Multi‐Functional Layered WS2 Nanosheets for Enhancing the Performance of Lithium–Sulfur Batteries

DOI: 10.1002/aenm.201601843 in order to restrict the loss of material. Furthermore, due to their scalability and flexibility, 3D flexible electronics (see Supporting Information (SI), where Table S1 contains a list) were considered revolutionary materials and were used in many fields such as imperceptible electronic devices, wearable electronic devices, and bionic technology. [11–13] Recently studies have shown the encapsulation of sulfur in the pores of carbon materials, such as meso-/microporous carbons,[11] cable-shaped carbon,[12] and carbon nanotubes/fibers,[13] can reduce the capacity fading. However, such nonpolar flexible carbon materials have a destructive disadvantage; they only have physical van der Waals (vdW) adsorption for polar Li2Sn, which leads to the facile detachment of Li2Sn from the carbon surface. [14] This proves that carbon-based materials alone cannot serve as the perfect host. In light of this new insight, various types of polar functional groups on carbon-based materials have been demonstrated to increase the interaction between Li2Sn species and the electrode; these materials can generally be categorized into three types: polymers (polyaniline, polypyrrole, poly(3,4ethylenedioxythiophene) (PEDOT)),[15] metal oxides (SiO2, TiO2, Al2O3, V2O5, MoO3), and transition-metal disulfides (TiS2, ZrS2,VS2). Zhang et al.[14] suggested that the moderate materials (such as TiS2, ZrS2, and VS2) are the best choices for battery electrodes. Recent research have revealed that the morphology of interconnected nanosheets, such as that found when MnO2 and MoS2 are vertically aligned on carbon materials, forms a 3D network producing electrodes with a stable cycling behavior. [13,17] Nevertheless, to obtain sophisticated pore structures and composites, tedious preparation methods were required for those materials, preventing them from having broad commercial applications. To further address the challenges and bring Li–S cells a step closer to commercialization, we obtained polar WS2 nanosheets deposited on carbon nanofibers (CNFs) using an efficient one-step hydrothermal reaction. Termed C@WS2, this is the first report of using this material as a composite electrode into Li–S batteries. In this flaky-structured C@WS2 composite electrode, dense WS2 nanosheets are wrapped around and anchored on the CNFs. Our experimental data was supported by a systematic simulation study, surveying WS2 with different binding strengths on Li2Sn species at different lithiation stages (n = 2, 3, 4, 6, 8). The nanoscale sulfur is firmly absorbed on the CNF surface not only through physical vdW forces, but also as a result of the WS2 polar functional groups. As a free-standing cathode material in Li–S batteries, the C@WS2 composite exhibits a high rate capability (≈1501 and 450 mA h g−1 are achieved at charge/discharge rates of 0.1 C and 3 C (1 C = 1675 mA h g−1), respectively) with outstanding Coulombic efficiencies (nearly 100%). The battery delivers Lithium–sulfur batteries, which exhibit a co-existence of high specific capacity (1675 mA h g−1) and energy density (2600 W h kg−1), have attracted increasing interest due to their low cost and environmentally friendly nature, both of which make them potentially applicable for electric vehicles (EVs) and large-scale stationary electric energy storage. [1,2] Unluckily, their implementation has been prevented by a series of reasons, including a weak cycle life and low capacities; these are caused by 1) the rapid dissolution of lithium polysulfide (Li2Sn) species into the electrolyte during the recharge, 2) the low electronic conductivity of both sulfur and lithium sulfide, and 3) the large volumetric expansion of sulfur (≈80%) upon lithiation. [3,4] Unquestionably, the poor performance of Li–S batteries is dominated by the issue of the soluble nature of the polysulfide (Sn) intermediates, which leads to their diffusion into the liquid electrolyte. This undesired phenomenon causes the mass transport of electroactive species, and it is termed the “polysulfide shuttle”. [4] To mitigate the detrimental effects of Sn solubility, over the past few decades, researchers have focused on two different aspects of the cell, following a logical train of thought. 1) For the electrolyte, some additives such as Li2S8 and LiNO3 have been added to slow down the polysulfide shuttle and improve Coulombic efficiency. [5] 2) In contrast, the majority of effort has been focused on the material used for the sulfur host; for example, conductive polymer matrices, such as polyaniline,[6] polypyrrole,[7] and poly(3,4-ethylenedioxythiophene),[8] have been shown to have a positive influence on increasing the cycle life of lithium–sulfur batteries. Although the above materials can relieve the effects of polysulfides shuttling, their ability to elongate the cell lifetime are yet far from perfect. To avoid corrosion on the current collector and reduce the production costs of the battery, aluminum was considered suitable for the current collector; it can be alternatively used in the place of Li–S electrodes. However, the role of a current collector can be for outstripping, transporting the current. Actually, 3D cathodes—such as sulfur–nickel foam (SNF)[9] and carbon nanotubes[10]—might be of interest for containing the active material and trapping the polysulfides during cycling,