Tandem Sulfur Cathodes with High Areal Capacity for Li-S Batteries
Authors: Chang Chi-Hao , Chung Sheng-Heng , Manthiram Arumugam 
Published: February 2020

Lithium-sulfur (Li-S) batteries are considered as one of the most potential candidates for next-generation energy storage systems. This is mainly due to the high theoretical capacity of sulfur (1675 mA h g-1). Moreover, sulfur is ubiquitous, cost-effective, and environmentally benign. However, the practical application of Li-S batteries could not be realized because the intrinsic insulating nature of sulfur and its end product, Li2S, leads to low electrochemical utilizations. Furthermore, the severe polysulfide (Li2Sn, 4 < n ≤ 8) diffusion from the cathode to the anode during the discharge/charge process gives rise to fast capacity degradation and poor cycling stability. Many strategies have been developed to address these challenges, e.g., use of carbon/sulfur composite cathodes, insertion of conducting interlayers, and application of functional separators. Nevertheless, the low sulfur loadings (< 2 mg cm-2) in the cathode still pose problems for the practical use of Li-S batteries owing to relatively low areal capacity.
To tackle these problems, we couple a conducting interlayer with an active material layer into a free-standing cathode with 4 mg cm-2 sulfur loadings by a facile 2-step vacuum-filtration process. We also employ a regular cathode size of 1.13 cm-2 instead of extremely small ones. The conducting interlayer containing single-wall carbon nanotubes (SWCNTs) and carbon nanofibers (CNFs) functions as a blocking/absorbing layer to confine the intermediate polysulfide species. The active material layer is prepared by simply mixing sulfur particles and CNFs and then vacuum-filtering on the top of the conducting interlayer. To further amplify sulfur loadings in the whole cathode, the continuous vacuum-filtration procedure is exploited to fabricate the tandem sulfur cathode where the sulfur loadings can easily increase from 4 mg cm-2 to 12 mg cm-2. Moreover, the tandem cathode is robust and highly flexible; it is able to fold, bend, roll up to form a cylinder, and recover to the original state without damage. The tandem cathode design not only significantly increases the sulfur loading in the whole cell but also provides physical voids for trapping intermediate polysulfide species. As a result, the Li-S cell utilizing the tandem sulfur cathode is able to exhibit excellent electrochemical performance even with an extremely high sulfur loading of 12 mg cm-2.
The electrochemical performances reveal that the Li-S cells with the tandem cathodes (the corresponding total sulfur loading in parentheses) with 4 (4.52 mg), 8 (9.04 mg), and 12 (13.56 mg) mg cm-2 sulfur loadings can deliver high initial discharge areal capacities of, respectively, 4, 7.5, and 10 mA h cm-2 at 0.1C rate. The high initial capacities can be attributed to the fact that the sulfur particles are well embedded into the CNF network as shown in the SEM images. After 100 cycles, the reversible areal capacities approach 2.5, 4.3, and 6 mA h cm-2. The outstanding cycle stability with high capacity retention rates of about 60 % may result from the design of the cathode configuration. The active material layer constructed by intertwining the CNFs to form porous reservoirs effectively holds the intermediate polysulfide species. The conducting SWCNT/CNF interlayer, on the other hand, intercepts and absorbs the polysulfides by its interconnected porous space. During cycling, the trapped active materials can be reactivated/reutilized. As the result, not only does the tandem sulfur cathode design extensively augment sulfur loadings but also effectively improve the electrochemical performance of the Li-S cells. Thus, this strategy offers a great leap closer toward the practical application of Li-S batteries in energy storage systems.

Figure 1