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Energy, Environmental, and Catalysis Applications
Microwave-Assisted Synthesis of CuS Hierarchical Nanosheets as Cathode Material for High-Capacity Rechargeable Magnesium Batteries Zhitao Wang, Souleymen Rafai, Chen Qiao, Jian Jia, Youqi Zhu, Xilan Ma, and Chuanbao Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20533 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 23, 2019
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ACS Applied Materials & Interfaces
Microwave-Assisted Synthesis of CuS Hierarchical Nanosheets as Cathode Material for High-Capacity Rechargeable Magnesium Batteries Zhitao Wang, Souleymen Rafai, Chen Qiao, Jian Jia, Youqi Zhu, Xilan Ma, and Chuanbao Cao* Research Center of Materials Science, Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green Applications, Beijing Institute of Technology, Beijing 100081, China ABSTRACT: Rechargeable magnesium batteries (rMBs) have been triggered as one of most promising next-generation energy storage devices with high energy and power density. However, the development of rMBs has been hampered by the lack of usable cathode materials with high capacity and cycling stability. Herein, we report an ultrarapid, cost-effective and scalable synthesis of ultrathin CuS hierarchical nanosheets by a one-step microwave-assisted preparation. Benefiting from the exceptional structural configuration, when served as cathode material for rMBs at room temperature, the CuS hierarchical nanosheets deliver a high reversible discharge capacity of 300 mAh g-1 at 20 mA g-1, remarkable rate capability (256.5 mAh g−1 at 50 mA g−1 and 237.5 mAh g−1 at 100 mA g−1), and excellent cycling stability (135 mAh g−1 at 200 mA g−1 over 200 cycles). To date, the obtained excellent electrochemical performances are superior to the most ever reported results for cathode materials of rMBs. 1
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KEYWORDS: CuS hierarchical nanosheets, cathode, high capacity, microwaveassisted synthesis, rechargeable magnesium batteries 1.
Introduction Nowadays, rechargeable magnesium batteries (rMBs) are rising as one of the most
auspicious energy storage technologies due to their low cost, high theoretical energy density, and no dendritic hazards1-3. However, a usable cathode material is the bottleneck to find their way for practical applications4. To date, the researchers’ work have developed many types of cathodes materials for rMBs, including transition metal oxides/sulfides, polyanion compounds, and even some organic materials5. However, only a few cathode hosts for rMBs showed reasonable reversible capacity and cycling stability. Among them, transition metal sulfides usually showed good cycling stability but with low capacity, such as Chevrel phases Mo6X8 (X = S, Se)6-7, MoS28-9, layered TiS210-11, Ti2S412 ,VS213 and VS414. Transition metal oxides, for examples, V2O515-16, VTi2.6O7.217, MnO218-20 and MoO321, generally exhibited higher voltage but unsatisfying cycling endurance due to sluggish electrode kinetics caused by the high polarizing ability of the divalent Mg2+. Till now, the most successful cathode material for rMBs is still the Chevrel phase reported by Aurbach’s group, which exhibits excellent reversibility and cyclability6. However, the low theoretical specific capacity (120 mAh g−1) of Chevrel phase making it energetically uncompetitive. So, the development of alternative materials with high capacity and long cycling stability, as well as novel synthesis approaches, are urgently required. 2
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As a very promising candidate material, CuS has shown encouraging electrochemical performances as cathodes material for rMBs as its high theoretical specific capacity (560 mAh g-1) 22-23. However, the CuS usually exhibits a reversible capacity that is much lower than its theoretical value at room temperature due to the Mg2+ trapping in the host material24. Thus, it is challenging to find an efficient way to boost its reversible capacity. Recently, Wu et al.25 compared two different sizes of CuS particles (about 100 nm) as cathode material for rMBs, which demonstrated that the small-sized particles deliver a relatively higher reversible capacity of 175 mAh g−1 at 50 mA g−1. In contrast, the capacity of large-sized particles of CuS is much lower (90 mAh g−1). In light of the facts above, the search of suitable nanostructures of CuS helps to increase its reversible capacity. On the other hand, the nanostructured downsizing of the cathode's active material has been proved to be an effective strategy to facilitate the magnesium ion insertion kinetics24,
26-28.
In this regard, nano-sized materials are
considered as a promising route to achieve comparable capacity at room temperature. However, only downsizing the active materials with simple nanostructures (nanoparticles, nanowires, nanosheets, etc.) usually induces severe agglomeration during cycling process, resulting the reversible capacity fading. In contrast, 3D hierarchical nanostructures not only inherit the advantages of the individual nanostructure but also have multiple collection advantages, such as coupling effect and synergistic effect29. Thus, the hierarchical CuS nanostructures could be an ideal choice to enhance the Mg2+ diffusion kinetics and to alleviate the polarization due to the 3
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significantly huge specific surface areas, robust mechanical strength, shortened paths for charge and mass transfer, and improved electric conductivity30-31. Therefore, it is greatly desirable to rationally design and fabricate hierarchical CuS nanostructures as cathodes for rMBs. To address the above challenges, this work reports an ultrafast, cost-effective, and scalable microwave-assisted synthesis of hierarchical CuS nanosheet assemblies (NSA). The CuS NSA consist of numerous self-assembled and wrinkled sheets with a large planar area (up to hundreds of nanometers) and ultrathin thickness about 4 nm. When used as cathode material for rMBs at room temperature, the well-crystalline CuS NSA exhibit a high reversible discharge capacity of 300 mAh g−1 at 20 mA g−1. Furthermore, the CuS NSA demonstrates excellent reversible specific capacity of 135 mAh g−1 over 200 cycles at high current density of 200 mA g−1. The excellent energy storage performances of CuS NSA are ascribed to their unique structural configuration, where the ultrathin and two-dimensional morphology maximizes the contact between the electrolyte and the cathode’s active material to reduce electron/ion migration barriers; the 3D hierarchical feature limits the self-aggregation of the sheets and accommodates the volume expansion for satisfying cyclic stability. Briefly, our work provides an effective route to optimize and exploit new types of electrode materials with high specific capacity and long-term cycling stability for rMBs. 2.
Experimental Section
2.1 Preparation of the electrolyte 4
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In this work, the electrolyte preparation was carried out involves three steps as reported in previous literature reports32-33. The whole procedure was conducted in a glovebox (
