In Situ Probing of the Catalyst-DMSO Interface in Li-O2 Cells

The Li-O₂ cell, with a theoretical energy density of ~11686 Wh/kg, is an attractive technology for improving upon the current generation of rechargeable batteries [1]. However, poor cyclablility and low energy efficiencies make the application of these cells impractical in their current state. Selection of suitable electrolytes and catalysts to increase round trip efficiencies, improve rate capability, reduce the generation of undesirable side products, and promote practical cycling are necessary.

Much literature has focused on selecting non-aqueous electrolyte solvents and electrode materials that yield lithium peroxide as the main discharge product. Ether-based electrolytes (i.e. dimethoxyethane, tetraglyme) have been employed for their improved stability over carbonates, but have still been shown to decompose upon cycling [2,3]. Our recent study shows that dimethyl sulfoxide (DMSO) -based electrolytes are both more stable than ethers and provide a kinetic advantage to the discharge and charge reactions [4]. The oxygen electrode material also has a profound effect on the cyclability of Li-O₂ cells. Carbon has been shown to degrade upon repeated cycling [5] and exhibits high overpotentials for the oxygen reduction (ORR) and oxygen evolution (OER) reactions. Noble metal catalysts (i.e. Pt, Pd, Au) supported on carbon can promote ORR at lower overpotentials than carbon alone and demonstrate better cycle stability [6].

Here we examine the catalyst-electrolyte interface using in-situ techniques coupled with ex-situ morphological characterization. The electrolyte solvent, DMSO, is considered a probe for studying the environment at the interface during cycling by surface-enhanced Raman spectroscopy (SERS). Pt, Pd, and Au containing all-metal oxygen electrodes prepared by galvanic displacement and electrodeposition are used, thus eliminating carbon. We find that the proximity of polar solvent molecules to the electrode surface increases during both the discharge and charge (somewhat counterintuitively). We find that the selection of noble metal catalysts has a significant effect on this solvent-surface behavior. DMSO orientation at the interface is also influenced by the applied electric field. Our results indicate that the formation of discharge products does not limit electrolyte access to the electrode surface over a single cycle but may do so upon repeated cycling, leading to selective deactivation. While lithium peroxide was detected in-situ, ex-situ microscopy confirms that the platelet morphology of the discharge products is thin and significantly porous. The morphology of the discharge products, influenced by the choice of catalyst, is also seen to affect the interfacial behavior during charge.

Electrolyte proximity (the formation of a double layer) and ion exchange during reactions were also studied by in-situ electrochemical impedance spectroscopy. The impedance of the oxygen electrodes exhibit minima which correspond to bulk ORR and intermediate OER potentials. These findings, in addition to cyclic voltammetry and long term galvanostatic cycling, complement the in-situ SERS data. This work represents the first of its kind to study the catalyst-electrolyte interface in-situ at a molecular level with a variety of catalysts and observe electrolyte migration in Li-O₂ cells. Our findings have great implications in understanding the catalytic mechanism of ORR and OER with noble metals in a range of electrolytes.

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[2] B. D. McCloskey, R. Scheffler, A. Speidel, G. Girishkumar, and A. C. Luntz, J. Phys. Chem. C, 2012, 116, 23897–23905.

[3] B. D. McCloskey, A. Speidel, R. Scheffler, D. C. Miller, V. Viswanathan, J. S. Hummelshøj, J. K. Nørskov, and A. C. Luntz, J. Phys. Chem. Lett., 2012, 3, 997–1001.

[4] F. S. Gittleson, R. C. Sekol, G. Doubek, M. Linardi, and A. D. Taylor, Phys. Chem. Chem. Phys., 2014.

[5] M. M. O. Thotiyl, S. A. Freunberger, Z. Peng, and P. G. Bruce, J. Am. Chem. Soc., 2013, 135, 494–500.

[6] Y.-C. Lu, H. A. Gasteiger, and Y. Shao-Horn, J. Am. Chem. Soc., 2011, 133, 19048–19051.