Identifying Descriptors for Solvent Stability in Non-Aqueous Li-O2 Batteries

A crucial challenge in developing rechargeable Li-O₂ batteries is the identification of stable solvents that are resistant to decomposition in the electrochemical environment of Li-O₂ discharge products.  Recent theoretical studies using higher order quantum chemical methods have offered excellent insights into various possible degradation mechanisms such as nucleophilic attack/substitution and hydrogen atom abstraction in the presence of O₂, O₂⁻ and O₂²⁻ anions [1-2]. Similarly, some experimental studies have analyzed various classes of solvents based on the reversibility of the O₂ ↔ O₂⁻ ↔ O₂²⁻ oxygen reduction reactions in them [3-4]. These investigations, however, do not explicitly take into account either the effect of the electrode potential, or any direct electrochemical interaction of the solvent with the deposited Li-O₂ discharge products. In this work, we attempt to identify suitable descriptor that can be used to test for solvent stability by building a unified picture of the energy levels of the solvent molecules, Li-O₂ discharge products and the electrode potential. Based on recent experimental results [5-7], we first hypothesize that although solvent degradation affects the yield of Li₂O₂ during discharge, the majority of solvent degradation occurs via a process of decomposition of solvent into its oxidized products during charging, which eventually leads to CO₂ evolution at higher potentials in non-aqueous Li-O₂ batteries. Detailed analyses of these findings lead us to conclude the presence of a near universal mechanism of solvent degradation during charging. Further, we create the case for number of electrons per O₂ evolved during the charging process to be the marker of cell performance, the ideal being 2e^-/O₂. Based on these concepts, we propose that the HOMO level would be a good descriptor for solvent stability and demonstrate that this correlates well the degree of rechargeability. Taking this analysis a step further, we demonstrate that Gutmann donor number is also a good empirical descriptor for solvent stability, which has been explained earlier in terms of Hard Soft Acid Base Theory. Employing these descriptors, we identify several solvents which could enhance the rechargeability of non-aqueous Li-O₂ batteries.

References:

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[4] Laoire, C. O.; Mukerjee, S.; Abraham, K. M.; Plichta, E. J.; Hendrickson, M. A. Influence of Nonaqueous Solvents on the Electrochemistry of Oxygen in the Rechargeable LithiumAir Battery. J. Phys. Chem. C 2010, 114, 9178–9186

[5] McCloskey, B. D.; Bethune, D. S.; Shelby, R. M.; Mori, T.; Scheffler, R.; Speidel, A.; Sherwood, M.; Luntz, A. C. Limitations in Rechargeability of Li-O2 Batteries and Possible Origins. J. Phys. Chem. Lett. 2012, 3, 3043–3047

[6] McCloskey, B. D.; Valery, A.; Luntz, A. C.; Gowda, S. R.; Wallraff, G. M.; Garcia, J. M.; Mori, T.; Krupp, L. E. Combining Accurate O2 and Li2O2 Assays to Separate Discharge and Charge Stability Limitations in Nonaqueous Li-O2 Batteries. J. Phys.Chem. Lett. 2013, 4, 2989–2993

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