Fibrous network of highly integrated carbon nanotubes/MoO3 composite bundles anchored with MoO3 nanoplates for superior lithium ion battery anodes
Graphical abstract
Introduction
High energy, power density, and long cycle life time of Li-ion batteries (LIBs) have been considered as key issues with the increasing demand for large-scale energy storage such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and smart grids [1], [2], [3]. Therefore, unceasing efforts have been made to design and synthesize anode materials with excellent Li ion storage properties for LIBs by various nanostructuring strategies [4], [5], [6]. Sophisticated-designed nanostructured anode materials are the fundamental approach for enhancing LIBs performance because these provide a large contact area with the electrolyte, possess short lithium and electron pathways, and can accommodate strain during cycles [7], [8], [9]. Another effective strategy for high performance anodes is to composite transitional metal oxides with carbonaceous materials such as graphite, amorphous carbon, carbon nanotube (CNT), and graphene. These carbonaceous materials could compensate the low electrical conductivity of metal oxides anodes and accommodate the large strain induced by Li ion diffusion during cycling [10], [11], [12], [13].
Among carbonaceous materials, CNTs are representative candidates for use in LIBs due to their unique electrochemical properties. CNTs have been reported to have a very high electrical conductivity of 107 S m−1 [14]. Additionally, the high aspect ratio of CNTs compared to the other carbons such as carbon black and graphite, enables to establish an electrical percolation network by incorporating CNTs with a lower weight loading as a composite material [15], [16], [17]. It could enable the penetration of liquid electrolyte easily into the structure during cycles, which promotes the electrochemical reaction of host materials. Therefore, various structured CNT- metal oxide composites like as yolk-shell CNT-(NiCo)O/C microsphere [18], SnOx embedded in carbon nanofiber/carbon nanotube composite [19], Three-dimensional interconnected network GeOx/CNTs composite spheres [20], CNT-C@TiO2 composites with 3D networks [21], Mesoporus TiO2 spheres interconnected by CNT [22], as anode materials have been introduced for LIBs. Although, numerous CNT composite materials were proposed as anodes, the capacity and cycling stability are still not quite satisfactory for practical use. It is due to the non-uniform distribution of CNTs caused by van der Waals forces, phase segregation between metal oxides, and an insufficient amount of CNTs composited with metal oxides.
Recently, molybdenum trioxides (MoO3) have been considered as promising anodes for electrochemical energy storage devices owing to their high theoretical specific capacity of 1117 mA h g−1 and high stability with a layered structure [23]. Each layer is composed of two sub-layers, each of which is formed by corner-sharing octahedra along [001] and [100]; the two sub-layers stack together by sharing the edges of the octahedra along [001]. An alternate stack of these layered sheets along [010] would lead to the formation of MoO3, where a van der Waals interaction would be the major binding force between the piled sheets. Therefore, stable layered MoO3 structure is able to act as temporary support for the intercalation of lithium ions during charge/discharge process [23], [24]. Furthermore, MoO3 as an environmentally friendly n-type semiconductor material with a band gap of 3.1 eV is a highly attractive anode material due to its high electrical conductivities compare to other metal oxides [25], [26].
In this study, we proposed for the first time a unique fibrous network of highly-integrated CNTs/MoO3 composite bundle anchored with MoO3 nanoplates for anodes in LIBs. By performing the pre-acid-treatments of both CNTs and PAN, dipole-dipole interactions and hydrogen bonding between CNTs and PAN could form MoO2(acac)2-PAN-CNTs complex, which allows for the formation of a stable jet during spinning. As a result, fibrous network of highly-integrated CNTs without being aggregated was successfully obtained after electrospinning process. Subsequently, the amorphous carbon with lower electrical conductivity compared to CNTs, induced by PAN decomposition was selectively removed during the heat-treatment. The highly integrated CNTs/MoO3 composite bundle constitute network and these are efficiently interconnected each other and also contacted with MoO3 nano-plates. As a consequence, high reversible capacity, long cycle life stability, and superior rate properties of anodes could be achieved using the unique nanostructure proposed in this study.
Section snippets
Sample preparation
Fibrous network of highly-integrated CNTs/MoO3 composite bundle anchored with MoO3 nanoplates or pure MoO3 flat-rods were prepared by electrospinning process and a subsequent different heat-treatments, respectively. First, the colloidal solution for electrospinning was prepared by dissolving 3.0 g of Mo(Ⅲ) acetylacetonate [MoO2(acac)2, Junsei, 98.0%] and 1.5 g of sulfonated polyacrylonitrile [PAN, Mw: 150,000, Sigma-Aldrich] in a mixed solution containing 20 mL of dimethylformamide [DMF, Samchun,
Results and discussion
Fibrous network of highly-integrated CNTs/MoO3 composite bundle anchored with MoO3 nanoplates was prepared by electrospinning process and subsequent simple heat-treatment. The detailed structure formation mechanism of the unique MoO3/CNT composite fibrous-network was described in Scheme 1. In general, it is difficult to form a stable jet using a solution containing a large amount of CNTs along with PAN and metal precursors during electrospinning, which is due to the CNTs aggregation caused by
Conclusions
In this study, a unique fibrous network of CNTs/MoO3 composite bundle anchored with MoO3 nanoplates were prepared by electrospinning process and subsequent simple heat-treatment. In general, it was difficult to synthesize highly integrated and uniformly distributed metal oxide/CNT composite nanofibers because of CNTs aggregation by van der Waals forces and the phase segregation between precursors in a solution, which makes unstable jet during spinning. In this study, by performing the
Declaration of interests
There is no conflict to declare.
Acknowledgements
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIP) (NRF-2018R1A4A1024691, NRF-2017M1A2A2087577, and NRF-2018R1D1A3B07042514).
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