Active MoSx Nanoparticle Pillars Embedded in Reduced Graphene Oxide As Anode Materials for High Performance Na-Ion Batteries
Authors: Kim Suji , Kim You Jin , Ryu Won-Hee 
Published: December 2020

As the use of large-scale energy storage devices (i.e. electric vehicles, smart grids.) are increased in recent years, research on the energy storage system is constantly being developed.1 Li-ion batteries are extensively used for energy applications. However, there are some obstacles to its application to large-scale energy systems, such as the limited lithium resources, unstable supply, and cost issues.2 Na-ion batteries have been considered as one of the most promising alternative to Li-ion batteries owing to the low-cost and abundance of sodium element. However, the ionic radius of Na+ (1.06 Å) is larger than Li+ (0.76 Å).3 It leads to severe structural deformation of electrode materials, accompanied by phase change or structural pulverization during cycling.4 To effectively accommodate the larger Na ion into the electrode structure and enhance the cell performance, we need to find suitable anode material for Na-ion batteries. There have been many attempts to develop the carbonaceous electrode materials for Na-ion batteries such as hard carbon, N-doped porous carbon fiber, and expanded graphite.5
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6 Among them, the expanded graphite alternative could be a good choice due to (i) extended interlayer space for proper accommodation of large Na ions and (ii) stable structural feature with long range order for sufficient stress relaxation during Na intercalation. However, side-reactions between Na ions and a lot of anion pillars (e.g. epoxide and hydroxyl) are often found in the carbon layers, resulted in degradation of the cell performances. Therefore, proper pillar candidate should be introduced to optimize the expanded graphite anode for high performance Na-ion batteries.

A representative layered 2-D molydisulfide (MoS2) has been recognized as a promising anode material for Na-ion batteries.7 The MoS2 materials operate via intercalation of sodium at the beginning and a subsequent four-electron conversion reactions.8 The S-Mo-S layer are stacked by van der Waals forces, providing a large interlayer spacing. The large interlayer spacing of MoS2 (~6.2 Å vs. 3.4 Å of graphite) could allow facile transport of Na+ during initial intercalation reaction.9 However, MoS2 has low electronic conductivity and internal strain during sodiation/desodiation. Therefore, many strategies have been considered to develop the cell performance with tailored design, morphology control, (nanocomposite, core-shell structure etc.), and hybridization with carbon materials (carbon nanofibers, amorphous carbon etc.)10
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In this presentation, we introduce the expanded graphite material employing MoSx as bi-functional conversion reaction anode and pillar species between graphite interlayers. The MoSx-expanded graphite anode material synthesized via self-assembly of poly-cationic molybdenum-sulfide source and negatively charged graphite oxide flakes. We demonstrate that the morphological and structural changes of crystalline MoSx pillars under the different thermal temperature in reduction conditions. We attempt to solve the irreversible capacity problems by chemically forming SEI-layer on the electrode surface. We also elucidate the electrochemical reaction mechanisms and structural changes during charge/discharge reaction.

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