Embedding amorphous lithium vanadate into carbon nanofibers by electrospinning as a high-performance anode material for lithium-ion batteries

https://doi.org/10.1016/j.jcis.2020.06.111Get rights and content

Highlights

  • Amorphous LiV3Ox was encapsulated into carbon nanofibers via electrospinning and subsequent annealing.

  • The amorphous LVO@CNFs demonstrated superior lithium storage performance compared with the crystalline LiV3O8.

  • The amorphous structure and carbon hybridization contributed to the enhanced electrochemical properties.

Abstract

We design and fabricate a novel hybrid with amorphous lithium vanadate (LiV3Ox, LVO for short) uniformly encapsulated into carbon nanofibers (denoted as LVO@CNFs) via an easy electrospinning strategy followed by proper postannealing. When examined for use as anode materials for lithium-ion batteries (LIBs), the optimized LVO@CNFs present a high discharge capacity of 603 mAh g−1 with a capacity retention as high as 90% after 200 cycles at 0.5 A g−1 and a high rate capacity of 326 mAh g−1 after 400 cycles even at a high rate of 5 A g−1. The superior electrochemical performance with excellent cycling stability and rate capability is attributed to the full encapsulation of the amorphous LVO into the conductive carbon nanofibers, which hold enlarged electrochemically active sites for lithium storage, facilitate the charge transfer, and efficiently alleviate the volume changes upon lithium insertion/extraction. More importantly, the current synthesis can be a general strategy to fabricate various alkaline earth metal vanadates, which is promising for developing advanced electrochemical energy storage devices.

Introduction

Lithium-ion batteries (LIBs) with higher energy density, better safety, and longer cycling lifespan are urgently required in the fast advancing market of electric vehicles, portable electronic products and large-scale stationary energy storage devices [1], [2]. To meet the demands, great efforts have been devoted to searching for advanced anode materials, including those of intercalation, alloy and conversion reaction types [3], [4], [5], [6]. Graphite and Li4Ti5O12 are typical intercalation-type anode materials used in commercial LIBs. However, both of them suffer from limited theoretical capacity (372 mAh g−1 for graphite, 175 mAh g−1 for Li4Ti5O12) and inappropriate Li+ insertion potential, resulting in insufficient security or limited energy density [7], [8]. The alloy-type anode materials can deliver higher theoretical capacities [9]. However, the fast capacity fading, which is caused by volume change induced pulverization problems, hinders their use in large commercial applications [10]. The conversion-type anode materials, such as transition metal oxides/sulfides, can deliver capacities 2–3 times that of graphite, but their poor energy efficiencies and high reaction voltages make them impractical for use in high energy battery systems [11].

Among various electrode materials for LIBs, lithium vanadates, such as LiVO3 [12], LiV3O8 [13], LiV2O5 [14] and Li3VO4 [15], have been applied as cathodes in LIBs, owing to their superior intercalation properties for alkaline ions [16]. In particular, LiV3O8 has been widely studied for decades as a promising substitute for the expensive and toxic cathode material of LiCoO2 [17]. Recently, LiV3O8 was also investigated as an anode for aqueous lithium batteries, but it suffers from severe capacity fading and poor rate capability upon cycling [18]. Based on our knowledge, there are few reports on using LiV3O8 as an anode for organic LIBs. To date, only Li3VO4 has been studied as an LIB anode, because of its good ionic conductivity, suitable working plateau (0.5–1.0 V vs. Li/Li+) and high theoretical capacity compared to graphite and Li4Ti5O12 [7], [19]. However, the intrinsic low electrical conductivity, large volume variation under deep discharging/charging conditions and poor reversible capacity restrict their applications. To overcome these drawbacks, various strategies have been developed to improve the electrochemical properties of lithium vanadate anodes, such as introducing conductive components (e.g., carbon [20], Ni foam [21]), and nanostructuring [22]. For example, Xu et al. synthesized porous N-doped carbon wrapped Li3VO4 nanoparticles, which demonstrated superior lithium storage performance, delivering a capacity of ~405 mAh g−1 at 0.1 A g−1 and 236.6 mAh g−1 after 1000 cycles at 4.0 A g−1 [23]. Liu et al. demonstrated that polypyrrole-coated LiV3O8 nanorods showed enhanced electrochemical performance as an anode for aqueous lithium batteries, compared to pristine LiV3O8 [24].

It is well recognized that the physicochemical properties of materials are highly dependent on their structures in the nano/microscale, and the rational structure design greatly relies on the synthetic strategies [25], [26]. Electrospinning is an easy method to prepare fibrous carbon-based nanocomposites with high electronic conductivity, and various exotic components can be easily encapsulated into the carbon matrices, which is a widely used strategy to fabricate electrode materials for electrochemical energy storage applications [27], [28]. In addition, it has been suggested that amorphous structures with a disordered lattice and defects can provide more active sites, multiple ion diffusion pathways, reversed lattice variations and volumetric changes upon cycling of ion insertion/extraction, thus enhancing the electrochemical performance of the electrodes [29], [30], [31].

Herein, we demonstrated one-pot fabrication of amorphous LiV3Ox within carbon nanofibers (denoted as LVO@CNFs) via easy electrospinning and the following heat-treatment. To the best of our knowledge, we are the first to apply the amorphous LiV3Ox as an anode and studied its lithium storage behavior for normal organic LIBs. Owing to the uniform and full encapsulation of the amorphous LVO into the conductive CNFs matrices, the LVO@CNFs showed much enhanced lithium storage performance, including high reversible capacity, superior cycling stability and rate capability, when compared with the crystalline LiV3O8 counterpart. Most importantly, the strategy can be commonly applied for the fabrication of various alkaline earth metal vanadates, which is promising for developing high-performance electrodes for advanced energy storage devices.

Section snippets

Materials and LVO@CNFs preparation

All the chemicals, including N,N-dimethylformamide (DMF, C3H7NO, 99.5%, J&K Chemical), vanadium (III) acetylacetonate (C15H21O6V, 97%, Macklin), lithium hydroxide monohydrate (LiOH·H2O, 98%, Alfa Aesar), polyvinylpyrrolidone (PVP, (C6H9NO)n, average Mw = 13000000, Macklin) and polyacrylonitrile (PAN, (C3H3N)x, average Mw = 150000, Macklin), were adopted as-received with no further purification.

LVO@CNFs were prepared by electrospinning and subsequent annealing at different temperatures.

Results and discussion

Fig. 1a compares the XRD patterns of the different LVO@CNFs, which were obtained by annealing the precursors at different temperatures under an Ar atmosphere. No well-defined diffraction peaks are observed, indicating the amorphous nature of the LVO in the LVO@CNFs [32]. The broad peak located at approximately 26.7° is attributed to the (0 0 2) plane of the graphitic carbon but with poor crystallization [33]. In contrast, it should be noted that the as-prepared amorphous LVO@C500 would convert to

Conclusion

In summary, we first demonstrated the lithium storage performance of LiV3O8 based materials as LIB anodes and provided a useful strategy to enhance their electrochemical properties by combining structure amorphorization and carbon hybridization. The as-prepared hybrid with amorphous LiV3Ox uniformly encapsulated into defective carbon nanofibers (i.e., LVO@CNFs) displayed high specific capacity, superior cycle stability and rate ability, delivering a capacity of 603 mAh g−1 after 200 cycles at

CRediT authorship contribution statement

Ting Liu: Conceptualization, Methodology, Investigation, Writing - original draft. Tianhao Yao: . Li Li: Writing - review & editing. Lei Zhu: Investigation, Data curation. Jinkai Wang: Investigation, Data curation. Fang Li: Investigation, Data curation. Hongkang Wang: Conceptualization, Methodology, Writing - review & editing, Supervision, Project administration, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant No. 51402232 and 51905236), China Postdoctoral Science Foundation (Grant No. 2019M663695), the State Key Laboratory of Electrical Insulation and Power Equipment (Grant No. EIPE19127), the Natural Science Foundation of Jiangsu Province (Grant No. BK20170314) and the Qinglan Engineering Project of Jiangsu Universities.

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