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W-doped VO2(B) nanosheets-built 3D networks for fast lithium storage at high temperatures
Electrochimica Acta 295 (2019) 393e400
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
W-doped VO2(B) nanosheets-built 3D networks for fast lithium storage at high temperatures Linqing Gu, Jing Wang, Junwei Ding, Bin Li*, Shubin Yang** Key Laboratory of Aerospace Advanced Materials and Performance of Ministry of Education, School of Materials Science & Engineering, Beihang University, 100191, Beijing, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 July 2018 Received in revised form 22 October 2018 Accepted 23 October 2018 Available online 26 October 2018
VO2 as one of the most prospective cathode materials applied for lithium-ion batteries, has appealed increasing attentions owing to its high theoretical specific capacity, unique tunnel structure, as well as low cost. Among multiple methods to optimize the electrochemical performances of VO2(B), heteroatomdoping has attracted increasing interests owing to its efficiently adjusting the microstructure and interplanar distances of VO2(B). Here, W-doped VO2(B) nanosheets-built 3D networks are obtained readily via a hydrothermal route by oxalic acid reduction of vanadium pentoxide. The resultant 3D networks possess ultrathin nanowalls, interconnected structure and enlarged tunnels, which are advantageous for the easy access of electrolyte and fast diffusion of lithium ions. As a consequence, high reversible specific capacity of 304 mAh g1, ultrahigh rate capability (200 mAh g1 at 2 A g1), and hightemperature electrochemical performances are achieved as the 3D networks are applied for lithium storage. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Lithium ion battery VO2 Nanosheets W doping 3D networks
1. Introduction VO2 as a prospective cathode material applied in lithium ion batteries (LIBs), has its own distinctive superiority in comparison to the widely commercialized LiCoO2 or LiFePO4 cathodes, such as its remarkably high specific capacity (theoretically 323 mAh g1 for one Li inserting in one VO2) [1] and abundant resources (compared to Co) [2]. Moreover, high and long voltage plateau caused by the tunnel structure of VO2 results in its better electrochemical properties (especially the rate capacity) than other vanadium oxides like V2O5 [3]. Among various phases of VO2 [4e10], monoclinic VO2(B) performs the highest electrochemical properties due to its unique structure composed of deformed [VO6] octahedra which are interconnected by sharing edges in bilayers V4O10, forming plenty of layer gaps to accommodate lithium ions inserting. In addition, VO2 with B phase contains perovskite-like cavities leading to the possibility of uniaxial lithium diffusion which is parallel to the orientation of [010], supplying the fast diffusion pathways of lithium ions inserting in and extracting from the active materials. Moreover, in the process of Liþ inserting, the volume of VO2(B) tends to expand
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (B. Li). https://doi.org/10.1016/j.electacta.2018.10.145 0013-4686/© 2018 Elsevier Ltd. All rights reserved.
along [010], buffering the cleavage of VO2(B) crystal which leads to its structure stability [1,11]. However, the bulk VO2(B) is still hampered by the low reversible capacity and rate performances due to the low utilization of VO2(B) and poor electron conductivity. Therefore, further developing monoclinic VO2(B) is important to realizie its actual application. Fabrication of VO2(B) with various nanostructures become an efficient strategy to enhance the electrochemical performances of VO2(B), such as synthesizing 0D particles, 1D wires and rods, 2D sheets and belts and 3D porous structures [12e28]. Nanostructured VO2(B) not only exhibits large surface area but also shortens the diffusion length of lithium ions and facilitats electrons to transport, which are imperative for high rate cathodes [29e32]. For instance, ultrathin VO2(B) nanosheets were obtained by a short time hydrothermal approach possessing a width of ~60 nm and the size of hundreds of nanometers. Due to the single crystalline structure and high specific surface area, such VO2(B) nanosheets performed highrate capabilities which were prospective in large-scale energy storage [33]. However, these 2D VO2(B) are still hindered by the drawbacks of self-aggregation and pulverization, causing low reversible capacity and cycling stability [26]. Additionally, inducing heteroatoms into monoclinic VO2(B) is another effective way to improve the electrochemical performances. For example, aluminum atoms were doped into VO2(B) nanobelts via a
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hydrothermal method by Al(NO3)3$9H2O as the precursor with significantly increased length and width of Al-doped VO2(B) nanobelts. When n(Al)/n(V) was up to 1/6.9, the Al-doped VO2(B) displayed the best capacity retention (initial 282 mAh g1 and retention of 71.6% for 50 cycles, 32 mAh g1) which was ascribed to the higher electrical conductivity and superior structural stability [34]. However, large atoms doped in VO2(B), which might increase the interplanar distances and consequently facilitating the conduction of lithium ions, have not been reported before. As a consequence, based on large atoms doped VO2(B), combining ultrathin 2D nanostructure-built 3D networks are expected to display the outstanding properties in LIBs, which is a challenging and prospective research. Herein, we employ a simple and speedy route to synthesize Wdoped VO2(B) nanosheets-built 3D networks via a facile hydrothermal method by oxalic acid reduction of vanadium pentoxide. The resultant 3D networks possess ultrathin nanowalls, interconnected structure and enlarged tunnels, which are advantageous for the easy access of electrolyte and fast diffusion of lithium ions. As a consequence, high reversible specific capacity of 304 mAh g1(current density 50 mA g1), ultrahigh rate capability (200 mAh g1 at 2 A g1), and high-temperature electrochemical performances (323 mAh g1 at 1 A g1, 70 C) are achieved as the 3D networks are applied for lithium storage. 2. Experimental section 2.1. Preparation In the synthetic process of W-doped VO2(B), 1.2 g of V2O5 (Acros, 99.6%) powders, 1.8 g of H2C2O4 2H2O (analytical reagent (AG)) and certain amount of H2WO4 (AG, 1 at.%, 2 at.% and 3 at.% W) were added into 40 mL of deionized water, generating an orange mixture. Being stirred for 1 h in an oil bath with temperature of 75 C, a blue solution with dark precipitates at the bottom of the beaker were obtained. After cooled down naturally to room temperature, the mixture was slowly poured into a 50 mL Teflon-lined reactor. The reaction lasted for 3 h at 180 C, followed by air-cooled to room temperature. Then the obtained mixture was filtrated by deionized water for at least three times to collect the dark blue precipitate, leaving transparent bright blue filtrate in the conical flasks which was the typical color of V4þ. Ultimately, those dark blue precipitates were freeze-dried for 24e30 h, aiming at getting the different doping contents of W-doped VO2(B). As a comparison, the 0 at.% WVO2(B) was prepared via the identical process except for the addition of tungstic acid. 2.2. Characterization The micro-morphology, composition and structure of W-doped VO2(B) was verified by field emission scanning electron microscope (FESEM, JEOL-7500), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD, Rigaku D/max2500PC) and transmission electron microscopy (TEM, JEOL, NEM-2100F). XRD was used the Cu Ka radiation in the angle range of 5e90 . The state of W-VO2(B) was operated by X-ray photoelectron spectroscopy (XPS) using Thermo escalab 250Xi equipped with a monochromatized Al Ka X-ray radiation at constant 200 eV pass energy, and high resolution 30 eV was over the sample. Raman spectrum was tested by iHR550 using a laser with the wavelength of 488 nm. 2.3. Electrochemical measurements W-doped cathode electrodes were fabricated by mixing the active material (W-VO2(B), 80% by weight), carbon black (10% by
weight), and polyvinylidene fluoride (PVDF, 10% by weight). Those three powders were scattered in the solvent of N-Methyl pyrrolidone (NMP) and spread evenly onto a copper foil. Then the foils were dried in a vacuum oven (DZF-6050) at 120 C for 12 h. The coin cells (CR2032) were fabricated in a glove box (German Mbraun), using polypropylene (PP, Celgard 2400) films as separators, lithium foils as the anode, and 1 M LiPF6 being scattered in 1:1:1 vol ratio of ethyl methyl carbonate (EMC), dimethyl carbonate (DMC) and ethylene carbonate (EC). Galvanostatic charge-discharge cycles were operated on a LAND system and the voltage window exerted on the cells was 1.5e3.5 V (vs Liþ/Li). Electrochemical impedance spectroscopy (EIS) tests were operated on an electrochemical workstation (Chenhua CHI760E China), in the frequency range of 100 kHz-0.01 Hz. Cyclic voltammograms (CV) profiles of W-doped VO2(B) were measured in an electrochemical workstation (Chenhua CHI760E China) with the voltage window of 1.5e3.5 V and the scanning speed of 0.1 mV s1. 6 cycles were conducted to form three loop curves. The half-cells cycled in various temperatures were carried out in a high low temperature test box (JK-80G) which was set for 25, 40, 55, and 70 C. 3. Results and discussion W-doped VO2(B) (denoted as W-VO2(B)) 3D networks built by VO2(B) ultrathin nanosheets were prepared via a one-step self-assembly hydrothermal process (Fig. 1c). During the fabrication process, raw V2O5 particles (Fig. 1d) with layered structure (Fig. 1a) were reduced by oxalic acid under the existence of tungstic acid which provided W atoms as the dopant. Induced by W atoms, WVO2(B) nanosheets (Fig. 1e) with tunnel structure (Fig. 1b) were generated, followed by being interconnected together to form the three-dimensional W-VO2(B) networks. The content of W atoms in W-VO2(B) could be adjusted by the amount of tungstic acid, thus 1e3 at.% W-VO2(B) were obtained. Moreover, the structure of the resultant W-doped VO2(B) were tunable by the content of doped W atoms. It was noteworthy that the as-fabricated 2 at.% W-VO2(B) exhibited a typical 3D networks structure composed of numerous ultrathin VO2(B) nanosheets (Fig. 1f), which was obviously distinctive from the nanorods-structured 1 at.% W-VO2(B) and nanoparticle-structured 0 at.% W-VO2(B). Accordingly, if not particularly illustrated, W-VO2(B) is pointed to 2 at.% W-doped VO2(B). Additionally, it should be pointed that a long time freezedrying strategy for more than 24 h is essential to form the WVO2(B) 3D networks. The structure of as-fabricated W-VO2(B) was observed by X-ray diffraction (XRD) patterns and Raman spectrum. In XRD patterns, three strong peaks located at 25.2 , 49.22 , and 45.24 are assigned to (110), (312) and (511) planes, which are indexed to the standard JCPDS card of NO.31e1438, with standard lattice constants of a ¼ 12.054 Å, b ¼ 3.693 Å, c ¼ 6.424 Å and b ¼ 106.6 (Fig. 2a) [22]. In addition, peaks of other vanadium oxides and tungsten oxides are not observed, suggesting that high purity and well crystallization of monoclinic VO2 with B phase as well as W atoms successfully doping. Furthermore, the peaks of Raman spectrum located at 141, 190, 262.3, 383, and 500 cm1 are also indicated to the existence of VO2(B) (Fig. 2b) [35,36]. To disclose the details of the crystal structure of W-VO2(B), several representative peaks at 25.22 , 45.24 , 49.22 and 59.26 are minutely analyzed in XRD patterns. It is found that these peaks obviously shift to low angles due to the increasing interplanar spacings of (110), (511), (312) and (404), further demonstrating that the vanadium atoms in VO2(B) are indeed substituted by tungsten atoms in the doping process. Notably, as shown in Fig. 2cef, offsets of the peaks evidently increase with the increment of doped W atoms, which might be attributed to the larger ionic radius of W6þ (0.60 Å) than V4þ
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Fig. 1. Schematic of the synthesis process of W-doped VO2(B). (a) Structure of V2O5 along [001] plane; (b) Structure of W-doped VO2(B) along [010] facet; (c) Schematic of the synthesis process of 2 at.% W-doped VO2(B); (d) the SEM image of raw V2O5 particles; (eef) SEM images of the obtained 2 at.% W-doped VO2(B).
Fig. 2. Structure of W-VO2(B). (a) XRD patterns and (b) Raman spectra of W-VO2(B); Magnified XRD patterns of W-VO2(B) at (c) 24.5e26 , (d) 43.5e46.5 , (e) 46e51 and (f) 58e61.
(0.58 Å). When the amount of doped W atoms is up to 3 at.%, the monoclinic VO2(B) converts to another vanadium oxide due to the
excessive deformation of [VO6] octahedra which is indexed to V6O13.
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The micro-morphology and structure of W-VO2(B) were conducted by field emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HR-TEM). The TEM image of W-VO2(B) shows ultrathin VO2(B) nanosheets with the size of 0.5e1 mm, which are interleaved together to build the W-doped VO2(B) 3D networks. By the way, the ultrafine particles dispersing on the nanosheet in Fig. 3a are caused by longtime ultrasonic treatment. The HR-TEM image (Fig. 3b and d) and corresponding selected-area electron diffraction patterns (SAED) (Fig. 3c and inset in 3d) disclose the preferred growth of certain facets with interplanar distances of 3.548 Å and 1.924 Å, which are assigned to (110) and (312) planes of W-VO2(B). To identify the existence and distribution of W atoms, energy dispersive spectroscopy (EDS) spectrum and according elemental mapping images were implemented as shown in Fig. 3eei. W-VO2(B) contains three elements of V, O and W remarked as red, green and purple, illustrating the homogeneous distribution of W atoms in W-VO2(B) nanosheets. The morphology and elemental distribution of 0, 1 and 3 at.% W-VO2(B) are seen in Fig. S1aeh. By calculating the distances in HRTEM images and electron diffraction spots of W-VO2(B), interplanar spacings of (200), (201), (110), (400), (310), (312) and (511) are 5.815 Å, 5.069 Å, 3.548 Å, 2.904 Å, 2.71 Å, 1.924 Å and 1.795 Å, respectively. Compared to the indexed standard JCPDS card NO.31e1438, all facets expand by different distances of 0.015e0.084 Å. Those expanded facets are in good agreement with the result of XRD patterns, verifying W atoms indeed doping into VO2(B). Consequently, W-VO2(B) endows enlarged tunnels caused
by increased interplanar distances which are parallel to [010] axis, in comparison to 0 at.% and 1 at.% W-VO2(B), possessing easier pathways for the transportation of lithium ions. X-ray photoelectron spectroscopy (XPS) tests were conducted to further verify the state of W in VO2(B). As seen in the survey spectrum, V, O and W elements are all detected, indicating that W atoms are successfully doped in the lattice of VO2(B) (Fig. 4a). Furthermore, the contents of W measured in VO2(B) are 0.76% (1 at.% W), 1.53% (2 at.% W) and 2.35% (3 at.% W)). The highresolution spectrum of W4f in Fig. 4b shows two distinguished peaks located at the binding energy of 35.4 eV and 36.8 eV, which are ascribed to the energy level of W4f7/2 and W4f5/2 of hexavalent tungsten [37e39], respectively. The intensity of peaks of W4f increases with the increased content of W atoms in W-VO2(B), which is able to be directly adjusted by the addition amount of tungstic acid, according with the result of XRD patterns. In addition, the doping of the W atoms can affect the state of V element. The highresolution spectrum of V2p3/2 is deconvoluted into two sub-peaks. The peak at 516 eV is assigned to V4þ in VO2(B) and another peak at 517.1 eV is related to V5þ in other vanadium oxides (V6O13, V2O5 et al.) (Fig. 4c) [40e42]. Apparently, the content of V5þ increases gradually from 1 to 3 at.% (Table S1), that might because the V atoms are replaced by W atoms. Deductions were further confirmed by the high-resolution O1s XPS spectrum (Fig. 4d), where exists WeO and VeO bonds [43e46], further demonstrating that the doped W atoms are placed in the positions of V atoms in [VO6] octahedra. By the way, another O1s energy level deconvolved
Fig. 3. Morphology and structure of W-VO2(B). (aed) were the TEM image, HRTEM images and SAED patterns of W-VO2(B), indicating the typical 2D nanosheets structured WVO2(B) and single crystalline structure with enlarged interplanar spacings; (e) EDS spectrum of W-VO2(B), showing the existence of tungsten element; (f) the STEM image of WVO2(B) and corresponding EDS elemental mapping of (g) V, (h) O and (i) W, illustrating the uniform distribution of these elements.
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Fig. 4. XPS spectra of W-doped VO2(B). (a) XPS survey spectra and high-resolution spectra of (b) W4f, (c) V2p3/2, and (d) O1s energy level of 0e3 at.% W-VO2(B).
into the ~531.7 eV is due to surface contamination [44]. The states of V were further verified by Raman spectrum. Introduced by W atoms, several new peaks appear at 282.2, 406.7, 693.2 and 990.6 cm1, which are indexed to other vanadium oxides whose V valence are higher than tetravalence [47], corresponding to the aforementioned results of XPS spectrum. The electrochemical performances of W-VO2(B) were initially investigated by cyclic voltammetry (CV) curves with the voltage of 1.5e3.5 V, and the scanning speed was 0.1 mV s1 (Fig. 5a). The main redox peaks located at 2.617 V and 2.521 V are assigned to the lithium-ion deintercalation and intercalation in W-VO2(B), and the other couple of weak peaks at 2.175 V and 1.861 V might be caused by other vanadium oxides. Moreover, the difference of redox peaks is 0.096 V, which is smaller than that of 0 at.% and 1 at.% W-VO2(B) (0.134 V and 0.124 V) as marked in Fig. S2a and S2c, indicating the superior reversibility of W-VO2(B). The electrochemical behaviors of W-VO2(B) were conducted by galvanostatic charge-discharge (GCD) measurements and the voltage window employed on cells was 1.5e3.5 V. As visibly seen in Fig. 5b, Fig. S2b and S2d, the main discharge and charge plateaus are located at ~2.5e2.6 V, agreeing with the results of CV, which are given rise to the intercalation and deintercalation of Liþ inserting into VO2(B) [48]:
VO2 þ xLiþ þ xe /LixVO2 (x 1)
(1)
Where x is the molar ratio of Li ion inserting into VO2(B). Additionally, the lower plateaus at ~2 V is assigned to other vanadium oxides. A high reversible capacity of 304 mAh g1 of W-VO2(B) is obtained at current density of 100 mA g1, which is higher than that of 0 at.% W-VO2(B) (264 mAh g1), 1 at.% W-VO2(B) (276 mAh g1) and 3 at.% W-VO2(B) (282 mAh g1), confirming that high lithium storage performance of the designed W-VO2(B) 3D networks. Additionally, there are two couples of redox peaks being observed in the charge-discharge profiles, which are parallel to those peaks in CV curves. Notably, the W-VO2(B) exhibits a superior rate capacity of 210 mAh g1 at current density of 1 A g1 and even over 200 mAh g1 at 2 A g1, which are higher than that of 0 at.% WVO2(B)(134 mAh g1, 2 A g1), 1 at.% W-VO2(B) (152 mAh g1, 2 A g1), 3 at.% W-VO2(B) (139 mAh g1, 2 A g1) (Fig. 5c) and those reported capacities of pure VO2(B) and modified VO2(B) at the same situation [26,49,50]. Moreover, the W-VO2(B) possesses a high capacity recover property, when current density is back to 100 mA g1, the specific capacity reverts to ~260 mAh g1. Furthermore, the as-obtained W-VO2(B) displays a high cycling stability. The capacity can maintain ~68.5% with 209 mAh g1 over
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Fig. 5. Electrochemical performances of W-VO2(B). (a) Cyclic voltammograms of W-VO2(B) at 0.1 mV s1 in the voltage range of 1.5e3.5 V; (b) charge-discharge profiles of W-VO2(B) at 100 mA g1 from 1.5 to 3.5 V vs Liþ/Li; (c) Rate performances of W-doped VO2(B) (0e3 at.% W); (d) Capacity retention and coulombic efficiency of W-doped VO2(B) (0e3 at.% W) at current density of 100 mA g1 for 200 cycles; (e) Cycle performances of W-doped VO2(B) (0e3 at.% W) under various temperatures from 25 to 70 C at 1 A g1; (f) Capacity maintenance of W-doped VO2(B) (0e3 at.% W) at 1 A g1, 70 C for 40 cycles.
200 cycles, which is higher than that of 0 at.% W-VO2(B) (~46.6%, 122 mAh g1), 1 at.% W-VO2(B) (~58.2%, 160 mAh g1) and 3 at.% WVO2(B) (~47.9%, 135 mAh g1), revealing the inhibition effect in capacity decay of W-doped VO2(B) nanosheets-built 3D networks (Fig. 5d). Different capacities in the same rate of 0.3 C are ascribed to the ultrathin nanowalls, interconnected structure and enlarged tunnels in W-VO2(B). Meanwhile, the differences of crystallinity and micro-morphology are the exterior factors affecting the electrochemical properties, which are caused by the interplanar spacing. As for 3 at.% W-VO2(B), excess doping of W atoms changes the crystal structure of VO2(B) to V6O13 followed by poor crystallinity and distorted [VO6] octahedra, leading to inferior electrochemical performances compared to 2 at.% W-VO2(B). What's more, W-VO2(B) exhibits a high Coulombic efficiency of over 98%, revealing its excellent reversible property. And the fluctuation of Coulombic efficiency is normal in semiconductor VO2(B) without addition of carbon materials like graphene or carbon nanotubes.
Such outstanding electrochemical performances of W-VO2(B) 3D networks are owing to its unique structure: the 3D networks favoring the access of electrolyte, the ultrathin 2D nanosheets shortening the lithium ion diffusion distance and W doping enlarging the tunnels of VO2(B) which could promote the transportation of lithium ions. More interestingly, the electrochemical performances of WVO2(B) at elevated temperatures were systematically evaluated by operating the half-cell at the given temperatures of 25, 40, 55, and 70 C. As depicted in Fig. 5e, capacities of W-VO2(B) increase from 205 mAh g1 to 315 mAh g1 with the temperature raising from 25 to 70 C, because of the contact and charge-transfer resistances being significantly reduced at high temperatures [22]. The ratio ((capacity (T)-capacity (25 C))/capacity (25 C) (T ¼ 40, 55, and 70 C)) of 0e3 at.% W-VO2(B) shows that the capacity of W-VO2(B) increases to the maximum, compared to 0 at.%, 1 at.% and 3 at.% WVO2(B), disclosing excellent electrochemical performances of W-
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Fig. 6. EIS performances of W-doped VO2(B) (0e3 at.% W). (a) Nyquist plots of W-VO2(B) before cycle and after 10 cycles from 100 kHz to 0.1 Hz. (b) The relation curves related to Z0 u1/2 of W-VO2(B) after 10 cycles.
VO2(B) at high temperatures (Fig. S3). More importantly, in the severe condition of 70 C, the W-VO2(B) could display a surprising capacity maintenance from 315 mAh g1 to 287 mAh g1 over 40 times of charging and discharging, which is superior to 0 at%, 1 at.%, and 3 at.% W-VO2(B) (Fig. 5f). Electrochemical impedance spectroscopy (EIS) characterizations of the half-cells with W-VO2(B) electrodes were evaluated to get further understanding of the W-doping effects on electrochemical performances of VO2(B). Every typical Nyquist plot of the as-fabricated W-VO2(B) before cycle or after 10 cycles consists of an almost semicircle from high frequency to middle frequency which is pointed to the charge transfer resistance (Rct) and a 45 slope line in low frequency being assigned to the Warburg impedance (W) (Fig. 6a). Intuitively, the Rct of the W-VO2(B) is lower than that of 0 at.%, 1 at.%, and 3 at.% W-VO2(B), indicating the fast electron transfer and lithium ion diffusion of W-VO2(B) [51,52]. More specifically, by fitting with Nyquist plots, the calculated Rct of WVO2(B) is 11.98 U, which is one fourth of that of 0 at.% VO2(B) (49.98 U) and 3 at.% VO2(B) (47.79 U), and one third of that of 1 at.% W-VO2(B) (29.96 U), manifesting faster electronic transfer during the process of charge-discharge (Table S2). As is known to us all, the diffusion of Liþ in solid state is the rate controlling step during lithium intercalation and de-intercalation into electrode materials. Here, we quantified the lithium ion diffusion coefficient (indicated as DLiþ ) by further analyzing the EIS curves of W-VO2(B) electrode, as shown in Fig. 6b. According to equation (2) [53,54]:
DLiþ ¼
R2 T 2 2A2 n4 F 4 C 2 s2
4. Conclusions In summary, W-doped VO2(B) nanosheets-built 3D networks were successfully prepared via a simple and speedy hydrothermal route by oxalic acid reduction of vanadium pentoxide under the existence of tungstic acid. Due to the doping of tungsten atoms, the obtained W-VO2(B) exhibited expanded tunnel structure and interconnected 3D networks composed of 2D ultrathin nanosheets. Such a unique structure can promote lithium storage in three parts: 1) ultrathin nanosheets possessing shortened lithium ion diffusion pathways; 2) 3D-networks providing easy access of electrolyte to promote fast Liþ diffusion; 3) enlarged tunnels leading to expanded space which is easier for lithium ions to transfer. As a consequence, the electrochemical performances of W-VO2(B) increase with the incremental interplanar distances caused by increasing doped W atoms. The optimized 2 at.% W-doped VO2(B) exhibited high rate capability (200 mAh g1 at 2 A g1), high reversible capacity (304 mAh g1 at 100 mA g1) and high-temperature electrochemical performances (323 mAh g1, 1 A g1 and 70 C). Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51572007, 51622203 and 51702010). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2018.10.145.
(2)
where R represents gas constant (8.314 J/(mol K)), T is pointed to degree Kelvin (298.15 K), A is the contact area of electrode and electrolyte (1.131 cm2), n is the amount of electrons transferred in the reaction occurring in the cathode electrode (1 mol), F is Faraday's constant (96485 C/mol), C represents the molarity of Liþ inserting in W-VO2(B) (4.87 104 mol m3), and s is Warburg coefficient (42.267, 8.492, 21.126 and 48.695 of 3 at.%, 2 at.%, 1 at.% and 0 at.% W-VO2(B), respectively) which is the slope in Fig. 6b. The diffusion coefficient (DLiþ ) of W-VO2(B) is 1.62 1013 cm2 s1, which is 2 orders of magnitude higher than that of 0 at.% W-VO2(B) (4.94 1015 cm2 s1) and 3 at.% W-VO2(B) (6.55 1015 cm2 s1), and 1 order higher than that of 1 at.% W-VO2(B) (2.62 1014 cm2 s1), revealing the fast lithium ion diffusion behavior in W-VO2(B).
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
W-doped VO2(B) nanosheets-built 3D networks for fast lithium storage at high temperatures Linqing Gu, Jing Wang, Junwei Ding, Bin Li*, Shubin Yang** Key Laboratory of Aerospace Advanced Materials and Performance of Ministry of Education, School of Materials Science & Engineering, Beihang University, 100191, Beijing, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 July 2018 Received in revised form 22 October 2018 Accepted 23 October 2018 Available online 26 October 2018
VO2 as one of the most prospective cathode materials applied for lithium-ion batteries, has appealed increasing attentions owing to its high theoretical specific capacity, unique tunnel structure, as well as low cost. Among multiple methods to optimize the electrochemical performances of VO2(B), heteroatomdoping has attracted increasing interests owing to its efficiently adjusting the microstructure and interplanar distances of VO2(B). Here, W-doped VO2(B) nanosheets-built 3D networks are obtained readily via a hydrothermal route by oxalic acid reduction of vanadium pentoxide. The resultant 3D networks possess ultrathin nanowalls, interconnected structure and enlarged tunnels, which are advantageous for the easy access of electrolyte and fast diffusion of lithium ions. As a consequence, high reversible specific capacity of 304 mAh g1, ultrahigh rate capability (200 mAh g1 at 2 A g1), and hightemperature electrochemical performances are achieved as the 3D networks are applied for lithium storage. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Lithium ion battery VO2 Nanosheets W doping 3D networks
1. Introduction VO2 as a prospective cathode material applied in lithium ion batteries (LIBs), has its own distinctive superiority in comparison to the widely commercialized LiCoO2 or LiFePO4 cathodes, such as its remarkably high specific capacity (theoretically 323 mAh g1 for one Li inserting in one VO2) [1] and abundant resources (compared to Co) [2]. Moreover, high and long voltage plateau caused by the tunnel structure of VO2 results in its better electrochemical properties (especially the rate capacity) than other vanadium oxides like V2O5 [3]. Among various phases of VO2 [4e10], monoclinic VO2(B) performs the highest electrochemical properties due to its unique structure composed of deformed [VO6] octahedra which are interconnected by sharing edges in bilayers V4O10, forming plenty of layer gaps to accommodate lithium ions inserting. In addition, VO2 with B phase contains perovskite-like cavities leading to the possibility of uniaxial lithium diffusion which is parallel to the orientation of [010], supplying the fast diffusion pathways of lithium ions inserting in and extracting from the active materials. Moreover, in the process of Liþ inserting, the volume of VO2(B) tends to expand
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (B. Li). https://doi.org/10.1016/j.electacta.2018.10.145 0013-4686/© 2018 Elsevier Ltd. All rights reserved.
along [010], buffering the cleavage of VO2(B) crystal which leads to its structure stability [1,11]. However, the bulk VO2(B) is still hampered by the low reversible capacity and rate performances due to the low utilization of VO2(B) and poor electron conductivity. Therefore, further developing monoclinic VO2(B) is important to realizie its actual application. Fabrication of VO2(B) with various nanostructures become an efficient strategy to enhance the electrochemical performances of VO2(B), such as synthesizing 0D particles, 1D wires and rods, 2D sheets and belts and 3D porous structures [12e28]. Nanostructured VO2(B) not only exhibits large surface area but also shortens the diffusion length of lithium ions and facilitats electrons to transport, which are imperative for high rate cathodes [29e32]. For instance, ultrathin VO2(B) nanosheets were obtained by a short time hydrothermal approach possessing a width of ~60 nm and the size of hundreds of nanometers. Due to the single crystalline structure and high specific surface area, such VO2(B) nanosheets performed highrate capabilities which were prospective in large-scale energy storage [33]. However, these 2D VO2(B) are still hindered by the drawbacks of self-aggregation and pulverization, causing low reversible capacity and cycling stability [26]. Additionally, inducing heteroatoms into monoclinic VO2(B) is another effective way to improve the electrochemical performances. For example, aluminum atoms were doped into VO2(B) nanobelts via a
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hydrothermal method by Al(NO3)3$9H2O as the precursor with significantly increased length and width of Al-doped VO2(B) nanobelts. When n(Al)/n(V) was up to 1/6.9, the Al-doped VO2(B) displayed the best capacity retention (initial 282 mAh g1 and retention of 71.6% for 50 cycles, 32 mAh g1) which was ascribed to the higher electrical conductivity and superior structural stability [34]. However, large atoms doped in VO2(B), which might increase the interplanar distances and consequently facilitating the conduction of lithium ions, have not been reported before. As a consequence, based on large atoms doped VO2(B), combining ultrathin 2D nanostructure-built 3D networks are expected to display the outstanding properties in LIBs, which is a challenging and prospective research. Herein, we employ a simple and speedy route to synthesize Wdoped VO2(B) nanosheets-built 3D networks via a facile hydrothermal method by oxalic acid reduction of vanadium pentoxide. The resultant 3D networks possess ultrathin nanowalls, interconnected structure and enlarged tunnels, which are advantageous for the easy access of electrolyte and fast diffusion of lithium ions. As a consequence, high reversible specific capacity of 304 mAh g1(current density 50 mA g1), ultrahigh rate capability (200 mAh g1 at 2 A g1), and high-temperature electrochemical performances (323 mAh g1 at 1 A g1, 70 C) are achieved as the 3D networks are applied for lithium storage. 2. Experimental section 2.1. Preparation In the synthetic process of W-doped VO2(B), 1.2 g of V2O5 (Acros, 99.6%) powders, 1.8 g of H2C2O4 2H2O (analytical reagent (AG)) and certain amount of H2WO4 (AG, 1 at.%, 2 at.% and 3 at.% W) were added into 40 mL of deionized water, generating an orange mixture. Being stirred for 1 h in an oil bath with temperature of 75 C, a blue solution with dark precipitates at the bottom of the beaker were obtained. After cooled down naturally to room temperature, the mixture was slowly poured into a 50 mL Teflon-lined reactor. The reaction lasted for 3 h at 180 C, followed by air-cooled to room temperature. Then the obtained mixture was filtrated by deionized water for at least three times to collect the dark blue precipitate, leaving transparent bright blue filtrate in the conical flasks which was the typical color of V4þ. Ultimately, those dark blue precipitates were freeze-dried for 24e30 h, aiming at getting the different doping contents of W-doped VO2(B). As a comparison, the 0 at.% WVO2(B) was prepared via the identical process except for the addition of tungstic acid. 2.2. Characterization The micro-morphology, composition and structure of W-doped VO2(B) was verified by field emission scanning electron microscope (FESEM, JEOL-7500), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD, Rigaku D/max2500PC) and transmission electron microscopy (TEM, JEOL, NEM-2100F). XRD was used the Cu Ka radiation in the angle range of 5e90 . The state of W-VO2(B) was operated by X-ray photoelectron spectroscopy (XPS) using Thermo escalab 250Xi equipped with a monochromatized Al Ka X-ray radiation at constant 200 eV pass energy, and high resolution 30 eV was over the sample. Raman spectrum was tested by iHR550 using a laser with the wavelength of 488 nm. 2.3. Electrochemical measurements W-doped cathode electrodes were fabricated by mixing the active material (W-VO2(B), 80% by weight), carbon black (10% by
weight), and polyvinylidene fluoride (PVDF, 10% by weight). Those three powders were scattered in the solvent of N-Methyl pyrrolidone (NMP) and spread evenly onto a copper foil. Then the foils were dried in a vacuum oven (DZF-6050) at 120 C for 12 h. The coin cells (CR2032) were fabricated in a glove box (German Mbraun), using polypropylene (PP, Celgard 2400) films as separators, lithium foils as the anode, and 1 M LiPF6 being scattered in 1:1:1 vol ratio of ethyl methyl carbonate (EMC), dimethyl carbonate (DMC) and ethylene carbonate (EC). Galvanostatic charge-discharge cycles were operated on a LAND system and the voltage window exerted on the cells was 1.5e3.5 V (vs Liþ/Li). Electrochemical impedance spectroscopy (EIS) tests were operated on an electrochemical workstation (Chenhua CHI760E China), in the frequency range of 100 kHz-0.01 Hz. Cyclic voltammograms (CV) profiles of W-doped VO2(B) were measured in an electrochemical workstation (Chenhua CHI760E China) with the voltage window of 1.5e3.5 V and the scanning speed of 0.1 mV s1. 6 cycles were conducted to form three loop curves. The half-cells cycled in various temperatures were carried out in a high low temperature test box (JK-80G) which was set for 25, 40, 55, and 70 C. 3. Results and discussion W-doped VO2(B) (denoted as W-VO2(B)) 3D networks built by VO2(B) ultrathin nanosheets were prepared via a one-step self-assembly hydrothermal process (Fig. 1c). During the fabrication process, raw V2O5 particles (Fig. 1d) with layered structure (Fig. 1a) were reduced by oxalic acid under the existence of tungstic acid which provided W atoms as the dopant. Induced by W atoms, WVO2(B) nanosheets (Fig. 1e) with tunnel structure (Fig. 1b) were generated, followed by being interconnected together to form the three-dimensional W-VO2(B) networks. The content of W atoms in W-VO2(B) could be adjusted by the amount of tungstic acid, thus 1e3 at.% W-VO2(B) were obtained. Moreover, the structure of the resultant W-doped VO2(B) were tunable by the content of doped W atoms. It was noteworthy that the as-fabricated 2 at.% W-VO2(B) exhibited a typical 3D networks structure composed of numerous ultrathin VO2(B) nanosheets (Fig. 1f), which was obviously distinctive from the nanorods-structured 1 at.% W-VO2(B) and nanoparticle-structured 0 at.% W-VO2(B). Accordingly, if not particularly illustrated, W-VO2(B) is pointed to 2 at.% W-doped VO2(B). Additionally, it should be pointed that a long time freezedrying strategy for more than 24 h is essential to form the WVO2(B) 3D networks. The structure of as-fabricated W-VO2(B) was observed by X-ray diffraction (XRD) patterns and Raman spectrum. In XRD patterns, three strong peaks located at 25.2 , 49.22 , and 45.24 are assigned to (110), (312) and (511) planes, which are indexed to the standard JCPDS card of NO.31e1438, with standard lattice constants of a ¼ 12.054 Å, b ¼ 3.693 Å, c ¼ 6.424 Å and b ¼ 106.6 (Fig. 2a) [22]. In addition, peaks of other vanadium oxides and tungsten oxides are not observed, suggesting that high purity and well crystallization of monoclinic VO2 with B phase as well as W atoms successfully doping. Furthermore, the peaks of Raman spectrum located at 141, 190, 262.3, 383, and 500 cm1 are also indicated to the existence of VO2(B) (Fig. 2b) [35,36]. To disclose the details of the crystal structure of W-VO2(B), several representative peaks at 25.22 , 45.24 , 49.22 and 59.26 are minutely analyzed in XRD patterns. It is found that these peaks obviously shift to low angles due to the increasing interplanar spacings of (110), (511), (312) and (404), further demonstrating that the vanadium atoms in VO2(B) are indeed substituted by tungsten atoms in the doping process. Notably, as shown in Fig. 2cef, offsets of the peaks evidently increase with the increment of doped W atoms, which might be attributed to the larger ionic radius of W6þ (0.60 Å) than V4þ
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Fig. 1. Schematic of the synthesis process of W-doped VO2(B). (a) Structure of V2O5 along [001] plane; (b) Structure of W-doped VO2(B) along [010] facet; (c) Schematic of the synthesis process of 2 at.% W-doped VO2(B); (d) the SEM image of raw V2O5 particles; (eef) SEM images of the obtained 2 at.% W-doped VO2(B).
Fig. 2. Structure of W-VO2(B). (a) XRD patterns and (b) Raman spectra of W-VO2(B); Magnified XRD patterns of W-VO2(B) at (c) 24.5e26 , (d) 43.5e46.5 , (e) 46e51 and (f) 58e61.
(0.58 Å). When the amount of doped W atoms is up to 3 at.%, the monoclinic VO2(B) converts to another vanadium oxide due to the
excessive deformation of [VO6] octahedra which is indexed to V6O13.
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The micro-morphology and structure of W-VO2(B) were conducted by field emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HR-TEM). The TEM image of W-VO2(B) shows ultrathin VO2(B) nanosheets with the size of 0.5e1 mm, which are interleaved together to build the W-doped VO2(B) 3D networks. By the way, the ultrafine particles dispersing on the nanosheet in Fig. 3a are caused by longtime ultrasonic treatment. The HR-TEM image (Fig. 3b and d) and corresponding selected-area electron diffraction patterns (SAED) (Fig. 3c and inset in 3d) disclose the preferred growth of certain facets with interplanar distances of 3.548 Å and 1.924 Å, which are assigned to (110) and (312) planes of W-VO2(B). To identify the existence and distribution of W atoms, energy dispersive spectroscopy (EDS) spectrum and according elemental mapping images were implemented as shown in Fig. 3eei. W-VO2(B) contains three elements of V, O and W remarked as red, green and purple, illustrating the homogeneous distribution of W atoms in W-VO2(B) nanosheets. The morphology and elemental distribution of 0, 1 and 3 at.% W-VO2(B) are seen in Fig. S1aeh. By calculating the distances in HRTEM images and electron diffraction spots of W-VO2(B), interplanar spacings of (200), (201), (110), (400), (310), (312) and (511) are 5.815 Å, 5.069 Å, 3.548 Å, 2.904 Å, 2.71 Å, 1.924 Å and 1.795 Å, respectively. Compared to the indexed standard JCPDS card NO.31e1438, all facets expand by different distances of 0.015e0.084 Å. Those expanded facets are in good agreement with the result of XRD patterns, verifying W atoms indeed doping into VO2(B). Consequently, W-VO2(B) endows enlarged tunnels caused
by increased interplanar distances which are parallel to [010] axis, in comparison to 0 at.% and 1 at.% W-VO2(B), possessing easier pathways for the transportation of lithium ions. X-ray photoelectron spectroscopy (XPS) tests were conducted to further verify the state of W in VO2(B). As seen in the survey spectrum, V, O and W elements are all detected, indicating that W atoms are successfully doped in the lattice of VO2(B) (Fig. 4a). Furthermore, the contents of W measured in VO2(B) are 0.76% (1 at.% W), 1.53% (2 at.% W) and 2.35% (3 at.% W)). The highresolution spectrum of W4f in Fig. 4b shows two distinguished peaks located at the binding energy of 35.4 eV and 36.8 eV, which are ascribed to the energy level of W4f7/2 and W4f5/2 of hexavalent tungsten [37e39], respectively. The intensity of peaks of W4f increases with the increased content of W atoms in W-VO2(B), which is able to be directly adjusted by the addition amount of tungstic acid, according with the result of XRD patterns. In addition, the doping of the W atoms can affect the state of V element. The highresolution spectrum of V2p3/2 is deconvoluted into two sub-peaks. The peak at 516 eV is assigned to V4þ in VO2(B) and another peak at 517.1 eV is related to V5þ in other vanadium oxides (V6O13, V2O5 et al.) (Fig. 4c) [40e42]. Apparently, the content of V5þ increases gradually from 1 to 3 at.% (Table S1), that might because the V atoms are replaced by W atoms. Deductions were further confirmed by the high-resolution O1s XPS spectrum (Fig. 4d), where exists WeO and VeO bonds [43e46], further demonstrating that the doped W atoms are placed in the positions of V atoms in [VO6] octahedra. By the way, another O1s energy level deconvolved
Fig. 3. Morphology and structure of W-VO2(B). (aed) were the TEM image, HRTEM images and SAED patterns of W-VO2(B), indicating the typical 2D nanosheets structured WVO2(B) and single crystalline structure with enlarged interplanar spacings; (e) EDS spectrum of W-VO2(B), showing the existence of tungsten element; (f) the STEM image of WVO2(B) and corresponding EDS elemental mapping of (g) V, (h) O and (i) W, illustrating the uniform distribution of these elements.
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Fig. 4. XPS spectra of W-doped VO2(B). (a) XPS survey spectra and high-resolution spectra of (b) W4f, (c) V2p3/2, and (d) O1s energy level of 0e3 at.% W-VO2(B).
into the ~531.7 eV is due to surface contamination [44]. The states of V were further verified by Raman spectrum. Introduced by W atoms, several new peaks appear at 282.2, 406.7, 693.2 and 990.6 cm1, which are indexed to other vanadium oxides whose V valence are higher than tetravalence [47], corresponding to the aforementioned results of XPS spectrum. The electrochemical performances of W-VO2(B) were initially investigated by cyclic voltammetry (CV) curves with the voltage of 1.5e3.5 V, and the scanning speed was 0.1 mV s1 (Fig. 5a). The main redox peaks located at 2.617 V and 2.521 V are assigned to the lithium-ion deintercalation and intercalation in W-VO2(B), and the other couple of weak peaks at 2.175 V and 1.861 V might be caused by other vanadium oxides. Moreover, the difference of redox peaks is 0.096 V, which is smaller than that of 0 at.% and 1 at.% W-VO2(B) (0.134 V and 0.124 V) as marked in Fig. S2a and S2c, indicating the superior reversibility of W-VO2(B). The electrochemical behaviors of W-VO2(B) were conducted by galvanostatic charge-discharge (GCD) measurements and the voltage window employed on cells was 1.5e3.5 V. As visibly seen in Fig. 5b, Fig. S2b and S2d, the main discharge and charge plateaus are located at ~2.5e2.6 V, agreeing with the results of CV, which are given rise to the intercalation and deintercalation of Liþ inserting into VO2(B) [48]:
VO2 þ xLiþ þ xe /LixVO2 (x 1)
(1)
Where x is the molar ratio of Li ion inserting into VO2(B). Additionally, the lower plateaus at ~2 V is assigned to other vanadium oxides. A high reversible capacity of 304 mAh g1 of W-VO2(B) is obtained at current density of 100 mA g1, which is higher than that of 0 at.% W-VO2(B) (264 mAh g1), 1 at.% W-VO2(B) (276 mAh g1) and 3 at.% W-VO2(B) (282 mAh g1), confirming that high lithium storage performance of the designed W-VO2(B) 3D networks. Additionally, there are two couples of redox peaks being observed in the charge-discharge profiles, which are parallel to those peaks in CV curves. Notably, the W-VO2(B) exhibits a superior rate capacity of 210 mAh g1 at current density of 1 A g1 and even over 200 mAh g1 at 2 A g1, which are higher than that of 0 at.% WVO2(B)(134 mAh g1, 2 A g1), 1 at.% W-VO2(B) (152 mAh g1, 2 A g1), 3 at.% W-VO2(B) (139 mAh g1, 2 A g1) (Fig. 5c) and those reported capacities of pure VO2(B) and modified VO2(B) at the same situation [26,49,50]. Moreover, the W-VO2(B) possesses a high capacity recover property, when current density is back to 100 mA g1, the specific capacity reverts to ~260 mAh g1. Furthermore, the as-obtained W-VO2(B) displays a high cycling stability. The capacity can maintain ~68.5% with 209 mAh g1 over
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Fig. 5. Electrochemical performances of W-VO2(B). (a) Cyclic voltammograms of W-VO2(B) at 0.1 mV s1 in the voltage range of 1.5e3.5 V; (b) charge-discharge profiles of W-VO2(B) at 100 mA g1 from 1.5 to 3.5 V vs Liþ/Li; (c) Rate performances of W-doped VO2(B) (0e3 at.% W); (d) Capacity retention and coulombic efficiency of W-doped VO2(B) (0e3 at.% W) at current density of 100 mA g1 for 200 cycles; (e) Cycle performances of W-doped VO2(B) (0e3 at.% W) under various temperatures from 25 to 70 C at 1 A g1; (f) Capacity maintenance of W-doped VO2(B) (0e3 at.% W) at 1 A g1, 70 C for 40 cycles.
200 cycles, which is higher than that of 0 at.% W-VO2(B) (~46.6%, 122 mAh g1), 1 at.% W-VO2(B) (~58.2%, 160 mAh g1) and 3 at.% WVO2(B) (~47.9%, 135 mAh g1), revealing the inhibition effect in capacity decay of W-doped VO2(B) nanosheets-built 3D networks (Fig. 5d). Different capacities in the same rate of 0.3 C are ascribed to the ultrathin nanowalls, interconnected structure and enlarged tunnels in W-VO2(B). Meanwhile, the differences of crystallinity and micro-morphology are the exterior factors affecting the electrochemical properties, which are caused by the interplanar spacing. As for 3 at.% W-VO2(B), excess doping of W atoms changes the crystal structure of VO2(B) to V6O13 followed by poor crystallinity and distorted [VO6] octahedra, leading to inferior electrochemical performances compared to 2 at.% W-VO2(B). What's more, W-VO2(B) exhibits a high Coulombic efficiency of over 98%, revealing its excellent reversible property. And the fluctuation of Coulombic efficiency is normal in semiconductor VO2(B) without addition of carbon materials like graphene or carbon nanotubes.
Such outstanding electrochemical performances of W-VO2(B) 3D networks are owing to its unique structure: the 3D networks favoring the access of electrolyte, the ultrathin 2D nanosheets shortening the lithium ion diffusion distance and W doping enlarging the tunnels of VO2(B) which could promote the transportation of lithium ions. More interestingly, the electrochemical performances of WVO2(B) at elevated temperatures were systematically evaluated by operating the half-cell at the given temperatures of 25, 40, 55, and 70 C. As depicted in Fig. 5e, capacities of W-VO2(B) increase from 205 mAh g1 to 315 mAh g1 with the temperature raising from 25 to 70 C, because of the contact and charge-transfer resistances being significantly reduced at high temperatures [22]. The ratio ((capacity (T)-capacity (25 C))/capacity (25 C) (T ¼ 40, 55, and 70 C)) of 0e3 at.% W-VO2(B) shows that the capacity of W-VO2(B) increases to the maximum, compared to 0 at.%, 1 at.% and 3 at.% WVO2(B), disclosing excellent electrochemical performances of W-
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399
Fig. 6. EIS performances of W-doped VO2(B) (0e3 at.% W). (a) Nyquist plots of W-VO2(B) before cycle and after 10 cycles from 100 kHz to 0.1 Hz. (b) The relation curves related to Z0 u1/2 of W-VO2(B) after 10 cycles.
VO2(B) at high temperatures (Fig. S3). More importantly, in the severe condition of 70 C, the W-VO2(B) could display a surprising capacity maintenance from 315 mAh g1 to 287 mAh g1 over 40 times of charging and discharging, which is superior to 0 at%, 1 at.%, and 3 at.% W-VO2(B) (Fig. 5f). Electrochemical impedance spectroscopy (EIS) characterizations of the half-cells with W-VO2(B) electrodes were evaluated to get further understanding of the W-doping effects on electrochemical performances of VO2(B). Every typical Nyquist plot of the as-fabricated W-VO2(B) before cycle or after 10 cycles consists of an almost semicircle from high frequency to middle frequency which is pointed to the charge transfer resistance (Rct) and a 45 slope line in low frequency being assigned to the Warburg impedance (W) (Fig. 6a). Intuitively, the Rct of the W-VO2(B) is lower than that of 0 at.%, 1 at.%, and 3 at.% W-VO2(B), indicating the fast electron transfer and lithium ion diffusion of W-VO2(B) [51,52]. More specifically, by fitting with Nyquist plots, the calculated Rct of WVO2(B) is 11.98 U, which is one fourth of that of 0 at.% VO2(B) (49.98 U) and 3 at.% VO2(B) (47.79 U), and one third of that of 1 at.% W-VO2(B) (29.96 U), manifesting faster electronic transfer during the process of charge-discharge (Table S2). As is known to us all, the diffusion of Liþ in solid state is the rate controlling step during lithium intercalation and de-intercalation into electrode materials. Here, we quantified the lithium ion diffusion coefficient (indicated as DLiþ ) by further analyzing the EIS curves of W-VO2(B) electrode, as shown in Fig. 6b. According to equation (2) [53,54]:
DLiþ ¼
R2 T 2 2A2 n4 F 4 C 2 s2
4. Conclusions In summary, W-doped VO2(B) nanosheets-built 3D networks were successfully prepared via a simple and speedy hydrothermal route by oxalic acid reduction of vanadium pentoxide under the existence of tungstic acid. Due to the doping of tungsten atoms, the obtained W-VO2(B) exhibited expanded tunnel structure and interconnected 3D networks composed of 2D ultrathin nanosheets. Such a unique structure can promote lithium storage in three parts: 1) ultrathin nanosheets possessing shortened lithium ion diffusion pathways; 2) 3D-networks providing easy access of electrolyte to promote fast Liþ diffusion; 3) enlarged tunnels leading to expanded space which is easier for lithium ions to transfer. As a consequence, the electrochemical performances of W-VO2(B) increase with the incremental interplanar distances caused by increasing doped W atoms. The optimized 2 at.% W-doped VO2(B) exhibited high rate capability (200 mAh g1 at 2 A g1), high reversible capacity (304 mAh g1 at 100 mA g1) and high-temperature electrochemical performances (323 mAh g1, 1 A g1 and 70 C). Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51572007, 51622203 and 51702010). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2018.10.145.
(2)
where R represents gas constant (8.314 J/(mol K)), T is pointed to degree Kelvin (298.15 K), A is the contact area of electrode and electrolyte (1.131 cm2), n is the amount of electrons transferred in the reaction occurring in the cathode electrode (1 mol), F is Faraday's constant (96485 C/mol), C represents the molarity of Liþ inserting in W-VO2(B) (4.87 104 mol m3), and s is Warburg coefficient (42.267, 8.492, 21.126 and 48.695 of 3 at.%, 2 at.%, 1 at.% and 0 at.% W-VO2(B), respectively) which is the slope in Fig. 6b. The diffusion coefficient (DLiþ ) of W-VO2(B) is 1.62 1013 cm2 s1, which is 2 orders of magnitude higher than that of 0 at.% W-VO2(B) (4.94 1015 cm2 s1) and 3 at.% W-VO2(B) (6.55 1015 cm2 s1), and 1 order higher than that of 1 at.% W-VO2(B) (2.62 1014 cm2 s1), revealing the fast lithium ion diffusion behavior in W-VO2(B).
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