Vertical Aligned Graphene/Metal Sulfides/N-Doped Carbon Ternary Sandwich Hierarchical Structure for Sodium Storage

Thursday, 1 June 2017: 09:20
Churchill C1 (Hilton New Orleans Riverside)
D. Xie (Zhejiang University), X. Xia (zhejiang University), and J. Tu (Zhejiang Univerity)
Nowadays, sodium ion batteries (SIBs), one of the realistic promising alternatives to the lithium ion batteries (LIBs), have drawn increasing attention because of the high availability, the low cost and the intercalation chemical similarity of sodium compared with lithium. However, many active materials in application of LIBs are invalid for SIBs as far as the different ionic radium (Na+ ,1.06 Å vs. Li+ ,0.76 Å). Thus, much effort has been taken to explore suitable host material to allow reversible and rapid sodium ion insertion or extraction, especially for advanced anode materials.

Fortunately, the transition metal dichalcogenides (TMDs) with the formula MoX2 (S or Se) and graphene-like structure are widely studied as anode materials for SIBs due to the unique two-dimensional layered structure and high specific capacity. Nevertheless, the practical application of the graphene-like TMDs is impeded by its inherent limitations. First, layered TMDs nanosheets are so easy to restack with the van der Waals interaction, resulting into the decrease of accessible active sites for sodium ions. Second, compared with carbon-based materials, especially, graphene or carbon nanotubes, the TMDs have lower electrical conductivity, which is not favor electrode reactions. Third, the volume change and structural variation during discharge-charge process cause the pulverization and loose contact of the active materials, resulting rapid capacity fading. Fourth, the TMDs-based anodes face the dissolution issue of polysulfides or polyselenides, resulting in fast capacity fading and low Coulombic efficiency during the discharger-charge process. To combat the aforementioned problems, in this present work, we have made double modifications on the MoX2 by introducing the vertical aligned graphene (VG) as support backbone and nitrogen-doped carbon (N-C) coating as outer protective layer via a simple two-step process. The curved MoX2 nanoflakes are controlled deposited on the VG skeleton, which the cloth fabric server as current collector, via a facile hydrothermal method; subsequently, the N-C layer uniformly coats on the MoX2 nanoflakes by self-polymerizing dopamine followed by a post carbonization process. These samples are directly assembled into SIBs without any binders or conductive materials, and exhibit excellent electrochemical performance for SIBs. For example, our synthesized VG/MoSe2/N-C ternary composite exhibits superior storage sodium performance, with high capacity (540 mAh g-1), enhanced high rate capability and long-term cycling stability (298 mAh g-1 at 2.0 A g-1 after 1000 cycles). These designed ternary sandwich VG/MoX2/N-C ternary sandwich hierarchical structures, which provide a flexible vertical aligned graphene to accommodate volume changes, introduces large efficient active areas, good conductivity, and omnibearing paths for sodium ion and electrons, contributes to their superior performances and enables its application in energy storage.