A New Approach to the Lithium-Oxygen Cell Via the Utilisation of Redox Shuttles

Lithium-O₂ cells, with a theoretical specific energy of around 5 times higher than lithium-ion cells have the potential to become the technology powering tomorrow’s electric vehicles. However, since they were first reported by Abraham and Jiang[1] they have faced problems and setbacks. The principal problem was of instability of organic non-aqueous electrolytes to the superoxide ion formed as an intermediate discharge product [2]. Ionic liquids are a promising alternative to traditional carbonate based electrolytes, and indeed recent works have shown that Pyr₁₄TFSI is relatively stable to superoxide [3–5]. Having identified a stable electrolyte we have then looked at the problem of electrode passivation by the insoluble and insulating discharge product lithium peroxide [6], which produces a significant reduction on the practical capacity.

 This work continues our previous study on the use of  ethylviologen triflate as a mediator/redox shuttle for the discharge reaction [7], with a study of mediator action by the shuttle molecule to achieve  a 2-electron reduction of oxygen to form  lithium peroxide away from the electrode surface.  This eliminates electrode passivation as shown in Fig. 1. We will also present an in-depth study into the mechanism of oxygen reduction by ethylviologen triflate and other candidate mediators.

The need for mediators for the charge reaction has also been stated by Bruce et al.,[8] who found  TTF to be a redox mediator in DMSO. Our own studies of TTF have found it to be unsuitable for use in in Pyr₁₄TFSI due to its lower oxidation potential, so we have focused on the use of other compounds to carry electrons back to the electrode while the charge reaction converts the lithium peroxide back to oxygen. 

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[4]         I.M. AlNashef, M. a. Hashim, F.S. Mjalli, M.Q.A. Ali, M. Hayyan, Tetrahedron Lett.51 (2010) 1976–1978.

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[6]         V. Viswanathan, K.S. Thygesen, J.S. Hummelshøj, J.K. Nørskov, G. Girishkumar, B.D. McCloskey, et al., J. Chem. Phys.135 (2011) 214704.

[7]         M.J. Lacey, J.T. Frith, J.R. Owen, Electrochem. Commun.26 (2013) 74–76.

[8]         Y. Chen, S.A. Freunberger, Z. Peng, O. Fontaine, P.G. Bruce, Nat. Chem. 5 (2013) 489–94.