Lithium-sulfur (Li–S) batteries have been intensively pursued as the next-generation batteries for electrical vehicles, electrified flight, and renewable energy storage. They offer several advantages such as high theoretical capacity, low cost, and environmental friendliness. Despite the various advantages of Li–S batteries, the large-scale commercialization has been plagued by some persistent obstacles, especially the poor cycle life and low practical energy density. These problems are mainly contributed by the insulating nature of sulfur/lithium sulfides and polysulfide shuttling. In the past ten years, various polysulfide-trapping materials have been developed to address the polysulfide shuttling, including advanced carbon materials, polymers, metal oxides, and a few others. Our view is that the binder is a critical, but often overlooked, component in the sulfur cathode that addresses some of the most key problems.1,2
Here, we challenge the conventional wisdom regarding polymer binders and instead offer new design rules governing their proper use in Li–S batteries. With these design rules in mind, we developed a new polymer binder that offers a facile, liquid-like ion-conduction mechanism along weakly-coordinating anions that are not immobilized onto the polymer backbone, and instead only associated through electrostatic interactions. In this manner, the polymer binder is an active component in the system, directly participating in ion transport (Fig. 1). We find that this strategy also aids in controlling polysulfide migration and thereby extends cycle-life.
<align="left">Experimental
CR2032-type coin cells were assembled to test the electrochemical performance of binders in Li–S batteries. The cell was made by stacking lithium metal, Celgard 2400, and the sulfur electrode in sequence. The electrolyte was prepared by dissolving 1 M LiTFSI and 0.2 M LiNO3 in DOL/DME (1:1 v/v). Electrochemical tests were conducted on a Biologic VMP3 potentiostat. The battery cycling tests were carried out between 1.8 V and 2.8 V at a rate of C/5 (1 C = 1675 mA h g-1).
<align="left">Results and Discussion
Fig. 1 provides a schematic representation of the sulfur electrodes with either passive polymer binders such as PVDF, or, alternatively, active polymer binders such as those we’ve innovated upon here. In this configuration, two drawbacks of the PVDF binder exist: it has been reported that PVDF tends to block the transport of Li+ ions, leading to low sulfur utilization; PVDF does not possess robust groups to suppress the migration and loss of mobile polysulfides into the electrolyte. In contrast, our binder exhibits high intrinsic ionic conductivity, which provides efficient transport of Li+ ions through the binder.3Meanwhile, residues along the polymer backbone interact strongly with polysulfides, arresting their migration in the cell.
The long-term cycling stability of the Li–S batteries with either binder was evaluated at C/5. Those with our polymer binder in place possessed significantly improved cycling stability over PVDF cells. This is due to the fact that PVDF does not possess absorption ability to polysulfides, which tend to migrate into the electrolyte or to the lithium anode. Thus, there are more side reactions happening at the lithium metal surface in PVDF cells, which consumes electrolyte and lithium.
<align="left">References
1. M. J. Lacey, F. Jeschull, K. Edström and D. Brandell, J. Phys. Chem. C, 118, 25890 (2016).
2. P. D. Frischmann, Y. Hwa, E. J. Cairns and B. A. Helms, Chem. Mater., 28, 760(2016).
3. A.-L. Pont, R. Marcilla, I. De Meatza, H. Grande and D. Mecerreyes, J. Power Sources, 188, 558(2009).
Fig. 1. Schematic representation of sulfur electrodes with either (a) passive or (b) active polymer binders incorporated into the composite.