The Influence of Electrolyte Salt and Solvent on the Morphology of Li2O2 Formed on Li-Air Cathodes

Li-air batteries are an exciting class of energy storage devices with exceptional theoretical capacities which could facilitate longer-range electrical vehicles if fully optimised systems are realised.¹ Energy storage in Li-air batteries proceeds via different mechanisms to those associated with conventional Li-ion batteries, necessitating detailed studies into the fundamental processes associated with discharge and charge. Recently, increasing attention has been devoted to understanding the role of the electrolyte in determining the performance of Li-air batteries. It has been conclusively shown that carbonate based electrolytes are not suitable for rechargeable systems due to the rapid accumulation of carbonate based species on the cathode.² These carbonates are particularly prevalent during the charging process due to decomposition of the electrolyte and reactions between the electrolyte, the primary discharge product Li₂O₂ and its intermediates. Numerous reports have revealed that characteristic Li₂O₂ toroids often form on the cathode surface during discharge in a variety of cathode/electrolyte systems.^{3, 4} Recently, Nazar et al. showed that the formation of these toroids is strongly related to the applied current for a given system (TEGDME/LiTFSI electrolyte and Super P carbon cathode).⁵ Their results show that at low applied currents, large crystalline Li₂O₂ toroids form on the cathode surface with a clear change to quasi-amorphous Li₂O₂ films at higher applied currents. They also found that the toroids were much more difficult to decompose during charge than the thin films, indicating that the nature of the Li₂O₂formed on discharge plays a key role in determining rechargeability.

In this work we have investigated the morphology and composition of discharge products formed on Super P cathodes using various different electrolyte solvent and salt combinations. We demonstrate that even at high applied currents (250 μA), electrolytes containing sulfolane as the electrolyte solvent show preferential Li₂O₂ toroid formation (Figure 1 a,b). In comparison, electrolytes using TEGDME (Figure 1 c,d) as the electrolyte solvent do not lead to the formation of Li₂O₂ toroids at the same applied currents.  The comparative performance of cells using the various electrolytes determined using galvanostatic charge-discharge methods were demonstrated to be linked to the morphology and areal coverage of the Li₂O₂ formed on the cathodes. The decomposition of these particles and films is visualized by conducting ex-situ SEM analysis at various stages of the charge process. The influence of catalyst addition (Pd, MnO_2, Co₃O₄) to Super P carbon cathodes on the morphology of Li₂O₂ formed on cathodes and thus their electrochemical performance was also probed. This report gives insight into the importance of understanding the formation and decomposition of Li₂O₂ on cathodes for realizing rechargeable high capacity Li-O₂ battery systems.   

Figure 1: SEM images of Super P cathodes discharged with an applied current of 250 μA using different electrolyte solvent/salt combinations.

1.             Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J. M. Nature materials 2011,11, (1), 19-29.

2.             McCloskey, B. D.; Bethune, D. S.; Shelby, R. M.; Girishkumar, G.; Luntz, A. C. The Journal of Physical Chemistry Letters 2011,2, (10), 1161-1166.

3.             Fan, W.; Cui, Z.; Guo, X. The Journal of Physical Chemistry C 2013. 117 (6), pp 2623–2627

4.             Mitchell, R. R.; Gallant, B. M.; Shao-Horn, Y.; Thompson, C. V. The Journal of Physical Chemistry Letters 2013, 1060-1064.

5.             Adams, B. D.; Radtke, C.; Black, R.; Trudeau, M. L.; Zaghib, K.; Nazar, L. F. Energy & Environmental Science 2013,6, (6), 1772-1778.

Acknowledgements: Financial support was provided by the European Union Seventh Framework Programme (FP7/2007-2013) project STABLE.