Hollow NiâCoSe2 Embedded in Nitrogen-Doped ... - ACS Publications

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Hollow Ni-CoSe2 Embedded in Nitrogen-Doped Carbon Nanocomposites Derived from MOFs for High Rate Anodes Wan Liu, Meng Shao, Weiqiang Zhou, Bo Yuan, Cong Gao, Hongfeng Li, Xiujie Xu, Huimin Chu, Yun Fan, Weina Zhang, Sheng Li, Junfeng Hui, Daidi Fan, and Fengwei Huo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08861 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018

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ACS Applied Materials & Interfaces

Hollow Ni-CoSe2 Embedded in Nitrogen-Doped Carbon Nanocomposites Derived from MOFs for High Rate Anodes Wan Liu1, Meng Shao2, Weiqiang Zhou2, Bo Yuan2, Cong Gao2, HongFeng Li2, Xiujie Xu2, Huimin Chu1, Yun Fan2, Weina Zhang2, Sheng Li2, Junfeng Hui*1, Daidi Fan*1, and Fengwei Huo*2

1. Shaanxi Key Laboratory of Degradable Biomedical Materials, Shaanxi R&D Center of Biomaterials and Fermentation Engineering, School of Chemical Engineering, Northwest University, Xi'an 710069, Shaanxi, PR China.

2. Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211800, China

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ABSTRACT

Developing high rate anode materials with large capacity for lithium ion batteries (LIBs) is quite necessary for the booming electric vehicles industry. The utilization of stable and conductive hollow structures for electrode composite materials could make the desired performances possible in the future. Thus, in this study, a hollow structured Ni-CoSe2 embedded in N-doped amorphous carbon nanocomposite (Ni-CoSe2@NC) has been successfully synthesized with MOFs as precursors. Such strategy integrates both the merits of the multi-components and the hollow structure: the latter could facilitate both mass and charge transport; and the former (the N-doped carbon) could not only offer plenty of surface defects, improving the surface capacitive contributions, but also stabilize the electrode structure during charge/discharge processes. As a result, the metal selenide composite delivers outstanding high rate properties with good stability as anode for LIBs. The structure and components design could also be extended to other anode composites in the future.

KEYWORDS: metal-organic frameworks, hollow nanostructure, nitrogen-doped carbon, lithium ion battery, anode

INTRODUCTION

As the air pollution becoming intensely serious, the development of environmentally


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friendly energy storage and utilization technologies is imperative.1-4 Rechargeable lithium ion batteries (LIBs) have been broadly applied in portable electronics and have potential applications in electric vehicles.5-6 The electrode material for LIBs is one of the most important determinants for their performance.7-9 During the past few years, transition metal selenides (TMS, such as CoSe2,10 ZnSe,11 and NixCo1-xSe12) as anode materials have attracted growing attention because of their relatively large theoretical capacity, affordability of the elements and relatively higher electrical conductivity than oxides or sulfides.13-14 However, the practical applications of TMS are limited due to their poor cycle stability and rate performance caused by the sluggish ion/electron-transport kinetics as well as the obvious volume change during the repetitive charging/discharging processes.15-16

To address the above two limitations, ideas of structural and component design have been put forward for preparing active materials. It has been proposed that hollow structural nanomaterials with functional shells and inner voids could not only provide large surface area but also facilitate both mass and charge transport.17-19 The construction of TMS with hollow structure is beneficial for improving its ion/electron transport and expanding its capacity. Metal-organic frameworks (MOFs), a class of porous crystalline materials, enjoy the advantages of large surface areas, controllable structures, permanent porosity, etc., and therefore they have usually been selected as precursors for constructing hollow structures.20-25 For example, Shao et al. reported the hollow Co3O4 dodecahedrons with controllable interiors by direct pyrolysis of ZIF-67.26 Such hollow dodecahedrons



exhibited a capacity high of 1550 mAh g-1 yet with unsatisfied rate performance. As the hollow structure suffers from structural collapse, it is usually difficult to effectively improve the stability of the electrode.

Thus, in addition to the structure design, coating a carbon layer on hollow structure could probably be a strategy to prevent electrode aggregation and collapse and to buffer volume change during Li+ insertion/extraction processes.27-29 Moreover, nitrogen (N) doping could generate plenty of defects because the electronegativity of N (3.5) is higher than that of C (3.0), which could further facilitate Li+ capacitive storage properties.30 Huo et al. reported the synthesis of mesoporous N-doped carbon hollow spheres,31 which showed superior reversible capability than mesoporous carbon hollow spheres structure. Zhang et al. employed ZIF-67 as a precursor to synthesize CoS2 nanoparticles in N-doped carbon nanotube hollow frameworks.32 Such composite enhanced Li ion storage capability with high specific capacities at various current densities. It was also reported that the Li ion storage capacity of N-doped graphene depended largely on the N-doping level.30 Although the hollow TMS and N-doped carbon composites can be directly generated from MOFs, the content of carbon and nitrogen as well as the carbon conductive skeleton structure are still expected to be further improved. Capacitance of LIBs during charge/discharge processes is comprised of pseudocapacitance and double electronic layer capacitance. The Ni-CoSe2 nanoparticles and NC with active sites could contribute the pseudocapacitance, whereas the double electronic layer capacitance can be demonstrated in the hollow structure. Amorphous carbon has lower level of sp2 hybrid orbital than graphitized carbon, which makes more defects exposed and contributes to the capacitance of Li ion storage.33-34 Herein, we developed a strategy to synthesize polyhedral hollow NiSe2-CoSe2 nanocomposites embedded in N-doped amorphous carbon (Ni-CoSe2@NC) to meet the challenges of TMS for LIBs anodes. This


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strategy employs zeolitic imidazolate framework 67 (ZIF-67) as template to construct the hollow structure, and polydopamine (PDA) as precursors to fabricate N-doped carbon through pyrolysis process. Several advantages are expected to be achieved with NiCoSe2@NC nanocomposites serving as anode active materials for LIBs. First, nanosized materials and hollow structures could provide high contact surface area between electrode materials and the electrolyte, which could shorten the transport paths of both electrons and Li ions, facilitating the reaction kinetics of redox reactions. Second, the carbon frameworks could effectively buffer the volume changes during charge/discharge processes for NiCoSe2 nanoparticles, enhancing structure stability. Moreover, rich defects N-doped amorphous carbon could enhance electrical conductivity and also provide active sites that may adsorb Li ions on the surface, contributing surface capacitive to the total capacity during high rate conditions. Thus, with the interactions and synergistic effects between metal selenides and N-doped amorphous carbon, the nanocomposites could achieve satisfied specific capacity, excellent rate performance and cycling stability as anode materials for LIBs.

EXPERIMENTAL SECTION

Synthesis of ZIF-67. In a typical synthesis, 1.455 g Co(NO3)2·6H2O and 1.642 g 2-methyl imidazole were dissolved in 80 mL methanol, respectively. After mixing the above two solutions, the mixture was kept for 24 h at room temperature. The product was collected by centrifugation at 7000 rpm for 5 min and washed by ethanol for 3~5 times. Finally, ZIF67 powder was dried at vacuum oven overnight.

Synthesis of hollow NiCo-LDH. First, 80 mg ZIF-67 powder was dispersed in 15 mL ethanol, and 150 mg Ni(NO3)2·6H2O was dissolved in 10 mL ethanol. Then, nickel nitrate



solution was quickly added into the ZIF-67 solution. The mixed solution under stirring was heated at 90 oC for 20 min. Finally, the product was collected by centrifugation and vacuum-dried overnight.

Synthesis of hollow NiCo-LDH@PDA. First, 50 mg NiCo-LDH was dispersed in 25 mL Tris buffer solution (pH=8.5), followed by addition of 50 mg dopamine (DA). The mixture was stirred for 24 h to obtain NiCo-LDH@PDA. The product was collected by centrifugation with DI water and ethanol, respectively, and then vacuum-dried overnight.

Synthesis of Ni-CoSe2 and Ni-CoSe2@NC hollow nanocomposites. The prepared NiCoLDH (or NiCo-LDH@PDA) was mixed with selenium powder under a mass ratio of 1:2 followed by being heated at 350 oC for 90 min with temperature increasing rate of 1 oC min-1.

Synthesis of NC derived from ZIF-67. The prepared ZIF-67 was mixed with selenium powder under a mass ratio of 1:2 followed by being heated at 600 oC for 120 min with temperature increasing rate of 5 oC min-1.

Electrochemical measurements. Electrochemical measurements were carried out at room temperature based on a coin-type half-cell. The slurry was prepared by mixing together the active materials, super-P carbon black and sodium carboxymethyl cellulose (CMC) under a mass ratio of 7:2:1 in water. The slurry was coated onto Cu foil and dried at room


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temperature. Cu foil was then transferred to vacuum oven at 60 oC for 6 h to remove solvent. The CR 2025 type coin cells were assembled in an argon-filled glove box (H2O

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