Electrospun vanadium-based oxides as electrode materials

Journal of Power Sources 395 (2018) 414–429

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Review article

Electrospun vanadium-based oxides as electrode materials Ceilidh F. Armer a b c

a,b

a

b,c,∗

, Joyce S. Yeoh , Xu Li

T

a,∗∗

, Adrian Lowe

College of Engineering and Computer Science, Australian National University, Canberra, ACT, 0200, Australia Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, Innovis #08-03, 138634, Singapore Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore, 117543, Singapore

H I GH L IG H T S

is an effective and versatile nanofibre fabrication method. • Electrospinning oxides are viable electrode materials for batteries. • Vanadium • Electrospun vanadium oxides are an expanding area of research for energy storage.

A R T I C LE I N FO

A B S T R A C T

Keywords: Electrospinning Vanadium oxide Lithium ion battery Metal ion battery Electrode materials

Electrospinning is a nano-fabrication technique that easily produces ceramic oxide nanofibres which can find numerous applications as energy storage materials, such as battery electrodes. Vanadium oxide is a viable alternative electrode material with tuneable oxidation states and a layered structure that can reversibly intercalate charge carriers. This review examines the use of vanadium oxide as an electrode material for metal ion batteries with focus on electrospun derivatives. Vanadium oxide-based electrodes are predominantly considered in lithium ion batteries given the amount of published literature in this context. The use of vanadium oxide in energy storage devices, while promising, is limited by its low structural stability and slow electrochemical kinetics associated with charge carrier intercalation resulting in poor cycle stability. Doping with other metallic element and incorporation of carbon derivatives in vanadium oxides can potentially improve its cycle stability and rate retention. Vanadium oxide-based electrodes for sodium ion and aluminium ion batteries are also discussed to highlight its versatility in alternative metal ion battery systems.

1. Energy storage and metal-ion battery principles The increase in energy consumption and the use of its associated sources along with population growth is a significant driving force for much of the scientific literature published in energy storage related research [1]. With the critical, and ultimately inevitable, transition from non-renewable energy sources to renewables has led to the ongoing development of advanced energy storage technologies. The increased demand for efficient energy conversion creates a necessity for low cost-efficient energy storage methods that provide consistent energy reliably. Current challenges associated with the development of these devices are linked to the research of supercapacitors, fuel cells, dye-sensitised solar cells, and batteries [2]. Fuel cells convert hydrogen or hydrogen-rich fuel via an electrochemical oxidation reaction into electrical energy. These types of cells

require the flow of hydrogen and oxygen which react in the presence of catalysts. Consequently, the catalysts, their supporting substrates and the proton exchange membrane are the most critical components [1]. Dye-sensitive solar cells convert energy from sunlight into electrical energy through the photovoltaic effect. These devices make use of advanced thin-film technologies which typically consist of a dye coated TiO2 film wedged between a glass plate with fluorine-doped tin oxide deposited on it and a platinum sheet [3]. Supercapacitors, also known as electric double-layer capacitors, are high power devices that are able to charge and discharge rapidly [4]. Due to their ability to rapidly transfer charge, their capacitance is dependent on the surface area of the electrode material that is accessible to the electrolyte. Typically, porous carbons are used in commercially available supercapacitors [5]. Batteries, in particular lithium ion batteries (LIBs), are widely used as power sources as they are compact with high energy density, high

∗ Corresponding author. Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, Innovis #08-03, 138634, Singapore. ∗∗ Corresponding author. College of Engineering and Computer Science, Australian National University, Canberra, ACT, 0200, Australia. E-mail addresses: [email protected] (X. Li), [email protected] (A. Lowe).

https://doi.org/10.1016/j.jpowsour.2018.05.076 Received 15 January 2018; Received in revised form 19 May 2018; Accepted 23 May 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.

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C.F. Armer et al.

discharge voltage, and good cycle performance [6]. They are market leaders in clean energy storage technologies as they can be made from non-toxic materials, have high energy-to-weight ratios and a long life cycle. Research and development breakthroughs with these energy storage systems are centred on the active materials in such devices and their advanced fabrication techniques [7]. These clean alternative energy devices represent an important step towards satisfying society's energy demands. Despite this, they are still under development and new breakthroughs are needed to improve these devices' performance in terms of power density, conversion efficiency, harvest efficiency, durability, and cost [1]. The development of these energy devices is intimately linked to the development of functional nanostructured materials and their efficient production. In the past decade, the demand for LIBs has eclipsed all other battery types including nickel-cadmium (NiCd), nickel-metal hydrides (NiMH), zinc, and alkaline batteries because of their higher energy densities. They occupy the majority of the battery market [8] and in 2010 held a market share of 11 billion dollars [9]. Initially lithium (Li) metal was used as a negative electrode material due to its very high specific capacity of 3860 mA h g-1, light equivalent weight of 6.94 g mol−1, and specific gravity of 0.53 g cm−3. It was replaced in the late 1970s and early 1980s due to safety and reactivity concerns with dendritic growth with graphite while lithiated transition metal oxides, typically LiMO2 (M = Co, Ni, Mn) were used for positive electrodes, that are capable of the reversible intercalation of Li-ions [8]. Sony commercialized LIBs developed by Asahi Chemical in 1991 with lithiated metal oxides and graphite as the electrode materials [10] which delivered a capacity of 150 mA h g-1 with a cell voltage of 3.7 V vs Li/ Li+ [11]. The energy densities achieved in LIBs out-perform the energy density requirements of all energy storage technologies. The Ragone plot in Fig. 1 compares energy density and power density of commercially available batteries and capacitors. This figure shows that LIBs possess the highest energy density of common battery technologies with high power LIBs which are almost comparable to supercapacitors. Developing suitable electrode materials for use in LIBs that are reliable, structurally stable and possess high cycle stability is an important area of research along with the development of reliable and cost-effective processing technologies. An electrochemical cell is the smallest unit of an electrochemical

Fig. 2. LIB components in a conventional commercial rechargeable cell design in which the Li-ions move through the ion conduction electrolyte and electrons move through the external circuit.

device that converts chemical energy to electrical energy. It is made up of two electrodes of differing electric potentials and when immersed in an electrolyte this potential difference is called the open circuit voltage. The discharging process, or lithiation, refers to the cathode where Liions enter the positive transition metal electrode (reduction) and are extracted from the negative electrode (oxidation). The lithiation sequence is schematically represented in Fig. 2 which shows the transfer of electrons to the positive electrode through an external circuit while the electrolyte acts as an ionic conductor facilitating Li-ion transport from the negative electrode. This lithiation process is represented in the forward reaction of Equation (1), where Li-ions are extracted from a graphitic negative electrode (LixCy) and inserted into a lithiated metal oxide positive electrode (LiMO2), with approximately 0.5 Li-ions (x) inserted and extracted per unit of LiMO2 [13]:

Li x Cy + Li (1 − x ) MO2 ↔ Cy + LiMO2

x ≈ 0.5, y = 6

(1)

In a rechargeable cell, the electrochemical reactions are reversible so both oxidative and reductive reactions can occur at the same electrode. Consequently, during the charging process, or delithiation, the roles of the electrodes switch so Li-ions move out of the positive electrode, extracted from cathodic transition metal, and into the negative electrode. The back reaction in Equation (1) represents this process. Though, by convention, the terms remain the same with oxidation occurring at the negative electrode and reduction occurring at the positive electrode during the spontaneous discharge process [10]. Currently used positive electrode materials include variations of the compounds LiCoO2 and LiNiO2 which have theoretical capacities of approximately 275 mA h g-1 [14]. These insertion electrode materials possess high electronic and ionic conductivities, vacant sites in their crystalline structures, high redox potentials, high chemical stability, low specific surface area, and low cost [15]. However, some state-of-art electrodes, such as spinel LiMn2O4, which is stable with cycling despite its small capacity, and olivine LiFePO4, possess a lower Li-ion diffusivity and electrical conductive than that of LiCoO2 [16]. Another notable cathodic compound is LiNiO2 for which a high degree of Li-ion reversibility can be obtained, albeit with safety issues [10]. Furthermore, vanadium phosphate, (Li3V2(PO4)3, a layered structure, possesses high rate capability [17] with incorporated with carbon, and a theoretical capacity of 197 mAh g−1 [18]. Focus on positive electrode materials is particularly important as they are the main limiting factor with the advancement of LIBs [19]. Materials of promise are spinel LiMn1.5Ni0.5O4 with theoretical a

Fig. 1. Ragone Plot comparing the energy and power density of commercial batteries and capacitors of various chemistries, adapted from Ref. [12]. 415

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capacity of 145 mA h g-1 [20,21], Li-rich layered transition metal oxides in the form (Li2MnO3)x(LiMO2)y where M = Mn, Ni, Co have shown capacities of 250 mA h g-1 [22,23]. Olivine structures, of the form, LiMPO4 where M = Fe, Mn, Co, Ni have capacities around 165 mA h g-1 [24–26] and show good high potential performance around 5 V vs Li/ Li+ [27]. With a similar theoretical capacity as LiFePO4, the silicate family of the form Li2MSiO4 for which M2+ is a transition metal oxide, has been explored as an electrode material [28–30]. However, the low electronic conductivity and low electrode potential limits the practical use of this material [31]. Vanadium oxides are a very promising family of materials for LIB electrodes. They are made up of a consecutively layered crystal structure that provides a means for energy storage via metal-ion intercalation, in addition to good catalytic activity for energy conversion. They are used in numerous technological applications including solar cells, sensors, electrical and optical switching devices, electrochromic and thermochromic devices, photocatalyts and supercapacitors [32–38]. V2O5, a common phase of vanadium oxides which will be discussed in more detail in Chapter 2, typically have a theoretical capacity of 294 mAh g−1 [39–41] which is higher than of the previously mentioned positive electrode materials. Nb2O5, chemically similar to V2O5, has only shown a capacity of 242 mAh g−1 [42]. Vanadium oxides are a competitive alternate electrode material candidate due to their open-layered structures that allows for the reversible intercalation of Li-ions [43] higher capacities than the materials discussed previously. This shows that vanadium oxides are a viable and worthwhile material to investigate for energy storage. Graphitic materials are typically used in various commercial energy storage devices as negative electrodes. Carbon-based negative electrodes served as a replacement for Li metal as the build-up of Li species, such as LiOH, was prevented somewhat due to the insertion of Li-ions. Carbon experiences little structural change upon Li-ion insertion/extraction and has contributed to the high-energy density and reasonable cycle life of commercial LIBs today [10]. Though carbon electrodes have a high discharge capacity of 370 mA h g-1, they are limited by a low insertion potential. This causes metal Li deposition onto the electrode surface and, over time, has a detrimental effect on battery safety and long-life cycle [15]. With increasing concerns regarding energy requirements of renewable energy sources, the expanding electric vehicle market, and high demand of portable devices, much research focus now encompasses alternate energy storage systems [44]. Research into alternate metalion battery systems is gaining traction due to the uneven distribution of Li metal, high material cost and low abundance compared to the other metal-ion candidates (Table 1). Sodium ion batteries (NIBs) are a strong contender for the replacement of Li-based batteries with sodium's oxidation potential, abundance, comparable reaction mechanisms during charge transfer to LIBs, absence of alloying with aluminium, and low cost [45,46]. Despite this, the gravimetric and volumetric densities of NIBs do not exceed LIBs due to the heavier Na-ion and its larger radius (102 p.m. vs 76 p.m.). This results in slower kinetics during cycling in addition to its lower reducing potential [46,47]. Consequently, electrode materials for

NIBs must possess appropriate structural characteristics to enhance electrochemical activity during sodiation. Potassium ion (K-ion) batteries are a relatively poor candidate due to its low theoretical capacity combined with the monovalent state of Kion indicating only one charge transfer per redox reaction like that of Li-ion and Na-ion. Despite this K-ion does possess a slightly lower redox potential than Na-ion suggesting higher cell voltages along with lower toxicity and moderate abundance making K-ion batteries a viable candidate alternative battery system [48–50]. For magnesium ion (Mg-ion), aluminium ion (Al-ion), and calcium ion (Ca-ion) batteries more than one charge transfer takes place per redox reaction allowing for higher energy densities and specific capacities. Mg-ion is a promising alternative metal-ion system due to its ionic radius (72 p.m. vs 76 p.m. for Li-ion), abundance and cost. Despite this, the choice of suitable electrolyte is limited in order to promote reversible dissolution and deposition of Mg metal. Additionally, the positive electrode material must be carefully selected due to the slow solid state diffusion of Mg-ions which produces substantial polarisation in crystalline materials [51]. It has been claimed that aluminium ion batteries (AIBs) are the most promising battery beyond lithium energy storage systems [52]. Aluminium (Al) is the most abundant of the metal-ion candidates and its volumetric capacity, 8046 mA h mL-1, is four times higher than that of Li-ion at 2062 mA h mL-1. Additionally, Al-ion has a smaller ionic radius than Li (53.5 p.m. vs 76 p.m.) and is capable of a three-electron redox reaction via the insertion/extraction of the trivalent Al-ion [9,53]. Its specific capacity) is lower than that of Li (2980 vs 3861 mA h g-1) though higher than other metals used in energy storage devices such as sodium (1165 mA h g-1) and magnesium (2205 mA h g-1) [52]. For these metal-ion systems to be viable, electrode materials must allow for ion mobility, especially for Na-ion, K-ion and Ca-ion, when considering ionic radius.

2. Vanadium oxides as an energy storage material 2.1. Vanadium oxide structures The diverse chemistry and catalytic performance of vanadium oxides is the result of a variety of oxidation states (2+ to 5+) and oxygen coordination geometries that are available. These geometries include octrahedral, tetrahedral, pentagonal pyramids and square pyramids which combine to share edge, faces, and corners in a large variety of structural arrangements [54]. The vanadium oxide system has up to 13 distinct phases with further variation in stoichiometry occurring in some structures forming compounds such as VO2, V2O5, V2O3, V6O11, and V7O13 making this materials family a potential electrode material for energy storage technologies [55]. To highlight the complexity of the vanadium oxide system, vanadium dioxide (VO2) is a typic polymorph binary compound with known structures of VO2(A), VO2(B), VO2(M) and VO2(R) [56]. As a typical thermochromic material, the rutile phase, VO2(R), is the most thermodynamically stable phase and possesses a reversible metal-insulator phase transition to a monoclinic phase, VO2(M), at 68 °C varying the

Table 1 Summary of the energy storage parameters of candidate metal-ion negative electrodes. Oxidation state

Lithium Li

Atomic Weight [g mol−1] Specific Capacity [mA h g−1] Volumetric Capacity [mA h cm−3] Standard Potential [V vs SHE] Abundance in crust [ppm] Ionic Radius [pm]

+

6.94 3862 2000 −3.04 18 76

Sodium

Magnesium

+

2+

Na

Mg

22.99 1165 1000 −2.71 22700 102

24.31 2205 3900 −2.36 23000 72

416

Aluminium Al

3+

26.98 2980 8000 −1.68 82000 53.5

Potassium K

+

39.10 685 600 −2.93 18400 138

Calcium Ca2+ 40.08 1340 2000 −2.87 41000 100

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stability, structure, and high energy density makes V2O5 a desirable material for use in electrochemical devices [73]. Additionally, V2O5 is capable of intercalating monovalent and multivalent cations making it a candidate for other metal-ion battery systems [51].

electrical, optical and magnetic properties. This variation in properties accompanying a reversible phase transition makes VO2(R) a potential candidate for applications in gas sensors, optical switching devices and optical data storage. The VO2(A) is metastable though it is absent during the preparation of the VO2 polymorph, as such, it has been less studied. VO2(B) is another metastable phase possessing one-dimensional tunnels within its structure which are capable of Li-ion intercalation [57–59] and irreversibly forms the stable rutile phase at 350 °C [60]. In terms of LIBs, VO2 is a useful material due to its low cost, layered structure allowing for rapid Li-ion intercalation/extraction, and excellent electronic conductivity which is a key property for high power performance [61]. For detailed discussion of the VO2 structure and Liion intercalation proportions see the review by Chernova et al. where a range of layered vanadium oxides is discussed [62].

2.3. Structural effects of doping vanadium oxide Doping materials with other elements is undertaken to introduce a percentage of other atoms into the crystal structure [74]. This allows for the introduction of mixed-valence states into the material and the modification of the properties of the host crystal lattice leading to improved electron conductivity [35]. The mixed conductivities of the host material and dopant is necessary for charge neutrality preservation during Li-ion intercalation/extraction. Additionally, oxygen vacancies, brought about by doping, have also been shown to increase conductivity in oxide materials [75]. Electrochemical properties of V2O5 can be improved with doping via an expansion of the crystal lattice providing increased opportunity for Li-ion intercalation [64]. The presence of increased V4+ in V2O5 is indicative of dislocations and vacancies which may facilitate charge transfer by providing a more open structure and additional dopant energy levels along with improved Li-ion transport [76], [77]. Zhan et al. doped V2O5 with Cr3+, prepared via a sol gel method and showed a prevention of an irreversible phase change of V2O5 with an improvement in cycling performance compared to that of pure V2O5 [64]. Dopant amounts require optimisation as the effects on structural variations and phases must be considered in conjunction with the effects on electrochemical performance. For example, Li compounds, such as Li2O, can form within the active electrode materials and cause large volume expansion and material decomposition which results in capacity fading [78].

2.2. The V2O5 structure V2O5 is made up of V5+ and some V4+ and is the most stable of the vanadium oxide structures preferentially forming when heat treated in air at elevated temperatures [63]. It has an orthorhombic unit cell with a space group of Pmmn and is made up of a layered structure consisting of stacks of distorted VO5 square pyramids that share edges to form zigzag double chains, as shown in Fig. 3. These layers are bonded in the z – direction, or the (001) direction, by weak van der Waals bonds between the vanadium and oxygen of neighbouring pyramids in adjacent layers [64,65]. Bilayered V2O5.nH2O is considered an amorphous form of crystalline V2O5 and consists of chains of distorted VO5 pyramids which are all facing the same direction. These chains occur in pairs forming a monoclinic structure with space group C2/m with a large interlayer distance of > 8 Å between the bilayers due to the presence of water rather than 4.4 Å for orthorhombic V2O5 [51]. Bilayered V2O5.nH2O are typically xerogels or aerogels, depending on the drying method, and can be made from ionic exchange [66,67], peroxides reaction with V2O5 [68,69], or the quenching of molten V2O5 [70,71]. Nanostructured V2O5 is of interest for use in LIBs as it possesses lower polarisation than micro structured V2O5 due to reduced particle size. Smaller particle size as offered by nanostructuring results in increased contact surface area with the electrode and electrolyte [6,72]. There is also an improvement in the Li-ion intercalation properties due to a decrease in diffusion distances. The combination of chemical

3. Electrospinning of vanadium oxide nanomaterials 3.1. The electrospinning technique Nanotechnology provides a means to produce very small and finely structured materials and devices. Consequently, processing techniques that reliably fabricate nanostructured materials are required. In order for fabrication of nanostructured materials to be convenient and financially viable, production methods need to be practical in terms of pressure and temperature requirements, simplicity, and safety [79,80]. These techniques should have tuneable parameters to enable the exertion of some control over the resulting morphology and structure. Solution-based methods include a range of nanoprocessing techniques typically via “top down” or “bottom up” procedures. A “top down” procedure is the decomposition of bulk-scale into nanoscale dimension, such as ball-milling or grinding. A “bottom up” procedure is the synthesis of nanomaterials from atoms or molecules, such as hydrothermal methods. Though not discussed in this review, other common liquid-state processing techniques for producing nanostructured materials include co-precipitation [81], sol-gel methods [82], micro-emulsions [83] and hydrothermal synthesis [84,85]. Electrospinning is an effective, scalable, and inexpensive top-down nano-fabrication technique for synthesizing one-dimensional fibres from sol gel solutions [73,86]. This method allows uniform fibres with nano-scale diameters to be formed without further purification [87]. The history behind the techniques is long with patents authored by Cooley and Morton in 1902 featuring devices used to spray liquids via electrostatic potential [88,89]. Producing ultrafine plastic fibres using electrostatic potential was first patented by Anton Formhals in 1934 [90] and in the 1960s Geoffrey Taylor made theoretical and experimental analyses of the nature of electrified droplets [91]. Electrospinning did not receive more than passing academic attention until the 1990s when the Reneker group conducted detailed analyses of the electrospinning process [92–97] while the Rutledge group examined

Fig. 3. Structural representation of two layers of V2O5, where blue = vanadium and red = oxygen, showing two individual layers consisting of edge and corner sharing VO5 distorted square pyramids. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 417

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syringe and collector is small as the fibres do not have enough time to solidify before depositing on the collector. Sol gels that have high polymer to precursor ratio, resulting in a higher sol viscosity, require longer distances between the syringe and collector for fibre solidification. The studies summarised in Tables 2 and 3 feature vanadium-based oxide fibres of consistent diameters with high aspect ratios. These vanadium-based oxide fibres were electrospun with potentials of 10–20 kV and distances of 10–20 cm between the needle and collectors. This indicates that these parameter ranges are reliable in the production of electrospun fibres containing vanadium oxide. After electrospinning, the as-spun material is typically subjected to calcination treatment. Fig. 5 summarises three morphological changes that can be controlled when calcining as-spun fibres such as hollow, hierarchical and single crystalline variations. It has been shown that heat treatment temperature plays an important role in determining fibre morphology and can affect structural characteristics and electrochemical performances of vanadium oxides [107,111,117,124].

theoretical aspects of jet instability [98–100], fibre diameter [101] and the formation of sub-micron fibres caused by whipping-fluid jets [102]. Since then electrospinning has received substantial academic attention in not only the production of polymer fibres but also in metal oxide ceramics fibres [103,104]. There are several types of precursor solutions used for preparing oxide- and polymer-based nanofibres with associated advantages and disadvantages. For example, using inorganic sols can result in fibres that are several hundred nanometres in diameter which may be too large for certain applications [105]. Controlling the diameter of electrospun nanofibres requires precise control of the composition of the precursor solution, generally an alkoxide or salt, and the spinning parameters. Specifically, varying the amount of poly(vinyl pyrrolidone) (PVP)-to-alkoxide precursor ratio in their starting solution, Li and Xia showed that the resulting diameter of their TiO2 fibres could be controlled [106]. A small PVP-to-alkoxide precursor ratio produced smaller fibre diameters while a larger ratio resulted in thicker fibres. The spinnability of the solution can be dependent on the polymer used, which enables different types of ceramic oxides to be produced by changing the sol gel precursor. Vanadium oxide fibre precursors include: vanadium isopropoxide (VO(OC3H7)3) [107–109], vanadyl acetylacetonate (VO(acac)3) [55,110,111], ammonium metavanadate (NH4VO3) [39,112–114], and V2O5 powder [40]. Commonly used polymers include: poly(vinyl acetate) (PVAc) [107,109], poly(methyl methacrylate) (PMMA) [39,108,115] and poly(vinyl alcohol) (PVA) [40,55,110,112–114,116,117]. The electrochemical performance of these studies featuring electrospun vanadium oxide will be discussed in Chapter 4, however the electrospinning parameters are summarised in Tables 2 and 3. During electrospinning, a strong electric field is applied to the tip of a capillary containing the precursor solution which is drawn into a droplet. An electrostatic field is applied between the capillary and a grounded collector which can be a flat plate or a rotating drum, as is the case in the experimental set up shown in Fig. 4. When the electrostatic force at the end of the capillary exceeds the surface tension of the droplet, a continuous fine jet of solution is ejected from the capillary and moves through the electric field to deposit on the collector [118]. The precursor solution in Fig. 4 is coloured yellow to represent a vanadium-based sol gel containing V5+. Elongation of the charged droplet expelled from the tip of the needle is caused by electrostatic repulsions experienced in the bends of the lengthening droplet into a fibre. The elongation and thinning of the fibre creates uniform fibres with nanometre-scale diameters [119]. The surface morphology and diameter of the fibres can be controlled by varying the set-up parameters and sol gel components [55]. Parameters of the starting solution include viscosity, surface tension, conductivity, polymer molecular weight, dipole moment, and solution permittivity. In addition to the simplicity of the sol gel method and inexpensive equipment, which is an advantageous trait for mass production [120], a standard solution can easily be doped with a secondary metal precursor prior to electrospinning. The setup parameters include solution feed rate, distance between syringe and collector, electric field, collector setup, and needle specifications. The environmental influences are humidity and temperature. A combination of these parameters will determine the quantity and quality of electrospun fibres. Berezina et al. conducted a study on electrospun vanadium oxide fibres using a theoretical method based on non-stationary electrodynamics [120]. They found that the potential difference, which is specified by the distance between the needle and substrate, is an important factor that can affect the qualitative and quantitative characteristics of the fibres. At high potentials solution splashing can take place where fibres did not reach the collector and there is an increased chance of fibre break down between the needle and collector. Conversely, at low potentials the occurrence of fibre defects increases, such as inconsistence diameters or morphologies. Fibre defects are also more likely when the distance between the

3.2. Electrospun vanadium oxide An early study featuring electrospun vanadium oxide was conducted by Viswanathamurthi et al. in 2003 in which V2O5 was produced [107]. It was observed that diameter shrinkage took place after heat treatment at both 400 and 500 °C indicating total polymer decomposition after heat treatment. Additionally, morphological changes could be controlled via heat treatment temperature with the fibres calcined at 500 °C possessing larger particle sizes and a higher degree of crystallinity compared to those at 400 °C. Wang et al. synthesized electrospun V2O5 with controllable morphologies of porous nanotubes, hierarchical nanofibres, and single-crystalline nanobelts via heat treatment in air at 400, 500 and 600 °C, respectively [Fig. 6(a–d)] [111]. Fig. 6(e) shows electrospun V2O5 from a sol gel consisting of the same polymer, PVP, as used by Wang et al. with a vanadium precursor of ammonium metavanadate used by Mai et al. [112]. These images show that similar fibre morphologies can be obtained using different vanadium precursors which highlights the versatility of the electrospinning method when fabricating V2O5 fibres, as evidenced in Tables 2 and 3. Electrospinning is typically used to reliably produce fibrous materials, but when the ratio of polymer to solvent is low resulting in a low viscosity sol gel, the material ejected from the syringe can break apart as it whips through the electric field. In a typical electrospinning operation, this breakage is undesirable, though if it is controlled and consistent, however, some interesting spherical morphologies can be produced. The hollow spherical hierarchical V2O5 structures, shown in Fig. 6(f), consist of nanostructured particles using a sol gel made up of vanadium oxytripropoxide, PMMA and a 1:1 ratio of dichloromethane (DCM)/Dimethylformamide (DMF) [125]. Spherical vanadium-based oxide structures can be produced via spray drying [126,127], ionmodulating methods [61], self-assembly [128,129], solverthermal [130] hydrothermal methods [131–133], and refluxing routes [134]. Nanoscale microspheres have been shown to be competitive candidates as electrode materials as they possess superior cycling stability and high discharge rate performance [135]. Hollow microspheres can compensate for localized volume expansion caused from Li-ion intercalation by alleviating induced strains and hence improve cycle stability. Electrospinning is a useful nanofabrication technique for energy storage as it can produce nanoparticle networks with high aspect ratios that provide improved contact surface area between electrode and electrolyte, higher electronic and ionic conductivities and improved facile diffusion pathways [55]. The relevance of this nanofabrication method due to its reliability and effectiveness in producing one dimensional nanostructures which are advantageous in applications such as supercapacitors [136], photocatalyts [35], sensors [87], tissue regeneration [137] and water filtration [80]. A wide range of metal oxides have been prepared via electrospinning for energy storage investigations including cobalt oxide [138], 418

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Table 2 Summary of electrospinning parameters for electrospun V2O5 where: vanadium oxytriisopropoxide = VOTIP, vanadium isopropoxide = VO(OC3H7)3, vanadyl acetylacetonate = VO(acac)3, ammonium metavanadate = (NH4VO3), EtOH = ethanol, DMF = N,N-dimethylformamide, oxalic acid = H2C2O4.2H2O. Material

Morphology/ diameter (nm)

V2O5

Nanowires/300-80 Nanorods/70-800

V2O5

Fibres/1000

H0.48V4O10

Fibres/250

V2O5

Nanotubes/560 Nanofibres/340 Nanobelts/300 Nanowires/100-200 Nanorods/50-100

V2O5 (with impurities VxO2)

T-V2O5 = template, S- V2O5 = no template

Tubular/1000,

V2O5

Nanofibres; 200400

V2O5

Fibres/40-70

V2O5

Nanofibres/500800

V2O5.nH2O

Fibres/350

V2O5-15 = 15 min heat treatment, V2O5-60 = 60 min

Porous nanofibres; 300-500

V2O5

Micro/nanorods, 300

Solution components: - Precursor(s) - Polymer - Solvent -

(VO(acac)3) PVA Acetic acid, EtOH VO(OC3H7)3 PVAc EtOH, H2O VO(OC3H7)3 PMMA Acetic acid, DMF, chloroform (VO(acac)3) PVP DMF (NH4VO3) PVA DI H2O (NH4VO3) PMMA DMF V2O5 powder PVA DI H2O, DMF, EtOH, H2C2O4.2H2O VOTIP PMMA DMF, chloroform (VO(acac)3) PVA acetic acid V2O5 powder PVA H2O2 V2O5 powder PVP H2C2O4.2H2O, DI H2O, DMF NH4VO3 PVP H2C2O4.2H2O, H2O, DMF

Applied voltage (kV

Sample Collector

Feed Rate (mL hr−1)

Distance to collector (cm)

Reference

10

Al foil collector

2.0

10

[55]

15

Plate

N/A

N/A

[107]

20

Al foil collector

N/A

20

[108]

15

Al foil collector

1.0

15

[111]

20

Al foil collector

1.0

20

[112]

25

Al plate

N/A

12

[39]

15

Al foil collector

1.0

15

[40]

20

Al surface

N/A

20

[115]

10

Al foil collector

2.0

10

[116]

15

Pt foil

0.15

15

[117]

12

Collector type not specified

0.88 mm min−1

10

[121]

20

Square stainlesssteel net

0.6

20

[41]

with open or layered structures and controllable morphologies as LIB electrode materials. The motivation for these studies lies with the advantages of using of one-dimensional nanostructures in LIBs combined with the simple, cost-effective and versatile production technique electrospinning. Electroactive materials possessing porous, hollow or hierarchical fibres are expected to exhibit the following advantages: efficient 1D electron transport in the longitudinal direction, increased Li-ion flux at the interface of the electrolyte/electrode, decrease in Liion diffusion length which promotes faster charge transfer and improved structural stability against volume expansion thus improving cycling stability. Electrospinning has been used in conjunction with hydrothermal treatment of the as-spun materials by Ban et al. in two separate studies. In the first study, single crystalline fibres with diameters of approximately 100 nm produced an initial discharge capacity of 120 mA h g−1 at 0.1 mA cm−2 in the 1.75–3.75 V vs Li/Li+ range [108]. It was conceded that this capacity faded with subsequent cycling and continuously smoothed galvanostatic capacity vs potential curves were produced suggesting little Li-ion intercalation within the layered structure. In the second study by Ban et al. the calcination temperature after the hydrothermal treatment was increased from 420 to 500 °C [115]. H0.48V4O10.2H2O with a monoclinic structure was formed after hydrothermal treatment of the as-spun materials and a low initial capacity of

nickel oxide [139], zinc oxide [140], calcium cobaltite [141], TiO2 composites [142,143], and Nb2O5 [42]. Despite this, vanadium oxides remain a competitive candidate due to their open-layered structures that allows for the reversible intercalation of Li-ions [43] and results in higher capacities than those offered by commercially used materials [144]. Additionally, vanadium sources are relatively abundant resulting in lower cost than the limited Co [145]. 4. Electrochemical performance of electrospun vanadium oxide V2O5 has been extensively studied as a positive electrode material due to its electrochemical nature and ease of Li-ion intercalation. Other vanadium oxide phases have received far less attention in comparison. The reader is referred elsewhere for studies featuring VO2 for LIBs [57,60,61,146–150]. This section examines the performance of electrospun V2O5 for LIBs and briefly discusses their potential in alternate metal ion systems, namely NIBs and AlBs. Additionally, doped vanadium oxides of various phases and the incorporation of a carbon source either pre- or post-electrospinning are also discussed. 4.1. Electrochemcial properties of electrospun V2O5 A series of studies on electrospun V2O5 is summarised in Table 4. These studies have produced electrospun vanadium oxide nanofibres 419

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Table 3 Summary of electrospinning parameters for electrospun doped V2O5 and carbon incorporated V2O5 where: vanadium oxytriisopropoxide = VOTIP, vanadium isopropoxide = VO(OC3H7)3, vanadyl acetylacetonate = VO(acac)3, ammonium metavanadate = (NH4VO3), EtOH = ethanol, DMF = N,N-dimethylformamide, oxalic acid = H2C2O4.2H2O. Material

Morphology/diameter (nm)

V2O5, V1.79Ba0.21O5, V1.81Ti0.19O5

Fibres/1000

V2O5, Al0.5-V2O5, Al1.0-V2O5

Nanofibres/100-200

LiV3O8

Fibres/1000

ß-Ag0.33V2O5

Nanorods/70

reduced graphene oxide V2O5 nanowires (GVO)

Nanowires/100

V2O5/graphitic nanotubes

Graphitic nanotubes encapsulating V2O5/150

Solution components: - Precursor(s) - Polymer - Solvent - VO(OC3H7)3, barium oxide, titanium isopropoxide - PVAc - EtOH - (VO(acac)3) and aluminium nitrate (Al(NO3)3) - PVA - Acetic acid, EtOH - (NH4VO3), lithium acetate - PVA - Distilled H2O - (NH4VO3) - PVA - DI H2O - VO(acac)3 - PVP - DMF - vanadium source unspecified - PVP - solvent unspecified

Applied voltage (kV)

Sample Collector

Feed Rate (mL hr−1)

Distance to collector (cm)

Reference

22

Al foil - drum

2.0

15

[109]

10

Al foil collector

1.5

10

[110]

15

Cu wire drum

2.0

15

[113]

18

Al foil collector

1.0

15

[114]

10

Al foil collector

N/A

20

[122]

N/A

N/A

N/A

N/A

[123]

The capacity decreased though it remained reversible above 240 mA h g-1 for 25 cycles despite the formation of the rock salt structure observed around the 10th cycle. The improved electrochemical performance was attributed to shorter diffusion length associated with the nanofibres and greater structural stability which lead to better capacity retention. Investigations on electrospun V2O5 show an interesting progression in potential window size variations. It is well known that V2O5 undergoes a series of phase changes with Li-ion intercalation and the potential window affects phase change formation and cycle stability [111,112,116,151]. Both Mai et al. [112] and Cheah et al. [151] showed that cycling between 2.0 and 4.0 V vs Li/Li+ yielded higher cycle stability, 68% and 74% respectively compared to 1.75–4.0 V vs Li/Li+ with retentions of 51% and 50% respectively. The increased stability was attributed to absence of the irreversible ω-Li3V2O5 rock salt phase which is formed at potentials less than 2.0 V vs Li/Li+ as shown previously by Ban et al. [108,115]. Structural characterisation of electrospun nanowires by Mai et al. showed that hierarchical nanowires were made of vanadium nanorods composed of predominantly V2O5 with VxO2 impurities [112]. It was proposed by the authors that this structure kept the contact area of the active materials, conductive additives and electrolyte large allowing high capacities to be reached. The first galvanostatic discharge and charge capacities when cycled within 2.0–4.0 V vs Li/Li+ were both 275 mA h g-1 indicating no irreversible capacity loss in the first cycle. Conversely, the first galvanostatic discharge/charge capacities when cycled in the 1.75–4.0 V vs Li/Li+ range were 361 mA h g-1 and 90 mA h g-1 respectively. Continued cycling showed that multistep discharge behaviours which were still present between for 2.0–4.0 V vs Li/ Li+ had disappeared when cycled in the 1.75–4.0 V vs Li/Li+ range owing to the irreversible formation of ω-Li3V2O5. After 50 cycles, the discharge capacities were 201 mA g-1 at 1.75–4.0 V vs Li/Li+ and 187 mA h g-1 at 2.0–4 V.0 vs Li/Li+ with higher efficiencies for the latter range. Cheah et al. produced electrospun single-phase polycrystalline V2O5 nanowires with an initial discharge capacity of 230 mA h g-1 within 2.0–4.0 V vs Li/Li+ and a cycle retention of 55% over 50 cycles at a current density of 35 mA g−1 [151]. Electrical impedance spectroscopy

Fig. 4. Experimental schematic of the electrospinning set up where the starting solution in the syringe was ejected at a constant rate over approximately 10 cm through the electric and deposited on an aluminium foil coated rotating drum collector.

Fig. 5. A schematic of calcination treatment of as-spun fibres which can produce a range of fibre morphologies.

110 mA h g-1 was measured when discharged to 1.75 V vs Li/Li+ which decayed rapidly with further cycling. The evolution of the structure on heating was studied and it was shown that nanobelts of V2O5 were irreversibly formed between 450 and 500 °C. Galvanostatic testing of V2O5 showed vastly improved initial discharge capacity of 350 mA h g-1 at a current density of 0.1 mA cm−2 within 1.75–3.75 V vs Li/Li+ with multi-step behaviour which is typical for a crystalline intercalation material. With subsequent galvanostatic cycles the potential steps were lost due to the formation of ω-LixV2O5 where lithium and vanadium ions become randomised due to the formation of a rock salt structure. 420

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Fig. 6. SEM images with inset scales of 500 nm of (a) as-spun fibres, (b) porous nanofibres calcined at 400 °C, (c) hierarchical nanofibres calcined at 500 °C and (d) single crystalline nanofibres calcined at 600 °C electrospun from a solution of vanadium (IV) acetylacetone, PVP and Dimethylformamide (DMF), reproduced from Ref. [111] with permission from Wiley, (e) electrospun hierarchical fibre composed of V2O5 nanorods from a solution of ammonium metavanadate, PVA and deionized water, reprinted with permission from Ref. [112]. Copyright (2010) American Chemical Society. (f) Spherical hollow hierarchical V2O5 structures produced from electrospinning from a low viscosity sol gel consisting of vanadium oxytripropoxide, PMMA and a 1:1 ratio of dichloromethane (DCM)/(DMF) by the authors [125].

4.0 V vs Li/Li+ at a high current density of 2000 mA g−1 over 250 cycles with stabilities of 80.5% and 85.4% respectively and final capacities of 106 and 78 mA h g-1. Capacity vs potential plots and galvanic cycling for the nanotubes are presented in Fig. 7(c and d). However, the electrospun nanobelts were only cycled between 2.0 and 4.0 V vs Li/Li+ and exhibited high cycle stability of 85% over 250 cycles at 2000 mA g−1 with a final capacity of 138 mA h g-1. This performance was attributed to fibre morphology and advantageous crystal orientation with suppression of the [001] direction which prevented crystal volume fluctuation and electrode aggregation. The reason as to why the nanobelts were not cycled in the reduced 2.5–4.0 V Li/Li+ range was not specified. Li et al. also investigated the potential window variation effects on the electrochemical performance of V2O5 that was heat treated at 400 °C for either 15 min (V2O5-15) or 60 min (V2O5-60) [121]. The electrochemical performance of V2O5-15 was investigated more extensively than V2O5-60 due the superiority of its performance with an initial discharge capacity of 150 mA h g-1 at 0.2 C (1 C = 147 mA g−1) over 2.5–4.0 V vs Li/Li+ and a capacity retention of 86.9% after 50 cycles. For the 2.0–4.0 V vs Li/Li+ range, the initial discharge capacity was 275 mA h g-1 at 0.2 C (1 C = 294 mA g−1) with a cycle retention of 73.8% over 50 cycles. V2O5-60 produced an initial capacity of 144 mA h g-1 at 0.2 C for 2.5–4.0 V vs Li/Li+. The improved

(EIS) analysis showed that the V2O5 nanowires had the potential to minimize diffusion barriers, and ionic and electronic resistances by decreasing the internal resistance and facilitating electrolyte accessibility in the positive electrode network. Electrospinning produced a single-phase V2O5 nanofibres with porous and randomly interconnected network that allowed increased contact with the electrolyte/electrode material and facilitated the movement of Li-ions into and from the vanadium-based cathode. It should also be noted that a high proportion of conductive agent and binder was used in the preparation of these electrodes with a ratio of 60:20:20. This likely affected cycle performance and capacity retention though it was not quantified in this study. Cycle stability was improved by Cheah et al. [116] and Wang et al. [111] by further reducing the potential range to 2.5–4.0 V vs Li/Li+ which prevented the formation of the γ-LixV2O5 phase by limiting the Li-ion insertion to 1 mol per unit formula. Cheah et al. presented an interesting analysis of both full- and half-cell results with spinel Li4Ti5O12 as the negative electrode and an operating potential of 1.8 V vs Li/Li+. Galvanostatic analysis over 30 cycles at 20 mA g−1 revealed capacity retention of 91% with an initial discharge of 140 mA h g-1 [Fig. 7(a and b)] [116]. Wang et al. synthesized electrospun vanadium nanostructures with morphologies that were dictated by calcination temperature [111]. The synthesized nanotubes and nanofibres were cycled between 2.5 and

Table 4 Electrochemical performance summary of electrospun V2O5, see Table 2 for morphology and synthesis parameters. Material

Discharge capacity 1st cycle (mA h g−1)

Rate (mA g−1, C)

Nth cycle/discharge capacity (mA h g−1)

Retention (%)

Potential range (V vs Li/Li+)

Reference

V2O5

300 230 N/A 120 Nanotubes; 131. Nanofibres; 92 Nanobelts; 95 275 390 T-V2O5: 242 T-V2O5: 174 S-V2O5: 207 S-V2O5: 138 139.4 350 charge: 140 377 V2O5-15: 150 V2O5-15: 275.2 V2O5-60: 144 V2O5-550 °C: 418.8

35 35 N/A 0.1 mA cm−2 2000 2000 2000 30 30 294 1175 294 1175 800 0.1 mA c−2 20 655 0.2 C (1 C = 147 mA g−1) 0.2 C (1 C = 29 mA g−1) 0.2 C (1 C = 147 mA g−1) 50

50/150 50/170 N/A N/A 250/105.6 250/78.1 250/137.8 50/187 50/201 300/218 300/148 300/162 300/77 100/133.9 n = 25/241 30/charge: 127 40/340 50/130.5 50,/204 N/A 50/180.5

50 74 N/A N/A 80.5 85.4 85 68 51 89 85 78 56 96 69% 91 90 87 74 N/A 43

1.75–4 2–4 N/A 1.75–3.75 2.5–4 2.5–4 2–4 2–4 1.75–4 2–4 2–4 2–4 2–4 2–4 1.75–4 2.5–4 −0.5–1 V vs Ag/AgCl 2.5–4 2–4 N/A 2–4

[55]

V2O5 H0.48V4O10 V2O5

V2O5 (with impurities VxO2) T-V2O5 = template S- V2O5 = no template V2O5 V2O5 V2O5 V2O5.nH2O V2O5-15 = 15 min heat treatment, V2O5-60 = 60 min V2O5

421

[107] [108] [111]

[112] [101]

[40] [115] [116] [117] [121]

[41]

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Fig. 7. (a) Capacity vs potential profiles of vanadium oxide nanofibres over 2.5–4 .0 V vs Li/Li+ at 20 mA g−1, (b) galvanostatic cycling of vanadium oxide nanofibres at 20 mA g−1 over 250 cycles at various current densities, reprinted with permission from Ref. [116]. Copyright (2013) American Chemical Society. (c) Capacity vs potential profiles and (d) galvanostatic cycling of V2O5 nanotubes at various current densities over 2.5–4.0 V vs Li/Li+, reproduced from Ref. [111] with permission from Wiley.

that constitute the porous nanofibers, promoted electrochemical activity and improved Li-ion kinetics. V2O5 micro/nanorods electrospun by Zhu et al. proved to be very competitive with 419 mA h g-1 at a current density of 50 mA g−1 over 2.0–4.0 V vs Li/Li+ [41]. However, the capacity consistently degraded to 181 mA h g-1 with a resulting cycle retention of 43%. No indication of the rate performance was included. The as-spun material was heat treated at 500 °C, 550 °C and 600 °C though it was not clear which temperature the electrochemical performance data was obtained from. It would have been interesting to have all data included to compare energy storage capability as a function of heat treatment. Pure electrospun V2O5 has proven to produce competitive performances via the investigation of:

electrochemical performance of V2O5-15 was attributed to residual carbon within the porous nanotubes measured at 0.37 wt% according to XPS analysis. Despite this, no discussion of morphological changes, such as d-spacing variations or XRD refinements to compare unit cell variations between the samples were presented. Yu et al. conducted their electrochemical testing of electrospun mesoporous V2O5 nanofibres in a three-electrode cell with an electrolyte made up of 1 M LiClO4 in propylene carbonate and Pt as the counter electrode [117]. Galvanostatic cycling was conducted at a potential range of −0.5–0.1 V vs Ag/AgCl with a high current density of 625 mA g−1. The discharge capacity was initially 372 mA h g-1, decreased by 7% in the tenth cycle and remained stable for the remaining 30 cycles. This reversible capacity is significantly higher than the theoretical capacity of 294 mA h g-1 for bulk V2O5 assuming two Li-ion intercalations per moiety of V2O5. This electrochemical performance was attributed to improved Li-ion intercalation kinetics, high surface area, short diffusion distances and the small amount residual carbon at 0.3 wt% in the electrode material resulting from the incomplete oxidation of the polymer polyvinyl pyrrolidone (PVP) in the starting solution. As the galvanostatic testing took place in a three-electrode cell, it is likely that this contributed to the excellent performance as the excess of electrolyte would have ensured a fresh and constant flow of Liions to the surface of the electrodes. Electrospun PMMA fibres were used as a template by Yu et al. to prepare V2O5 which was removed after heat treating at 400 °C for 6 h [39]. The resulting porous nanostructured fibres were cycled over the 2.0–4.0 V vs Li/Li+ potential range for 300 cycles at both 294 and 1175 mA g−1 current densities with respective capacities of 218 and 162 mA h g-1, and corresponding impressive cycle retentions of 89% and 78%. The improvement in electrochemical performance between templated and non-template fibres indicates the extra structural stability provided by the template is beneficial for Li-ion intercalation kinetics. Despite this improvement, it does introduce another experimental step into preparation process which may negate the simplicity of the electrospinning method. Yan et al. and Zhu et al. added oxalic acid to the precursor solution prior to electrospinning in order to facilitate the dissolution of the vanadium precursor in the starting solution [40,41]. Fibres produced by Yan et al. showed impressive cycle stability over 100 cycles, when cycled at 800 mA g−1 over 2.0–4.0 V vs Li/Li+, with an initial capacity of 139 mA h g-1 and cycle retention of 96% at the 100th cycle. This performance was attributed to the porous nanostructured fibre that increased the electrolyte/electrode interface and consequently reduced charge-transfer resistances. In addition, the reduced size of particles,

- Crystallinity, fibre morphology and electrochemical performance can be varied and controlled through adjusting heat treatment temperature. - The extent and reversibility of lithiation could be controlled through varying operational potential windows with one Li-ion intercalation providing high cycle stable with low capacity, three Li-ion intercalations resulting in the opposite trends and two Li-ion intercalations proving to be most researched mode of V2O5 with a compromise of these aspects. 4.2. Electrochemical properties of doped electrospun V2O5 Doping is easily achieved in electrospinning as the dopant precursor can be added into the electrospinning solution resulting in a homogenously distributed dopant. Several studies featured doped electrospun vanadium oxide and the results are summarised in Table 5. Cheah et al. has examined the effects of aluminium (Al3+) dopant on the electrochemical performance of V2O5 at two different loadings, Al0.5V2O5 and Al1.0V2O5 [110]. Characterisation showed porous nanofibres and Rietveld refinement revealed an increase in the c parameters between undoped and Al3+ doped V2O5 which was attributed to Al-ions residing between the V2O5 layers. Initial discharge capacities of pure V2O5, Al0.5V2O5, and Al1.0V2O5 at 35 mA g−1 were 316, 250, and 350 mA h g-1 with cycle efficiencies of 43%, 63%, and 85% respectively over 50 cycles. Galvanostatic cycling at 35 mA h g-1 over 2.0–4.0 V vs Li/Li+ at both room temperature and 55 °C revealed the highest capacity of Al1.0V2O5 indicating that the Al-ion provides thermal stability to V2O5. Both dopant loadings of Al3+ delivered improved cycle efficiency compared to pure V2O5 at current densities of 35 and 350 mA g−1 when cycled at room temperature and 55 °C for 1.75–4.0 V vs Li/Li+. This 422

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Table 5 Electrochemical performance summary of electrospun doped V2O5 and carbon incorporated V2O5, see Table 3 for morphology and synthesis parameters. Material

Discharge capacity 1st cycle (mA h g−1)

Rate (mA g−1, C)

Nth cycle/discharge capacity (mA h g−1)

Retention (%)

Potential range (V vs Li/Li+)

Reference

V2O5, V1.79Ba0.21O5, V1.81Ti0.19O5 V2O5

158 after C-rate 128 after C-rate 213 after C-rate Ambient 316 Ambient 144 At 55 °C 285 Ambient 250 Ambient 240 At 55 °C 360 Ambient 350 Ambient 208 At 55 °C 350. 103 250 175 225

50 50 50 0.1 C 1C 0.1 C 0.1 C 1C 0.1 C 0.1 C 1C 0.1 C 60 20 100 0.2 C

25/102 25/128 25/194 50/Ambient 136 50/Ambient 68 50/At 55 °C 120 50/Ambient 158 50/Ambient 144 50/At 55 °C 180 50/Ambient 298 50/Ambient 146 50/At 55 °C 231 50/72 30/180 30/125 N/A

65 100 91 43 70 40 63 60 50 85 70 66 70 72 71 N/A

2–4 2–4 2–4 2–4 2–4 2–4 2–4 2–4 2–4 2–4 2–4 2–4 0.4–1.6 2–3.6 2–3.6 2–4

[109]

224 (reversible capacity)

150

200/211

92

2–4

Al0.5V2O5,

Al1.0V2O5

LiV3O8 ß-Ag0.33V2O5 reduced graphene oxide V2O5 nanowires (GVO) V2O5/graphitic nanotubes

(1 C = 350 mA g1)

(1 C = 350 mA g1)

(1 C = 350 mA g1)

(50 mA g−1)

suggests that the Al3+ dopant stabilizes the V2O5 structure in such a way that the irreversible phase change to the ω-LixV2O5 rock salt form is prevented or hindered. The electrochemical performance of this Al3+ doped electrospun vanadium oxide is quite competitive compared to the other electrospun V2O5 studies examined in this review, especially considering the wide potential range. Wu et al. synthesized ß-Ag0.33V2O5 using as-spun electrospun vanadium oxide for hydrothermal treatment with AgNO3 [114]. Electrospinning before hydrothermal treatment allowed increased contact area between the as-spun fibres and AgNO3 permitting a complete reaction to produce pure ß-Ag0.33V2O5. Galvanostatic analysis showed improved cycle performance compared to other silver vanadium oxide nanostructures referenced in their study. At 20 and 100 mA g−1 the initial capacities were 250 and 175 mA h g-1 respectively with cycle efficiencies of 72% and 71% over 30 cycles in the 2.0–3.6 V vs Li/Li+ range. A previous report by the authors investigated the structural and electrochemical performance of electrospun V2O5 doped with approximately 10 atomic % (at%) Ba2+ and Ti4+ forming V1.79Ba0.21O5 and V1.81Ti0.19O5 respectively [109]. It was found that the cycle stability of the doped V2O5 was improved compared to pure V2O5 with capacity retentions of 65% for pure V2O5 from 158 mA h g-1, 100% for V1.79Ba0.21O5 with 128 mA h g-1 and 91% from 213 mA h g-1 for V1.81Ti0.19O5 over 25 cycles at 50 mA g−1. These cycle stabilities were achieved after a C-rate test. The Ti4+ dopant was also observed to improve the electrochemical performance while the Ba2+ dopant at 10 at% did not improve capacity. The poor capacities measured with the V1.79Ba0.21O5 were attributed to active site blockage within the V2O5 layers. This section showed that the competitive performance of doped electrospun V2O5 demonstrated that doping presents a potential path for the improvement of V2O5 performance as an electrode material via electrospinning due to the relative ease of dopant addition to the starting solution combined with the interesting role that the dopant plays within the V2O5 structure.

[110]

[113] [114] [122] [123]

nanosheets. Initial discharge capacity of 103 mA h g-1 was obtained at 60 mA g−1 over 50 cycles with a cycle efficiency of 70% over 0.4–1.6 V vs Li/Li+. The electrochemical results measured with the hierarchical LiV3O8 out-performed other aqueous LIBs systems referenced within their study. This enhanced performance was attributed to the electrospun nature of the nanosheets that made up the nanofibres which promotes increased diffusion of the electrolyte and Li-ions to the fibre surface, a higher amount of active sites available in nanoscale sheets, and fibre morphology preventing aggregation of electrode material. As for mixed transition-metal oxides, where there is a significant variation in the vanadium oxidation states, interesting properties are demonstrated due to multiple valences, synergetic effects between different metals and good electric conductivities. Xiang et al. prepared various nanotube structures of MxV2O8 (M = CO, Ni, Fe) via electrospinning with empirical formulas Co3V2O8, Ni3V2O8 and FeVO4 [152]. Only the electrochemical results for Co3V2O8 were presented and showed a high reversible capacity of 900 mA h g-1 after 2000 cycles at 5 A g−1. It would have been interesting to see XPS data to determine the variation in oxidations states and what role they played in the resultant electrochemical performance. 4.4. Electrospun vanadium oxides for other metal-ion batteries Vanadium oxide has attracted attention as an electrode material for NIBs due to its layered structure and adjustable interlayer spacing. The open structure of VOx allows reversible capacity with Na-ion insertion/ extraction. VO2 has been studied as an NIB electrode material as NaxVO2 octahedral or trigonal symmetries [45] while Dider et al. showed reversible sodiation of NaxVO2 for the range ½ ≤ x ≤ 1 [153]. Chao et al. grew VO2 arrays coated with graphene quantum dots onto a graphene network with resultant Na-ion intercalation of 306 mA h g-1 at current density of 100 mA g−1 [154]. An early sodiation study of V2O5 was conducted by Bach et al. in which a vanadium bronze, β-Na0.33V2O5, was prepared via ion exchange with a NaCl aqueous solution and V2O51.6H2O xerogel followed by heat treatment at 550 °C [155]. Since Bach et al., pure V2O5 has been prepared for NIB electrodes via electrochemical deposition [156], and V2O5 hollow nanospheres [157] and single crystalline nanobelts [158] via a solvothermal process. Amorphous and crystalline V2O5 has been investigated by Uchaker et al. via sol-gel processing and electrochemical deposition [159] and by Liu et al. via anodic electrochemical deposition on graphite paper [107]. Amorphous V2O5, prepared by Liu et al. recorded a capacity of 241 mA h g-1 compared to 120 mA h g-1 for the crystalline counterpart at 23.6 mA g−1 (0.1 C) [160]. In both cases

4.3. Other doped electrospun vanadium oxide phases for lithium ion batteries All studies discussed up to this point have been focussed on the positive electrode for typical LIBs with organic electrolytes. Liang et al. electrospun LiV3O8 nanofibres and examined their suitability as a negative electrode material in aqueous LIBs [113]. Structural characterisation revealed pure monoclinic LiV3O8 nanofibres composed of 423

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Monoclinic VO2(B) forms a tunnel like structure which is capable of not only reversibly inserting and extracting Li-ions but also Al ions. Jayaprakash et al. investigated V2O5 nanowires prepared hydrothermally, as a positive electrode against Al metal in 1-ethyl-3-methylimidazolium containing AlCl3 electrolyte. V2O5 delivered an initial capacity of 305 mA h g-1 at 125 mA g−1 and maintained a capacity of 273 mA hg−1 after 20 cycles within 0.02–2.5 V vs Al/Al3+ [183]. The results of this study were put into question when Reed et al. showed that the electrochemical activity of V2O5 within 0.005–1.5 V vs Al/Al3+ was due to the reactivity of the stainless steel current collector and not the V2O5 [184]. The V2O5 xerogel in Reed et al.’s investigation was prepared via solvent exchange followed by processing into a positive electrode using an Al negative electrode with the same electrolyte as Jayaprakash et al. Wang et al. prepared binder-free V2O5-based electrodes, in which the active materials were prepared via an in situ hydrothermal deposition method on a Ni foam current collector [185]. This group showed that the binder-free electrode electrodes delivered an initial discharge capacity of 239 mA h g-1 in the range 0.02–2.5 vs Al/Al3+ compared to 46 mA h g-1 and 86.5 mA h g-1 for electrodes made with polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) respectively. They also showed that using the standard binder for electrode preparation, PVDF, was inappropriate as it reacted with acidic AlCl3/[1-butyl-3methylimidazolium](BMIM)Cl ionic electrolyte. Rather, PTFE was shown to be unreactive and consequently a better choice. Chiku et al. showed that a V2O5/C composite could deliver an initial discharge capacity of 150 mA h g-1 at 22 mA g−1 (C/20 where 1 C = 442 mA g−1) [186]. A Mo current collector with an electrolyte composition of AlCl3, dipropylsulfone and toluene (1:10:5 mol ratio) was used though no specifications regarding electrochemical cell components were supplied. The V2O5 active material was prepared by combining metal vanadium powder in hydrogen peroxide with acetylene black and acetone. A detailed study featuring the reversibility of Al-ions in V2O5 was conducted by Gu et al. [187]. They showed that the intercalation of Alions resulted in the reduction of V5+ to V4+ and TEM revealed the formation of an amorphous layer on the nanowire surfaces. The V2O5 was hydrothermally prepared and a Ni foam current collector was used in preparation of half cells with an Al metal negative electrode. It was shown that the storage of Al-ions into V2O5 is a combination of intercalation/extraction and phase transition reactions with the formation of a 10 nm amorphous layer on the V2O5 nanowires. Despite the recent rapid develop of AlBs, the energy density still remains an issue [188]. The charge storage capability is approximately half that of LIBs and the energy density is about one quarter of LIBs resulting in AlBs that would need to be four times heavier. It is anticipated that this issue can mitigated with the exploration of novel positive electrode materials. This section showed that the use of V2O5 in NIBs and AlBs is a promising direction for research of alternate metal ion battery systems beyond LIBs. It is worth noting that:

it was observed that amorphous V2O5 possessed a more open framework resulting in an effective diffusion network and fast faradaic reactions. For pre-inserted Na-V2O5, Muller-Bouvet et al. performed an in depth study of the phases formed in Na//α-V2O5 and provided extensive structural characterisation [161]. Additionally, Cai et al. showed that hydrothermally prepared Na0.28V2O5 could be used for both LIBs and NIBs [162]. As for composites, Ali et al. prepared a V2O5/C composite with nanosized particles via a hydrothermal process followed by ball milling [163]. Na-V2O5 reduced graphene oxide (rG-O) was prepared by Park et al. via a hydrothermal method and investigated as a positive electrode for NIBs with a discharge capacity of 150 mA h g-1 for the composite and 50 mA h g-1 without [164]. Additionally, an uniform coating of nanoporous carbon encapsulating V2O5 in a V2O5/C composite was prepared by ambient hydrolysis deposition by Raju et al. [165]. Electrospinning of NIB electrode materials has produced improved electrochemical performance attributed to the advantages offered by 1D nanostructures. The use of metal and relevant alloys, such as Sn [166], Sb [167], Ge [168] and SnSb [169] have shown suitable Na-ion insertion potentials and high theoretical capacities. Despite this, large volume changes and structural variations limit their practical use in NIBs. Graphitic carbons have shown to poorly store Na-ions due to a small interlayer distance of d(002) = 0.334 nm [46]. Non-graphitic carbons, such as hollow carbon nanowires [170] and nanospheres [171] has been considered to be the most suitable candidate material. This presents opportunity for electrospinning to be a prevalent production method of carbon composite NIB electrode materials due the ease of production and variety of morphologies available [47]. Examples of investigated nanofibrous electrospun materials including: Fe3O4/C and MoS2 nanoplates embedded in electrospun carbon nanofibres (CNFs) [172,173], Li4Ti5O12 and NaxCy nanoparticles embedded in electrospun CNFs [174] along with flexible and light weight free-standing electrospun electrodes have been investigated [175]. There is limited research featuring electrospun materials for NIB and at this stage in the writing there were no studies featuring electrospun vanadium oxide for NIBs. However, the rapid development with this alternate energy storage variation is proving promising. It should be noted that in depth discussions of the previous studies have been omitted as they fall outside the scope of this review. Despite this, the study of electrospun materials for NIBs presents as viable route of investigation for energy storage beyond LIBs. The study of the use of Al metal as a negative electrode material has been investigated for several decades now, with Gifford and Palmisano in 1988 who studied the electrochemical effects of graphite and Al metal in a 1,2-dimethyl-3-propylimidazolium chloride electrolyte containing AlCl3 [176]. FeS2 has shown to be beneficial as a positive electrode for AIBs given its low cost and low equivalent weight while copper hexacyanoferrate nanoparticles were able to reversibly intercalate Al-ions in an aqueous solution [177]. Naturally, several studies have investigated graphite electrodes and have shown that Al-ion can be reversibly inserted into the graphitic layers [178–180]. The effects of Al-ion insertion into TiO2 nanotube arrays has also been investigated using a three-electrode cell showing that the Al-ions caused the reduction of Ti4+ to Ti3+ and the importance of Cl− in facilitating this process was emphasised [181]. Using a slightly different strategy, Hudak investigated polypyrrole films as positive electrodes for AIBs and the electrochemical effect of cycling chloroaluminate anions contained within the electrolyte [182]. Similar to NIBs, electrospun vanadium oxide is limited indicating that there is substantial room for investigation of electrospun vanadium oxide as electrode materials for aluminium ion batteries (AIB). Vanadium oxide fabricated via other means has been investigated as an AlB electrode material. Monoclinic VO2(B) nanoscale rods were prepared by Wang et al. via a low temperature hydrothermal method [53]. Processing into an electrode material for Al-based coin cells revealed capacities above 116 mA h g-1 after 100 cycles at 50 mA g−1.

- The importance of an open framework of the V2O5 layers to promote enhanced mobility of Na-ions and Al-ions, - There is little to no research of electrospun vanadium oxides as an electrode material in metal ion battery systems beyond LIBs, - The above observations combined with the potential of vanadiumbased oxides in LIBs presents a promising route of investigation. 4.5. Incorporation of carbon into vanadium oxides The incorporation of carbon or a derivative thereof, with another material combines the long life cycle of carbon with the defining features of the other additive [189]. While V2O5 has drawn wide attention as an electrode material, its poor capacity retention and rate performance, caused by its low electronic conductivity and low Li-ion 424

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However, the improvement was not significant, with the composite experiencing continuous capacity decrease over 30 cycles at 0.2 C. The value associated with 1 C in terms of mA h g−1 was not specified. Consequently, it is assumed here that 1 C is equal to the approximate theoretical value of V2O5 for the intercalation of 2 mol Li-ions which is approximately 300 mA h g-1. A rate performance test revealed that the composite did show good rate capability of 25 mA h g-1 at 5 C though both samples did not show any electrochemical response at 10 C [Fig. 8(b)]. Coulombic efficiencies were shown to be very high for the composite, around 98% over the entire rate performance tests which totalled 60 cycles with 95% for the pure V2O5 nanowires. Long term galvanostatic cycling was not included in this study. The improvement was attributed to increased conductivity due to the presence of graphene which also facilitated volume control during Li-ion movement in the composite material. This study shows that there is room for improving electrospun vanadium oxide performance in LIBs through the incorporation of carbon. Kong et al. also produced a V2O5 composite using electrospun V2O5 nanosheets that were encapsulated in chemical vapour-deposited graphitic carbon layers [123]. The resultant nanotubes were woven into flexible free-standing sheets and then used directly as positive electrodes for LIBs. A capacity of 224 mA h g-1 was obtained at 150 mA g−1 (0.5 C) with a Coulombic efficiency of 92%. Cycle retention after 200 cycles was 211 mA h g-1 with a cycle stability of 91.7%. Additionally, the composite possessed excellent rate capability with a capacity of 90 mA h g-1 obtained at a current density of 30 A g−1 (100 C). The good performance was attributed to not only improved conductivity afforded by the graphitic carbon but also due to the absence of the standard electrode components (binder, conductive carbon, current collector foil). This study is an interesting and valid addition to free-standing electrode device development.

diffusion rate have hindered the development of this material [190]. Producing composites of vanadium oxide has been shown to be an effective way to improve its electrochemical performance [37]. The introduction of activated carbon, carbon nanotubes or carbon shells have stabilized vanadium oxide nanostructures [191]. Graphene-based materials have shown to improve electrochemical conductivity, thermal stability and Li-ion diffusion rates in LIB systems. In some cases, graphene oxide is functionalized in order to introduce hydrophilic groups (-COOH < -C=O, -OH) to the surface of the graphene sheets which act as anchoring sites for the vanadium oxide nanostructures [191]. Carbon coating is another effective method for improving electrochemical performance of electrodes by forming protective layers on the active materials. A coating can also improve structural stability during cycling as carbon can act as a barrier to suppress pulverization and aggregation of active particles. Additionally, carbon has high electronic conductivity and it hence improves the conductance of composite materials [192]. While electrospinning of vanadium oxide with subsequent carbon treatment is not particularly prevalent in the literature, other methods for producing carbon incorporated vanadium oxides are briefly discussed. Channu et al. fabricated vanadium oxide nanostructures via hydrothermal method using vanadyl(IV) sulphate (VOSO4) and either sucrose or functionalized graphene oxide as a carbon coating source [191]. It was observed that the graphene oxide-coated vanadium oxide outperformed the sucrose-derived carbon-coated vanadium oxide which was attributed to higher surface area and higher conductivity of the former. The following select studies represent a wide range of morphologies and carbon sources that result in improved electrochemical performance: V2O5/CNT microspheres using spray-drying multi-walled carbon nanotubes (MWCNT) with V2O5 prepared via a hydrothermal method [6,126,127], V2O5 nanosheets anchored on graphene using the slow hydrolysis of vanadyl triisobutoxide [193], V2O5 nanoparticles and CNTs integrated into a porous sheet using an icetemplating assembly [194], and functionalized CNTs coated with V2O5 nanoparticles via hydrolysis of vanadium oxy-tripropoxide [195]. In addition, VO2 has also been incorporated into a vertically aligned nanobelt forest grown on a metal foam and graphene scaffold which showed remarkable stability [196]. Interestingly, a couple studies presented in Section 2.2 attributed the presence of residual carbon after heat treatment from the polymer component of the electrospinning solution to an improvement in electrochemical performance [117,121]. It is then logical that the incorporation of carbon into electrospinning is a viable method to improve both capacity and cycle stability. V2O5 nanowires electrospun by Pham-Cong et al. were mixed with reduced graphene oxide to create a composite that possessed the advantages of both vanadium oxide and graphene [Fig. 8(a)] [122]. Electrochemical performance of the composite compared to the pure V2O5 nanowires was improved over the 2.0–4.0 V vs Li/Li+ range.

5. Outlook and future directions This review explored the production of nanostructured one-dimensional electrospun vanadium oxides and their potential for use in metalion batteries, particularly in LIBs. Electrospun V2O5 was predominantly examined though other vanadium oxide stoichiometries, both electrospun or otherwise, were briefly introduced to highlight the versatility of this material. The studies analysed in this review presented a mix of cases where electrospun vanadium oxide was produced in either a single-step process (i.e. just electrospinning) or a two-step process (i.e. electrospinning and hydrothermal treatment). It was shown that pure vanadium oxide produced in a two-step process had limited cycle stability [108,115], except in the case where electrospun silver doped vanadium oxide produced good cycle stability [114]. Controlling the amount of Li-ions intercalated into the vanadium oxide via potential range variation presented an evolution in cycle stability investigations. Common aspects of these studies discussed in this review were the open particle network produced from electrospinning providing increased contact between the positive electrode and a decrease in Li-ion diffusion pathways. Despite the competitive performance of doped electrospun V2O5, there is a significant lack of these materials present in the literature. This highlights a strategy for the improvement of V2O5 as an electrode material via electrospinning due to the relative ease of dopant addition to the starting solution combined with the interesting role that the dopant plays within the V2O5 structure. It is suggested that the future of energy storage lies with the development of both electrode materials and metal-ion systems beyond Li. Al-based systems are proving to be quite competitive though electrolyte studies have shown that performance is limited in aqueous electrolytes due to the formation of resistive oxide layers on the Al electrode. This leads to polarisation and low cell voltage. The oxide layer on the Al metal electrode can be dissolved using alkaline electrolytes though this can lead to corrosion of the electrode itself. Strategies for overcoming

Fig. 8. (a) TEM image of graphene covered V2O5 particles, (b) galvanostatic cycling at 0.2 C followed by C-rate test for V2O5 fibres and graphene oxide V2O5 (GVO) from Ref. [122] with permission from Elsevier. 425

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this corrosion include alloying the Al negative electrode, low operational temperatures, increased current density and corrosion inhibitors [9]. Non-aqueous electrolytes, such as inorganic and ionic molten salts, have proven to be more practical offering a wider electrochemical window. In particular, room temperature ionic liquids containing a mixture of aluminium chloride and imidazolium salts have shown high aluminium dissolution and plating efficiencies. The reader is referred to for an extensive discussion with further reading in regards to electrolyte studies [197,198]. When the considering the chemistry behind the alternative metal-ion systems, the most promising are Al-ion, despite the electrolyte questions, and Na-ion. Due to the promise that Al-ion systems show, it is likely electrospun vanadium oxides will play a role in the development of the alternate metal ion system. For electrospinning to be a viable and large-scale nanomaterial fabrication technique, the complete preparation methodology needs to be as simple as possible whilst producing materials that have competitive performance. The interest in the electrospinning technique has steadily grown over the years as solutions for new energy storage technologies are being sought. Overall, electrospinning has proven to be a reliable method that can be conducted in room temperature with the continual improvement of electrochemical performance of electrospun vanadium oxides. The versatility of this facile method for producing a wide range of high quality nanofibres has shown that electrospinning will continue to play an important role in development of nanomaterials, in particular vanadium oxide, in energy storage technology.

http://dx.doi.org/10.1021/acsami.5b02618. [18] Q. Chen, T. Zhang, X. Qiao, D. Li, J. Yang, Li3V2(PO4)3/C nanofibers composite as a high performance cathode material for lithium-ion battery, J. Power Sources 234 (2013) 197–200, http://dx.doi.org/10.1016/j.jpowsour.2013.01.164. [19] R. Marom, S.F. Amalraj, N. Leifer, D. Jacob, D. Aurbach, A review of advanced and practical lithium battery materials, J. Mater. Chem. 21 (2011) 9938–9954, http:// dx.doi.org/10.1039/C0JM04225K. [20] K.M. Shaju, P.G. Bruce, Nano-LiNi0.5Mn1.5O4 spinel: a high power electrode for Li-ion batteries, Dalton Trans. (2008) 5471–5475, http://dx.doi.org/10.1039/ B806662K. [21] Y. Talyosef, B. Markovsky, R. Lavi, G. Salitra, D. Aurbach, D. Kovacheva, M. Gorova, E. Zhecheva, R. Stoyanova, Comparing the behavior of nano- and microsized particles of LiMn1.5Ni0.5O4 spinel as cathode materials for Li-Ion batteries, J. Electrochem. Soc. 154 (2007) A682–A691. [22] C.S. Johnson, N. Li, C. Lefief, J.T. Vaughey, M.M. Thackeray, Synthesis, characterization and electrochemistry of lithium battery electrodes: xLi2MnO3·(1 −x) LiMn0.333Ni0.333Co0.333O2 (0 ≤ x ≤ 0.7), Chem. Mater. 20 (2008) 6095–6106, http://dx.doi.org/10.1021/cm801245r. [23] F. Amalraj, D. Kovacheva, M. Talianker, L. Zeiri, J. Grinblat, N. Leifer, G. Goobes, B. Markovsky, D. Aurbach, Synthesis of integrated cathode materials xLi2MnO3⋅ ( 1 − x ) LiMn1/3Ni1/3Co1/3O2 ( x = 0.3 , 0.5 , 0.7 ) and studies of their electrochemical behavior, J. Electrochem. Soc. 157 (2010) A1121–A1130. [24] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, Phospho-olivines as positiveelectrode materials for rechargeable lithium batteries, J. Electrochem. Soc. 144 (1997) 1188–1194. [25] K.-S. Park, S.B. Schougaard, J.B. Goodenough, Conducting-polymer/iron-redoxcouple composite cathodes for lithium secondary batteries, Adv. Mater. 19 (2007) 848–851, http://dx.doi.org/10.1002/adma.200600369. [26] Y.-H. Huang, J.B. Goodenough, High-rate LiFePO4 lithium rechargeable battery promoted by electrochemically active polymers, Chem. Mater. 20 (2008) 7237–7241, http://dx.doi.org/10.1021/cm8012304. [27] S. Okada, S. Sawa, M. Egashira, J. Yamaki, M. Tabuchi, H. Kageyama, T. Konishi, A. Yoshino, Cathode properties of phospho-olivine LiMPO4 for lithium secondary batteries, Proc. 10th Int. Meet. Lithium Batter, vols. 97–98, 2001, pp. 430–432, , http://dx.doi.org/10.1016/S0378-7753(01)00631-0. [28] A. Nytén, A. Abouimrane, M. Armand, T. Gustafsson, J.O. Thomas, Electrochemical performance of Li2FeSiO4 as a new Li-battery cathode material, Electrochem. Commun. 7 (2005) 156–160, http://dx.doi.org/10.1016/j.elecom. 2004.11.008. [29] R. Dominko, M. Bele, M. Gaberšček, A. Meden, M. Remškar, J. Jamnik, Structure and electrochemical performance of Li2MnSiO4 and Li2FeSiO4 as potential Libattery cathode materials, Electrochem. Commun. 8 (2006) 217–222, http://dx. doi.org/10.1016/j.elecom.2005.11.010. [30] A.R. West, F.P. Glasser, Preparation and crystal chemistry of some tetrahedral Li3PO4-type compounds, J. Solid State Chem. 4 (1972) 20–28, http://dx.doi.org/ 10.1016/0022-4596(72)90127-2. [31] B.L. Ellis, K.T. Lee, L.F. Nazar, Positive electrode materials for Li-Ion and LiBatteries, Chem. Mater. 22 (2010) 691–714, http://dx.doi.org/10.1021/ cm902696j. [32] J.-S. Huang, C.-Y. Chou, M.-Y. Liu, K.-H. Tsai, W.-H. Lin, C.-F. Lin, Solution-processed vanadium oxide as an anode interlayer for inverted polymer solar cells hybridized with ZnO nanorods, Org. Electron. 10 (2009) 1060–1065, http://dx. doi.org/10.1016/j.orgel.2009.05.017. [33] M. Benmoussa, A. Outzourhit, A. Bennouna, E. Ameziane, Electrochromism in sputtered V2O5 thin films: structural and optical studies, Thin Solid Films 405 (2002) 11–16, http://dx.doi.org/10.1016/S0040-6090(01)01734-5. [34] A. Jin, W. Chen, Q. Zhu, Z. Jian, Multi-electrochromism behavior and electrochromic mechanism of electrodeposited molybdenum doped vanadium pentoxide films, Electrochim. Acta 55 (2010) 6408–6414, http://dx.doi.org/10.1016/j. electacta.2010.06.047. [35] Z. Zhang, C. Shao, L. Zhang, X. Li, Y. Liu, Electrospun nanofibers of V-doped TiO2 with high photocatalytic activity, J. Colloid Interface Sci. 351 (2010) 57–62, http://dx.doi.org/10.1016/j.jcis.2010.05.067. [36] B.-H. Kim, C.H. Kim, K.S. Yang, A. Rahy, D.J. Yang, Electrospun vanadium pentoxide/carbon nanofiber composites for supercapacitor electrodes, Electrochim. Acta 83 (2012) 335–340, http://dx.doi.org/10.1016/j.electacta.2012.07.093. [37] X. Chen, B. Zhao, Y. Cai, M.O. Tadé, Z. Shao, Amorphous V–O–C composite nanofibers electrospun from solution precursors as binder- and conductive additivefree electrodes for supercapacitors with outstanding performance, Nanoscale 5 (2013), http://dx.doi.org/10.1039/c3nr04484j 12589. [38] S. Li, Y. Li, K. Qian, S. Ji, H. Luo, Y. Gao, P. Jin, Functional fiber mats with tunable diffuse reflectance composed of electrospun VO2/PVP composite fibers, ACS Appl. Mater. Interfaces 6 (2014) 9–13, http://dx.doi.org/10.1021/am405006g. [39] J.J. Yu, J. Yang, W.B. Nie, Z.H. Li, E.H. Liu, G.T. Lei, Q.Z. Xiao, A porous vanadium pentoxide nanomaterial as cathode material for rechargeable lithium batteries, Electrochim. Acta 89 (2013) 292–299, http://dx.doi.org/10.1016/j.electacta. 2012.11.032. [40] B. Yan, X. Li, Z. Bai, M. Li, L. Dong, D. Xiong, D. Li, Superior lithium storage performance of hierarchical porous vanadium pentoxide nanofibers for lithium ion battery cathodes, J. Alloy. Comp. 634 (2015) 50–57, http://dx.doi.org/10.1016/j. jallcom.2015.01.292. [41] C. Zhu, J. Shu, X. Wu, P. Li, X. Li, Electrospun V2O5 micro/nanorods as cathode materials for lithium ion battery, J. Electroanal. Chem.. (n.d.). doi:10.1016/j.jelechem.2015.11.013. [42] A.L. Viet, M.V. Reddy, R. Jose, B.V.R. Chowdari, S. Ramakrishna, Nanostructured Nb2O5 polymorphs by electrospinning for rechargeable lithium batteries, J. Phys.

References [1] Z. Dong, S.J. Kennedy, Y. Wu, Electrospinning materials for energy-related applications and devices, J. Power Sources 196 (2011) 4886–4904, http://dx.doi. org/10.1016/j.jpowsour.2011.01.090. [2] S. Cavaliere, S. Subianto, I. Savych, D.J. Jones, J. Roziere, Electrospinning: designed architectures for energy conversion and storage devices, Energy Environ. Sci. 4 (2011) 4761–4785, http://dx.doi.org/10.1039/C1EE02201F. [3] B. O'Regan, M. Gratzel, A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films, Nature 353 (1991) 737–740, http://dx.doi.org/10. 1038/353737a0. [4] A. González, E. Goikolea, J.A. Barrena, R. Mysyk, Review on supercapacitors: technologies and materials, Renew. Sustain. Energy Rev. 58 (2016) 1189–1206, http://dx.doi.org/10.1016/j.rser.2015.12.249. [5] A.G. Pandolfo, A.F. Hollenkamp, Carbon properties and their role in supercapacitors, J. Power Sources 157 (2006) 11–27, http://dx.doi.org/10.1016/j. jpowsour.2006.02.065. [6] M. Qin, J. Liu, S. Liang, Q. Zhang, X. Li, Y. Liu, M. Lin, Facile synthesis of multiwalled carbon nanotube–V2O5 nanocomposites as cathode materials for Li-ion batteries, J. Solid State Electrochem. 18 (2014) 2841–2846, http://dx.doi.org/10. 1007/s10008-014-2543-7. [7] S. Holmberg, A. Perebikovsky, L. Kulinsky, M. Madou, 3-D micro and nano technologies for improvements in electrochemical power devices, Micromachines 5 (2014) 171–203, http://dx.doi.org/10.3390/mi5020171. [8] A. Vlad, N. Singh, C. Galande, P.M. Ajayan, Design considerations for unconventional electrochemical energy storage architectures, Adv. Energy Mater 5 (2015), http://dx.doi.org/10.1002/aenm.201402115 1402115. [9] L. Fu, N. Li, Y. Liu, W. Wang, Y. Zhu, Y. Wu, Advances of aluminum based energy storage systems, Chin. J. Chem. 35 (2017) 13–20, http://dx.doi.org/10.1002/cjoc. 201600655. [10] J.-K. Park, Principles and Applications of Lithium Secondary Batteries, WILEYVCH Verlag GmbH & Co. KGaA, Weinheim, Germany, n.d. [11] J.-M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359–367, http://dx.doi.org/10.1038/35104644. [12] International Energy Agency, Technology Roadmap: Electric and Plug-in Hybrid Electric Vehicles, Intermational Energy Agency, Paris, 2011. [13] B. Scrosati, J. Garche, Lithium batteries: status, prospects and future, J. Power Sources 195 (2010) 2419–2430, http://dx.doi.org/10.1016/j.jpowsour.2009.11. 048. [14] N. Nitta, F. Wu, J.T. Lee, G. Yushin, Li-ion battery materials: present and future, Mater. Today 18 (2015) 252–264, http://dx.doi.org/10.1016/j.mattod.2014.10. 040. [15] J. Garcia-Martinez, Nanotechnology for the Energy Challenge, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Padstow, 2010. [16] M. Park, X. Zhang, M. Chung, G.B. Less, A.M. Sastry, A review of conduction phenomena in Li-ion batteries, J. Power Sources 195 (2010) 7904–7929, http:// dx.doi.org/10.1016/j.jpowsour.2010.06.060. [17] L. Chen, B. Yan, J. Xu, C. Wang, Y. Chao, X. Jiang, G. Yang, Bicontinuous structure of Li3V2(PO4)3 clustered via carbon nanofiber as high-performance cathode material of Li-Ion batteries, ACS Appl. Mater. Interfaces 7 (2015) 13934–13943,

426

Journal of Power Sources 395 (2018) 414–429

C.F. Armer et al.

Chem. C 114 (2010) 664–671, http://dx.doi.org/10.1021/jp9088589. [43] D. McNulty, D.N. Buckley, C. O'Dwyer, Synthesis and electrochemical properties of vanadium oxide materials and structures as Li-ion battery positive electrodes, J. Power Sources 267 (2014) 831–873, http://dx.doi.org/10.1016/j.jpowsour.2014. 05.115. [44] F. Ambroz, T.J. Macdonald, T. Nann, Trends in aluminium-based intercalation batteries, Adv. Energy Mater (2017), http://dx.doi.org/10.1002/aenm. 201602093 1602093-n/a. [45] D. Hamani, M. Ati, J.-M. Tarascon, P. Rozier, NaxVO2 as possible electrode for Naion batteries, Electrochem. Commun. 13 (2011) 938–941, http://dx.doi.org/10. 1016/j.elecom.2011.06.005. [46] L. Weihan, Z. Linchao, W. Ying, Y. Yan, Nanostructured electrode materials for lithium-ion and sodium-ion batteries via electrospinning, Sci. CHINA Mater 59 (2016) 287–321 https://doi.org/10.1007/s40843-016-5039-6. [47] H.-G. Wang, S. Yuan, D.-L. Ma, X.-B. Zhang, J.-M. Yan, Electrospun materials for lithium and sodium rechargeable batteries: from structure evolution to electrochemical performance, Energy Environ. Sci. 8 (2015) 1660–1681, http://dx.doi. org/10.1039/C4EE03912B. [48] Z. Jian, W. Luo, X. Ji, Carbon electrodes for K-Ion batteries, J. Am. Chem. Soc. 137 (2015) 11566–11569, http://dx.doi.org/10.1021/jacs.5b06809. [49] S. Komaba, T. Hasegawa, M. Dahbi, K. Kubota, Potassium intercalation into graphite to realize high-voltage/high-power potassium-ion batteries and potassiumion capacitors, Electrochem. Commun. 60 (2015) 172–175, http://dx.doi.org/10. 1016/j.elecom.2015.09.002. [50] K. Beltrop, S. Beuker, A. Heckmann, M. Winter, T. Placke, Alternative electrochemical energy storage: potassium-based dual-graphite batteries, Energy Environ. Sci. (2017), http://dx.doi.org/10.1039/C7EE01535F. [51] A. Moretti, S. Passerini, Bilayered nanostructured V2O5·nH2O for metal batteries, Adv. Energy Mater 6 (2016), http://dx.doi.org/10.1002/aenm.201600868 1600868-n/a. [52] Z.A. Zafar, S. Imtiaz, R. Razaq, S. Ji, T. Huang, Z. Zhang, Y. Huang, J.A. Anderson, Cathode materials for rechargeable aluminum batteries: current status and progress, J. Mater. Chem. A 5 (2017) 5646–5660, http://dx.doi.org/10.1039/ C7TA00282C. [53] W. Wang, B. Jiang, W. Xiong, H. Sun, Z. Lin, L. Hu, J. Tu, J. Hou, H. Zhu, S. Jiao, A new cathode material for super-valent battery based on aluminium ion intercalation and deintercalation, Sci. Rep. 3 (2013) 3383. [54] S. Surnev, M. Ramsey, F. Netzer, Vanadium oxide surface studies, Prog. Surf. Sci. 73 (2003) 117–165, http://dx.doi.org/10.1016/j.progsurf.2003.09.001. [55] Y.L. Cheah, N. Gupta, S.S. Pramana, V. Aravindan, G. Wee, M. Srinivasan, Morphology, structure and electrochemical properties of single phase electrospun vanadium pentoxide nanofibers for lithium ion batteries, J. Power Sources 196 (2011) 6465–6472, http://dx.doi.org/10.1016/j.jpowsour.2011.03.039. [56] J. Hou, J. Zhang, Z. Wang, Z. Zhang, Z. Ding, Structural transition of VO2 (A) nanorods studied by vibrational spectroscopies, RSC Adv. 4 (2014), http://dx.doi. org/10.1039/c4ra00585f 18055. [57] E. Baudrin, G. Sudant, D. Larcher, B. Dunn, J.-M. Tarascon, Preparation of nanotextured VO2[B] from vanadium oxide aerogels, Chem. Mater. 18 (2006) 4369–4374, http://dx.doi.org/10.1021/cm060659p. [58] D.W. Murphy, P.A. Christian, F.J. DiSalvo, J.N. Carides, J.V. Waszczak, Lithium incorporation by V 6 O 13 and related vanadium (+4, +5) oxide cathode materials, J. Electrochem. Soc. 128 (1981) 2053–2060. [59] W. Li, J.R. Dahn, D.S. Wainwright, Rechargeable lithium batteries with aqueous electrolytes, Science 264 (1994) 1115, http://dx.doi.org/10.1126/science.264. 5162.1115. [60] C. Tsang, A. Manthiram, Synthesis of nanocrystalline VO 2 and its electrochemical behavior in lithium batteries, J. Electrochem. Soc. 144 (1997) 520–524. [61] C. Niu, J. Meng, C. Han, K. Zhao, M. Yan, L. Mai, VO2 nanowires assembled into hollow microspheres for high-rate and long-life lithium batteries, Nano Lett. 14 (2014) 2873–2878, http://dx.doi.org/10.1021/nl500915b. [62] N.A. Chernova, M. Roppolo, A.C. Dillon, M.S. Whittingham, Layered vanadium and molybdenum oxides: batteries and electrochromics, J. Mater. Chem. 19 (2009) 2526–2552, http://dx.doi.org/10.1039/B819629J. [63] C. Wu, H. Wei, B. Ning, Y. Xie, New vanadium oxide nanostructures: controlled synthesis and their smart electrical switching properties, Adv. Mater. 22 (2010) 1972–1976, http://dx.doi.org/10.1002/adma.200903890. [64] S.Y. Zhan, C.Z. Wang, K. Nikolowski, H. Ehrenberg, G. Chen, Y.J. Wei, Electrochemical properties of Cr doped V2O5 between 3.8 V and 2.0 V, Solid State Ionics 180 (2009) 1198–1203, http://dx.doi.org/10.1016/j.ssi.2009.05.020. [65] T. Zhai, H. Liu, H. Li, X. Fang, M. Liao, L. Li, H. Zhou, Y. Koide, Y. Bando, D. Golberg, Centimeter-long V2O5 nanowires: from synthesis to field-emission, electrochemical, electrical transport, and photoconductive properties, Adv. Mater. 22 (2010) 2547–2552, http://dx.doi.org/10.1002/adma.200903586. [66] J.K. Bailey, G.A. Pozarnsky, M.L. Mecartney, The direct observation of structural development during vanadium pentoxide gelation, J. Mater. Res. 7 (1992) 2530–2537, http://dx.doi.org/10.1557/JMR.1992.2530. [67] H.-K. Park, W.H. Smyrl, M.D. Ward, V 2 O 5 xerogel films as intercalation hosts for lithium: I. Insertion stoichiometry, site concentration, and specific energy, J. Electrochem. Soc. 142 (1995) 1068–1073. [68] B. Alonso, J. Livage, Synthesis of vanadium oxide gels from peroxovanadic acid solutions: a 51V NMR study, J. Solid State Chem. 148 (1999) 16–19, http://dx.doi. org/10.1006/jssc.1999.8283. [69] C.J. Fontenot, J.W. Wiench, M. Pruski, G.L. Schrader, Vanadia gel synthesis via peroxovanadate precursors. 2. Characterization of the gels, J. Phys. Chem. B 105 (2001) 10496–10504, http://dx.doi.org/10.1021/jp010351f. [70] V. Petkov, P.N. Trikalitis, E.S. Bozin, S.J.L. Billinge, T. Vogt, M.G. Kanatzidis,

[71]

[72]

[73]

[74]

[75]

[76]

[77]

[78] [79] [80]

[81] [82]

[83]

[84] [85]

[86] [87]

[88] [89] [90] [91] [92]

[93] [94]

[95]

[96]

[97]

[98]

[99]

427

Structure of V2O5·nH2O xerogel solved by the atomic pair distribution function technique, J. Am. Chem. Soc. 124 (2002) 10157–10162, http://dx.doi.org/10. 1021/ja026143y. M. Nabavi, C. Sanchez, J. Livage, Structure and properties of amorphous V2O5, Philos. Mag. Part B 63 (1991) 941–953, http://dx.doi.org/10.1080/ 13642819108205549. A. Pan, J.-G. Zhang, Z. Nie, G. Cao, B.W. Arey, G. Li, S. Liang, J. Liu, Facile synthesized nanorod structured vanadium pentoxide for high-rate lithium batteries, J. Mater. Chem. 20 (2010) 9193, http://dx.doi.org/10.1039/c0jm01306d. N.L. Lala, R. Jose, M.M. Yusoff, S. Ramakrishna, Continuous tubular nanofibers of vanadium pentoxide by electrospinning for energy storage devices, J. Nanoparticle Res. 14 (2012), http://dx.doi.org/10.1007/s11051-012-1201-1. G. Liu, S. Liu, Q. Lu, H. Sun, Z. Xiu, Synthesis and characterization of Bi(VO4)1-m (PO4)m nanofibers by electrospinning process with enhanced photocatalytic activity under visible light, RSC Adv. 4 (2014) 33695–33701, http://dx.doi.org/10. 1039/C4RA04107K. E. Uchaker, G. Cao, The role of intentionally introduced defects on electrode materials for alkali-ion batteries, Chem. Asian J. 10 (2015) 1608–1617, http://dx. doi.org/10.1002/asia.201500401. H. Zeng, D. Liu, Y. Zhang, K.A. See, Y.-S. Jun, G. Wu, J.A. Gerbec, X. Ji, G.D. Stucky, Nanostructured Mn-Doped V2O5 cathode material fabricated from layered vanadium jarosite, Chem. Mater. 27 (2015) 7331–7336, http://dx.doi. org/10.1021/acs.chemmater.5b02840. Z. Li, C. Zhang, C. Liu, H. Fu, X. Nan, K. Wang, X. Li, W. Ma, X. Lu, G. Cao, Enhanced electrochemical properties of Sn-doped V2O5 as a cathode material for lithium ion batteries, Electrochim. Acta 222 (2016) 1831–1838, http://dx.doi. org/10.1016/j.electacta.2016.11.174. L. Huang, Q. Wei, R. Sun, L. Mai, Nanowire electrodes for advanced lithium batteries, Front. Energy Res. 2 (2014), http://dx.doi.org/10.3389/fenrg.2014.00043. B. Yan, L. Liao, Y. You, X. Xu, Z. Zheng, Z. Shen, Single-crystalline V2O5 Ultralong Nanoribbon Waveguides, vol. 21, (2009), pp. 2436–2440. S. Ramakrishna, R. Jose, P.S. Archana, A.S. Nair, R. Balamurugan, J. Venugopal, W.E. Teo, Science and engineering of electrospun nanofibers for advances in clean energy, water filtration, and regenerative medicine, J. Mater. Sci. 45 (2010) 6283–6312, http://dx.doi.org/10.1007/s10853-010-4509-1. A.H. Lu, E.E. Salabas, Schüth, Magnetic Nanoparticles: Synthesis, protection, Functionalization, and Application, n.D. I.A. Rahman, V. Padavettan, Synthesis of silica nanoparticles by sol-gel: size-dependent properties, surface modification, and applications in silica-polymer nanocomposites—a review, J. Nanomater. 2012 (2012) 8. J.A. López Pérez, M.A. López Quintela, J. Mira, J. Rivas, S.W. Charles, Advances in the preparation of magnetic nanoparticles by the microemulsion method, J. Phys. Chem. B 101 (1997) 8045–8047, http://dx.doi.org/10.1021/jp972046t. G. Demazeau, Solvothermal processes: a route to the stabilization of new materials, J. Mater. Chem. 9 (1999) 15–18, http://dx.doi.org/10.1039/A805536J. A. Rabenau, The role of hydrothermal synthesis in preparative chemistry, Angew Chem. Int. Ed. Engl. 24 (1985) 1026–1040, http://dx.doi.org/10.1002/anie. 198510261. W.-E. Teo, R. Inai, S. Ramakrishna, Technological advances in electrospinning of nanofibers, Sci. Technol. Adv. Mater. 12 (2011) 013002. V. Modafferi, S. Trocino, A. Donato, G. Panzera, G. Neri, Electrospun V2O5 composite fibers: synthesis, characterization and ammonia sensing properties, Thin Solid Films 548 (2013) 689–694, http://dx.doi.org/10.1016/j.tsf.2013.03. 137. J.F. Cooley, Apparatus for electrically dispersing fluids, https://www.google.com/ patents/US692631, (1902). W.J. Morton, Method of dispersing fluids, https://www.google.com/patents/ US705691, (1902). F. Anton, Process and apparatus for preparing artificial threads, https://www. google.com/patents/US1975504, (1934). Electrically driven jets, Proc. R. Soc. Lond. Math. Phys. Sci 313 (1969) 453, http:// dx.doi.org/10.1098/rspa.1969.0205. J. Doshi, D.H. Reneker, Electrospinning process and applications of electrospun fibers, Sel. Pap. Spec. Tech. Sess. Electrost. Polym. Process. Charge Monit. 1993 IEEE Ind. Appl. Soc. Meet 35 (1995) 151–160, http://dx.doi.org/10.1016/03043886(95)00041-8. Darrell H. Reneker, Iksoo Chun, Nanometre diameter fibres of polymer, produced by electrospinning, Nanotechnology 7 (1996) 216. H. Fong, I. Chun, D. Reneker, Beaded nanofibers formed during electrospinning, Polymer 40 (1999) 4585–4592, http://dx.doi.org/10.1016/S0032-3861(99) 00068-3. D.H. Reneker, A.L. Yarin, H. Fong, S. Koombhongse, Bending instability of electrically charged liquid jets of polymer solutions in electrospinning, J. Appl. Phys. 87 (2000) 4531–4547, http://dx.doi.org/10.1063/1.373532. S. Koombhongse, W. Liu, D.H. Reneker, Flat polymer ribbons and other shapes by electrospinning, J. Polym. Sci., Part B: Polym. Phys. 39 (2001) 2598–2606, http:// dx.doi.org/10.1002/polb.10015. A.L. Yarin, S. Koombhongse, D.H. Reneker, Bending instability in electrospinning of nanofibers, J. Appl. Phys. 89 (2001) 3018–3026, http://dx.doi.org/10.1063/1. 1333035. M.M. Hohman, M. Shin, G. Rutledge, M.P. Brenner, Electrospinning and electrically forced jets. I. Stability theory, Phys. Fluids 13 (2001) 2201–2220, http:// dx.doi.org/10.1063/1.1383791. M.M. Hohman, M. Shin, G. Rutledge, M.P. Brenner, Electrospinning and electrically forced jets. II. Applications, Phys. Fluids 13 (2001) 2221–2236, http://dx. doi.org/10.1063/1.1384013.

Journal of Power Sources 395 (2018) 414–429

C.F. Armer et al.

batteries, RSC Adv. 4 (2014), http://dx.doi.org/10.1039/c4ra01316f 21018. [127] Q. Li, Y. Chen, J. He, F. Fu, F. Qi, J. Lin, W. Zhang, Carbon nanotube modified V 2 O 5 porous microspheres as cathodes for high-performance lithium-ion batteries, Energy Technol. (2017), http://dx.doi.org/10.1002/ente.201600435. [128] D. Zhu, H. Liu, L. Lv, Y.D. Yao, W.Z. Yang, Hollow microspheres of V2O5 and Cudoped V2O5 as cathode materials for lithium-ion batteries, Scripta Mater. 59 (2008) 642–645, http://dx.doi.org/10.1016/j.scriptamat.2008.05.020. [129] A.-M. Cao, J.-S. Hu, H.-P. Liang, L.-J. Wan, Self-assembled vanadium pentoxide (V2O5) hollow microspheres from nanorods and their application in lithium-ion batteries, Angew. Chem. Int. Ed. 44 (2005) 4391–4395, http://dx.doi.org/10. 1002/anie.200500946. [130] H.-Y. Li, C. Wei, L. Wang, Q.-S. Zuo, X. Li, B. Xie, Hierarchical vanadium oxide microspheres forming from hyperbranched nanoribbons as remarkably high performance electrode materials for supercapacitors, J. Mater. Chem. (2015), http:// dx.doi.org/10.1039/C5TA06088E. [131] C. Zheng, L. Zeng, M. Wang, H. Zheng, M. Wei, Synthesis of hierarchical ZnV 2 O 4 microspheres and its electrochemical properties, CrystEngComm 16 (2014) 10309–10313, http://dx.doi.org/10.1039/C4CE01445F. [132] F.K. Butt, C. Cao, Q. Wan, P. Li, F. Idrees, M. Tahir, W.S. Khan, Z. Ali, M.J.M. Zapata, M. Safdar, X. Qu, Synthesis, evolution and hydrogen storage properties of ZnV2O4 glomerulus nano/microspheres: a prospective material for energy storage, Int. J. Hydrogen Energy 39 (2014) 7842–7851, http://dx.doi.org/ 10.1016/j.ijhydene.2014.03.033. [133] X.-P. Yan, J. Chen, J.-M. Liu, P.-P. Zhou, X. Liu, Z.-X. Su, Fabrication of hollow VO2 microspheres by a facile template-free process, Mater. Lett. 64 (2010) 278–280, http://dx.doi.org/10.1016/j.matlet.2009.10.061. [134] H.-E. Wang, D.-S. Chen, Y. Cai, R.-L. Zhang, J.-M. Xu, Z. Deng, X.-F. Zheng, Y. Li, I. Bello, B.-L. Su, Facile synthesis of hierarchical and porous V2O5 microspheres as cathode materials for lithium ion batteries, J. Colloid Interface Sci. 418 (2014) 74–80, http://dx.doi.org/10.1016/j.jcis.2013.12.011. [135] E. Uchaker, N. Zhou, Y. Li, G. Cao, Polyol-mediated solvothermal synthesis and electrochemical performance of nanostructured V2O5 hollow microspheres, J. Phys. Chem. C 117 (2013) 1621–1626, http://dx.doi.org/10.1021/jp310641k. [136] G. Wee, H.Z. Soh, Y.L. Cheah, S.G. Mhaisalkar, M. Srinivasan, Synthesis and electrochemical properties of electrospun V2O5 nanofibers as supercapacitor electrodes, J. Mater. Chem. 20 (2010) 6720–6725, http://dx.doi.org/10.1039/ C0JM00059K. [137] S. Peng, G. Jin, L. Li, K. Li, M. Srinivasan, S. Ramakrishna, J. Chen, Multi-functional electrospun nanofibres for advances in tissue regeneration, energy conversion & storage, and water treatment, Chem. Soc. Rev. (2016), http://dx.doi.org/ 10.1039/C5CS00777A. [138] H. Guan, C. Shao, S. Wen, B. Chen, J. Gong, X. Yang, A novel method for preparing Co3O4 nanofibers by using electrospun PVA/cobalt acetate composite fibers as precursor, Mater. Chem. Phys. 82 (2003) 1002–1006, http://dx.doi.org/10.1016/ j.matchemphys.2003.09.003. [139] H. Guan, C. Shao, S. Wen, B. Chen, J. Gong, X. Yang, Preparation and characterization of NiO nanofibres via an electrospinning technique, Inorg. Chem. Commun. 6 (2003) 1302–1303, http://dx.doi.org/10.1016/j.inoche.2003.08.003. [140] X. Yang, C. Shao, H. Guan, X. Li, J. Gong, Preparation and characterization of ZnO nanofibers by using electrospun PVA/zinc acetate composite fiber as precursor, Inorg. Chem. Commun. 7 (2004) 176–178, http://dx.doi.org/10.1016/j.inoche. 2003.10.035. [141] K.A. Sekak, A. Lowe, Structural and thermal characterization of calcium cobaltite electrospun nanostructured fibers, J. Am. Ceram. Soc. 94 (2011) 611–619, http:// dx.doi.org/10.1111/j.1551-2916.2010.04106.x. [142] P. Zhu, Y. Wu, M.V. Reddy, A. Sreekumaran Nair, B.V.R. Chowdari, S. Ramakrishna, Long term cycling studies of electrospun TiO2 nanostructures and their composites with MWCNTs for rechargeable Li-ion batteries, RSC Adv. 2 (2012) 531–537, http://dx.doi.org/10.1039/C1RA00514F. [143] Y. Tang, S. Fu, K. Zhao, L. Teng, G. Xie, Fabrication of TiO2 micro-/nano-spheres embedded in nanofibers by coaxial electrospinning, Mater. Res. Bull. 78 (2016) 11–15, http://dx.doi.org/10.1016/j.materresbull.2016.02.018. [144] L. Mai, X. Xu, L. Xu, C. Han, Y. Luo, Vanadium oxide nanowires for Li-ion batteries, J. Mater. Res. 26 (2011) 2175–2185, http://dx.doi.org/10.1557/jmr.2011. 171. [145] M. Ganesan, Studies on the effect of titanium addition on LiCoO2, Ionics 15 (2009) 609–614, http://dx.doi.org/10.1007/s11581-008-0310-4. [146] M.J. Powell, P. Marchand, C.J. Denis, J.C. Bear, J.A. Darr, I.P. Parkin, Direct and continuous synthesis of VO2 nanoparticles, Nanoscale 7 (2015) 18686–18693, http://dx.doi.org/10.1039/C5NR04444H. [147] M. Lübke, N. Ding, M.J. Powell, D.J.L. Brett, P.R. Shearing, Z. Liu, J.A. Darr, VO2 nano-sheet negative electrodes for lithium-ion batteries, Electrochem. Commun. 64 (2016) 56–60, http://dx.doi.org/10.1016/j.elecom.2016.01.013. [148] C.V. Subba Reddy, E.H. Walker Jr., S.A. Wicker Sr., Q.L. Williams, R.R. Kalluru, Synthesis of VO2 (B) nanorods for Li battery application, Curr. Appl. Phys. 9 (2009) 1195–1198, http://dx.doi.org/10.1016/j.cap.2009.01.012. [149] I. Mjejri, N. Etteyeb, F. Sediri, Mesoporous vanadium oxide nanostructures: hydrothermal synthesis, optical and electrochemical properties, Ceram. Int. 40 (2014) 1387–1397, http://dx.doi.org/10.1016/j.ceramint.2013.07.020. [150] N. Li, W. Huang, Q. Shi, Y. Zhang, L. Song, A CTAB-assisted hydrothermal synthesis of VO2(B) nanostructures for lithium-ion battery application, Ceram. Int. 39 (2013) 6199–6206, http://dx.doi.org/10.1016/j.ceramint.2013.01.039. [151] Y.L. Cheah, N. Gupta, S.S. Pramana, V. Aravindan, G. Wee, M. Srinivasan, Morphology, structure and electrochemical properties of single phase electrospun vanadium pentoxide nanofibers for lithium ion batteries, J. Power Sources 196 (2011) 6465–6472, http://dx.doi.org/10.1016/j.jpowsour.2011.03.039.

[100] Y.M. Shin, M.M. Hohman, M.P. Brenner, G.C. Rutledge, Experimental characterization of electrospinning: the electrically forced jet and instabilities, Polymer 42 (2001) 09955–09967, http://dx.doi.org/10.1016/S0032-3861(01)00540-7. [101] S.V. Fridrikh, J.H. Yu, M.P. Brenner, G.C. Rutledge, Controlling the fiber diameter during electrospinning, Phys. Rev. Lett. 90 (2003) 144502, http://dx.doi.org/10. 1103/PhysRevLett.90.144502. [102] Y.M. Shin, M.M. Hohman, M.P. Brenner, G.C. Rutledge, Electrospinning: a whipping fluid jet generates submicron polymer fibers, Appl. Phys. Lett. 78 (2001) 1149–1151, http://dx.doi.org/10.1063/1.1345798. [103] A. Greiner, J.H. Wendorff, Electrospinning: a fascinating method for the preparation of ultrathin fibers, Angew. Chem. Int. Ed. 46 (2007) 5670–5703, http:// dx.doi.org/10.1002/anie.200604646. [104] R. Ramaseshan, S. Sundarrajan, R. Jose, S. Ramakrishna, Nanostructured ceramics by electrospinning, J. Appl. Phys. 102 (2007), http://dx.doi.org/10.1063/1. 2815499 111101. [105] D. Li, J.T. McCann, Y. Xia, M. Marquez, Electrospinning: a simple and versatile technique for producing ceramic nanofibers and nanotubes, J. Am. Ceram. Soc. 89 (2006) 1861–1869, http://dx.doi.org/10.1111/j.1551-2916.2006.00989.x. [106] D. Li, Y. Xia, Fabrication of titania nanofibers by electrospinning, Nano Lett. 3 (2003) 555–560, http://dx.doi.org/10.1021/nl034039o. [107] P. Viswanathamurthi, Vanadium pentoxide nanofibers by electrospinning, Scripta Mater. 49 (2003) 577–581, http://dx.doi.org/10.1016/S1359-6462(03)00333-6. [108] C. Ban, M.S. Whittingham, Nanoscale single-crystal vanadium oxides with layered structure by electrospinning and hydrothermal methods, Solid State Ionics 179 (2008) 1721–1724, http://dx.doi.org/10.1016/j.ssi.2008.01.037. [109] C.F. Armer, M. Lübke, M.V. Reddy, J.A. Darr, X. Li, A. Lowe, Phase change effect on the structural and electrochemical behaviour of pure and doped vanadium pentoxide as positive electrodes for lithium ion batteries, J. Power Sources 353 (2017) 40–50, http://dx.doi.org/10.1016/j.jpowsour.2017.03.121. [110] Y.L. Cheah, V. Aravindan, S. Madhavi, Improved elevated temperature performance of Al-Intercalated V2O5 electrospun nanofibers for lithium-ion batteries, ACS Appl. Mater. Interfaces 4 (2012) 3270–3277, http://dx.doi.org/10.1021/ am300616k. [111] H. Wang, D. Ma, Y. Huang, X. Zhang, Electrospun V2O5 nanostructures with controllable morphology as high-performance cathode materials for lithium-ion batteries, Chem. Eur J. 18 (2012) 8987–8993, http://dx.doi.org/10.1002/chem. 201200434. [112] L. Mai, L. Xu, C. Han, X. Xu, Y. Luo, S. Zhao, Y. Zhao, Electrospun ultralong hierarchical vanadium oxide nanowires with high performance for lithium ion batteries, Nano Lett. 10 (2010) 4750–4755, http://dx.doi.org/10.1021/ nl103343w. [113] L. Liang, M. Zhou, Y. Xie, Electrospun hierarchical LiV3O8 nanofibers assembled from nanosheets with exposed 100 facets and their enhanced performance in aqueous lithium-ion batteries, Chem. Asian J. 7 (2012) 565–571, http://dx.doi. org/10.1002/asia.201100757. [114] Y. Wu, P. Zhu, X. Zhao, M.V. Reddy, S. Peng, B.V.R. Chowdari, S. Ramakrishna, Highly improved rechargeable stability for lithium/silver vanadium oxide battery induced viaelectrospinning technique, J. Mater. Chem. 1 (2013) 852–859, http:// dx.doi.org/10.1039/C2TA00042C. [115] C. Ban, N.A. Chernova, M.S. Whittingham, Electrospun nano-vanadium pentoxide cathode, Electrochem. Commun. 11 (2009) 522–525, http://dx.doi.org/10.1016/ j.elecom.2008.11.051. [116] Y.L. Cheah, V. Aravindan, S. Madhavi, Synthesis and enhanced lithium storage properties of electrospun V2O5 nanofibers in full-cell assembly with a spinel Li4Ti5O12 anode, ACS Appl. Mater. Interfaces 5 (2013) 3475–3480, http://dx.doi. org/10.1021/am400666n. [117] D. Yu, C. Chen, S. Xie, Y. Liu, K. Park, X. Zhou, Q. Zhang, J. Li, G. Cao, Mesoporous vanadium pentoxide nanofibers with significantly enhanced Li-ion storage properties by electrospinning, Energy Environ. Sci. 4 (2011) 858–861, http://dx.doi. org/10.1039/C0EE00313A. [118] J.E. Panels, Y.L. Joo, Incorporation of vanadium oxide in silica nanofiber mats via electrospinning and sol-gel synthesis, J. Nanomater. 2006 (2006) 1–10, http://dx. doi.org/10.1155/JNM/2006/41327. [119] S. Beke, A review of the growth of V2O5 films from 1885 to 2010, Thin Solid Films 519 (2011) 1761–1771, http://dx.doi.org/10.1016/j.tsf.2010.11.001. [120] O.Y. Berezina, D.A. Kirienko, N.P. Markova, A.L. Pergament, Synthesis of vanadium pentoxide micro- and nanofibers by electrospinning, Tech. Phys. 60 (2015) 1361–1366, http://dx.doi.org/10.1134/S1063784215090054. [121] Z. Li, G. Liu, M. Guo, L.-X. Ding, S. Wang, H. Wang, Electrospun porous vanadium pentoxide nanotubes as a high-performance cathode material for lithium-ion batteries, Electrochim. Acta 173 (2015) 131–138, http://dx.doi.org/10.1016/j. electacta.2015.05.057. [122] D. Pham-Cong, K. Ahn, S.W. Hong, S.Y. Jeong, J.H. Choi, C.H. Doh, J.S. Jin, E.D. Jeong, C.R. Cho, Cathodic performance of V2O5 nanowires and reduced graphene oxide composites for lithium ion batteries, Curr. Appl. Phys. 14 (2014) 215–221, http://dx.doi.org/10.1016/j.cap.2013.10.022. [123] D. Kong, X. Li, Y. Zhang, X. Hai, B. Wang, X. Qiu, Q. Song, Q.-H. Yang, L. Zhi, Encapsulating V2O5 into carbon nanotubes enables the synthesis of flexible highperformance lithium ion batteries, Energy Environ. Sci. 9 (2016) 906–911, http:// dx.doi.org/10.1039/C5EE03345D. [124] H. Liu, D. Tang, Synthesis of ZnV2O6 powder and its cathodic performance for lithium secondary battery, Mater. Chem. Phys. 114 (2009) 656–659, http://dx. doi.org/10.1016/j.matchemphys.2008.10.055. [125] C.F. Armer, J.S. Yeoh, A. Lowe, Unpublished work, (n.d.). [126] X. Jia, L. Zhang, R. Zhang, Y. Lu, F. Wei, Carbon nanotube-penetrated mesoporous V2O5 microspheres as high-performance cathode materials for lithium-ion

428

Journal of Power Sources 395 (2018) 414–429

C.F. Armer et al.

[152] J. Xiang, X.-Y. Yu, U. Paik, General synthesis of vanadium-based mixed metal oxides hollow nanofibers for high performance lithium-ion batteries, J. Power Sources 329 (2016) 190–196, http://dx.doi.org/10.1016/j.jpowsour.2016.08.079. [153] C. Didier, M. Guignard, J. Darriet, C. Delmas, O′3–NaxVO2 system: a superstructure for Na1/2VO2, Inorg. Chem. 51 (2012) 11007–11016, http://dx.doi. org/10.1021/ic301505e. [154] D. Chao, C. Zhu, X. Xia, J. Liu, X. Zhang, J. Wang, P. Liang, J. Lin, H. Zhang, Z.X. Shen, H.J. Fan, Graphene quantum dots coated VO2 arrays for highly durable electrodes for Li and Na ion batteries, Nano Lett. 15 (2015) 565–573, http://dx. doi.org/10.1021/nl504038s. [155] S. Bach, N. Baffier, J. Pereira-Ramos, R. Messina, Electrochemical sodium intercalation in Na0.33V2O5 bronze synthesized by a sol-gel process, Solid State Ionics 37 (1989) 41–49, http://dx.doi.org/10.1016/0167-2738(89)90285-3. [156] S. Tepavcevic, H. Xiong, V.R. Stamenkovic, X. Zuo, M. Balasubramanian, V.B. Prakapenka, C.S. Johnson, T. Rajh, Nanostructured bilayered vanadium oxide electrodes for rechargeable sodium-ion batteries, ACS Nano 6 (2012) 530–538, http://dx.doi.org/10.1021/nn203869a. [157] D.W. Su, S.X. Dou, G.X. Wang, Hierarchical orthorhombic V2O5 hollow nanospheres as high performance cathode materials for sodium-ion batteries, J. Mater. Chem. 2 (2014) 11185–11194, http://dx.doi.org/10.1039/C4TA01751J. [158] D. Su, G. Wang, Single-crystalline bilayered V2O5 nanobelts for high-capacity sodium-ion batteries, ACS Nano 7 (2013) 11218–11226, http://dx.doi.org/10. 1021/nn405014d. [159] E. Uchaker, Y.Z. Zheng, S. Li, S.L. Candelaria, S. Hu, G.Z. Cao, Better than crystalline: amorphous vanadium oxide for sodium-ion batteries, J. Mater. Chem. 2 (2014) 18208–18214, http://dx.doi.org/10.1039/C4TA03788J. [160] S. Liu, Z. Tong, J. Zhao, X. Liu, J. Wang, X. Ma, C. Chi, Y. Yang, X.-X. Liu, Y. Li, Rational selection of amorphous or crystalline V2O5 cathode for sodium-ion batteries, Phys. Chem. Chem. Phys. (2016), http://dx.doi.org/10.1039/ C6CP04064K. [161] D. Muller-Bouvet, R. Baddour-Hadjean, M. Tanabe, L.T.N. Huynh, M.L.P. Le, J.P. Pereira-Ramos, Electrochemically formed alpha’-NaV2O5: a new sodium intercalation compound, Electrochim. Acta 176 (2015) 586–593, http://dx.doi.org/ 10.1016/j.electacta.2015.07.030. [162] Y. Cai, J. Zhou, G. Fang, G. Cai, A. Pan, S. Liang, Na0.282V2O5: a high-performance cathode material for rechargeable lithium batteries and sodium batteries, J. Power Sources 328 (2016) 241–249, http://dx.doi.org/10.1016/j.jpowsour.2016. 08.016. [163] G. Ali, J. Lee, S.H. Oh, B.W. Cho, K.-W. Nam, K.Y. Chung, Investigation of the Na intercalation mechanism into nanosized V2O5/C composite cathode material for Na-Ion batteries, ACS Appl. Mater. Interfaces 8 (2016) 6032–6039, http://dx.doi. org/10.1021/acsami.5b11954. [164] B. Park, S.M. Oh, Y.K. Jo, S.-J. Hwang, Efficient electrode material of restacked Na–V2O5–graphene nanocomposite for Na-ion batteries, Mater. Lett. 178 (2016) 79–82, http://dx.doi.org/10.1016/j.matlet.2016.04.161. [165] V. Raju, J. Rains, C. Gates, W. Luo, X. Wang, W.F. Stickle, G.D. Stucky, X. Ji, Superior cathode of sodium-ion batteries: orthorhombic V2O5 nanoparticles generated in nanoporous carbon by ambient hydrolysis deposition, Nano Lett. 14 (2014) 4119–4124, http://dx.doi.org/10.1021/nl501692p. [166] Y. Xu, Y. Zhu, Y. Liu, C. Wang, Electrochemical performance of porous carbon/tin composite anodes for sodium-ion and lithium-ion batteries, Adv. Energy Mater 3 (2013) 128–133, http://dx.doi.org/10.1002/aenm.201200346. [167] J. Qian, Y. Chen, L. Wu, Y. Cao, X. Ai, H. Yang, High capacity Na-storage and superior cyclability of nanocomposite Sb/C anode for Na-ion batteries, Chem. Commun. 48 (2012) 7070–7072, http://dx.doi.org/10.1039/C2CC32730A. [168] V.L. Chevrier, G. Ceder, Challenges for Na-ion negative electrodes, J. Electrochem. Soc. 158 (2011) A1011–A1014. [169] A. Darwiche, M.T. Sougrati, B. Fraisse, L. Stievano, L. Monconduit, Facile synthesis and long cycle life of SnSb as negative electrode material for Na-ion batteries, Electrochem. Commun. 32 (2013) 18–21, http://dx.doi.org/10.1016/j.elecom. 2013.03.029. [170] Y. Cao, L. Xiao, M.L. Sushko, W. Wang, B. Schwenzer, J. Xiao, Z. Nie, L.V. Saraf, Z. Yang, J. Liu, Sodium ion insertion in hollow carbon nanowires for battery applications, Nano Lett. 12 (2012) 3783–3787, http://dx.doi.org/10.1021/ nl3016957. [171] K. Tang, L. Fu, R.J. White, L. Yu, M.-M. Titirici, M. Antonietti, J. Maier, Hollow carbon nanospheres with superior rate capability for sodium-based batteries, Adv. Energy Mater 2 (2012) 873–877, http://dx.doi.org/10.1002/aenm.201100691. [172] L. Wang, Y. Yu, P.C. Chen, D.W. Zhang, C.H. Chen, Electrospinning synthesis of C/ Fe3O4 composite nanofibers and their application for high performance lithiumion batteries, J. Power Sources 183 (2008) 717–723, http://dx.doi.org/10.1016/j. jpowsour.2008.05.079. [173] C. Zhu, X. Mu, P.A. van Aken, Y. Yu, J. Maier, Single-layered ultrasmall nanoplates of MoS2 embedded in carbon nanofibers with excellent electrochemical performance for lithium and sodium storage, Angew. Chem. Int. Ed. 53 (2014) 2152–2156, http://dx.doi.org/10.1002/anie.201308354. [174] J. Liu, K. Tang, K. Song, P.A. van Aken, Y. Yu, J. Maier, Tiny Li4Ti5O12 nanoparticles embedded in carbon nanofibers as high-capacity and long-life anode materials for both Li-ion and Na-ion batteries, Phys. Chem. Chem. Phys. 15 (2013) 20813–20818, http://dx.doi.org/10.1039/C3CP53882F. [175] W. Li, L. Zeng, Z. Yang, L. Gu, J. Wang, X. Liu, J. Cheng, Y. Yu, Free-standing and

[176]

[177]

[178]

[179]

[180]

[181]

[182]

[183]

[184] [185]

[186]

[187]

[188]

[189]

[190]

[191]

[192]

[193]

[194]

[195]

[196]

[197]

[198]

429

binder-free sodium-ion electrodes with ultralong cycle life and high rate performance based on porous carbon nanofibers, Nanoscale 6 (2014) 693–698, http:// dx.doi.org/10.1039/C3NR05022J. P.R. Gifford, J.B. Palmisano, An aluminum/chlorine rechargeable cell employing a room temperature molten salt electrolyte, J. Electrochem. Soc. 135 (1988) 650–654. S. Liu, G.L. Pan, G.R. Li, X.P. Gao, Copper hexacyanoferrate nanoparticles as cathode material for aqueous Al-ion batteries, J. Mater. Chem. 3 (2015) 959–962, http://dx.doi.org/10.1039/C4TA04644G. H. Sun, W. Wang, Z. Yu, Y. Yuan, S. Wang, S. Jiao, A new aluminium-ion battery with high voltage, high safety and low cost, Chem. Commun. 51 (2015) 11892–11895, http://dx.doi.org/10.1039/C5CC00542F. S. Jiao, H. Lei, J. Tu, J. Zhu, J. Wang, X. Mao, An industrialized prototype of the rechargeable Al/AlCl3-[EMIm]Cl/graphite battery and recycling of the graphitic cathode into graphene, Carbon 109 (2016) 276–281, http://dx.doi.org/10.1016/j. carbon.2016.08.027. J.V. Rani, V. Kanakaiah, T. Dadmal, M.S. Rao, S. Bhavanarushi, Fluorinated natural graphite cathode for rechargeable ionic liquid based aluminum–ion battery, J. Electrochem. Soc. 160 (2013) A1781–A1784. Y. Liu, S. Sang, Q. Wu, Z. Lu, K. Liu, H. Liu, The electrochemical behavior of Cl− assisted Al3+ insertion into titanium dioxide nanotube arrays in aqueous solution for aluminum ion batteries, Electrochim. Acta 143 (2014) 340–346, http://dx.doi. org/10.1016/j.electacta.2014.08.016. N.S. Hudak, Chloroaluminate-doped conducting polymers as positive electrodes in rechargeable aluminum batteries, J. Phys. Chem. C 118 (2014) 5203–5215, http://dx.doi.org/10.1021/jp500593d. N. Jayaprakash, S.K. Das, L.A. Archer, The rechargeable aluminum-ion battery, Chem. Commun. 47 (2011) 12610–12612, http://dx.doi.org/10.1039/ C1CC15779E. L.D. Reed, E. Menke, The roles of V2O5 and stainless steel in rechargeable Al–ion batteries, J. Electrochem. Soc. 160 (2013) A915–A917. H. Wang, Y. Bai, S. Chen, X. Luo, C. Wu, F. Wu, J. Lu, K. Amine, Binder-free V2O5 cathode for greener rechargeable aluminum battery, ACS Appl. Mater. Interfaces 7 (2015) 80–84, http://dx.doi.org/10.1021/am508001h. M. Chiku, H. Takeda, S. Matsumura, E. Higuchi, H. Inoue, Amorphous vanadium oxide/carbon composite positive electrode for rechargeable aluminum battery, ACS Appl. Mater. Interfaces 7 (2015) 24385–24389, http://dx.doi.org/10.1021/ acsami.5b06420. S. Gu, H. Wang, C. Wu, Y. Bai, H. Li, F. Wu, Confirming reversible Al3+ storage mechanism through intercalation of Al3+ into V2O5 nanowires in a rechargeable aluminum battery, Energy Storage Mater. (n.d.). doi:10.1016/j.ensm.2016.09. 001. Y. Delai, L. Bin, L. Gaoqing Max, W. Lianzhou, Will new aluminum-ion battery be a game changer? Sci. Bull. 60 (2015) 1042–1044 https://doi.org/10.1007/s11434015-0808-x. L. Zhang, A. Aboagye, A. Kelkar, C. Lai, H. Fong, A review: carbon nanofibers from electrospun polyacrylonitrile and their applications, J. Mater. Sci. 49 (2014) 463–480, http://dx.doi.org/10.1007/s10853-013-7705-y. S.H. Choi, Y.C. Kang, Uniform decoration of vanadium oxide nanocrystals on reduced graphene-oxide balls by an aerosol process for lithium-ion battery cathode material, Chem. Eur J. 20 (2014) 6294–6299, http://dx.doi.org/10.1002/chem. 201400134. V.S. Reddy Channu, D. Ravichandran, B. Rambabu, R. Holze, Carbon and functionalized graphene oxide coated vanadium oxide electrodes for lithium ion batteries, Appl. Surf. Sci. 305 (2014) 596–602, http://dx.doi.org/10.1016/j.apsusc. 2014.03.140. G. Zhou, J. Ma, L. Chen, Selective carbon coating techniques for improving electrochemical properties of NiO nanosheets, Electrochim. Acta 133 (2014) 93–99, http://dx.doi.org/10.1016/j.electacta.2014.03.161. Z.-F. Li, H. Zhang, Q. Liu, Y. Liu, L. Stanciu, J. Xie, Hierarchical nanocomposites of vanadium oxide thin film anchored on graphene as high-performance cathodes in Li-Ion batteries, ACS Appl. Mater. Interfaces 6 (2014) 18894–18900, http://dx. doi.org/10.1021/am5047262. J. Cheng, G. Gu, Q. Guan, J.M. Razal, Z. Wang, X. Li, B. Wang, Synthesis of a porous sheet-like V2O5-CNT nanocomposite using an ice-templating “bricks-andmortar” assembly approach as a high-capacity, long cyclelife cathode material for lithium-ion batteries, J. Mater. Chem. 4 (2016) 2729–2737, http://dx.doi.org/10. 1039/C5TA10414A. M. Sathiya, A.S. Prakash, K. Ramesha, J. Tarascon, A.K. Shukla, V2O5-Anchored carbon nanotubes for enhanced electrochemical energy storage, J. Am. Chem. Soc. 133 (2011) 16291–16299, http://dx.doi.org/10.1021/ja207285b. G. Ren, M.N.F. Hoque, X. Pan, J. Warzywoda, Z. Fan, Vertically aligned VO2(B) nanobelt forest and its three-dimensional structure on oriented graphene for energy storage, J. Mater. Chem. 3 (2015) 10787–10794, http://dx.doi.org/10.1039/ C5TA01900A. S. Xia, X.-M. Zhang, K. Huang, Y.-L. Chen, Y.-T. Wu, Ionic liquid electrolytes for aluminium secondary battery: influence of organic solvents, J. Electroanal. Chem. 757 (2015) 167–175, http://dx.doi.org/10.1016/j.jelechem.2015.09.022. H. Wang, S. Gu, Y. Bai, S. Chen, F. Wu, C. Wu, High-voltage and noncorrosive ionic liquid electrolyte used in rechargeable aluminum battery, ACS Appl. Mater. Interfaces 8 (2016) 27444–27448, http://dx.doi.org/10.1021/acsami.6b10579.