Efficient Synthesis of Graphene Nanoscrolls for Fabricating Sulfur-Loaded Cathode and Flexible Hybrid Interlayer Toward High Performance Li−S Batteries

Abstract

Graphene oxide; Sulfur/carbon composite; Graphene nanoscroll; Metal oxides nanowires; Interlayer; Li−S batteries Nowadays, the growing grievous environmental problems together with the exhaustion of energy resources put urgent demands for developing high energy density devices, whereas traditional lithium-ion batteries (LIBs) are getting close to their energy density. Consideration of the factors including capacity, resource and environment, lithium sulfur (Li-S) batteries are the most potential lithium secondary batteries.[1] However, many drawbacks need to be overcome before its industrial application, such as low electronic conductivity of sulfur, serious shuttling effects of dissolution polysulfides, volume change of electrode during charging-discharging process and problems such as lithium dendrites, passivation and corrosion of lithium anode. This work aims to improve the properties of Li-S batteries by preparation of sulfur/carbon composite cathode materials and building high-efficiency interlayer .[2-4] Firstly, a modified lyophilization approach is developed and used for highly efficient transformation of 2D graphene oxide sheet into 1D graphene nanoscroll (GNS) with high topological transforming efficiency (94%). Because of the unique open tubular structure and large specific surface area (545 m2 g−1), GNS is utilized for the first time as a porous cathode scaffold for encapsulating sulfur with a high loading (81 wt%). The interlayer space and pores of obtained GNS provide large sites for sulfur loading, and this unique structure can accommodate volume expansion and prevent the dissolution of polysulfides. As a result, the composite deliver good rate performance and cycling stability (the capacity decay is only 0.04% per cycle after 400 cylces at 2C).[5] Furthermore, a flexible self-standing hybrid interlayer is engineered by intertwining one-dimensional (1D) metal oxides nanowires (NWs) with GNS into a robust interconnected 3D network to be used as both an upper current collector and physical/chemical polysulfide-trapper. Combining GNS with MnO2, V2O5 and MoO3 respectively, free-standing membranes are obtained to serve as interlayers for Li-S battery (namely, GNSMn, GNSV and GNSMo). The highly conductive and open GNS framework provides bicontinuous transfer channels for rapid electron and ion transport across the cathode/separator interface. Meanwhile, the metal oxides nanowires act as ideal redox mediators to dynamically block polysulfide dissolution and facilitate their conversion into sulfides via formation of active intermediate polythionate complexes. Even though GNSV is used as an interlayer for lithium sulfur battery with bare sulfur as cathodethese cells with 70 wt% sulfur content in the whole cathode have significantly improved performance with a long cycling life (1000 cycles), low capacity decay (0.041% per cycle), and considerable areal capacity (>4 mA h cm- 2), even at a high sulfur loading of 5.5 mg cm- 2. This novel interlayer design strategy has great potential for promoting the practical use of Li–S batteries.[6] In summary, the work described here shows a potential of GNS as a promising functional carbon material to realize high sulfur loading and electrochemically stable electrode configuration aimed toward developing high-performance Li−S batteries. Figure 1 (a) SEM images and (b) TEM images of GNS, (c) schematic illustration on the advantages in entrapping polysulfides of the S@GNS (insert: typical colors of the cycled S@GNS and Gs-S cathodes after soaking in a mixture of DOL/DME for 24 h), (d) schematic model of the cell (insert: photos of the free-standing and flexible GNSM interlayer).