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VNb9O25 nanowires with superior electrochemical property towards lithium ion batteries
Ceramics International 45 (2019) 18111–18114
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Short communication
VNb9O25 nanowires with superior electrochemical property towards lithium ion batteries
T
Cheng Jiang1, Tingting Liu1, Nengbing Long∗, Xing Cheng, Na Peng, Jundong Zhang, Runtian Zheng, Haoxiang Yu, Jie Shu∗∗ Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo, 315211, Zhejiang, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Lithium ion batteries Anode material VNb9O25 Nanowires Electrochemical property
Niobium-based compounds are extensively studied as anode materials for LIBs on account of the great structural stability and high theoretical capacity. Among them, VNb9O25 has an attractive theoretical capacity (416 mAh g−1). Unfortunately, low electronic conductivity and Li+ diffusivity hinder its practical application. To overcome above barriers, VNb9O25 nanowires were synthesized directly by electrospinning method. As expected, VNb9O25 nanowires deliver an outstanding capacity retention (92.1% after 200 cycles) and a robust rate property (90.7 mAh g−1 at 1000 mA g−1). Thus, as-electrospun VNb9O25 nanowires are alternative promising anode materials for LIBs.
1. Introduction The growth of advanced energy storage systems is propitious to strengthen the protection of environment and reduce the dependence on non-renewable energy resources [1–6]. Over the past decades, lithium ion batteries (LIBs) have become ubiquitous power sources for portable electronic devices thanks to their rechargeability and excellent energy density. In addition, the applications of LIBs are being extended to the large-scale energy storage system, such as wind and solar electricity-generating systems, electric and hybrid electric vehicles [7–11]. As we all known, as an integral component of LIBs, anode material has a fundamental influence on the characteristic of LIBs. Currently, commercialized graphite anode cannot satisfy the stringent requirements of LIBs. Firstly, the graphite possesses poor rate performance due to the slow Li+ diffusivity. On the other hand, the low operating potential of graphite results in the formation of dendritic lithium at extreme conditions, thus causing the security hidden danger [12–15]. For solving above deficiencies of graphite, Li4Ti5O12 as a popular titanate-based material has been widely researched because of its great structural stability and safety characteristics. Nevertheless, the practical application of Li4Ti5O12 is restricted owing to the low specific capacity of 175 mAh g−1 [16–19]. Therefore, developing novel and utility anode materials with superior property is vital to the advancement of next-generation large-power LIBs.
Niobium-based compounds (Nb2O5 [20], LiNb3O8 [21], TiNb2O7 [22], TiNb24O62 [23], FeNb11O29 [24] and so on) are an attractive group as anode materials for LIBs on account of their great structural stability (presence of ReO3-structure) and high theoretical capacity (rich redox chemistry). Additionally, the high working potentials (∼1.6 V) of niobium-based anode materials can prevent the formation of lithium dendrite and solid electrolyte interphase (SEI), ensuring greater safety than graphite-based anode materials [25,26]. Among niobium-based compounds, VNb9O25 has an attractive theoretical capacity (416 mAh g−1). Unfortunately, similar to other niobium-based compounds, the electrochemical kinetics of VNb9O25 is limited [27,28]. It has commonly been acknowledged that nano-crystallization is a simple and effective modification strategy to enhance the kinetics of VNb9O25 [29–34]. Nanostructures can dramatically decrease the average diffusion distance of Li+ and offer a short electronic pathway, thus leading to the enhancement of the rate property and cyclability of anode materials [35–37]. In this work, cheap and simple electrospinning method is chosen to generate VNb9O25 nanowires [38,39]. The VNb9O25 nanowires demonstrate outstanding electrochemical performance, including excellent rate property (90.7 mAh g−1 at 1000 mA g−1) and capacity retention (92.1% after 200 cycles). Based on the experimental datum, VNb9O25 nanowires as promising anode materials for LIBs can be substitutes for commercialized anodes.
∗
Corresponding author. Corresponding author. E-mail addresses: [email protected] (N. Long), [email protected] (J. Shu). 1 These authors contributed equally to this work. ∗∗
https://doi.org/10.1016/j.ceramint.2019.05.217 Received 23 April 2019; Received in revised form 17 May 2019; Accepted 21 May 2019 Available online 22 May 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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2. Experimental Firstly, C10H5NbO20, NH4VO3 and H2C2O4 (1 : 1: 9 by stoichiometric proportion) were fully dissolved in a little deionized water and 40 mL ethanol to obtain a uniform mixture. After that, a sol-gel solution was acquired by adding 4 g polyvinylpyrrolidone (PVP) to above mixture and it was stirred for 24 h. Then, the resulting solution was transferred to a plastic injector. Under voltage of 25 kV, the solution was sprayed onto the collector (stainless steel foil) at a flow rate of 15 μL min−1. Subsequently, VNb9O25 nanowires were formed by calcination of electrospun precursor at 680 °C for 20 h. X-ray diffraction (XRD, model D/max-2500 system), scanning electron microscopy (SEM, HITACHI S-4800) and transmission electron microscope (TEM, JEOL-2100F) are exploited to characterize purity, morphology and crystal structures of samples. The electrochemical characterization of VNb9O25 nanowires was measured with CR2032type coin cells, which were produced in an argon-filled glove box. A mixture of VNb9O25 nanowires, polyvinylidene fluoride (PVDF) and acetylene black was used as the working electrode. Galvanostatic cycles were tested in LAND battery testing system. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were collected on CHI660B and CHI660E electrochemical working stations, respectively.
3. Results and discussion Fig. 1a intuitively depicts the synthesis route of VNb9O25 nanowires. It shows that the spinning machine sprays nanofibers on collector (stainless steel foil). Afterwards, for the sake of getting terminal products (VNb9O25 nanowires), the as-obtained samples are calcined at high temperature. As shown in Fig. 1b, the typical peaks of VNb9O25 nanowires, corresponding to the (130), (101), (240), (121), (231), (250), (260), (251), (370), (002) and (471) crystallographic planes of VNb9O25 (JCPDS Card No. 49–0289), suggest that precursor nanofibers are thorough converted to VNb9O25 nanowires with great crystallinity. As revealed from the inset of Fig. 1b, the tetragonal VNb9O25 is composed of corner-sharing VO4 tetrahedra and NbO6 octahedra. In 3 × 3 ReO3-type NbO6 blocks, the central NbO6 polyhedron is bonded to four
Fig. 2. (a) SEM image of VNb9O25 precursor; (b) SEM images of nanowires in different magnifications; (c) TEM image of VNb9O25 nanowires, and the inset shows a schematic illustration of ion/electron transfer in nanostructures; (d) HRTEM image of the VNb9O25 nanowires.
NbO6 polyhedra. And the remaining four NbO6 polyhedra share corner with one VO4, respectively. Furthermore, the Rietveld refinement results (Fig. 1c) of VNb9O25 nanowires further prove the correctness of aforementioned conclusion from XRD pattern. The morphology of samples is examined by using SEM, TEM and HRTEM (Fig. 2a–d). As shown in Fig. 2a, the size of the nanofibers ranges from 850 to 950 nm. Meanwhile, smooth surface of nanofibers can be observed. After calcinations, the VNb9O25 nanowires show a reticulate framework, which consists of rough and interlaced nanowires with diameters of 500–700 nm (Fig. 2b). As can be seen in the inset of Fig. 2b, it is obvious that the nanowires are assembled from nanoparticles with size around ∼90 nm. Moreover, the TEM image (Fig. 2c) further proves that the VNb9O25 nanowires are entirely formed by interconnected small-sized nanoparticles. As presented in Fig. 2d,
Fig. 1. (a) Schematic of fabrication routes of VNb9O25 nanowires; (b) XRD pattern of VNb9O25 nanowires; inset of (b) is the crystal structural model of VNb9O25 nanowires; (c) the Rietveld refinement profile of VNb9O25 nanowires. 18112
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Fig. 3. (a) The cycling stabilities of VNb9O25 nanowires and VNb9O25 bulks at 100 mA g−1; (b) CV patterns of VNb9O25 nanowires; the inset shows the charge/ discharge profiles of VNb9O25 nanowires; (c) rate performance of VNb9O25 nanowires; (d) EIS spectra of VNb9O25 nanowires and VNb9O25 bulks; the insets show the equivalent circuit and the relation between Z′ and ω−0.5.
continuous and defined lattice fringe is found. Besides, the adjoining fringes distance is measured (0.496 nm), and the result is in agreement with the (130) interplane spacing of VNb9O25. Fig. 3a illustrates the long cycling stabilities of VNb9O25 nanowires and VNb9O25 bulks under the current density of 100 mA g−1. The results show that the cyclic stability of VNb9O25 nanowires is better than VNb9O25 bulks. The charge specific capacity of VNb9O25 nanowires at first cycle is 207.2 mAh g−1 at 100 mA g−1. After the 200 cycles, it stabilizes at 190.9 mAh g−1 (capacity retention of 92.1%). However, VNb9O25 bulks remain only 47.4% of capacity retention after 200 cycles. It demonstrates that nanostructured materials own the superior values in successive cycles. Fig. 3b displays typical CV curves for VNb9O25 nanowires in the first three scans at 0.1 mV s−1. Upon first cathodic sweep, two peaks at 2.21 and 1.93 V can be clearly observed, which can be ascribed to the variation of valence state from V5+ to V3+, corresponding to the two plateaus at 2.22 and 1.99 V (inset of Fig. 3b). Apparently, in subsequent two cycles of cathodic process, the reduction peak at 2.21 V in the first cycle disappears, which can be due to the partially irreversible conversion of V5+ to V3+ [40,41]. In addition, two anodic/cathodic pairs of peaks are centered at 1.46/1.29 V and 1.78/1.59 V, which are related to Nb4+/Nb3+ and Nb5+/Nb4+, corresponding to the charge/discharge platforms between 1.40 and 2 V (inset of Fig. 3b) [42,43]. In order to insight into the high current capability of VNb9O25 nanowires, the charge rate performance is presented (Fig. 3c) with progressively increasing current densities (100, 200, 300, 400, 500, 600, 900 and 1000 mA g−1). The charge specific capacity of VNb9O25 nanowires reaches 117.3 mAh g−1 at 600 mA g−1. Even at 1000 mA g−1, the charge specific capacity still remains at 90.7 mAh g−1, which is indicative of a robust rate capability for VNb9O25 nanowires. To better research the nanowires kinetics, the EIS measurement for the VNb9O25 nanowires and VNb9O25 bulks is performed (Fig. 3d). The Nyquist plot is showed in Fig. 3d. Obviously, the spectrum displays a semicircle and a slope in high frequency region and low frequency region, respectively. Moreover, the two regions represent the charge transfer resistance (Rct) (semicircle) and the Li+ diffusion of the electrode materials (slope), respectively. Here, as illustrated in the inset of Fig. 3d, the two spectra are simulated using a fitting equivalent circuit. It is noticed that the VNb9O25 nanowires exhibit the Rct of 50.7 Ω which
is smaller than that of VNb9O25 bulks (111.3 Ω). Besides, the DLi+ values are gained by calculations. Compared with VNb9O25 bulks (7.97 × 10−15 cm2 s−1), the VNb9O25 nanowires possess higher DLi+ value of 3.36 × 10−14 cm2 s−1. The above results indicate that the VNb9O25 nanowires have great electrons transportation and Li+ diffusion behaviors due to the existence of nanostructure [44–46]. 4. Conclusion In conclusion, VNb9O25 nanowires as new lithium storage materials are successfully obtained by using electrospinning technique. VNb9O25 nanowires show a reticulate framework consisted of interlaced nanowires, which are assembled from nanoparticles. Due to the unique nanoscale morphology, VNb9O25 nanowires possess remarkable electrochemical performance. After 200 cycles, VNb9O25 nanowires maintain an outstanding reversible capacity of 190.9 mAh g−1 with a capacity decay of 0.0395% per cycle at 100 mA g−1. Furthermore, the charge specific capacity can reach 90.7 mAh g−1 even at 1000 mA g−1. In summary, VNb9O25 nanowires provide a satisfactory electrochemical property for LIBs. All the promotions can be ascribed to the existence of nanostructure, which is able to boost the Li+/electron transfer. Acknowledgements This work is supported by NSAF (U1830106) and K.C. Wong Magna Fund in Ningbo University. References
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Short communication
VNb9O25 nanowires with superior electrochemical property towards lithium ion batteries
T
Cheng Jiang1, Tingting Liu1, Nengbing Long∗, Xing Cheng, Na Peng, Jundong Zhang, Runtian Zheng, Haoxiang Yu, Jie Shu∗∗ Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo, 315211, Zhejiang, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Lithium ion batteries Anode material VNb9O25 Nanowires Electrochemical property
Niobium-based compounds are extensively studied as anode materials for LIBs on account of the great structural stability and high theoretical capacity. Among them, VNb9O25 has an attractive theoretical capacity (416 mAh g−1). Unfortunately, low electronic conductivity and Li+ diffusivity hinder its practical application. To overcome above barriers, VNb9O25 nanowires were synthesized directly by electrospinning method. As expected, VNb9O25 nanowires deliver an outstanding capacity retention (92.1% after 200 cycles) and a robust rate property (90.7 mAh g−1 at 1000 mA g−1). Thus, as-electrospun VNb9O25 nanowires are alternative promising anode materials for LIBs.
1. Introduction The growth of advanced energy storage systems is propitious to strengthen the protection of environment and reduce the dependence on non-renewable energy resources [1–6]. Over the past decades, lithium ion batteries (LIBs) have become ubiquitous power sources for portable electronic devices thanks to their rechargeability and excellent energy density. In addition, the applications of LIBs are being extended to the large-scale energy storage system, such as wind and solar electricity-generating systems, electric and hybrid electric vehicles [7–11]. As we all known, as an integral component of LIBs, anode material has a fundamental influence on the characteristic of LIBs. Currently, commercialized graphite anode cannot satisfy the stringent requirements of LIBs. Firstly, the graphite possesses poor rate performance due to the slow Li+ diffusivity. On the other hand, the low operating potential of graphite results in the formation of dendritic lithium at extreme conditions, thus causing the security hidden danger [12–15]. For solving above deficiencies of graphite, Li4Ti5O12 as a popular titanate-based material has been widely researched because of its great structural stability and safety characteristics. Nevertheless, the practical application of Li4Ti5O12 is restricted owing to the low specific capacity of 175 mAh g−1 [16–19]. Therefore, developing novel and utility anode materials with superior property is vital to the advancement of next-generation large-power LIBs.
Niobium-based compounds (Nb2O5 [20], LiNb3O8 [21], TiNb2O7 [22], TiNb24O62 [23], FeNb11O29 [24] and so on) are an attractive group as anode materials for LIBs on account of their great structural stability (presence of ReO3-structure) and high theoretical capacity (rich redox chemistry). Additionally, the high working potentials (∼1.6 V) of niobium-based anode materials can prevent the formation of lithium dendrite and solid electrolyte interphase (SEI), ensuring greater safety than graphite-based anode materials [25,26]. Among niobium-based compounds, VNb9O25 has an attractive theoretical capacity (416 mAh g−1). Unfortunately, similar to other niobium-based compounds, the electrochemical kinetics of VNb9O25 is limited [27,28]. It has commonly been acknowledged that nano-crystallization is a simple and effective modification strategy to enhance the kinetics of VNb9O25 [29–34]. Nanostructures can dramatically decrease the average diffusion distance of Li+ and offer a short electronic pathway, thus leading to the enhancement of the rate property and cyclability of anode materials [35–37]. In this work, cheap and simple electrospinning method is chosen to generate VNb9O25 nanowires [38,39]. The VNb9O25 nanowires demonstrate outstanding electrochemical performance, including excellent rate property (90.7 mAh g−1 at 1000 mA g−1) and capacity retention (92.1% after 200 cycles). Based on the experimental datum, VNb9O25 nanowires as promising anode materials for LIBs can be substitutes for commercialized anodes.
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Corresponding author. Corresponding author. E-mail addresses: [email protected] (N. Long), [email protected] (J. Shu). 1 These authors contributed equally to this work. ∗∗
https://doi.org/10.1016/j.ceramint.2019.05.217 Received 23 April 2019; Received in revised form 17 May 2019; Accepted 21 May 2019 Available online 22 May 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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2. Experimental Firstly, C10H5NbO20, NH4VO3 and H2C2O4 (1 : 1: 9 by stoichiometric proportion) were fully dissolved in a little deionized water and 40 mL ethanol to obtain a uniform mixture. After that, a sol-gel solution was acquired by adding 4 g polyvinylpyrrolidone (PVP) to above mixture and it was stirred for 24 h. Then, the resulting solution was transferred to a plastic injector. Under voltage of 25 kV, the solution was sprayed onto the collector (stainless steel foil) at a flow rate of 15 μL min−1. Subsequently, VNb9O25 nanowires were formed by calcination of electrospun precursor at 680 °C for 20 h. X-ray diffraction (XRD, model D/max-2500 system), scanning electron microscopy (SEM, HITACHI S-4800) and transmission electron microscope (TEM, JEOL-2100F) are exploited to characterize purity, morphology and crystal structures of samples. The electrochemical characterization of VNb9O25 nanowires was measured with CR2032type coin cells, which were produced in an argon-filled glove box. A mixture of VNb9O25 nanowires, polyvinylidene fluoride (PVDF) and acetylene black was used as the working electrode. Galvanostatic cycles were tested in LAND battery testing system. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were collected on CHI660B and CHI660E electrochemical working stations, respectively.
3. Results and discussion Fig. 1a intuitively depicts the synthesis route of VNb9O25 nanowires. It shows that the spinning machine sprays nanofibers on collector (stainless steel foil). Afterwards, for the sake of getting terminal products (VNb9O25 nanowires), the as-obtained samples are calcined at high temperature. As shown in Fig. 1b, the typical peaks of VNb9O25 nanowires, corresponding to the (130), (101), (240), (121), (231), (250), (260), (251), (370), (002) and (471) crystallographic planes of VNb9O25 (JCPDS Card No. 49–0289), suggest that precursor nanofibers are thorough converted to VNb9O25 nanowires with great crystallinity. As revealed from the inset of Fig. 1b, the tetragonal VNb9O25 is composed of corner-sharing VO4 tetrahedra and NbO6 octahedra. In 3 × 3 ReO3-type NbO6 blocks, the central NbO6 polyhedron is bonded to four
Fig. 2. (a) SEM image of VNb9O25 precursor; (b) SEM images of nanowires in different magnifications; (c) TEM image of VNb9O25 nanowires, and the inset shows a schematic illustration of ion/electron transfer in nanostructures; (d) HRTEM image of the VNb9O25 nanowires.
NbO6 polyhedra. And the remaining four NbO6 polyhedra share corner with one VO4, respectively. Furthermore, the Rietveld refinement results (Fig. 1c) of VNb9O25 nanowires further prove the correctness of aforementioned conclusion from XRD pattern. The morphology of samples is examined by using SEM, TEM and HRTEM (Fig. 2a–d). As shown in Fig. 2a, the size of the nanofibers ranges from 850 to 950 nm. Meanwhile, smooth surface of nanofibers can be observed. After calcinations, the VNb9O25 nanowires show a reticulate framework, which consists of rough and interlaced nanowires with diameters of 500–700 nm (Fig. 2b). As can be seen in the inset of Fig. 2b, it is obvious that the nanowires are assembled from nanoparticles with size around ∼90 nm. Moreover, the TEM image (Fig. 2c) further proves that the VNb9O25 nanowires are entirely formed by interconnected small-sized nanoparticles. As presented in Fig. 2d,
Fig. 1. (a) Schematic of fabrication routes of VNb9O25 nanowires; (b) XRD pattern of VNb9O25 nanowires; inset of (b) is the crystal structural model of VNb9O25 nanowires; (c) the Rietveld refinement profile of VNb9O25 nanowires. 18112
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Fig. 3. (a) The cycling stabilities of VNb9O25 nanowires and VNb9O25 bulks at 100 mA g−1; (b) CV patterns of VNb9O25 nanowires; the inset shows the charge/ discharge profiles of VNb9O25 nanowires; (c) rate performance of VNb9O25 nanowires; (d) EIS spectra of VNb9O25 nanowires and VNb9O25 bulks; the insets show the equivalent circuit and the relation between Z′ and ω−0.5.
continuous and defined lattice fringe is found. Besides, the adjoining fringes distance is measured (0.496 nm), and the result is in agreement with the (130) interplane spacing of VNb9O25. Fig. 3a illustrates the long cycling stabilities of VNb9O25 nanowires and VNb9O25 bulks under the current density of 100 mA g−1. The results show that the cyclic stability of VNb9O25 nanowires is better than VNb9O25 bulks. The charge specific capacity of VNb9O25 nanowires at first cycle is 207.2 mAh g−1 at 100 mA g−1. After the 200 cycles, it stabilizes at 190.9 mAh g−1 (capacity retention of 92.1%). However, VNb9O25 bulks remain only 47.4% of capacity retention after 200 cycles. It demonstrates that nanostructured materials own the superior values in successive cycles. Fig. 3b displays typical CV curves for VNb9O25 nanowires in the first three scans at 0.1 mV s−1. Upon first cathodic sweep, two peaks at 2.21 and 1.93 V can be clearly observed, which can be ascribed to the variation of valence state from V5+ to V3+, corresponding to the two plateaus at 2.22 and 1.99 V (inset of Fig. 3b). Apparently, in subsequent two cycles of cathodic process, the reduction peak at 2.21 V in the first cycle disappears, which can be due to the partially irreversible conversion of V5+ to V3+ [40,41]. In addition, two anodic/cathodic pairs of peaks are centered at 1.46/1.29 V and 1.78/1.59 V, which are related to Nb4+/Nb3+ and Nb5+/Nb4+, corresponding to the charge/discharge platforms between 1.40 and 2 V (inset of Fig. 3b) [42,43]. In order to insight into the high current capability of VNb9O25 nanowires, the charge rate performance is presented (Fig. 3c) with progressively increasing current densities (100, 200, 300, 400, 500, 600, 900 and 1000 mA g−1). The charge specific capacity of VNb9O25 nanowires reaches 117.3 mAh g−1 at 600 mA g−1. Even at 1000 mA g−1, the charge specific capacity still remains at 90.7 mAh g−1, which is indicative of a robust rate capability for VNb9O25 nanowires. To better research the nanowires kinetics, the EIS measurement for the VNb9O25 nanowires and VNb9O25 bulks is performed (Fig. 3d). The Nyquist plot is showed in Fig. 3d. Obviously, the spectrum displays a semicircle and a slope in high frequency region and low frequency region, respectively. Moreover, the two regions represent the charge transfer resistance (Rct) (semicircle) and the Li+ diffusion of the electrode materials (slope), respectively. Here, as illustrated in the inset of Fig. 3d, the two spectra are simulated using a fitting equivalent circuit. It is noticed that the VNb9O25 nanowires exhibit the Rct of 50.7 Ω which
is smaller than that of VNb9O25 bulks (111.3 Ω). Besides, the DLi+ values are gained by calculations. Compared with VNb9O25 bulks (7.97 × 10−15 cm2 s−1), the VNb9O25 nanowires possess higher DLi+ value of 3.36 × 10−14 cm2 s−1. The above results indicate that the VNb9O25 nanowires have great electrons transportation and Li+ diffusion behaviors due to the existence of nanostructure [44–46]. 4. Conclusion In conclusion, VNb9O25 nanowires as new lithium storage materials are successfully obtained by using electrospinning technique. VNb9O25 nanowires show a reticulate framework consisted of interlaced nanowires, which are assembled from nanoparticles. Due to the unique nanoscale morphology, VNb9O25 nanowires possess remarkable electrochemical performance. After 200 cycles, VNb9O25 nanowires maintain an outstanding reversible capacity of 190.9 mAh g−1 with a capacity decay of 0.0395% per cycle at 100 mA g−1. Furthermore, the charge specific capacity can reach 90.7 mAh g−1 even at 1000 mA g−1. In summary, VNb9O25 nanowires provide a satisfactory electrochemical property for LIBs. All the promotions can be ascribed to the existence of nanostructure, which is able to boost the Li+/electron transfer. Acknowledgements This work is supported by NSAF (U1830106) and K.C. Wong Magna Fund in Ningbo University. References
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