(Invited) Multi-Electron Reduction of Perfluorinated Gases: The 8-Electron Sulfur Hexafluoride System As a Model Reaction
Authors: Li Yuanda , Khurram Aliza , He Mingfu , Gallant Betar M. 
Published: February 2020

High-potential, high-capacity redox reactions are of fundamental interest in the development of novel primary and secondary battery chemistries. To date, the most widely explored “advanced” nonaqueous cathode reactions for Li or Na batteries involve two-electron transfers: nonaqueous O2 reduction in Li-O2 batteries (2 e-/O2) and reduction of elemental sulfur (16 e-/S8 or a net 2 e-/S), both of which are based on redox with the chalcogen family [1-3]. In this talk, we report on redox of molecules from a different family: those containing halogen ligands, with a particular focus on the model perfluorinated gas, sulfur hexafluoride (SF6) as an illustrative example.

Perfluorinated gases in general, and SF6 in particular, are conventionally described as highly stable [4-5], yet can yield high thermodynamic potentials vs. Li metal with large theoretical numbers of electrons transferred (e.g. SF6 + 8Li --> 6LiF + Li2S, Eo
r = 3.69 V vs. Li/Li+) [6]. In recent work, we have developed a Li-SF6 primary battery that employs a gas-to-solid reaction at the positive electrode, analogous in some aspects to a Li-O2 battery that similarly operates on a dissolved-gas cathode reaction. In such a configuration, we have demonstrated that SF6, which is virtually chemically inert against Li metal at room temperature, can be activated at an electrified interface consisting of a simple carbon electrode in a nonaqueous electrolyte (0.3 M LiClO4 in tetraethyleneglycol dimethyl ether (TEGDME)). Upon activation, SF6 readily undergoes continued reduction and reaction with Li+ ions, resulting in the hypothesized 8-electron reduction of the central S6+ to Li2S and the concurrent expulsion of all fluoride ligands to form stoichiometric LiF. The reaction stoichiometry has been validated using a suite of techniques, including pressure-coupled discharge measurements, solid phase analysis (XRD, FTIR and XPS), liquid-phase analysis (19F and 1H NMR), and electrochemical characterization. The reduction potential, which varies from ~2.0 – 2.4 V vs. Li/Li+depending on the electrolyte solvent, reflects significant overpotentials (~1 V) which are unusual among the currently studied “advanced” multi-electron reactions.

In this talk, we will examine the behavior of the Li-SF6 reduction reaction as a function of parameters including electrolyte solvent, anion composition, and salt concentration, in order to gain mechanistic insight into the dramatic SF6 gas-to-solid phase transformation. Specifically, we characterize the electrolyte-dependence of the reduction onset potential, electron consumption and Coulombic efficiency, capacity, and nature of the solid phase (chemistry and morphology) and discuss the degree of “tunability” achievable by tailoring the reaction environment. Possible descriptors governing the growth process of the solid phase (LiF), resulting morphology, and corresponding capacity will be examined and compared to the theory developed in a sister field, that of O2-to-Li2O2 transformations in Li-O2 batteries [7-8]. In addition, we discuss current understanding of the origin of such large overpotentials involving “inert” molecules in electrochemical systems, and outline challenges to be addressed in the development of a practical system. The possibility to broaden this reaction class to include potentially reversible reactions based on halogenated molecules will be discussed.

[1] Lu, Y. C., Gallant, B. M., Kwabi, D. G., Harding, J. R., Mitchell, R. R., Whittingham, M. S. & Shao-Horn, Y. Energ Environ Sci
6, 750-768, (2013).

[2] Aurbach, D., McCloskey, B. D., Nazar, L. F. & Bruce, P. G. Nature Energy
1, 16128-16139 (2016).

[3] Bruce, P. G., Freunberger, S. A., Hardwick, L. J. & Tarascon, J. M. Nat Mater
11, 19-29, (2012).

[4] Seppelt, K. Chem Rev
115, 1296-1306, (2015).

[5] Zamostna, L., Braun, T. & Braun, B. Angew Chem Int Edit
53, 2745-2749, (2014).

[6] Groff, E. G. & Faeth, G. M. Journal of Hydronautics
12, 63-70 (1978).

[7] Aetukuri, N. B., McCloskey, B. D., Garcia, J. M., Krupp, L. E., Viswanathan, V. & Luntz, A. C. Nat Chem
7, 50-56, (2015).

[8] Johnson, L., Li, C. M., Liu, Z., Chen, Y. H., Freunberger, S. A., Ashok, P. C., Praveen, B. B., Dholakia, K., Tarascon, J. M. & Bruce, P. G. Nat Chem
6, 1091-1098, (2014).

Figure 1