Constraints of the Shuttle Mechanism in Li-S Batteries

Lithium-sulfur cathodes seem to be promising due to their high theoretical specific capacity. The L₂S_m species formed by the reduction of elemental sulfur are soluble in most commonly used solvents and diffuse continuously to the Li anodes where they are chemically reduced. As a result, it is impossible to fully recharge the Li–S battery. Therefore, a protective barrier is required to retain the polysulfides from reacting with the anode.

LiNO₃ forms a passivation film on anodes for Li batteries and therefore is a widely used additive in Li-sulfur battery systems, especially in ethereal solutions. In order to avoid LiNO₃ reduction on the high surface area cathodes the potential window must be limited to 1.7V vs. metallic lithium (e.g., those which include activated carbon elements)[1, 2].

Our follow-up work intends to enhance the importance of the cutoff voltage and its ramifications. We show that when the cathode’s potential is lowered below 1.9 V vs. Li/Li NO₃⁻ is reduced to NO₂⁻ followed by formation of a surface film demonstrated by Electrochemical Quartz Crystal Microbalance (EQCM).

Consequently, reduction of NO₃⁻ anions on the cathode side leads to pronounced capacity fading of Li-S cells. Massive nitrate consumption on the cathode side leads to a remarkable capacity fading of Li-S cells due to the well-known shuttle mechanism. Therefore, the potential range of sulfur cathodes in Li-S cells needs to be limited, so the lowest potential should be higher than 1.8 V vs. Li. This means of course giving up some of the cathode’s capacity.

 Another strategy intended to suppress the shuttle mechanism is the use of electrolyte solutions in which the polysulfide species are insoluble or only slightly soluble. Park et al.[3] showed that Li₂S_m dissolution was effectively suppressed in the ionic liquid (IL) electrolytes with bis(trifluoromethylsulfonyl)imide (TFSI) anions and demonstrated reversible cycling of S–C composite electrodes, prepared by employing nano-sized activated carbon Ketjen black (KB, EC600JD) as a carbon matrix in such electrolytes. They also concluded that the poor performance of Li–S cells (fast capacity fading) in IL electrolytes containing bis(fluorosulfonyl)imide (FSI) anions is the result of decomposition of these anions, because of their reaction with polysulfides. We showed that that this conclusion is not fully correct and demonstrated a very stable cycling of Li–S cells which contain FSI-based IL electrolytes. Our Li-S cells with FSI based IL electrolytes demonstrated a capacity close to the theoretical value, at elevated temperatures [4]. We concluded that the compatibility of the structure of the carbon matrix and the anions of the IL electrolyte is very important for the operation of Li–S cells based on IL electrolyte systems. We show in the presentation that a key factor for the performance of S-C composite cathodes in the FSI-based IL electrolytes is the formation of protecting surface films which behave like solid electrolyte interphase  (SEI), during the initial discharge at potential lower 1.5V vs. Li/Li⁺. SEI type surface films facilitate Li ions desolvation processes and prevents the dissolution of soluble Li polysulfides from the cathode to solution phase. We suggested that reversible lithiation of S-C cathodes occurs according to a quasi-solid-state mechanism including full desolvation of lithium ions at the interphase between the cathode surface and the electrolyte solution. According to the proposed mechanism, the cycling behavior of the Li-S cells is highly dependent on the sulfur loading in the composite electrodes. The best performance and very high initial Columbic efficiency was demonstrated by the S-C electrodes with fully filled micropores. Thus, the quasi-solid-state reaction of Li ions with sulfur encapsulated in the micropores occurs in the solvent free environment, and the SEI type surface films formed, facilitate an easy desolvation process. We demonstrated that FSI anions in solutions ensure a better passivation of Li anodes than TFSI anions, preventing an infinite charging process in the initial cycles of Li-S cells, even in the absence of LiNO₃.

A promising approach to suppress the shuttle mechanism in Li-S cells is the use of polar substrates for the sulfur moieties such as metal-oxides [5]. We describe the consequences of introducing polar materials into the cathodes or the separator.

References

1. S. S. Zhang, J. Electrochem. Soc., 159, A920 (2012).
2. A. Rosenman, R. Elazari, G. Salitra, D. Aurbach, A. Garsuch  J. Electrochem. Soc., 162, A470 (2015).
3. J. Park, K. Ueno, N. Tachikawa, K. Dokko, M. Watanabe, J. Phys. Chem. C, 117, 20531 (2013).
4. G. Salitra, E. Markevich, A. Rosenman, Y. Talyosef, D. Aurbach, A. Garsuch, ChemElectroChem, 1, 1492 (2014).
5. S. Evers, T. Yim, L. F. Nazar, J. Phys. Chem. C, 116, 19653 (2012).