carbon and product Li₂O₂ particles. With this novel approach, the formation and decomposition of Li₂O₂ is no longer confined to the carbon surface. The Li₂O₂blocking effect in the discharge process was greatly depressed. Meanwhile, the redox shuttle mechanism effectively lowers the overpotential during the charging process. Thus, the capacity and cyclability of the battery was improved dramatically.

Some other solution phase catalysts based on the redox shuttle mechanism, such as ferrocene-derived compounds and iodine, will also be discussed. The efficiency of various redox couples to oxidize the Li₂O₂during charging is compared with an oxygen monitoring method. We also show some evidence of side reactions caused by some certain redox shuttle compounds.

 Reference:

(1)           Y.H. Chen, S.A. Freunberger, Z.Q. Peng, O. Fontaine & P.G. Bruce, "Charging a Li-O-2 battery using a redox mediator". Nat. Chem., 2013, 5(6),489-494.

(2)           H.-D. Lim, H. Song, J. Kim, H. Gwon, Y. Bae, K.-Y. Park, J. Hong, H. Kim, T. Kim, Y.H. Kim, X. Lepró, R. Ovalle-Robles, R.H. Baughman & K. Kang, "Superior Rechargeability and Efficiency of Lithium–Oxygen Batteries: Hierarchical Air Electrode Architecture Combined with a Soluble Catalyst". Angewandte Chemie International Edition, 2014, 53(15),3926-3931.

(3)           B.J. Bergner, A. Schurmann, K. Peppler, A. Garsuch & J. Janek, "TEMPO: A Mobile Catalyst for Rechargeable Li-O2 Batteries". J. Am. Chem. Soc., 2014, 136(42),15054-15064.

(4)           D. Sun, Y. Shen, W. Zhang, L. Yu, Z. Yi, W. Yin, D. Wang, Y. Huang, J. Wang, D. Wang & J.B. Goodenough, "A Solution-Phase Bifunctional Catalyst for Lithium–Oxygen Batteries". J. Am. Chem. Soc., 2014, 136(25),8941-8946.