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Sodium-oxygen batteries are becoming of increasing interest in the research community as they are able to overcome some of the difficulties associated with lithium-oxygen batteries. The interpretation of the processes governing the discharge and charge of these batteries, however, has been under debate since their early development. In this work we combine different electrochemical methods to build up a model of the discharge product formation and decomposition. We initially analyze the formation and decomposition of the discharge products by means of electrochemical impedance spectroscopy. After that, and for the first time, oxygen electrode processes in Na-O2 cells are analyzed by means of electrochemical quartz crystal microbalance experiments. Based on the combination of these two techniques it is possible to evidence the stabilization of the discharge products in the electrolyte prior to their precipitation. The deposition of passivating products that cannot be stripped off during charge is also demonstrated. Cyclic voltammetry experiments at different potential limits further confirm these passivation reactions. In conclusion, this work provides an accurate picture of the mechanism of the Na-O2 cell reactions by combining different electrochemical techniques.

A new route to enhance the performance of Na-O2 cells is demonstrated. Redox mediators (such as ethyl viologen) are shown to facilitate the discharge reaction, producing an increased capacity (due to suppressed electrode passivation), higher discharge potential (due to faster kinetics) and stable cycling.

Non-aqueous Li-O2 batteries are promising for next-generation energy storage. New battery chemistries based on LiOH, rather than Li2 O2 , have been recently reported in systems with added water, one using a soluble additive LiI and the other using solid Ru catalysts. Here, the focus is on the mechanism of Ru-catalyzed LiOH chemistry. Using nuclear magnetic resonance, operando electrochemical pressure measurements, and mass spectrometry, it is shown that on discharging LiOH forms via a 4 e- oxygen reduction reaction, the H in LiOH coming solely from added H2 O and the O from both O2 and H2 O. On charging, quantitative LiOH oxidation occurs at 3.1 V, with O being trapped in a form of dimethyl sulfone in the electrolyte. Compared to Li2 O2 , LiOH formation over Ru incurs few side reactions, a critical advantage for developing a long-lived battery. An optimized metal-catalyst-electrolyte couple needs to be sought that aids LiOH oxidation and is stable towards attack by hydroxyl radicals.

The parasitic reactions associated with reduced oxygen species and the difficulty in achieving the high theoretical capacity have been major issues plaguing development of practical nonaqueous Li-O

Lithium–oxygen battery development is hampered by degradation reactions initiated by superoxide, which is formed in the pathway of oxygen reduction to peroxide. This work demonstrates that the superoxide lifetime is drastically decreased upon addition of ethyl viologen, which catalyses the reduction of superoxide to peroxide.

Non‐aqueous Li–O2 batteries are promising for next‐generation energy storage. New battery chemistries based on LiOH, rather than Li2O2, have been recently reported in systems with added water, one using a soluble additive LiI and the other using solid Ru catalysts. Here, the focus is on the mechanism of Ru‐catalyzed LiOH chemistry. Using nuclear magnetic resonance, operando electrochemical pressure measurements, and mass spectrometry, it is shown that on discharging LiOH forms via a 4 e− oxygen reduction reaction, the H in LiOH coming solely from added H2O and the O from both O2 and H2O. On charging, quantitative LiOH oxidation occurs at 3.1 V, with O being trapped in a form of dimethyl sulfone in the electrolyte. Compared to Li2O2, LiOH formation over Ru incurs few side reactions, a critical advantage for developing a long‐lived battery. An optimized metal‐catalyst–electrolyte couple needs to be sought that aids LiOH oxidation and is stable towards attack by hydroxyl radicals.

The phase changes that occur during lithium extraction from LiCoPO

Redox mediators hold significant promise in reducing the large overpotentials pervasive upon charging of lithium-oxygen (Li-O2) cells. Cobalt bis(terpyridine) (Co(Terp)2) was investigated as mediator of the Li2O2 oxidation reaction using electrochemical, XRD and mass spectrometry measurements and benchmarked against tetrathiafulvalene (TTF). Significant reductions in reversible potential versus Li+/Li are measured for Co(Terp)2 and TTF from diglyme to Pyr14TFSI:diglyme to Pyr14TFSI, attributable to upward shift in the Li+/Li, due to weakening Li+ solvation in this solvent order. The lowering of the reversible potentials has noticeable gains on the kinetics of the charge reaction and greater reduction in charge overpotential are observed with the cobalt complex. However, probing the efficacy of the discharge and charge processes using differential electrochemical mass spectrometry reveal that Co(Terp)2 undergoes CoII to CoI reduction on cell discharge which interferes with the desired O2 reduction; furthermore less than 25% of the O2 consumed on discharge is recovered on charge. On the other hand, TTF allows the ideal 2 e-/O2 on discharge and enables up to 32% O2 recovery on charge. CO2 is a significant charging product as voltages becomes greater than 4.0 V vs. Li+/Li because of electrolyte decomposition. The methodology here developed is valuable to critically evaluate the effectiveness of redox agents in Li-O2 batteries.

In-situ Raman spectroscopy is applied, for the first time, to elucidate the reaction products of oxygen reduction in two types of ionic liquids: 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (C2mimTFSI) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Pyr14TFSI). The degradation of C2mimTFSI by superoxide attack is evidenced by the appearance of bands characteristic of amide and carbonate compounds. On the contrary, Pyr14TFSI is found to be resistant towards degradation. It is observed that superoxide is the first product of oxygen reduction in Pyr14TFSI, and the formation of Li2O2 is observed at longer times. Keywords: Lithium–oxygen batteries, Ionic liquids, Oxygen reduction reaction

A novel design of the non-aqueous lithium air cell is presented with a demonstration of a new reaction concept, involving a soluble redox shuttle to catalyse oxygen reduction. In principle, this can relieve the requirement for fast diffusion of molecular oxygen from the air interface to the positive electrode. To demonstrate this concept, ethyl viologen ditriflate was dissolved in BMPTFSI, reduced at a carbon electrode and regenerated by aspiration with oxygen. Useful shuttle behaviour, confirmed by several reduction–oxidation cycles, was observed in the case where the electrolyte contained at least 0.3 M lithium salt. The beneficial effect of the salt was attributed to its critical role in converting superoxide, which would otherwise destroy the shuttle, into the more desirable product of oxygen reduction, lithium peroxide. Keywords: Li-air, Viologen, Redox shuttle, Oxygen reduction, Catalysis, Ionic liquid

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. [1] K.M. Abraham, Z. Jiang, J. Electrochem. Soc.143 (1996) 1–5. [2] F. Mizuno, S. Nakanishi, Y. Kotani, S. Yokoishi, H. Iba, Electrochemistry. 78 (2010) 403–405. [3] J.T. Frith, N. Garcia-araez, A.E. Russell, J.R. Owen, Prep.(n.d.). [4] I.M. AlNashef, M. a. Hashim, F.S. Mjalli, M.Q.A. Ali, M. Hayyan, Tetrahedron Lett.51 (2010) 1976–1978. [5] S. Randström, G.B. Appetecchi, C. Lagergren, A. Moreno, S. Passerini, Electrochim. Acta. 53 (2007) 1837–1842. [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.

Direct electrochemical oxygen reduction in non-aqueous electrolytes results in the deposition of an insoluble product on the electrode surface, thereby blocking access of oxygen to the surface for continued reaction (Fig. 1). Once the surface is completely covered, the reaction stops and the discharge is ended. Alternatively, in the presence of a soluble redox shuttle, oxygen reduction can occur homogeneously in solution near the air interface (Fig. 2). This has the effect of displacing oxygen reduction away from the electrode surface, which remains active to regenerate the shuttle to the reduced form. A further possibility is mediation of the oxygen reduction reaction in order to favour the formation of lithium peroxide rather than radical products that may degrade the electrolyte (Fig. 3). Given a suitable shuttle reagent, this scheme may have several potential advantages, for example, Avoidance of electrode passivation by insoluble lithium oxides An increased, chemical driving force for oxygen adsorption into the electrolyte Increased mass transfer to the electrode surface Enhanced kinetics of lithium peroxide formation Minimisation of side reactions that may degrade the electrolyte We will present results on Ethyl Viologen acting as a shuttle, including its reduction at the electrode, its reactions with oxygen, and its stability to the products of oxygen reduction under various conditions. Acknowledgement This work was supported by the European Union’s Seventh Framework Programme under EC-GA No. 265971 ‘LABOHR’ and the University of Southampton.