Regular Article
Embedding CoMoO4 nanoparticles into porous electrospun carbon nanofibers towards superior lithium storage performance

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Abstract

CoMoO4 nanoparticles have been successfully in-situ formed and simultaneously embedded within the porous carbon nanofibers (CoMoO4/CNFs) via a facile electrospinning-annealing strategy. The porous CoMoO4/CNFs exhibit a specific surface area of 255.3 m2/g and a pore volume of 0.52 cc/g with average pore diameter of 43.5 nm. The carbon content in the CoMoO4/CNFs can be readily controlled by adjusting the annealing temperature. When examined as anode materials for lithium ion batteries (LIBs), the CoMoO4/CNFs demonstrate superior electrochemical performance, delivering a high reversible capacity of 802 mA h/g after 200 cycles at 200 mA/g and a high-rate capacity of 574 mA h/g at 2000 mA/g. The excellent lithium storage behavior can be attributed to the incorporation of CoMoO4 nanoparticles into the porous N-doped graphitic carbon nanofibers, which efficiently buffer the volume changes of CoMoO4 upon lithiation/delithiation and maintain the overall electrode conductivity/integrity.

Introduction

Lithium ion batteries (LIBs) have been considered as one of the most promising energy storage systems owing to their high energy density, no memory effects, long cycle life and environmental benignity, which have been extensively used in various electrical fields ranging from portable electronics such as mobile phones to large-scale devices including electric vehicles and smart-grid [1], [2], [3], [4], [5]. Commercial LIBs using graphite anode, which possesses a low specific theoretical capacity of only 372 mA h/g, can’t satisfy the fast expanding energy-storage demand [5], [6], [7], [8]. To fulfill the requirement on large-scale applications, development of alternative electrode materials with higher specific power and energy densities has been receiving increasing attentions [9], [10], [11], [12].

Among various candidate anode materials, transition metal molydates have emerged as prospective materials for energy storage [13], [14], [15], [16], [17], [18]. As a representative, CoMoO4 has attracted great interests owing to its high theoretical capacity of 980 mA h/g [19], [20], [21], [22]. However, the CoMoO4 anode suffers from intrinsic insufficient conductivity and relatively severe volume changes upon lithiation/delithiation, which result in the poor electrochemical properties and thus hinder its practical application. To address these problems, rational structure design and efficient material synthesis have been focused in order to develop novel CoMoO4 nanostructures with high energy storage capability and excellent cycling stability [23]. For example, Zhang et al. fabricated CoMoO4 submicrometer particles with interconnected networks through thermolysis of polymer-metal precursor solution [19]. Chen et al. demonstrated the fabrication of hierarchical three-dimensional CoMoO4/polypyrrole core-shell nanowire arrays on carbon cloth by a two-step solution based route [24]. Dong et al. prepared various micro/nano-structured CoMoO4 via pH-dependent solvothermal approach and the as-prepared hollow hydrangea-like CoMoO4 electrode displayed superior electrochemical properties [25]. All of the as-prepared CoMoO4 nanostructures demonstrated great improvement on lithium storage properties, but which were all prepared via solution-based synthetic strategies such as hydrothermal and solvothermal methods, which showed limitations on large scale fabrication. Therefore, development of efficient synthetic route with delicate nanostructure design and easiness to scale-up is still challenging.

Electrospinning is an efficient facile technique to fabricate one-dimensional nanostructures with delicate composition control, which can be easily scaled up [26], [27], [28]. To the best of our knowledge, electrospinning has been rarely used for synthesis of CoMoO4 nanostructures. Only Xu et al. have demonstrated the electrospinning synthesis of CoMoO4(@graphene) nanofibers, but which were obtained by only annealing in air [20]. The as-obtained pure CoMoO4 nanofibers can only deliver a capacity of 321 mA h/g after 85 cycles, while the CoMoO4@graphene nanofibers delivered a reversible capacity of 735 mA h/g after 200 cycles at a current density of 100 mA/g. Apparently, introduction of carbon-based nanomaterials can greatly improve the lithium storage performance. It’s noteworthy that the polymers used for electrospinning can be excellent carbon source, such as polyvinyl pyrrolidone (PVP) and polyacrylonitrile (PAN) with higher N content can be used for fabrication of N-doped carbon nanofibers with higher electric conductivity, but which can only be obtained by annealing under inert atmosphere at high temperature [29], [30]. At this temperature, the as-produced carbon is reductive, which would reduce or react with the oxidized metal species and cannot be used to directly synthesis CoMoO4 nanomaterials in the carbon matrices.

Herein, we successfully developed the synthesis of porous CoMoO4/CNFs by electrospinning followed by a two-step annealing strategy. The first annealing under inert atmosphere allowed the carbonization of polymer forming graphitic crystalline carbon nanofibers with the in-situ generated transition metal nanoparticles as catalysts, while the second annealing in air allowed the oxidation of Co-Mo intermediates by consuming partial carbon matrices, which resulted in the formation of porous nanostructure. The carbon content in the CoMoO4/CNFs can be tuned by the annealing temperature. When used as LIB anode, the CoMoO4/CNFs demonstrated superior electrochemical behaviors due to the novel porous interconnected nanostructures.

Section snippets

Materials synthesis

All chemicals, including polyacrylonitrile (PAN, (C3H3N)n, Macklin), N,N-dimethyformamide (DMF, C3H7NO, Macklin), cobalt(II) acetate (C4H6CoO4, Co(ac)2, Macklin) and molybdenyl acetylacetonate (C10H14MoO6, Mo(acac)2, Macklin), were used as received without any further treatment. In a typical synthesis of the CoMoO4/CNFs, electrospinning and subsequent annealing were used. First, a precursor solution for electrospinning was prepared by dissolving 1.0 g PAN, 2 mmol Co(ac)2 and 2 mmol Mo(acac)2 in

Results and discussion

Fig. 1a schematically illustrates the synthetic procedure of fibrous CoMoO4/CNFs via electrospinning and subsequent annealing. First, precursor nanofibers of Mo(acac)2/Co(Ac)2/PAN were prepared via electrospinning (Fig. S1), which were subsequently annealed at 700 °C for 2 h in Ar atmosphere. During this process, PAN carbonized and then reduced or reacted with the Co/Mo sources, in-situ forming Co/MoC nanoparticles which were encapsulated into the carbon nanofiber matrices (denote as

Conclusions

In summary, we have demonstrated the in-situ formation of CoMoO4 nanoparticles within porous carbon nanofibers (denote as CoMoO4/CNFs), in which CoMoO4 nanoparticles were cross-linked by the porous N-doped carbon nanofibers, via electrospinning and subsequent annealing under inert/air atmospheres. The annealing in inert atmosphere allowed the carbonization of the PAN with graphitic crystallization under the catalytic effect of the in-situ generated Co/Mo species, which were produced through

Acknowledgments

This work was supported by the Natural Science Basis Research Plan in Shaanxi Province of China (No. 2018JM5085), State Key Laboratory for Modification of Chemical Fibers and Polymer Materials (Grant No. KF1806) of Donghua University, and the Key Laboratory Construction Program of Xi'an Municipal Bureau of Science and Technology (201805056ZD7CG40). H.W. appreciates the support of the Tang Scholar Program from the Cyrus Tang Foundation. We thank Dr. Chao Li from the Instrument Analysis Center of

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