A cathode for Li-ion batteries made of vanadium oxide on vertically aligned carbon nanotube arrays/graphene foam
Graphical abstract
Introduction
Lithium-ion batteries (LIBs) plays a vital role in energy storage and conversion, particularly in mobile devices and electric vehicles due to its high energy density, fast charge–discharge rate, and long cycle life [1], [2]. Since its first discovery as a LIB cathode in 1976, vanadium oxide (V2O5) has attracted significant interest as cathode for LIBs because of its high theoretical capacity [3], [4]. It has an orthorhombic layered structure which can hold up to 3Li-ions per V2O5, resulting in a high theoretical specific capacity of 440 mAh g−1 with the potential window of 2.0–4.0 V [5]. However, there are several drawbacks for V2O5 cathode. Firstly, the structural changes are accompanied by multiple phase transitions (LixV2O5) as lithiation goes on [6], [7], including α (x < 0.01), ε (0.35 < x < 0.7), δ (0.9 < x ≤ 1), γ (0 < x ≤ 2), and ω (2 < x ≤ 3) phase [6], [8], [9], [10], [11]. Secondly, the co-existence of different phases within a single particle in the charge–discharge process induces large lattice strains, which causes irreversible structural damage and poor cyclability. Lastly, V2O5 also suffers a poor electronic conductivity (10−3–10−5 S cm−1) and a low ions diffusion coefficient (10−12–10−15 cm2 s−1) [12], which greatly limits its electrochemical performance and further practical applications. Therefore, it is essential to overcome these obstacles in designing V2O5 cathodes for LIBs. Nanosizing and hybridizing V2O5 with other materials are usually used to achieve this goal. Nanomaterials with the large surface area and short diffusion lengths for electrons and ions have attracted widespread attention [13], [14]. V2O5 nanorod arrays [15], nanofibers [16], nanoflakes [17], [18], nanowires [19], and nanocapsules [10] have been demonstrated to exhibit the improved performance. Moreover, the electronic conductivity of V2O5 could be enhanced by integration with carbon nanomaterials to form a conductive network [12], [20], [21], [22], [23], [24], [25], [26], [27]. For example, V2O5/carbon nanotube (CNT) composites have been reported as electrodes for supercapacitors [28], [29], [30], [31] and LIBs [32], [23], [33], [34], [35], [20] with significantly improved performance achieved. Furthermore, coating conducting polymer onto the surface of V2O5 has also been employed to prohibit the active material from dissolution and reduce the irreversible loss during consecutive charge/discharge process [12], [36], [37], [38].
Recently, the development of free-standing electrodes containing electroactive materials became an intensively studied area to achieve highly integrated electrodes with improved mechanical properties [39], [40], [41], [42], [43], [44], [45], [46]. By eliminating the use of metal current collectors and insulating binders which greatly reduce the conductivity of the electrode, higher energy densities and better rate performances can be delivered [47], [48]. The light-weight 3D porous graphene foam (GF) with its high electrical conductivity, mechanical robustness and porous structure is among the popular choices for a free-standing electrode substrate [42]. The growth of CNTs on GF can further maximize the surface area of the electrode that available to incorporate with electroactive materials [49], [50], [51], [52]. However, due to the requirement of explicit and well-controlled synthetic procedures, a free-standing composite of V2O5 on VA-CNTs/GF as a robust cathode for LIBs has not been reported.
In this work, we report a lightweight and free-standing PEDOT-V2O5-VA-CNTs/GF electrode for LIBs. The VA-CNTs were grown on GF, forming well defined regular porous structure with a large surface area for the uniform deposition of V2O5 nanobelts and provides appropriate intertube voids for the volume expansion/extraction during the charging and discharging processes, at the same time acting as a conductive backbone. In addition, conductive PEDOT layer was deposited uniformly on V2O5-VA-CNTs/GF facilitating the electron and ion transfer and protecting the active material. PEDOT-V2O5-VA-CNTs/GF electrode has a specific capacity of 428.6 and 296.8 mAh g−1 at the rate of 0.1C and 1C, respectively. A capacity of 113.3 mAh g-1can be retained after 1000 cycles at 5C. First principles calculations indicate the VA-CNTs not only improve the electronic conductivity of the V2O5 composite, but also facilitate Li-ion adsorption in the interface of CNTs and V2O5 nanoparticles, which lead to the outstanding Li-ion storage and conversion behaviour. Our research can provide insight into the enhancement of V2O5-based materials’ performance in energy storage devices especially in terms of rate performance and cycle life, and strategies for the development of free-standing electrodes.
Section snippets
Materials
All reagents were used without further purification. Aluminium oxide (Al2O3), cobalt (Ⅱ) acetate (Co(Ac)2, 99.9995%), iron (II) acetate (Fe(Ac)2, 99.99%) were obtained from Sigma-Aldrich. 3, 4-Ethlenedioxythiophene (EDOT, 97%) and acetonitrile (99%) purchased from Alfa Aesar Chemical Co., Ltd. Commercial V2O5, oxalic acid dehydrate (H2C2O4·H2O, 99.5%), iron (III) chloride anhydrous (FeCl3, 97%) and 30% H2O2 were used in this experiment and purchased from Sinopharm Chemical Reagent Co., Ltd. All
Results and discussion
Fig. 1 demonstrates the fabrication process of PEDOT-V2O5-VA-CNTs/GF electrode. Porous GF was deposited on the surface of Ni foam by CVD method, and then deposited with an Al2O3 buffer layer and Fe-Co catalysts layer. The VA-CNTs were grown by a Plasma Enhanced Chemical Vapour Deposition (PECVD) system. After etching away Ni template by HCl, VA-CNTs/GF was oxidized by HNO3 to improve its hydrophilicity. V2O5 nanobelts can be successfully synthesized on VA-CNTs/GF by hydrothermal method, and
Conclusion
In summary, a novel lightweight, robust and self-standing PEDOT-V2O5-VA-CNTs/GF electrode has been successfully fabricated and applied as a cathode for Li-ion batteries. Instead of forming a thick coating layer around, the V2O5 nanobelts disperse uniformly among the CNTs forest without severe aggregations. The PEDOT-V2O5-VA-CNTs/GF delivered a reversible capacity of 296.8 mAh g−1 at 1C, and has capacity retention of 113.3 mAh g−1 at 5C after 1000 cycles. The free-standing electrode exhibited
Acknowledgements
This work was supported by Natural Science Foundation of China (Grant No. 51502135 and 51302079) and the Natural Science Foundation of Human Province (2017JJ1008). The authors thank to Prof. Shen Zexiang, Nanyang Technological University, Singapore for his guidance.
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Contributed equally to this work.