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Facile Synthesis of V2O5 Hollow Spheres as Advanced Cathodes for High-Performance Lithium-Ion Batteries Xingyuan Zhang 1 , Jian-Gan Wang 1, *, Huanyan Liu 1 , Hongzhen Liu 1 and Bingqing Wei 1,2, * 1

2

*

State Key Laboratory of Solidification Processing, Center for Nano Energy Materials, School of Materials Science and Engineering, Northwestern Polytechnical University and Shaanxi Joint Lab of Graphene (NPU), Xi’an 710072, China; [email protected] (X.Z.); [email protected] (H.L.); [email protected] (H.L.) Department of Mechanical Engineering, University of Delaware, Newark, DE 19716, USA Correspondence: [email protected] (J.-G.W.); [email protected] (B.W.); Tel.: +86-29-8846-0204 (J.-G.W.)

Academic Editor: Maryam Tabrizian Received: 14 December 2016; Accepted: 16 January 2017; Published: 18 January 2017

Abstract: Three-dimensional V2 O5 hollow structures have been prepared through a simple synthesis strategy combining solvothermal treatment and a subsequent thermal annealing. The V2 O5 materials are composed of microspheres 2–3 µm in diameter and with a distinct hollow interior. The as-synthesized V2 O5 hollow microspheres, when evaluated as a cathode material for lithium-ion batteries, can deliver a specific capacity as high as 273 mAh·g−1 at 0.2 C. Benefiting from the hollow structures that afford fast electrolyte transport and volume accommodation, the V2 O5 cathode also exhibits a superior rate capability and excellent cycling stability. The good Li-ion storage performance demonstrates the great potential of this unique V2 O5 hollow material as a high-performance cathode for lithium-ion batteries. Keywords: vanadium pentoxide; lithium-ion batteries; hollow spheres; cathode

1. Introduction Rechargeable lithium-ion batteries (LIBs) are considered to be one of the most promising energy storage devices for portable devices and electric vehicles (EVs) owing to their high energy densities, environmental friendliness, and long cycle lifetime [1–4]. However, the challenges of high cost and the low capacity of the currently used cathode materials hinder their widespread applications [5,6]. In order to obtain high specific energy, considerable efforts have been devoted to developing materials with a higher specific capacity than the current commercial ones, such as LiCoO2 (140 mAh·g−1 ), LiMn2 O4 (146 mAh·g−1 ), and LiFePO4 (176 mAh·g−1 ) [7,8]. Among the alternative candidates, vanadium pentoxide (V2 O5 ) has drawn considerable attention as a cathode material for lithium-ion batteries because of its advantages of layered structure, low cost, rich abundance in nature, and a high theoretical capacity of about 294 mAh·g−1 (intercalation/deintercalation of two Li+ ions between 2.0 and 4.0 V) [9–11]. Nevertheless, the poor cycling stability, low electronic conductivity, and low lithium ion diffusion rate restrict its practical application [12–14]. Developing nanostructured materials has been demonstrated to be an effective method to address these critical issues because nanomaterials have a large surface area to provide more reaction sites and can effectively shorten the diffusion distance of lithium ions during the insertion/extraction, thereby resulting in an enhanced cycling performance and rate capability [15,16]. A variety of V2 O5 nanostructures, such as nanourchins [17], nanotubes [18], nanospikes [10], nanorods [8], nanowires [19–21], and nanobelts [22–25], have been fabricated, and improved Materials 2017, 10, 77; doi:10.3390/ma10010077

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electrochemicalperformance performancehas hasbeen been achieved with these materials when used as cathodes for electrochemical achieved with these materials when used as cathodes for LIBs. LIBs. In particular, hollow structures are rather favorable because the unique hollow structure can In particular, hollow structures are rather favorable because the unique hollow structure can effectively effectively buffer the volume of expansion the V2O5 cathode to improve the cycling performance buffer the volume expansion the V2 Oof 5 cathode to improve the cycling performance [9,26–28]. [9,26–28]. Lou et al. reported a carbon-templating to synthesize V2Ospheres 5 hollow spheres with an Lou et al. reported a carbon-templating method tomethod synthesize V2 O5 hollow with an excellent excellent electrochemical performance [9]. However, the complex synthesis procedures and the electrochemical performance [9]. However, the complex synthesis procedures and the expensive expensive reactant materials limit the scale-up production. Therefore, it remains a great challenge to reactant materials limit the scale-up production. Therefore, it remains a great challenge to controllably controllably synthesize V2O5 hollow structures through a simple and low-cost method. synthesize V2 O5 hollow structures through a simple and low-cost method. In this work, we report a facile one-step solvothermal method to controllably synthesize V2O5 In this work, we report a facile one-step solvothermal method to controllably synthesize V2 O5 hollow microspheres by employing a copolymer surfactant of poly(ethylene hollow microspheres by employing a copolymer surfactant of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (P123) as a soft template and oxide)-block-poly(ethylene oxide) (P123) as a soft template and low-cost NH4 VO3 as the vanadium low-cost NH4VO3 as the vanadium source. The effects of the annealing temperature on the source. The effects of the annealing temperature on the crystallinity, morphology, and electrochemical crystallinity, morphology, and electrochemical properties of V2O5 have been studied. The optimized properties of V2 O5 have been studied. The optimized V2 O5 hollow microspheres, when used as LIB V2O5 hollow microspheres, when used as LIB cathodes, exhibit a satisfactory lithium-ion storage cathodes, exhibit a satisfactory lithium-ion storage performance. performance.

2. Discussion and Results 2. Discussion and Results 2.1. Structure and Morphology of the V2 O5 Microspheres 2.1. Structure and Morphology of the V2O5 Microspheres Figure 1 shows the typical SEM images of the as-prepared samples. As shown in Figure 1a, 1 showswas the typical SEMofimages of the as-prepared samples. As shown Figure 1a, the V2Figure O5 precursor composed three-dimensional (3D) microspheres with in diameters inthe the V 2O5 precursor was composed of three-dimensional (3D) microspheres with diameters in the range range of two to three microns. The magnified image in Figure 1a2 indicates that the microspheres were of two to three microns. The magnified image in Figure 1a2 indicates that the microspheres were assembled with petal-like nanosheets. The hollow structure can be clearly observed in some broken assembled with petal-like nanosheets. The hollow structure can be clearly observed in some broken microspheres (Figure 1a3). V2 O5 was obtained by annealing the precursors, and the morphology microspheres (Figure 1a3). V2O5 was obtained by annealing the precursors, and the morphology evolution with the annealing temperature is exhibited in Figure 1b–d. The microsphere morphology evolution with the annealing temperature is exhibited in Figure 1b–d. The microsphere morphology was well maintained within the annealing temperature range of 300–500 ◦ C. However, the building was well maintained within the annealing temperature range of 300–500 °C. However, the building blocks changed with the temperature. At a low annealing temperature of 300 ◦ C (V2 O5 -300), as shown blocks changed with the temperature. At a low annealing temperature of 300 °C (V2O5-300), as in Figure 1b2, the petal-like nanosheet structure was preserved on the V2 O5 microsphere surface, shown in Figure 1b2, the petal-like nanosheet structure was preserved on the V2O5 microsphere which was almost identical to that of the precursors. As the temperature increased to 400 ◦ C (V2 O5 -400), surface, which was almost identical to that of the precursors. As the temperature increased to 400 °C the surface of the V2 O5 hollow microspheres became constructed by interconnected nanoparticles (V2O5-400), the surface of the V2O5 hollow microspheres became constructed by interconnected (Figure 1c2). The nanoparticles became larger irregular structures at a higher temperature of 500 ◦ C nanoparticles (Figure 1c2). The nanoparticles became larger irregular structures at a higher (V2 O5 -500, Figure 1d2). The morphology evolution can be attributed to the different growth rate and temperature of 500 °C (V2O5-500, Figure 1d2). The morphology evolution can be attributed to the crystallization degree of V2 O5 at different annealing temperatures [27]. different growth rate and crystallization degree of V2O5 at different annealing temperatures [27].

Figure 1. Cont.

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Figure 1. 1. SEM SEM images images of of the the (a1–a3) (a1–a3) V 2O5 precursor and V2O5 microspheres annealed at 300 °C Figure 2O and V V22O55 microspheres annealed at 300 300 ◦°C Figure 1. SEM images of the (a1–a3) VV2 O C 55 precursor and (b1–b3); 400 °C (c1–c3); and 500 °C (d1–d3). ◦ ◦ (b1–b3); 400 400 °C (c1–c3); and and 500 500 °C (b1–b3); C (c1–c3); C (d1–d3).

The crystallographic crystallographic structure structure of of the samples was characterized The the samples samples was was characterized characterized by by X-ray X-raypowder powderdiffraction diffraction The crystallographic structure of the by X-ray powder diffraction (XRD). Figure 2a shows the XRD patterns of V2O5. Clearly, the main diffraction peaks can be well (XRD). Figure 2a shows the XRD patterns of V 2O5. Clearly, the main diffraction peaks can be well (XRD). Figure 2a shows the XRD patterns of V O . Clearly, the main diffraction peaks can be well 2 5 indexed to the orthorhombic phase of V2O5 (JCPDS Card No. 41-1426). In addition, the characteristic indexed to to the the orthorhombic orthorhombicphase phaseof ofVV2O O5 (JCPDS (JCPDS Card Card No. No. 41-1426). 41-1426). In In addition, addition, the the characteristic characteristic indexed 2 5 peaks showed a stronger diffraction intensity and narrower shape at an elevated annealing peaksshowed showed stronger a stronger diffraction intensity and narrower shape at an elevated annealing peaks diffraction intensity andatnarrower at an elevated temperature, temperature, asuggesting a higher crystallinity a highershape temperature [16,29].annealing Raman spectra were temperature, suggesting a higheratcrystallinity at a higher[16,29]. temperature [16,29]. Raman spectra were suggesting a higher a higher Raman spectra were employed employed to furthercrystallinity confirm the structure of temperature the samples. As shown in Figure 2b, the samples sharedto employed to furtherstructure confirm the structure of the AsFigure shown2b, in Figure 2b, theshared samples shared further confirm ofare the samples. Assamples. shown in samples identical Ramanthe peaks, which characteristic of orthorhombic V2O5the [29,30]. The typicalidentical Raman identical Raman peaks, which are characteristic of orthorhombic V 2O5 [29,30]. The typical Raman Raman which are characteristic of orthorhombic The typical 2 OV-O-V 5 [29,30]. 2 O5 peaks peaks, V2O5 included the skeleton bent vibrations of V the bonds at 145 Raman and 193peaks cm−1−1 ,Vthe peaks V2the O5 included the vibrations skeleton bent vibrations of the 145 V-O-V and 193 vibration cm , the −1 ,145 included skeleton of 281 the V-O-V and bonds 193 cmat the bending vibration of bent the V=O bonds at and 402bonds cm−1−1, at the bending vibrations ofbending V3=O at 302 cm−1−1 bending vibration of theand V=O bonds and 402vibrations cm , the bending vibrations V−13=O at 302 cm 1at 281bending −of 1 and −1 of theV-O-V V=O bonds 402 cm− at 302atcm bonds and bondsatat281 480 cm−1 , and the, the stretching vibrations ofofVV 3=O bonds 527 cm , V-O-V V2=O bonds 3 =O −1, V2=O bonds and V-O-V bonds at 480 cm , and the stretching vibrations of V 3 =O bonds at 527 cm −1 and the stretching vibrations −1. atat480 of V3 =O bonds at 527 cm−1 , V2 =O bonds at 701 cm−1 , 701cm cm−1−1, ,and V=O bonds at 995 cm−1 at 701 cm , and V=O bonds at 995 cm . − 1 and V=O bonds at 995 cm .

Figure 2. XRD patterns (a) and Raman spectra (b) of the V2O5 samples. Figure XRDpatterns patterns(a) (a)and andRaman Ramanspectra spectra(b) (b)of ofthe theVV2O O5 samples. Figure 2.2. XRD 2 5 samples.

2.2. Electrochemical Performance 2.2. Electrochemical Performance 2.2. Electrochemical Performance The electrochemical performance of the V2O5 samples was first evaluated using the cyclic The electrochemical performance offirst the V2CV O5 samples was first evaluated using the cyclic voltammogram (CV). Figure 3 shows theof curves of was the Vfirst 2O5-400 cathode using at a scan of The electrochemical performance thefive V2 O5 samples evaluated therate cyclic −1 voltammogram (CV). Figure 3 shows theto first five CV curves of theof V2redox O5-400peaks cathode at aobserved scan rateatof 0.1 mV·s in a potential range from 2.0 4.0 V. Four main pairs were voltammogram (CV). Figure 3 shows the first five CV curves of the V2 O5 -400 cathode at a scan 0.1 mV·s−1 in a potential range from 2.0 to 2.26/2.51 4.0 V. Four main pairs ofwhich redox are peaks were observed around 3.36/3.45, 3.16/3.34, and V,Four respectively, associated theat rate of 0.13.59/3.64, mV·s−1 in a potential range from 2.0 to 4.0 V. main pairs of redox peaks were with observed around 3.59/3.64, 3.36/3.45, 3.16/3.34, and 2.26/2.51 V, respectively, are associated with the reversible lithium intercalation/deintercalation into/from V 2O5 towhich form α-Li xV2O5, ε-Li xV2O 5, at around 3.59/3.64, 3.36/3.45, 3.16/3.34, and 2.26/2.51 V, respectively, which are associated with reversible lithium intercalation/deintercalation into/from V2O5 to form α-LixVthe 2O5,CV ε-Li xV2O5, δ-LixV2O5, and γ-LixV 2O5, as expressed in Equations (1)–(4) [25,26,31,32]. In addition, curves δ-LixV2O5, and γ-LixV2O5, as expressed in Equations (1)–(4) [25,26,31,32]. In addition, the CV curves

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theMaterials reversible lithium intercalation/deintercalation into/from V2 O5 to form α-Lix V2 O5 , ε-Lix4V O , 2017, 10, 77 of29 5 δ-LiMaterials V O , and γ-Li V O , as expressed in Equations (1)–(4) [25,26,31,32]. In addition, the CV x 2 52017, 10, 77 x 2 5 4 of 9 curves overlapped, indicating the highly reactive reversibility and cycling good cycling stability werewere well well overlapped, indicating the highly reactive reversibility and good stability of the of were well overlapped, indicating the highly reactive reversibility and good cycling stability of the thecathode. cathode. − cathode. (1) (1) V2 O5 + xLi+ + + xe ↔ α-Li V2 O5 (x < 0.1) V2O5 + xLi + xe− ↔ α-LixV2Ox 5 (x < 0.1) (1) + + + xe −− ↔ α-LixV2O5 (x < 0.1) V 2O 5 + xLi V O + xLi + xe ↔ ε-Li V O (0.35 < x < 0.7) + − x 2 5 2 5 V2O5 + xLi + xe ↔ ε-LixV2O5 (0.35 < x < 0.7) (3) (2) V2O5 + xLi+ ++xe− ↔−ε-LixV2O5 (0.35 < x < 0.7) (3) − ↔ δ-Li V O55 ++xLi xLi+ + + ↔ δ-Li xV 2 O5

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