- Email: [email protected]
Enhanced electrochemical performance of electrospun V2O5 nanotubes as cathodes for lithium ion batteries
Accepted Manuscript Enhanced electrochemical performance of electrospun V2O5 nanotubes as cathodes for lithium ion batteries Yindan Liu, Dayong Guan, Guohua Gao, Xing Liang, Wei Sun, Kun Zhang, Wenchao Bi, Guangming Wu PII:
S0925-8388(17)32590-2
DOI:
10.1016/j.jallcom.2017.07.214
Reference:
JALCOM 42630
To appear in:
Journal of Alloys and Compounds
Received Date: 24 April 2017 Revised Date:
21 July 2017
Accepted Date: 22 July 2017
Please cite this article as: Y. Liu, D. Guan, G. Gao, X. Liang, W. Sun, K. Zhang, W. Bi, G. Wu, Enhanced electrochemical performance of electrospun V2O5 nanotubes as cathodes for lithium ion batteries, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.07.214. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT
Enhanced electrochemical performance of electrospun V2O5 nanotubes as cathodes for lithium ion batteries Yindan Liu, Dayong Guan, Guohua Gao*, Xing Liang, Wei Sun, Kun Zhang, Wenchao Bi and
RI PT
Guangming Wu*
Shanghai Key Laboratory of Special Artificial Microstructure materials and technology, School of
University, Shanghai, 200092, China.
M AN U
E-mail: [email protected]; wugm@ tongji.edu.cn
SC
Physics Science and Engineering, College of Electronics and Information Engineering, Tongji
ABSTRACT
Hollow porous vanadium pentoxide nanotubes (VNTs) have been fabricated by using electrospinning and successive sintering process. By controlling the calcination time, we synthesized the cathode made of the vanadium pentoxide with good electrochemical performance
TE D
and nanotubes morphology after sintering at 400 °C for 2 hours in air. The optimized VNTs electrode displays improved electrochemical performance with good specific discharge capacities, cycling durability (capacity retaining 72.5% at 100 cycles), and the improved high-rate performance (186 mA h g-1 at 1000 mA g-1). This can be attributed to the reasons following: The
EP
special nanostructure with pores and hollow morphology could shorten the lithium ion (Li+) diffusing distance, increase the electrochemical activity, provide higher specific surface areas to
AC C
enlarge the contact area between electrolyte and electrode, and endure the volume changes during Li-ions insertion and extraction process. This electrode can potentially provide as good cathode materials for improved electrochemical performance lithium ion batteries (LIBs). Keywords: Lithium ion batteries; Cathode materials; Electrospinnning; Vanadium oxide; Hollow porous nanostructure
1
ACCEPTED MANUSCRIPT
1. Introduction As the energy issues getting more and more serious, researchers are searching for alternative energies such as solar, ocean, wind, geothermal. Then the energy storage would need to be high-efficient and environmentally friendly. Among various energy storage systems, lithium ion
RI PT
batteries (LIBs) stand out due to their high energy density, long cycle life and excellent safety[1-3]. However, the energy and power density of the electrode materials are not qualified for the application in electrical vehicles (EVs) and hybrid electrical vehicles (HEVs)[4, 5]. Cathode, one essential part of the electrode, is of great importance for the LIBs performance[6, 7]. Therefore, it
SC
is critical to develop different cathode materials of LIBs to meet with the growing energy demand. Cathode materials have been studied for many years. And vanadium pentoxide (V2O5) has
M AN U
become one of the potential cathode materials because many advantages it owns: low cost, abundance[8, 9]. Besides, V2O5 has specially layered structure, which makes it a typical intercalation cathode materials[10, 11], and higher theoretical capacity (294 mA h g-1) than those of LiCoO2 (140 mA h g-1)[12], LiMn2O4 (148 mA h g-1)[13] and LiFeO4 (170 mA h g-1)[14]. All the values of the theoretical capacity above are corresponding to two lithium ions participating in the inter/deintercalation process. Nevertheless, intrinsic poor electronic conductivity, low Li+ diffusion coefficient, and irreversible structural changes upon deep charge and discharge, and
TE D
some other inherent defects of V2O5 would result in poor rate capability and severe capacity fading to limit their viability in practical LIBs applications[15-17]. One dimensional (1D) nanostructure with shorten Li+ diffusion pathway, increased contact
EP
area between the active material and the electrolyte, the reduced charge transfer resistance, and the structural stability which could improve the strain relaxation to withstand the volume expansion during Li+ insertion/extraction process[18-20]. Thus, materials with 1D nanostructure could solve
AC C
the problems of V2O5 applied on the LIBs. Porous hollow V2O5 nanotubes (VNTs) not only owns the advantages of 1D nanostructure, but also provides pores with different aperture and hollow structure. This structure can improve the penetration of electrolyte, increase the contact surface area between electrolyte and active material and accommodate the volume variations via additional void space during cycling[21-23]. Electrospinning is a convenient and versatile method to prepare ultralong hierarchical nanofibers with controllable lengths, diameters, compositions, and complex architectures such as core-shell, hollow, multilayer, aligned, or porous[24, 25]. Polymers[26], metals[27], ceramics[28], alloys[29], and composites[30] can all be formed into NFs. The principle is that the polymer 2
ACCEPTED MANUSCRIPT
SC
RI PT
solution is charged when the high voltage is on[25, 31, 32]. The charged solution exists mainly two kinds of forces: electrical force (Fe) and solution inherent forces (Fsi) including the surface tension and viscosity of the fluid. Structure of Talyor cone forms at the nozzle tip (Fe=Fsi)[33-35]. Then the jet is ejected from the tip of the nozzle (Fe>Fsi). The solvent begins to evaporate quickly and the jet is stretched further under the Fe. Finally, a solid nonwoven fiber mat is formed on the collector. Many factors can have influence on the properties of the prepared NFs: the inherent parameters of polymer solution, the voltage applied, solution feed rate, distance from the nozzle to the collector and the surrounding environment[31, 34, 36]. Electrospinning technique has many other applications except for LIBs, such as tissue regeneration, energy conversion and storage, water treatment and electrochemical capacitors, etc.[37, 38]. Thus, the electrospun products are playing a more and more important role in materials science and engineering. Therefore, electrospinning NFs are meaningful to improve the electrochemical performance of cathode materials for LIBs. Herein, we have fabricated the porous nanotubes by electrospinning. The functional V2O5 sol was the precursor material and followed the sintering process. Indeed, the as-prepared porous
M AN U
vanadium nanotubes (VNTs) exhibited good electrochemical performance: specific capacity retaining 72.5 % of original capacity (297 mA h g-1) for the VNTs sintered for two hours (VNTs-2). This can be explained by the hollow porous nanostructure which can mitigate the volume changes during cycling, shorten the transportation pathway of Li ions, and provide large contact area between electrolyte and active materials.
TE D
2. Experimental 2.1 Synthesis of the V2O5 sol
Firstly, we prepared the functional V2O5 sol composed of VOx oligomers based on our previous researches [39, 40]. Briefly, the producing process of electrospun NFs was as following:
EP
raw V2O5 powder, benzyl alcohol and isopropanol were mixed together at a molar ratio of 1:4:40 to get a suspension. Then, heating the obtained suspension at 110 °C under condensate reflux for
AC C
6 hours. Finally, the unreacted V2O5 was filtrated out to obtain the remaining yellowy sol which was concentrated by 2/3 volume during heating reflux. The heating reflux process would be about 2 hours and the V2O5 content wan about 40 mg mL-1. 2.2 Fabrication and characterization of VNTs We used the V2O5 sol to prepare the electrospinning precursor solution, in which the functional V2O5 sol, acetic acid and poly(vinylpyrrolidone) (PVP, Mw~ 1,300,000) were mixed together in a weight ratio of 10:7:3 and under vigorous stirring for one night to acquire a homogeneous green electrospinning precursor solution. The electrospinning process is as 3
ACCEPTED MANUSCRIPT illustrated in the Fig. 1. The precursor solution was loaded into a 10 mL syringe of the electrospinning and set up the system using a 23 G stainless needle. The high voltage and distance between the aluminum foil collector and needle were about 15 kV and 13 cm, respectively. The flow rate was set as 2 mL h-1. In the high electric field, the NFs were collected on the aluminum foil as shown in Fig. 1. To obtain the porous VNTs structure, the NFs were annealed in air for 1/3,
RI PT
1 and 2 hours with a heating rate of 1 °C/min at 400 °C and denoted as 1/3-V2O5, 1-V2O5 and 2-V2O5, respectively. 2.3 Material characterization
Morphological features of as-prepared V2O5 were studied using field-emission scanning
SC
electron microscope (FESEM, Philips-XL-30FEG), transmission electron microscopy (TEM, JEOL-1230). The weight loss was studied by thermogravimetric analysis (TGA). The crystal 1.5406 Å) between 10 - 60 o. 2.4 Electrochemical characterization
M AN U
phase structure was examined using RigakuD/Max-C diffractometer with Cu-Kα radiation (λ =
VNTs cathodes were prepared by mixing sintered VNTs, carbon black as a conducting agent and Poly(vinylidene fluride ) (PVDF, Sigma-Aldrich) as a binder at a weight ratio of 7:2:1 in N-Methylpyridine (NMP) solvent, which were uniformly pasted on aluminum foils after stirring
TE D
for one night. Cutting into disks (diameter 12 mm) after being dried in a vacuum at 120 oC for 8 h and the loaded materials on aluminum foils were ~ 1.4 mg. The cells were assembled in an argon-filled glove box with water and oxygen contents less than 1 ppm. The assembled cells were based on the configuration of vanadium oxide materials /electrolyte/Li with a liquid electrolyte (1
EP
M LiPF6 in ethylene carbonate (EC) / dimethyl carbonate (DMC) (volume ratio 1:1). A microporous membrane (Celgard 2500) was used as a separator and lithium metal as the counter electrode. Then the cellsare aged for 24 h before electrochemical measurement. Charge/discharge
AC C
and cyclic voltammetry (CV) tests were performed at a scanning rate of 0.1 mV s-1 and the test instruments were a LAND cell-testing system and CHI660C (Chenghua, Shanghai) electrochemical workstation within the potential range of 2 ~ 4 V vs. Li. Electrochemical impedance spectroscopy (EIS) measurements were performed to understand the electrochemical kinetics at frequencies between 100 kHz and 0.01 Hz using cycled cells at an AC signal of 5 mV. The high-rate performance was investigated at current densities of 100, 200, 500, 1000 mA g-1. The tests above were performed at the room temperature, and specific capacity were calculated based on the mass of active materials. 4
ACCEPTED MANUSCRIPT 3.
Results and discussion To investigate the morphologies and further examine the nanostructure of VNTs, SEM
measurements were carried out and as shown in the Fig.1. The SEM image reveals that the as-spun VNFs are 300 ~ 700 nm in diameter with the length 30 ~ 70 µm long. The morphology of the NFs displays a relatively smooth surface, no pores.
RI PT
To investigate the thermal decomposition of the as-spun NFs, TGA (Thermogravimetric Analysis) was performed as a reference of choosing sintering temperature (Fig. 2). As we can see, the NFs precursor undergoes significant weight-loss during the heat treatment process. The
SC
mass-loss below 200 °C could be attributed to the evaporation of moisture and residual solvent. There is a dramatic weight loss based on a broad exothermic peak near 260 °C in the DSC curve,
M AN U
corresponding to the decomposition of V2O5 sol and the degradation of PVP. PVP has two degradation mechanisms involved in both intra- and intermolecular-transfer reactions [41]. The second mass-loss is accompanied by an exothermic peak at 427 °C. This could be the oxidation of carbon and release of carbon monoxide by the decomposition of polymeric residuals. Until the temperature up to ~ 455 °C, PVP decomposes completely. Thus, we choose the 400 oC as the
TE D
calcinating temperature which could ensure the most PVP to be removed by varying the sintering time. Thus, the temperature of 400 °C might result in incomplete oxidation of PVP which would remain some residual carbon [42]. The residual carbon on the VNTs could be beneficial to improve the electrical conductivity of the materials [42-44]. These results could be confirmed by
EP
the TGA curves of three sintered samples: 1/3-V2O5, 1-V2O5, 2-V2O5 (Fig. 2b). As the sintering temperature raised, weight loss begin to increase until 650 °C. The decomposition of 2-V2O5 is
AC C
one-step process and has the most weight retention (97.2%) compared to the 1-V2O5 and the 1/3-V2O5 (95.3 % and 88.8 %, respectively). However, the decomposition of the 1-V2O5 and the 1/3-V2O5 have two steps, which could be that PVP isn’t degraded completely into carbon compared to that of the 2-V2O5 [10]. Subsequent sintering of these fibers at 400 oC (1 oC/min) was performed based on the analysis of TGA-DSC curves. To investigate the influence of sintering holding time on the morphology of the NFs and the crystallization of V2O5, we set a holding time gradient: 20 min, 1 hour, 2 hours. The corresponding SEM images are given in Fig.3. These SEM images revealed the 5
ACCEPTED MANUSCRIPT morphology changes from smooth, continuous NFs to hollow porous VNTs, which is similar to the papers reported [43, 45]. After annealing for 20 min, the average diameter is about 400 nm and the continuous 1D nanostructure is retained. But the 1/3-V2O5 shows the rough surface with some pores and has no obvious nanoparticles on it (Fig. 3a). However, the hollow structure has been formed as Fig. 3a inset shown, which means that the nanotube morphology is formed at this moment. The porosity could facilitate the electrolyte penetration and electrode materials activation,
RI PT
which could improve the electrochemical performance. Emerging hollow structure with bigger, more pores and bigger diameters of nanotubes, but V2O5 grain still small after sintering 1 hour, which means that more PVP decomposed and the NFs matrix is also made up of VOx particles (Fig. 3b and inset) [16]. After 2 hours sintering, the 2-V2O5 (Fig. 3c) shows the perfect hollow
SC
porous VNTs with bigger pores, which indicates PVP has been mostly removed. Thus, the morphology of nanostructure can be controlled by adjusting the holding time of the high temperature[46]. This might obtain higher specific surface areas to enlarge the contact area
M AN U
between electrode and electrolyte and facilitate the electrochemical activity of the cathode. TEM images also shows the detailed morphology and interior structure of the sintered VNTs, which is consistent with the SEM analysis: the thickness of nanotubes getting smaller, V2O5 grains growing bigger and more obvious hollow structure as the sintering time increasing (Fig. 4). Meantime, the hollow property is good for decreasing the Li-ion-diffusion pathway and
TE D
relieves the volume expansion during Li+ insertion/extraction process.
The VNTs formation process is shown in Fig. 4g. When the temperature rises to 400 oC slowly, the residual solvent is removed completely, PVP is oxidized into CO2, the VOx particles grow and diffuse outside of VNTs, simultaneously. Sintering for 20 min, the 1/3-V2O5 has no
EP
enough time to finish the process mentioned above and displays little pores, incomplete hollow structure and no obvious V2O5 grains. After one hour, the VNTs structure obviously appeared. After treatment for 2 hours, PVP has almost completely decomposed and nanoparticles have
AC C
grown big enough to connect to each other to form bigger pores and obvious hollow structure. The possible formation mechanism of VNTs as following: the VOx nanoparticles near the
surface of the as-spun NFs are oxidized and PVP was oxidized into CO2 during the annealing process. Therefore, there is a concentration gradient after the dealing process above [10, 47]. Then, the VOx particles inside the NFs would migrate to the surface. PVP decomposes into CO2 to help the formation of pores and also force the VOx particles to compress to form nonuniform V2O5 particles [10, 43]. Thus, there are two forces existing in the as-spun nanofibers. One is the swelling force formed by the evaporation of gas and the other is the contraction force of the 6
ACCEPTED MANUSCRIPT composite nanofibers due to the viscosity of the polymer. If the swelling force is larger than the shrinkage force, the nanotube structure could be obtained[36]. And the weight ratio of V : PVP is 0.56 : 1 which is close to the 0.61 : 1 studied by Shengjie Peng team[36]. Finally, the V2O5 grains connect to each other to form the hollow VNTs structure which has various pore size and nonuniform V2O5 particles. These results are consistent with the researches before [10, 11, 43]. The obtained VNTs are synthesized with high porosity and shorten Li+ diffusion pathway. These
RI PT
would be important to improve the electrochemical properties which show the advantages of the 1D hierarchical nanostructure.
As the analysis above shows, the products we prepared displayed a porous morphology and the porosity size is bigger with longer calcinating time. Therefore, to understand more clearly the
SC
porous properties of three samples, we tested the N2 adsorption/desorption isotherms [36, 37]. The result indicatesed that these samples belonged to the micro-macroporous materials. This could be
M AN U
supported by the inset figure, which shows that pore distribution is mainly centered in the range of 60 - 100 nm. Furthermore, the BET surface area and total pore volumes of 1/3-V2O5, 1-V2O5, 2-V2O5 are 12.9 m2 g-1, 16.5 m2 g-1 and 25.7 m2 g-1, respectively. Thus, the sample with more sintering time has larger surface area. Because that hollower nanostructure and bigger pore size formed. These results are consistent with the morphology analysis above, which are beneficial to increase the contact surface between electrode and electrolyte and shorten the transmission path of Li-ions. Finally, we can achieve the goal of improving the electrochemical properties by this
TE D
method.
The annealed samples with different heating time are characterized by XRD (X-ray diffraction) pattern to compare the crystalline-degree. As shown in Fig. 6, there is no much
EP
obvious difference in peak positions among the three samples, and no secondary phase is observed. All of the diffraction peaks of three samples are indexed to an orthorhombic phase V2O5 (space group: Pmmn (no.59); a = 11.515, b = 3.565, c = 4.372 Å; JCPDS card no.: 41-1426), agreeing
AC C
with the literatures [10, 48]. And the XRD pattern indicates that the V2O5 crystallization increases with heating time.
To investigate the electrochemical properties of the three prepared V2O5 electrodes, relevant
electrochemical measurements were carried out and given in Fig.7. Fig. 7a shows the second, third and fifth CV curves of the 1/3-V2O5. The 1/3-V2O5 has three obvious reduction peaks: 3.32 V, 3.10V, 2.14V (vs. Li/Li+). These potentials correspond to the phase transformations from α-V2O5 to ɛ-Li0.5V2O5, δ-Li0.5V2O5 and γ-Li0.5V2O5, respectively. The reaction formulas are as following [11]: 7
ACCEPTED MANUSCRIPT V2O5 + 0.5 Li+ + 0.5 e-
Li0.5V2O5
(1)
Li1.0V2O5
(2)
Li1.0V2O5 + 1.0 Li+ + 1.0 e-
Li2.0V2O5
(3) .
RI PT
Li0.5V2O5 + 0.5 Li+ + 0.5 e-
During the anodic scan, the three oxidization peaks appear at 2.58, 3.27, and 3.47 V owing to the extraction of the two Li ions, forming Li1.0V2O5, Li0.5V2O5 and V2O5, respectively [49]. Moreover, the reduction and oxidation peaks still obviously exist for following cycles. Fig. 7b, 7c shows CV curves of the 1-V2O5 and the 2-V2O5. Similarly, they have obvious reduction / oxidation peaks:
SC
3.33, 3.13, 2.20 / 2.57, 3.28, 3.47 V and 3.31, 3.10, 2.17 / 2.62, 3.34, 3.51 V for the 1-V2O5 and the 2-V2O5, respectively. This indicates that there are the corresponding phase transformations like
M AN U
that of the 1/3-V2O5. However, we can see that the current intensity of charge-discharge is higher for the 2-V2O5 than that of the 1-V2O5 and the 1/3-V2O5. It can be that the 2-V2O5 has better electric conductivity, which results from the carbon composed from PVP. The 1-V2O5 and the 1/3-V2O5 has more residual organics due to incompletely decomposition of PVP, which results in lower electric conductivity. This conclusion is consistent with the TGA analysis above. Therefore, the improved performance is mainly attributed to the residual carbon improving the electric conductivity and the hollow porous nanostructures providing shorten Li+ transport length, more
TE D
contact areas between electrode and electrolyte.
Fig. 8 shows galvanotactic discharge-charge profiles of half-cells (Li / VNTs) for the second, third and fifth cycle of the 1/3-V2O5, the 1-V2O5 and the 2-V2O5 at 1 C rate (1 C = 100
EP
mA g-1) between 2.0 and 4.0 V vs. Li. As Fig. 8a shows, the 1/3-V2O5 has three obvious discharge plateaus at ~3.3, ~3.2, and 2.2 V vs. Li, which corresponds to the phase transformations from α-V2O5 to ɛ-Li0.5V2O5, δ-Li0.5V2O5 and γ-Li0.5V2O5 [45, 50]. And the corresponding charge
AC C
process also has three stable plateaus which attribute to the extraction of Li+ previously intercalated into the cathode materials. The second discharge capacity can reach 225 mA h g-1 and lowers than the theoretical value of 294 mA h g-1. The third and fifth discharge capacities are 223 and 220 mA h g-1. As for the 1-V2O5 and the 2-V2O5, their charge-discharge curves are similar to the 1/3-V2O5. The multiple charge-discharge plateaus are obvious and can be retained relatively well after 5 cycles. The noticeable is that the discharge capacities of the 2-V2O5 reaches 297, 290, 285 mA h g-1 for the 2nd, the 3rd and the fifth discharge capacity, respectively. These values are higher than that of the 1/3-V2O5 and the 1-V2O5. The reasons may be: The bigger and more pores would shorten the transport distance of Li+ and increase the contact area between the electrode and 8
ACCEPTED MANUSCRIPT the electrolyte; As analyzed in the TGA, the 2-V2O5 has carbon with less residual organics of incomplete decomposition of PVP, which can improve the electric conductivity of cathode materials. Fig. 9a illustrates the plot of the three samples’ discharge capacity vs. cycle number obtained from the galvanostatic discharge/charge curves of the cells. They are also carried out
RI PT
between 2.0 and 4.0 V vs. Li at 1 C rate (100 mA g-1). The 2-V2O5 delivers an initial discharge capacity of 286 mA h g-1 which responds to insert 2 mol of lithium. The initial capacities of 1-V2O5 and 1/3-V2O5 are 236 and 221 mA h g-1, respectively. After 100 cycles, the capacity retention of 2-V2O5 is ~ 72.5%, which is higher than that of the other two samples (67.2% and 46.1% for 1-V2O5, 1/3-V2O5, respectively) and also higher than that of some papers reported [10,
SC
24]. The discharge capacity continues to be failing when the cells tested to 150 cycles from 100 cycles (Fig.9a inset). From the results, we can believe that the electrode material with better
M AN U
crystallinity and perfect nanotube morphology is key to obtain good capacity retention after the manifold cycles[46]. Varying constant C-rates performance (Fig. 9b) is carried out, which is an important prerequisite for cathode materials. The cells are tested at 1 C, 2 C, 5 C, and 10 C and then back to 1 C between 2.0 and 4.0 V. As we can see, the 2-V2O5 always shows better rate performance due to the higher discharge capacity than that of the 1/3-V2O5 and the 1-V2O5 at various current densities. The 2-V2O5 delivers discharge capacities of ~ 186 mA h g-1 which is higher than 180 and 155 mA h g-1 of the 1/3-V2O5 and the 1-V2O5 at 10 C, respectively. The
TE D
capacity degradation would be explained like the following reasons. The specific porous nanostructure has large contact area with electrolyte which might increase the dissolution or the reactivity of the material with the electrolyte[45, 51]. This makes three samples undergo obviously capacity degradation. However, the 1/3-V2O5 has more residual organic due to the incomplete
EP
decomposition which makes the electron conductivity and the cycling performance poor. Moreover, the residual organics and the calcinating time may have influence on the crystalline of
AC C
the V2O5 which may also result in the decrease of capacity[52]. Multiple cycles might make the shape changes of the nanostructures or slightly pulverization of original V2O5 nanaoparticles make the
nanotubes
damaged
and
unconnected
with
current
collector
during
the
Li+
intercalation/extraction process[53]. Together all the factors mentioned above, the 2-V2O5 has shown the best high-rate performance during the testing though the capacity decay happens. To ensure the retention of the porous nanostructures morphology after cycling, SEM was performed as Fig. 10 shows. Hence, the cycled cells were opened in an argon-filled glovebox/ Then, the electrode was washed with DEC and dried. As comparison, fresh prepared electrodes are tested to get corresponding SEM images in Fig.10. The shape of the three samples are 9
ACCEPTED MANUSCRIPT maintained well obviously. This can be attributed to the porous nanostructures which can effectively alleviate the nanostructure strain to prevent the structural collapse and the agglomeration or the self-assembly of the nanomaterials during the process of the intercalation/deintercalation of Li+. Thus, the nanostructure of the samples is relatively stable. Besides, it can reduce the distances of Li+ fluxing across the interface, help 1D electron transport along the longitudinal direction and increase the specific surface areas and the contact areas
RI PT
between electrolyte and electrode materials[11, 22, 23]. These are also the reasons of the 2-V2O5 with better electrochemical performance.
To further clarify the influence of sintering time toward cathode materials, electrochemical impedance spectroscopy (EIS) measurements (Fig. 11) were performed. From the figure, we can
SC
see that all the Nyquist plots are composed of a depressed semicircle and a slope. The depressed semicircle and the slope are in the region of high-to-medium frequencies and low frequencies,
M AN U
respectively. The former one is related to the charge transfer resistance (Rct). And the latter slope is associated with the Li+ diffusion inside electrodes. If the diameter of the semicircle is smaller, the value of Rct will be smaller [15, 54, 55]. A lower charge transfer resistance induces Li+ ion and electron to transfer more quickly. This would result in the enhancements in electrode reaction kinetics and obtaining high-rate performance. Thus, we can clearly know that the 2-V2O5 (295 Ω) possesses a much lower resistance than that of the 1/3-V2O5 (526 Ω) and the 1-V2O5 (407 Ω). This is originated from PVP’s inadequately decomposition, which can enhance the Rct [42, 43]. The
TE D
2-V2O5 possesses sloping straight line with a bigger phase angle of greater than 45 degrees to the real axis (Z') in the low frequency, which may be resulted from the hollow porous nanostructure of nanotubes benefitting for more Li+ diffusion inside electrodes. These results indicate the 2-V2O5
4. Conclusions
EP
has better rate performance due to it can afford more charges to transport with smaller resistance.
AC C
1D porous VNTs are synthesized via simple electrospinning technique taking VOx oligomers as the electrospinning precursor. Electrochemical tests indicate that electrodes composed of electrospun VNTs delivered good initial discharge capacity of 297 mA h g-1 which is close to the theoretical capacity and the capacity was retained 72.5% after 100 charge/discharge cycles for the 2-V2O5. The retentions of the 1-V2O5 and the 1/3-V2O5 are 62.5% and 46.5%, respectively. More importantly, this study demonstrates that the hollow porous nanostructure can effectively increase the electroactive surface area and decrease the diffusion pathway of Li+ which can enhance the electrochemical properties. And this kind electrode may upgrade the overall
10
ACCEPTED MANUSCRIPT efficiency of the LIBs. Thus, it can be also suitable for the synthesis of other electrochemical materials requiring various controlled morphologies by the facile technology. Acknowledgements The authors acknowledge the financial support of the financial support from the National Natural Science Foundation of China (U1503292, 51472182, 11404213), National Key Research
RI PT
and Development Program of China (2017YFA0204600) and Fundamental Research Funds for
SC
the Central Universities.
M AN U
References
[1] L. Mai, X. Tian, X. Xu, L. Chang, L. Xu, Nanowire Electrodes for Electrochemical Energy Storage Devices, Chem. Rev. 114 (2014‐ 11828-11862.
TE D
[2] L. Yu, H. Hu, H.B. Wu, X.W.D. Lou, Complex Hollow Nanostructures: Synthesis and Energy Related Applications, Adv. Mater. (2017‐ 1604563-1604602. [3] X.Y. Yu, L. Yu, X.W.D. Lou, Hollow Nanostructures of Molybdenum Sulfides for
EP
Electrochemical Energy Storage and Conversion, Small Methods 1 (2017‐ 1600020-1600028.
AC C
[4] K.T. Lee, J. Cho, Roles of nanosize in lithium reactive nanomaterials for lithium ion batteries, Nano Today 6 (2011‐ 28-41. [5] Z.-F. Li, H. Zhang, Q. Liu, Y. Liu, L. Stanciu, J. Xie, Hierarchical Nanocomposites of Vanadium Oxide Thin Film Anchored on Graphene as High-Performance Cathodes in Li-Ion Batteries, ACS Appl. Mater. Interfaces 6 (2014‐ 18894-18900.
11
ACCEPTED MANUSCRIPT [6] H. Li, Z. Wang, L. Chen, X. Huang, Research on Advanced Materials for Li ion Batteries, Adv. Mater. 21 (2009‐ 4593-4607. [7] W. Shi, X. Rui, J. Zhu, Q. Yan, Design of nanostructured hybrid materials based on carbon
RI PT
and metal oxides for li ion batteries, J. Phys. Chem. C 116 (2012‐ 26685-26693. [8] X. Wang, W. Jia, L. Wang, Y. Huang, Y. Guo, Y. Sun, D. Jia, W. Pang, Z. Guo, X. Tang,
Simple in situ synthesis of carbon-supported and nanosheet-assembled vanadium oxide for
SC
ultra-high rate anode and cathode materials of lithium ion batteries, J. Mater. Chem. 4 (2016‐
M AN U
13907-13915.
[9] Y. Song, T.Y. Liu, B. Yao, T.Y. Kou, D.Y. Feng, X.X. Liu, Y. Li, Amorphous Mixed Valence Vanadium Oxide/Exfoliated Carbon Cloth Structure Shows a Record High Cycling Stability, Small 13 (2017‐ 1700067-1700075.
TE D
[10] Z. Li, G. Liu, M. Guo, L.-X. Ding, S. Wang, H. Wang, Electrospun porous vanadium pentoxide nanotubes as a high-performance cathode material for lithium-ion batteries, Electrochim. Acta 173 (2015‐ 131-138.
EP
[11] B. Yan, X. Li, Z. Bai, M. Li, L. Dong, D. Xiong, D. Li, Superior lithium storage performance
AC C
of hierarchical porous vanadium pentoxide nanofibers for lithium ion battery cathodes, J. Alloys
Compd. 634 (2015‐ 50-57.
[12] S. Kalluri, M. Yoon, M. Jo, S. Park, S. Myeong, J. Kim, S.X. Dou, Z. Guo, J. Cho, Surface Engineering Strategies of Layered LiCoO2 Cathode Material to Realize High Energy and High Voltage Li Ion Cells, Adv. Energy Mater. 7 (2017‐ 1601507-1601528.
12
ACCEPTED MANUSCRIPT [13] F. Lai, X. Zhang, H. Wang, S. Hu, X. Wu, Q. Wu, Y. Huang, Z. He, Q. Li, Three-dimension hierarchical Al2O3 nanosheets wrapped LiMn2O4 with enhanced cycling stability as cathode material for lithium ion batteries, ACS Appl. Mater. Interfaces 8 (2016‐ 21656-21665.
RI PT
[14] A. Tron, Y.N. Jo, S.H. Oh, Y.D. Park, J. Mun, Surface modification of the LiFePO4 cathode for the aqueous rechargeable lithium ion battery, ACS Appl. Mater. Interfaces 9 (2017‐ 12391-12399.
SC
[15] X. Liang, G. Gao, Y. Liu, T. Zhang, G. Wu, Synthesis and characterization of Fe-doped
M AN U
vanadium oxide nanorods and their electrochemical performance, J. Alloys Compd. 715 (2017‐ 374-383.
[16] G.-H. An, D.-Y. Lee, H.-J. Ahn, Carbon-encapsulated hollow porous vanadium-oxide nanofibers for improved lithium storage properties, ACS Appl. Mater. Interfaces 8 (2016‐
TE D
19466-19474.
[17] X. Liang, G. Gao, G. Wu, Synthesis and characterization of hollow and core-shell structured V 2 O 5 microspheres and their electrochemical properties, J. Alloys Compd.
EP
(2017‐, doi:10.1016/j.jallcom.2017.06.295.
AC C
[18] J.S. Cho, J.-K. Lee, Y.C. Kang, Graphitic carbon-coated FeSe2 hollow nanosphere-decorated reduced graphene oxide hybrid nanofibers as an efficient anode material for sodium ion batteries, Sci. rep. 6 (2016‐ 23699, doi:10.1038/srep23699. [19] Q. Wei, F. Xiong, S. Tan, L. Huang, E.H. Lan, B. Dunn, L. Mai, Porous One Dimensional Nanomaterials: Design, Fabrication and Applications in Electrochemical Energy Storage, Adv. Mater. 29 (2017‐, doi:10.1002/adma.201602300. 13
ACCEPTED MANUSCRIPT [20] X. Liu, C. Chen, Y. Zhao, B. Jia, A review on the synthesis of manganese oxide nanomaterials and their applications on lithium-ion batteries, J. Nanomater. 2013 (2013‐. [21] Y. Chen, Y. Wang, H. Yang, H. Gan, X. Cai, X. Guo, B. Xu, M. Lü, A. Yuan, Facile
RI PT
synthesis of porous hollow Co3O4 microfibers derived-from metal-organic frameworks as an advanced anode for lithium ion batteries, Ceramics International 43 (2017‐ 9945-9950.
[22] D. Ma, Y. Li, P. Zhang, A.J. Cooper, A.M. Abdelkader, X. Ren, L. Deng, Mesoporous
SC
Li1.2Mn0.54Ni0.13Co0.13O2 nanotubes for high-performance cathodes in Li-ion batteries, J.
M AN U
Power Sources 311 (2016‐ 35-41.
[23] W. Lei, S. Liu, W.-H. Zhang, Porous hollow carbon nanofibers derived from multi-walled carbon nanotubes and sucrose as anode materials for lithium-ion batteries, RSC Adv. 7 (2017‐ 224-230.
TE D
[24] Y.L. Cheah, N. Gupta, S.S. Pramana, V. Aravindan, G. Wee, M. Srinivasan, Morphology, structure and electrochemical properties of single phase electrospun vanadium pentoxide nanofibers for lithium ion batteries, J. Power Sources 196 (2011‐ 6465-6472.
EP
[25] S. Cavaliere, S. Subianto, I. Savych, D.J. Jones, J. Rozière, Electrospinning: designed
AC C
architectures for energy conversion and storage devices, Energy Environ. Sci. 4 (2011‐ 4761-4785.
[26] T.D. Brown, P.D. Dalton, D.W. Hutmacher, Melt electrospinning today: An opportune time for an emerging polymer process, Prog. Polym. Sci. 56 (2016‐ 116-166.
14
ACCEPTED MANUSCRIPT [27] Z. Liu, R. Guo, J. Meng, X. Liu, X. Wang, Q. Li, L. Mai, Facile electrospinning formation of carbon-confined metal oxide cube-in-tube nanostructures for stable lithium storage, Chem. Commun. (2017‐, doi:10.1039/C7CC03727A.
RI PT
[28] J.-W. Liang, W.-Y. Wang, C.-L. Chung, Preparation of fluorescent ceramic nanofibers by electrospinning and heat treatment, Electronics Packaging (ICEP‐, 2017 International Conference on. IEEE (2017‐ 573-576, doi:10.23919/ICEP.2017.7939451.
SC
[29] J. Kim, H.M. Mousa, C.H. Park, C.S. Kim, Enhanced corrosion resistance and
M AN U
biocompatibility of AZ31 Mg alloy using PCL/ZnO NPs via electrospinning, Appl. Surf. Sci. 396 (2017‐ 249-258.
[30] C. Harito, R. Porras, D.V. Bavykin, F.C. Walsh, Electrospinning of in situ and ex situ synthesized polyimide composites reinforced by titanate nanotubes, J. Appl. Polym. Sci. 134
TE D
(2017‐.
[31] G. Sun, L. Sun, H. Xie, J. Liu, Electrospinning of nanofibers for energy applications, Nanomaterials 6 (2016‐ 129.
EP
[32] C.B.J. Manuel, V.G.L. Jesús, S.M. Aracely, Electrospinning for Drug Delivery Systems:
AC C
Drug Incorporation Techniques, Electrospinning-Material, Techniques, and Biomedical Applications, InTech. (2016‐,doi:10.5772/65939. [33] E.S. Pampal, E. Stojanovska, B. Simon, A. Kilic, A review of nanofibrous structures in lithium ion batteries, J. Power Sources 300 (2015‐ 199-215. [34] J.-W. Jung, C.-L. Lee, S. Yu, I.-D. Kim, Electrospun nanofibers as a platform for advanced secondary batteries: a comprehensive review, J. Mater. Chem. A 4 (2016‐ 703-750. 15
ACCEPTED MANUSCRIPT [35] W. Teo, R. Inai, S. Ramakrishna, Technological advances in electrospinning of nanofibers, Sci. Technol. Adv. Mater. 12 (2011‐ 013002. [36] L. Li, S. Peng, Y. Cheah, P. Teh, J. Wang, G. Wee, Y. Ko, C. Wong, M. Srinivasan,
capacitors, Chem. -Eur. J. 19 (2013‐ 5892-5898.
RI PT
Electrospun porous NiCo2O4 nanotubes as advanced electrodes for electrochemical
[37] S. Peng, L. Li, Y. Hu, M. Srinivasan, F. Cheng, J. Chen, S. Ramakrishna, Fabrication of
SC
Spinel One-Dimensional Architectures by Single-Spinneret Electrospinning for Energy Storage
M AN U
Applications, ACS Nano 9 (2015‐ 1945-1954.
[38] S. Peng, G. Jin, L. Li, K. Li, M. Srinivasan, S. Ramakrishna, J. Chen, Multi-functional electrospun nanofibres for advances in tissue regeneration, energy conversion & storage, and water treatment, Chem. Soc. Rev. 45 (2016‐ 1225-1241.
TE D
[39] X. Liang, G. Gao, G. Wu, H. Yang, Synthesis and characterization of novel hierarchical starfish-like vanadium oxide and their electrochemical performance, Electrochim. Acta 188 (2016‐ 625-635.
EP
[40] Y. Wu, G. Gao, G. Wu, Self-assembled three-dimensional hierarchical porous V 2 O
AC C
5/graphene hybrid aerogels for supercapacitors with high energy density and long cycle life, J. Mater. Chem. 3 (2015‐ 1828-1832. [41] S.J. Azhari, M.A. Diab, Thermal degradation and stability of poly(4-vinylpyridine‐ homopolymer and copolymers of 4-vinylpyridine with methyl acrylate, Polym. Degrad. Stabil. 60 (1998‐ 253-256.
16
ACCEPTED MANUSCRIPT [42] D. Yu, C. Chen, S. Xie, Y. Liu, K. Park, X. Zhou, Q. Zhang, J. Li, G. Cao, Mesoporous vanadium pentoxide nanofibers with significantly enhanced Li-ion storage properties by electrospinning Energy Environ. Sci. 4 (2011‐ 858-861.
RI PT
[43] H.G. Wang, D.L. Ma, Y. Huang, X.B. Zhang, Electrospun V2O5 nanostructures with controllable morphology as high-performance cathode materials for lithium-ion batteries, Chem. Eur. J. 18 (2012‐ 8987-8993.
SC
[44] Z. Wang, X. Liu, M. Lv, P. Chai, Y. Liu, J. Meng, Preparation of Ferrite MFe2O4 (M = Co,
M AN U
Ni‐ Ribbons with Nanoporous Structure and Their Magnetic Properties, J. Phys. Chem. B 112 (2008‐ 11292-11297.
[45] Y.L. Cheah, R. von Hagen, V. Aravindan, R. Fiz, S. Mathur, S. Madhavi, High-rate and elevated temperature performance of electrospun V2O5 nanofibers carbon-coated by plasma
TE D
enhanced chemical vapour deposition, Nano Energy 2 (2013‐ 57-64. [46] R. Ostermann, D. Li, Y. Yin, J.T. McCann, Y. Xia, V2O5 Nanorods on TiO2 Nanofibers: A New Class of Hierarchical Nanostructures Enabled by Electrospinning and Calcination, Nano
EP
Lett. 6 (2006‐ 1297-1302.
AC C
[47] X. Chen, K.M. Unruh, C. Ni, B. Ali, Z. Sun, Q. Lu, J. Deitzel, J.Q. Xiao, Fabrication, Formation Mechanism, and Magnetic Properties of Metal Oxide Nanotubes via Electrospinning and Thermal Treatment, J. Phys. Chem. C 115 (2011‐ 373-378. [48] K. Dewangan, N.N. Sinha, P.G. Chavan, P.K. Sharma, A.C. Pandey, M.A. More, D.S. Joag, N. Munichandraiah, N.S. Gajbhiye, Synthesis and characterization of self-assembled
17
ACCEPTED MANUSCRIPT nanofiber-bundles of V2O5: their electrochemical and field emission properties, Nanoscale 4 (2012‐ 645-651. [49] M. Sathiya, A.S. Prakash, K. Ramesha, J.M. Tarascon, A.K. Shukla, V2O5-anchored
RI PT
carbon nanotubes for enhanced electrochemical energy storage, J. Am. Chem. Soc. 133 (2011‐ 16291-16299.
[50] Y.L. Cheah, V. Aravindan, S. Madhavi, Improved Elevated Temperature Performance of
SC
Al-Intercalated V2O5 Electrospun Nanofibers for Lithium-Ion Batteries, ACS Appl. Mater.
M AN U
Interfaces 4 (2012‐ 3270-3277.
[51] J.J. Yu, J. Yang, W.B. Nie, Z.H. Li, E.H. Liu, G.T. Lei, Q.Z. Xiao, A porous vanadium pentoxide nanomaterial as cathode material for rechargeable lithium batteries, Electrochim. Acta 89 (2013‐ 292-299.
TE D
[52] L. Mai, L. Xu, C. Han, X. Xu, Y. Luo, S. Zhao, Y. Zhao, Electrospun Ultralong Hierarchical Vanadium Oxide Nanowires with High Performance for Lithium Ion Batteries, Nano Lett. 10 (2010‐ 4750-4755.
EP
[53] D. Chao, X. Xia, J. Liu, Z. Fan, C.F. Ng, J. Lin, H. Zhang, Z.X. Shen, H.J. Fan, A
AC C
V2O5/conductive-polymer core/shell nanobelt array on three-dimensional graphite foam: a high-rate, ultrastable, and freestanding cathode for lithium-ion batteries, Adv. Mater. 26 (2014‐ 5794-5800.
[54] Y. Liu, X. Zhou, Y. Guo, Effects of fluorine doping on the electrochemical properties of LiV3O8 cathode material, Electrochim. Acta 54 (2009‐ 3184-3190.
18
ACCEPTED MANUSCRIPT [55] C.J. Zhang, S.H. Park, S.E. O'Brien, A. Seral-Ascaso, M. Liang, D. Hanlon, D. Krishnan, A. Crossley, N. McEvoy, J.N. Coleman, Liquid Exfoliation of Interlayer Spacing-Tunable 2D Vanadium Oxide Nanosheets: High Capacity and Rate Handling Li-ion Battery Cathodes,
AC C
EP
TE D
M AN U
SC
RI PT
Nano Energy 39 (2017‐ 151-161.
19
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Fig. 1 Schematic illustration the process of electrospinning and the SEM image of the as-spun NFs. Fig.2 The thermos-gravimetric analyses (TGA) of (a) the as-spun nanofibers and (b) the sintered samples: 1/3-V2O5, 1-V2O5, 2-V2O5. Fig.3 FE-SEM images of sintered (a) 1/3-V2O5, (b) 1-V2O5, (c) 2-V2O5. Fig.4 TEM images of sintered (a-b) 1/3-V2O5, (c-d) 1-V2O5, (e-f) 2-V2O5, And (g) VNTs formation process through NFs post-calcianted at 400°C in air for 1/3 hour, 1hour, and 2 hours. Fig.5 N2 adsorption/desorption isotherms of the V2O5 nanomaterials with different sintering time. Inset is the corresponding pore size distribution. Fig.6 X-ray diffraction (XRD) patterns of the 1/3-V2O5, 1-V2O5, and 2-V2O5 were obtained at 400 °C for various holding time. Fig.7 Cyclic voltammograms of first five cycles of (a) 1/3-V2O5, (b) 1-V2O5 and (c) 2-V2O5 cells at a scan rate of 0.1 mV s-1 between 2 ~ 4 V vs. Li at 100 mA g-1, respectively. Fig.8 Galvanostatic charge-discharge profiles of first five cycles of (a) 1/3-V2O5, (b) 1-V2O5 and (c) 2-V2O5 cells between 2 ~ 4 V at 100 mA g-1, respectively. Fig.9 (a) Cycling performance of 2-V2O5, 1-V2O5 and 1/3-V2O5 cells at a current density of 100 mA g-1 between 2.0-4.0 V; (b) cycling of 2-V2O5, 1-V2O5 and 1/3-V2O5 cells at various C rates. All voltages are reported vs. Li/Li+. Fig.10 FE-SEM images of three VNTs samples ((a) 1/3-V2O5, (b) 1-V2O5, and (c) 2-V2O5) before and after electrochemical tests. Fig.11 Nyquist plots of all the three samples after 100 cycles at a current density of 1 A g-1.
1
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Fig.1
2
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Fig.2
3
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Fig.3
4
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Fig.4
5
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Fig.5
6
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Fig.6
7
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Fig.7
8
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Fig.8
9
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Fig.9
10
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Fig.10
11
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Fig.11
12
ACCEPTED MANUSCRIPT
Enhanced electrochemical performance of electrospun V2O5 nanotubes as cathodes for lithium ion batteries Yindan Liu, Dayong Guan, Guohua Gao*, Xing Liang, Wei Sun, Kun Zhang, Wenchao Bi and
RI PT
Guangming Wu*
Shanghai Key Laboratory of Special Artificial Microstructure materials and technology, School of
University, Shanghai, 200092, China.
M AN U
E-mail: [email protected]; wugm@ tongji.edu.cn
SC
Physics Science and Engineering, College of Electronics and Information Engineering, Tongji
Highlights
Hollow porous V2O5 nanotubes are prepared by electrospinning and sintering process.
V2O5 products with various morphology are obtained by different sintering duration.
The obtained V2O5 nanotubes display improved electrochemical performance.
AC C
EP
TE D
S0925-8388(17)32590-2
DOI:
10.1016/j.jallcom.2017.07.214
Reference:
JALCOM 42630
To appear in:
Journal of Alloys and Compounds
Received Date: 24 April 2017 Revised Date:
21 July 2017
Accepted Date: 22 July 2017
Please cite this article as: Y. Liu, D. Guan, G. Gao, X. Liang, W. Sun, K. Zhang, W. Bi, G. Wu, Enhanced electrochemical performance of electrospun V2O5 nanotubes as cathodes for lithium ion batteries, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.07.214. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT
Enhanced electrochemical performance of electrospun V2O5 nanotubes as cathodes for lithium ion batteries Yindan Liu, Dayong Guan, Guohua Gao*, Xing Liang, Wei Sun, Kun Zhang, Wenchao Bi and
RI PT
Guangming Wu*
Shanghai Key Laboratory of Special Artificial Microstructure materials and technology, School of
University, Shanghai, 200092, China.
M AN U
E-mail: [email protected]; wugm@ tongji.edu.cn
SC
Physics Science and Engineering, College of Electronics and Information Engineering, Tongji
ABSTRACT
Hollow porous vanadium pentoxide nanotubes (VNTs) have been fabricated by using electrospinning and successive sintering process. By controlling the calcination time, we synthesized the cathode made of the vanadium pentoxide with good electrochemical performance
TE D
and nanotubes morphology after sintering at 400 °C for 2 hours in air. The optimized VNTs electrode displays improved electrochemical performance with good specific discharge capacities, cycling durability (capacity retaining 72.5% at 100 cycles), and the improved high-rate performance (186 mA h g-1 at 1000 mA g-1). This can be attributed to the reasons following: The
EP
special nanostructure with pores and hollow morphology could shorten the lithium ion (Li+) diffusing distance, increase the electrochemical activity, provide higher specific surface areas to
AC C
enlarge the contact area between electrolyte and electrode, and endure the volume changes during Li-ions insertion and extraction process. This electrode can potentially provide as good cathode materials for improved electrochemical performance lithium ion batteries (LIBs). Keywords: Lithium ion batteries; Cathode materials; Electrospinnning; Vanadium oxide; Hollow porous nanostructure
1
ACCEPTED MANUSCRIPT
1. Introduction As the energy issues getting more and more serious, researchers are searching for alternative energies such as solar, ocean, wind, geothermal. Then the energy storage would need to be high-efficient and environmentally friendly. Among various energy storage systems, lithium ion
RI PT
batteries (LIBs) stand out due to their high energy density, long cycle life and excellent safety[1-3]. However, the energy and power density of the electrode materials are not qualified for the application in electrical vehicles (EVs) and hybrid electrical vehicles (HEVs)[4, 5]. Cathode, one essential part of the electrode, is of great importance for the LIBs performance[6, 7]. Therefore, it
SC
is critical to develop different cathode materials of LIBs to meet with the growing energy demand. Cathode materials have been studied for many years. And vanadium pentoxide (V2O5) has
M AN U
become one of the potential cathode materials because many advantages it owns: low cost, abundance[8, 9]. Besides, V2O5 has specially layered structure, which makes it a typical intercalation cathode materials[10, 11], and higher theoretical capacity (294 mA h g-1) than those of LiCoO2 (140 mA h g-1)[12], LiMn2O4 (148 mA h g-1)[13] and LiFeO4 (170 mA h g-1)[14]. All the values of the theoretical capacity above are corresponding to two lithium ions participating in the inter/deintercalation process. Nevertheless, intrinsic poor electronic conductivity, low Li+ diffusion coefficient, and irreversible structural changes upon deep charge and discharge, and
TE D
some other inherent defects of V2O5 would result in poor rate capability and severe capacity fading to limit their viability in practical LIBs applications[15-17]. One dimensional (1D) nanostructure with shorten Li+ diffusion pathway, increased contact
EP
area between the active material and the electrolyte, the reduced charge transfer resistance, and the structural stability which could improve the strain relaxation to withstand the volume expansion during Li+ insertion/extraction process[18-20]. Thus, materials with 1D nanostructure could solve
AC C
the problems of V2O5 applied on the LIBs. Porous hollow V2O5 nanotubes (VNTs) not only owns the advantages of 1D nanostructure, but also provides pores with different aperture and hollow structure. This structure can improve the penetration of electrolyte, increase the contact surface area between electrolyte and active material and accommodate the volume variations via additional void space during cycling[21-23]. Electrospinning is a convenient and versatile method to prepare ultralong hierarchical nanofibers with controllable lengths, diameters, compositions, and complex architectures such as core-shell, hollow, multilayer, aligned, or porous[24, 25]. Polymers[26], metals[27], ceramics[28], alloys[29], and composites[30] can all be formed into NFs. The principle is that the polymer 2
ACCEPTED MANUSCRIPT
SC
RI PT
solution is charged when the high voltage is on[25, 31, 32]. The charged solution exists mainly two kinds of forces: electrical force (Fe) and solution inherent forces (Fsi) including the surface tension and viscosity of the fluid. Structure of Talyor cone forms at the nozzle tip (Fe=Fsi)[33-35]. Then the jet is ejected from the tip of the nozzle (Fe>Fsi). The solvent begins to evaporate quickly and the jet is stretched further under the Fe. Finally, a solid nonwoven fiber mat is formed on the collector. Many factors can have influence on the properties of the prepared NFs: the inherent parameters of polymer solution, the voltage applied, solution feed rate, distance from the nozzle to the collector and the surrounding environment[31, 34, 36]. Electrospinning technique has many other applications except for LIBs, such as tissue regeneration, energy conversion and storage, water treatment and electrochemical capacitors, etc.[37, 38]. Thus, the electrospun products are playing a more and more important role in materials science and engineering. Therefore, electrospinning NFs are meaningful to improve the electrochemical performance of cathode materials for LIBs. Herein, we have fabricated the porous nanotubes by electrospinning. The functional V2O5 sol was the precursor material and followed the sintering process. Indeed, the as-prepared porous
M AN U
vanadium nanotubes (VNTs) exhibited good electrochemical performance: specific capacity retaining 72.5 % of original capacity (297 mA h g-1) for the VNTs sintered for two hours (VNTs-2). This can be explained by the hollow porous nanostructure which can mitigate the volume changes during cycling, shorten the transportation pathway of Li ions, and provide large contact area between electrolyte and active materials.
TE D
2. Experimental 2.1 Synthesis of the V2O5 sol
Firstly, we prepared the functional V2O5 sol composed of VOx oligomers based on our previous researches [39, 40]. Briefly, the producing process of electrospun NFs was as following:
EP
raw V2O5 powder, benzyl alcohol and isopropanol were mixed together at a molar ratio of 1:4:40 to get a suspension. Then, heating the obtained suspension at 110 °C under condensate reflux for
AC C
6 hours. Finally, the unreacted V2O5 was filtrated out to obtain the remaining yellowy sol which was concentrated by 2/3 volume during heating reflux. The heating reflux process would be about 2 hours and the V2O5 content wan about 40 mg mL-1. 2.2 Fabrication and characterization of VNTs We used the V2O5 sol to prepare the electrospinning precursor solution, in which the functional V2O5 sol, acetic acid and poly(vinylpyrrolidone) (PVP, Mw~ 1,300,000) were mixed together in a weight ratio of 10:7:3 and under vigorous stirring for one night to acquire a homogeneous green electrospinning precursor solution. The electrospinning process is as 3
ACCEPTED MANUSCRIPT illustrated in the Fig. 1. The precursor solution was loaded into a 10 mL syringe of the electrospinning and set up the system using a 23 G stainless needle. The high voltage and distance between the aluminum foil collector and needle were about 15 kV and 13 cm, respectively. The flow rate was set as 2 mL h-1. In the high electric field, the NFs were collected on the aluminum foil as shown in Fig. 1. To obtain the porous VNTs structure, the NFs were annealed in air for 1/3,
RI PT
1 and 2 hours with a heating rate of 1 °C/min at 400 °C and denoted as 1/3-V2O5, 1-V2O5 and 2-V2O5, respectively. 2.3 Material characterization
Morphological features of as-prepared V2O5 were studied using field-emission scanning
SC
electron microscope (FESEM, Philips-XL-30FEG), transmission electron microscopy (TEM, JEOL-1230). The weight loss was studied by thermogravimetric analysis (TGA). The crystal 1.5406 Å) between 10 - 60 o. 2.4 Electrochemical characterization
M AN U
phase structure was examined using RigakuD/Max-C diffractometer with Cu-Kα radiation (λ =
VNTs cathodes were prepared by mixing sintered VNTs, carbon black as a conducting agent and Poly(vinylidene fluride ) (PVDF, Sigma-Aldrich) as a binder at a weight ratio of 7:2:1 in N-Methylpyridine (NMP) solvent, which were uniformly pasted on aluminum foils after stirring
TE D
for one night. Cutting into disks (diameter 12 mm) after being dried in a vacuum at 120 oC for 8 h and the loaded materials on aluminum foils were ~ 1.4 mg. The cells were assembled in an argon-filled glove box with water and oxygen contents less than 1 ppm. The assembled cells were based on the configuration of vanadium oxide materials /electrolyte/Li with a liquid electrolyte (1
EP
M LiPF6 in ethylene carbonate (EC) / dimethyl carbonate (DMC) (volume ratio 1:1). A microporous membrane (Celgard 2500) was used as a separator and lithium metal as the counter electrode. Then the cellsare aged for 24 h before electrochemical measurement. Charge/discharge
AC C
and cyclic voltammetry (CV) tests were performed at a scanning rate of 0.1 mV s-1 and the test instruments were a LAND cell-testing system and CHI660C (Chenghua, Shanghai) electrochemical workstation within the potential range of 2 ~ 4 V vs. Li. Electrochemical impedance spectroscopy (EIS) measurements were performed to understand the electrochemical kinetics at frequencies between 100 kHz and 0.01 Hz using cycled cells at an AC signal of 5 mV. The high-rate performance was investigated at current densities of 100, 200, 500, 1000 mA g-1. The tests above were performed at the room temperature, and specific capacity were calculated based on the mass of active materials. 4
ACCEPTED MANUSCRIPT 3.
Results and discussion To investigate the morphologies and further examine the nanostructure of VNTs, SEM
measurements were carried out and as shown in the Fig.1. The SEM image reveals that the as-spun VNFs are 300 ~ 700 nm in diameter with the length 30 ~ 70 µm long. The morphology of the NFs displays a relatively smooth surface, no pores.
RI PT
To investigate the thermal decomposition of the as-spun NFs, TGA (Thermogravimetric Analysis) was performed as a reference of choosing sintering temperature (Fig. 2). As we can see, the NFs precursor undergoes significant weight-loss during the heat treatment process. The
SC
mass-loss below 200 °C could be attributed to the evaporation of moisture and residual solvent. There is a dramatic weight loss based on a broad exothermic peak near 260 °C in the DSC curve,
M AN U
corresponding to the decomposition of V2O5 sol and the degradation of PVP. PVP has two degradation mechanisms involved in both intra- and intermolecular-transfer reactions [41]. The second mass-loss is accompanied by an exothermic peak at 427 °C. This could be the oxidation of carbon and release of carbon monoxide by the decomposition of polymeric residuals. Until the temperature up to ~ 455 °C, PVP decomposes completely. Thus, we choose the 400 oC as the
TE D
calcinating temperature which could ensure the most PVP to be removed by varying the sintering time. Thus, the temperature of 400 °C might result in incomplete oxidation of PVP which would remain some residual carbon [42]. The residual carbon on the VNTs could be beneficial to improve the electrical conductivity of the materials [42-44]. These results could be confirmed by
EP
the TGA curves of three sintered samples: 1/3-V2O5, 1-V2O5, 2-V2O5 (Fig. 2b). As the sintering temperature raised, weight loss begin to increase until 650 °C. The decomposition of 2-V2O5 is
AC C
one-step process and has the most weight retention (97.2%) compared to the 1-V2O5 and the 1/3-V2O5 (95.3 % and 88.8 %, respectively). However, the decomposition of the 1-V2O5 and the 1/3-V2O5 have two steps, which could be that PVP isn’t degraded completely into carbon compared to that of the 2-V2O5 [10]. Subsequent sintering of these fibers at 400 oC (1 oC/min) was performed based on the analysis of TGA-DSC curves. To investigate the influence of sintering holding time on the morphology of the NFs and the crystallization of V2O5, we set a holding time gradient: 20 min, 1 hour, 2 hours. The corresponding SEM images are given in Fig.3. These SEM images revealed the 5
ACCEPTED MANUSCRIPT morphology changes from smooth, continuous NFs to hollow porous VNTs, which is similar to the papers reported [43, 45]. After annealing for 20 min, the average diameter is about 400 nm and the continuous 1D nanostructure is retained. But the 1/3-V2O5 shows the rough surface with some pores and has no obvious nanoparticles on it (Fig. 3a). However, the hollow structure has been formed as Fig. 3a inset shown, which means that the nanotube morphology is formed at this moment. The porosity could facilitate the electrolyte penetration and electrode materials activation,
RI PT
which could improve the electrochemical performance. Emerging hollow structure with bigger, more pores and bigger diameters of nanotubes, but V2O5 grain still small after sintering 1 hour, which means that more PVP decomposed and the NFs matrix is also made up of VOx particles (Fig. 3b and inset) [16]. After 2 hours sintering, the 2-V2O5 (Fig. 3c) shows the perfect hollow
SC
porous VNTs with bigger pores, which indicates PVP has been mostly removed. Thus, the morphology of nanostructure can be controlled by adjusting the holding time of the high temperature[46]. This might obtain higher specific surface areas to enlarge the contact area
M AN U
between electrode and electrolyte and facilitate the electrochemical activity of the cathode. TEM images also shows the detailed morphology and interior structure of the sintered VNTs, which is consistent with the SEM analysis: the thickness of nanotubes getting smaller, V2O5 grains growing bigger and more obvious hollow structure as the sintering time increasing (Fig. 4). Meantime, the hollow property is good for decreasing the Li-ion-diffusion pathway and
TE D
relieves the volume expansion during Li+ insertion/extraction process.
The VNTs formation process is shown in Fig. 4g. When the temperature rises to 400 oC slowly, the residual solvent is removed completely, PVP is oxidized into CO2, the VOx particles grow and diffuse outside of VNTs, simultaneously. Sintering for 20 min, the 1/3-V2O5 has no
EP
enough time to finish the process mentioned above and displays little pores, incomplete hollow structure and no obvious V2O5 grains. After one hour, the VNTs structure obviously appeared. After treatment for 2 hours, PVP has almost completely decomposed and nanoparticles have
AC C
grown big enough to connect to each other to form bigger pores and obvious hollow structure. The possible formation mechanism of VNTs as following: the VOx nanoparticles near the
surface of the as-spun NFs are oxidized and PVP was oxidized into CO2 during the annealing process. Therefore, there is a concentration gradient after the dealing process above [10, 47]. Then, the VOx particles inside the NFs would migrate to the surface. PVP decomposes into CO2 to help the formation of pores and also force the VOx particles to compress to form nonuniform V2O5 particles [10, 43]. Thus, there are two forces existing in the as-spun nanofibers. One is the swelling force formed by the evaporation of gas and the other is the contraction force of the 6
ACCEPTED MANUSCRIPT composite nanofibers due to the viscosity of the polymer. If the swelling force is larger than the shrinkage force, the nanotube structure could be obtained[36]. And the weight ratio of V : PVP is 0.56 : 1 which is close to the 0.61 : 1 studied by Shengjie Peng team[36]. Finally, the V2O5 grains connect to each other to form the hollow VNTs structure which has various pore size and nonuniform V2O5 particles. These results are consistent with the researches before [10, 11, 43]. The obtained VNTs are synthesized with high porosity and shorten Li+ diffusion pathway. These
RI PT
would be important to improve the electrochemical properties which show the advantages of the 1D hierarchical nanostructure.
As the analysis above shows, the products we prepared displayed a porous morphology and the porosity size is bigger with longer calcinating time. Therefore, to understand more clearly the
SC
porous properties of three samples, we tested the N2 adsorption/desorption isotherms [36, 37]. The result indicatesed that these samples belonged to the micro-macroporous materials. This could be
M AN U
supported by the inset figure, which shows that pore distribution is mainly centered in the range of 60 - 100 nm. Furthermore, the BET surface area and total pore volumes of 1/3-V2O5, 1-V2O5, 2-V2O5 are 12.9 m2 g-1, 16.5 m2 g-1 and 25.7 m2 g-1, respectively. Thus, the sample with more sintering time has larger surface area. Because that hollower nanostructure and bigger pore size formed. These results are consistent with the morphology analysis above, which are beneficial to increase the contact surface between electrode and electrolyte and shorten the transmission path of Li-ions. Finally, we can achieve the goal of improving the electrochemical properties by this
TE D
method.
The annealed samples with different heating time are characterized by XRD (X-ray diffraction) pattern to compare the crystalline-degree. As shown in Fig. 6, there is no much
EP
obvious difference in peak positions among the three samples, and no secondary phase is observed. All of the diffraction peaks of three samples are indexed to an orthorhombic phase V2O5 (space group: Pmmn (no.59); a = 11.515, b = 3.565, c = 4.372 Å; JCPDS card no.: 41-1426), agreeing
AC C
with the literatures [10, 48]. And the XRD pattern indicates that the V2O5 crystallization increases with heating time.
To investigate the electrochemical properties of the three prepared V2O5 electrodes, relevant
electrochemical measurements were carried out and given in Fig.7. Fig. 7a shows the second, third and fifth CV curves of the 1/3-V2O5. The 1/3-V2O5 has three obvious reduction peaks: 3.32 V, 3.10V, 2.14V (vs. Li/Li+). These potentials correspond to the phase transformations from α-V2O5 to ɛ-Li0.5V2O5, δ-Li0.5V2O5 and γ-Li0.5V2O5, respectively. The reaction formulas are as following [11]: 7
ACCEPTED MANUSCRIPT V2O5 + 0.5 Li+ + 0.5 e-
Li0.5V2O5
(1)
Li1.0V2O5
(2)
Li1.0V2O5 + 1.0 Li+ + 1.0 e-
Li2.0V2O5
(3) .
RI PT
Li0.5V2O5 + 0.5 Li+ + 0.5 e-
During the anodic scan, the three oxidization peaks appear at 2.58, 3.27, and 3.47 V owing to the extraction of the two Li ions, forming Li1.0V2O5, Li0.5V2O5 and V2O5, respectively [49]. Moreover, the reduction and oxidation peaks still obviously exist for following cycles. Fig. 7b, 7c shows CV curves of the 1-V2O5 and the 2-V2O5. Similarly, they have obvious reduction / oxidation peaks:
SC
3.33, 3.13, 2.20 / 2.57, 3.28, 3.47 V and 3.31, 3.10, 2.17 / 2.62, 3.34, 3.51 V for the 1-V2O5 and the 2-V2O5, respectively. This indicates that there are the corresponding phase transformations like
M AN U
that of the 1/3-V2O5. However, we can see that the current intensity of charge-discharge is higher for the 2-V2O5 than that of the 1-V2O5 and the 1/3-V2O5. It can be that the 2-V2O5 has better electric conductivity, which results from the carbon composed from PVP. The 1-V2O5 and the 1/3-V2O5 has more residual organics due to incompletely decomposition of PVP, which results in lower electric conductivity. This conclusion is consistent with the TGA analysis above. Therefore, the improved performance is mainly attributed to the residual carbon improving the electric conductivity and the hollow porous nanostructures providing shorten Li+ transport length, more
TE D
contact areas between electrode and electrolyte.
Fig. 8 shows galvanotactic discharge-charge profiles of half-cells (Li / VNTs) for the second, third and fifth cycle of the 1/3-V2O5, the 1-V2O5 and the 2-V2O5 at 1 C rate (1 C = 100
EP
mA g-1) between 2.0 and 4.0 V vs. Li. As Fig. 8a shows, the 1/3-V2O5 has three obvious discharge plateaus at ~3.3, ~3.2, and 2.2 V vs. Li, which corresponds to the phase transformations from α-V2O5 to ɛ-Li0.5V2O5, δ-Li0.5V2O5 and γ-Li0.5V2O5 [45, 50]. And the corresponding charge
AC C
process also has three stable plateaus which attribute to the extraction of Li+ previously intercalated into the cathode materials. The second discharge capacity can reach 225 mA h g-1 and lowers than the theoretical value of 294 mA h g-1. The third and fifth discharge capacities are 223 and 220 mA h g-1. As for the 1-V2O5 and the 2-V2O5, their charge-discharge curves are similar to the 1/3-V2O5. The multiple charge-discharge plateaus are obvious and can be retained relatively well after 5 cycles. The noticeable is that the discharge capacities of the 2-V2O5 reaches 297, 290, 285 mA h g-1 for the 2nd, the 3rd and the fifth discharge capacity, respectively. These values are higher than that of the 1/3-V2O5 and the 1-V2O5. The reasons may be: The bigger and more pores would shorten the transport distance of Li+ and increase the contact area between the electrode and 8
ACCEPTED MANUSCRIPT the electrolyte; As analyzed in the TGA, the 2-V2O5 has carbon with less residual organics of incomplete decomposition of PVP, which can improve the electric conductivity of cathode materials. Fig. 9a illustrates the plot of the three samples’ discharge capacity vs. cycle number obtained from the galvanostatic discharge/charge curves of the cells. They are also carried out
RI PT
between 2.0 and 4.0 V vs. Li at 1 C rate (100 mA g-1). The 2-V2O5 delivers an initial discharge capacity of 286 mA h g-1 which responds to insert 2 mol of lithium. The initial capacities of 1-V2O5 and 1/3-V2O5 are 236 and 221 mA h g-1, respectively. After 100 cycles, the capacity retention of 2-V2O5 is ~ 72.5%, which is higher than that of the other two samples (67.2% and 46.1% for 1-V2O5, 1/3-V2O5, respectively) and also higher than that of some papers reported [10,
SC
24]. The discharge capacity continues to be failing when the cells tested to 150 cycles from 100 cycles (Fig.9a inset). From the results, we can believe that the electrode material with better
M AN U
crystallinity and perfect nanotube morphology is key to obtain good capacity retention after the manifold cycles[46]. Varying constant C-rates performance (Fig. 9b) is carried out, which is an important prerequisite for cathode materials. The cells are tested at 1 C, 2 C, 5 C, and 10 C and then back to 1 C between 2.0 and 4.0 V. As we can see, the 2-V2O5 always shows better rate performance due to the higher discharge capacity than that of the 1/3-V2O5 and the 1-V2O5 at various current densities. The 2-V2O5 delivers discharge capacities of ~ 186 mA h g-1 which is higher than 180 and 155 mA h g-1 of the 1/3-V2O5 and the 1-V2O5 at 10 C, respectively. The
TE D
capacity degradation would be explained like the following reasons. The specific porous nanostructure has large contact area with electrolyte which might increase the dissolution or the reactivity of the material with the electrolyte[45, 51]. This makes three samples undergo obviously capacity degradation. However, the 1/3-V2O5 has more residual organic due to the incomplete
EP
decomposition which makes the electron conductivity and the cycling performance poor. Moreover, the residual organics and the calcinating time may have influence on the crystalline of
AC C
the V2O5 which may also result in the decrease of capacity[52]. Multiple cycles might make the shape changes of the nanostructures or slightly pulverization of original V2O5 nanaoparticles make the
nanotubes
damaged
and
unconnected
with
current
collector
during
the
Li+
intercalation/extraction process[53]. Together all the factors mentioned above, the 2-V2O5 has shown the best high-rate performance during the testing though the capacity decay happens. To ensure the retention of the porous nanostructures morphology after cycling, SEM was performed as Fig. 10 shows. Hence, the cycled cells were opened in an argon-filled glovebox/ Then, the electrode was washed with DEC and dried. As comparison, fresh prepared electrodes are tested to get corresponding SEM images in Fig.10. The shape of the three samples are 9
ACCEPTED MANUSCRIPT maintained well obviously. This can be attributed to the porous nanostructures which can effectively alleviate the nanostructure strain to prevent the structural collapse and the agglomeration or the self-assembly of the nanomaterials during the process of the intercalation/deintercalation of Li+. Thus, the nanostructure of the samples is relatively stable. Besides, it can reduce the distances of Li+ fluxing across the interface, help 1D electron transport along the longitudinal direction and increase the specific surface areas and the contact areas
RI PT
between electrolyte and electrode materials[11, 22, 23]. These are also the reasons of the 2-V2O5 with better electrochemical performance.
To further clarify the influence of sintering time toward cathode materials, electrochemical impedance spectroscopy (EIS) measurements (Fig. 11) were performed. From the figure, we can
SC
see that all the Nyquist plots are composed of a depressed semicircle and a slope. The depressed semicircle and the slope are in the region of high-to-medium frequencies and low frequencies,
M AN U
respectively. The former one is related to the charge transfer resistance (Rct). And the latter slope is associated with the Li+ diffusion inside electrodes. If the diameter of the semicircle is smaller, the value of Rct will be smaller [15, 54, 55]. A lower charge transfer resistance induces Li+ ion and electron to transfer more quickly. This would result in the enhancements in electrode reaction kinetics and obtaining high-rate performance. Thus, we can clearly know that the 2-V2O5 (295 Ω) possesses a much lower resistance than that of the 1/3-V2O5 (526 Ω) and the 1-V2O5 (407 Ω). This is originated from PVP’s inadequately decomposition, which can enhance the Rct [42, 43]. The
TE D
2-V2O5 possesses sloping straight line with a bigger phase angle of greater than 45 degrees to the real axis (Z') in the low frequency, which may be resulted from the hollow porous nanostructure of nanotubes benefitting for more Li+ diffusion inside electrodes. These results indicate the 2-V2O5
4. Conclusions
EP
has better rate performance due to it can afford more charges to transport with smaller resistance.
AC C
1D porous VNTs are synthesized via simple electrospinning technique taking VOx oligomers as the electrospinning precursor. Electrochemical tests indicate that electrodes composed of electrospun VNTs delivered good initial discharge capacity of 297 mA h g-1 which is close to the theoretical capacity and the capacity was retained 72.5% after 100 charge/discharge cycles for the 2-V2O5. The retentions of the 1-V2O5 and the 1/3-V2O5 are 62.5% and 46.5%, respectively. More importantly, this study demonstrates that the hollow porous nanostructure can effectively increase the electroactive surface area and decrease the diffusion pathway of Li+ which can enhance the electrochemical properties. And this kind electrode may upgrade the overall
10
ACCEPTED MANUSCRIPT efficiency of the LIBs. Thus, it can be also suitable for the synthesis of other electrochemical materials requiring various controlled morphologies by the facile technology. Acknowledgements The authors acknowledge the financial support of the financial support from the National Natural Science Foundation of China (U1503292, 51472182, 11404213), National Key Research
RI PT
and Development Program of China (2017YFA0204600) and Fundamental Research Funds for
SC
the Central Universities.
M AN U
References
[1] L. Mai, X. Tian, X. Xu, L. Chang, L. Xu, Nanowire Electrodes for Electrochemical Energy Storage Devices, Chem. Rev. 114 (2014‐ 11828-11862.
TE D
[2] L. Yu, H. Hu, H.B. Wu, X.W.D. Lou, Complex Hollow Nanostructures: Synthesis and Energy Related Applications, Adv. Mater. (2017‐ 1604563-1604602. [3] X.Y. Yu, L. Yu, X.W.D. Lou, Hollow Nanostructures of Molybdenum Sulfides for
EP
Electrochemical Energy Storage and Conversion, Small Methods 1 (2017‐ 1600020-1600028.
AC C
[4] K.T. Lee, J. Cho, Roles of nanosize in lithium reactive nanomaterials for lithium ion batteries, Nano Today 6 (2011‐ 28-41. [5] Z.-F. Li, H. Zhang, Q. Liu, Y. Liu, L. Stanciu, J. Xie, Hierarchical Nanocomposites of Vanadium Oxide Thin Film Anchored on Graphene as High-Performance Cathodes in Li-Ion Batteries, ACS Appl. Mater. Interfaces 6 (2014‐ 18894-18900.
11
ACCEPTED MANUSCRIPT [6] H. Li, Z. Wang, L. Chen, X. Huang, Research on Advanced Materials for Li ion Batteries, Adv. Mater. 21 (2009‐ 4593-4607. [7] W. Shi, X. Rui, J. Zhu, Q. Yan, Design of nanostructured hybrid materials based on carbon
RI PT
and metal oxides for li ion batteries, J. Phys. Chem. C 116 (2012‐ 26685-26693. [8] X. Wang, W. Jia, L. Wang, Y. Huang, Y. Guo, Y. Sun, D. Jia, W. Pang, Z. Guo, X. Tang,
Simple in situ synthesis of carbon-supported and nanosheet-assembled vanadium oxide for
SC
ultra-high rate anode and cathode materials of lithium ion batteries, J. Mater. Chem. 4 (2016‐
M AN U
13907-13915.
[9] Y. Song, T.Y. Liu, B. Yao, T.Y. Kou, D.Y. Feng, X.X. Liu, Y. Li, Amorphous Mixed Valence Vanadium Oxide/Exfoliated Carbon Cloth Structure Shows a Record High Cycling Stability, Small 13 (2017‐ 1700067-1700075.
TE D
[10] Z. Li, G. Liu, M. Guo, L.-X. Ding, S. Wang, H. Wang, Electrospun porous vanadium pentoxide nanotubes as a high-performance cathode material for lithium-ion batteries, Electrochim. Acta 173 (2015‐ 131-138.
EP
[11] B. Yan, X. Li, Z. Bai, M. Li, L. Dong, D. Xiong, D. Li, Superior lithium storage performance
AC C
of hierarchical porous vanadium pentoxide nanofibers for lithium ion battery cathodes, J. Alloys
Compd. 634 (2015‐ 50-57.
[12] S. Kalluri, M. Yoon, M. Jo, S. Park, S. Myeong, J. Kim, S.X. Dou, Z. Guo, J. Cho, Surface Engineering Strategies of Layered LiCoO2 Cathode Material to Realize High Energy and High Voltage Li Ion Cells, Adv. Energy Mater. 7 (2017‐ 1601507-1601528.
12
ACCEPTED MANUSCRIPT [13] F. Lai, X. Zhang, H. Wang, S. Hu, X. Wu, Q. Wu, Y. Huang, Z. He, Q. Li, Three-dimension hierarchical Al2O3 nanosheets wrapped LiMn2O4 with enhanced cycling stability as cathode material for lithium ion batteries, ACS Appl. Mater. Interfaces 8 (2016‐ 21656-21665.
RI PT
[14] A. Tron, Y.N. Jo, S.H. Oh, Y.D. Park, J. Mun, Surface modification of the LiFePO4 cathode for the aqueous rechargeable lithium ion battery, ACS Appl. Mater. Interfaces 9 (2017‐ 12391-12399.
SC
[15] X. Liang, G. Gao, Y. Liu, T. Zhang, G. Wu, Synthesis and characterization of Fe-doped
M AN U
vanadium oxide nanorods and their electrochemical performance, J. Alloys Compd. 715 (2017‐ 374-383.
[16] G.-H. An, D.-Y. Lee, H.-J. Ahn, Carbon-encapsulated hollow porous vanadium-oxide nanofibers for improved lithium storage properties, ACS Appl. Mater. Interfaces 8 (2016‐
TE D
19466-19474.
[17] X. Liang, G. Gao, G. Wu, Synthesis and characterization of hollow and core-shell structured V 2 O 5 microspheres and their electrochemical properties, J. Alloys Compd.
EP
(2017‐, doi:10.1016/j.jallcom.2017.06.295.
AC C
[18] J.S. Cho, J.-K. Lee, Y.C. Kang, Graphitic carbon-coated FeSe2 hollow nanosphere-decorated reduced graphene oxide hybrid nanofibers as an efficient anode material for sodium ion batteries, Sci. rep. 6 (2016‐ 23699, doi:10.1038/srep23699. [19] Q. Wei, F. Xiong, S. Tan, L. Huang, E.H. Lan, B. Dunn, L. Mai, Porous One Dimensional Nanomaterials: Design, Fabrication and Applications in Electrochemical Energy Storage, Adv. Mater. 29 (2017‐, doi:10.1002/adma.201602300. 13
ACCEPTED MANUSCRIPT [20] X. Liu, C. Chen, Y. Zhao, B. Jia, A review on the synthesis of manganese oxide nanomaterials and their applications on lithium-ion batteries, J. Nanomater. 2013 (2013‐. [21] Y. Chen, Y. Wang, H. Yang, H. Gan, X. Cai, X. Guo, B. Xu, M. Lü, A. Yuan, Facile
RI PT
synthesis of porous hollow Co3O4 microfibers derived-from metal-organic frameworks as an advanced anode for lithium ion batteries, Ceramics International 43 (2017‐ 9945-9950.
[22] D. Ma, Y. Li, P. Zhang, A.J. Cooper, A.M. Abdelkader, X. Ren, L. Deng, Mesoporous
SC
Li1.2Mn0.54Ni0.13Co0.13O2 nanotubes for high-performance cathodes in Li-ion batteries, J.
M AN U
Power Sources 311 (2016‐ 35-41.
[23] W. Lei, S. Liu, W.-H. Zhang, Porous hollow carbon nanofibers derived from multi-walled carbon nanotubes and sucrose as anode materials for lithium-ion batteries, RSC Adv. 7 (2017‐ 224-230.
TE D
[24] Y.L. Cheah, N. Gupta, S.S. Pramana, V. Aravindan, G. Wee, M. Srinivasan, Morphology, structure and electrochemical properties of single phase electrospun vanadium pentoxide nanofibers for lithium ion batteries, J. Power Sources 196 (2011‐ 6465-6472.
EP
[25] S. Cavaliere, S. Subianto, I. Savych, D.J. Jones, J. Rozière, Electrospinning: designed
AC C
architectures for energy conversion and storage devices, Energy Environ. Sci. 4 (2011‐ 4761-4785.
[26] T.D. Brown, P.D. Dalton, D.W. Hutmacher, Melt electrospinning today: An opportune time for an emerging polymer process, Prog. Polym. Sci. 56 (2016‐ 116-166.
14
ACCEPTED MANUSCRIPT [27] Z. Liu, R. Guo, J. Meng, X. Liu, X. Wang, Q. Li, L. Mai, Facile electrospinning formation of carbon-confined metal oxide cube-in-tube nanostructures for stable lithium storage, Chem. Commun. (2017‐, doi:10.1039/C7CC03727A.
RI PT
[28] J.-W. Liang, W.-Y. Wang, C.-L. Chung, Preparation of fluorescent ceramic nanofibers by electrospinning and heat treatment, Electronics Packaging (ICEP‐, 2017 International Conference on. IEEE (2017‐ 573-576, doi:10.23919/ICEP.2017.7939451.
SC
[29] J. Kim, H.M. Mousa, C.H. Park, C.S. Kim, Enhanced corrosion resistance and
M AN U
biocompatibility of AZ31 Mg alloy using PCL/ZnO NPs via electrospinning, Appl. Surf. Sci. 396 (2017‐ 249-258.
[30] C. Harito, R. Porras, D.V. Bavykin, F.C. Walsh, Electrospinning of in situ and ex situ synthesized polyimide composites reinforced by titanate nanotubes, J. Appl. Polym. Sci. 134
TE D
(2017‐.
[31] G. Sun, L. Sun, H. Xie, J. Liu, Electrospinning of nanofibers for energy applications, Nanomaterials 6 (2016‐ 129.
EP
[32] C.B.J. Manuel, V.G.L. Jesús, S.M. Aracely, Electrospinning for Drug Delivery Systems:
AC C
Drug Incorporation Techniques, Electrospinning-Material, Techniques, and Biomedical Applications, InTech. (2016‐,doi:10.5772/65939. [33] E.S. Pampal, E. Stojanovska, B. Simon, A. Kilic, A review of nanofibrous structures in lithium ion batteries, J. Power Sources 300 (2015‐ 199-215. [34] J.-W. Jung, C.-L. Lee, S. Yu, I.-D. Kim, Electrospun nanofibers as a platform for advanced secondary batteries: a comprehensive review, J. Mater. Chem. A 4 (2016‐ 703-750. 15
ACCEPTED MANUSCRIPT [35] W. Teo, R. Inai, S. Ramakrishna, Technological advances in electrospinning of nanofibers, Sci. Technol. Adv. Mater. 12 (2011‐ 013002. [36] L. Li, S. Peng, Y. Cheah, P. Teh, J. Wang, G. Wee, Y. Ko, C. Wong, M. Srinivasan,
capacitors, Chem. -Eur. J. 19 (2013‐ 5892-5898.
RI PT
Electrospun porous NiCo2O4 nanotubes as advanced electrodes for electrochemical
[37] S. Peng, L. Li, Y. Hu, M. Srinivasan, F. Cheng, J. Chen, S. Ramakrishna, Fabrication of
SC
Spinel One-Dimensional Architectures by Single-Spinneret Electrospinning for Energy Storage
M AN U
Applications, ACS Nano 9 (2015‐ 1945-1954.
[38] S. Peng, G. Jin, L. Li, K. Li, M. Srinivasan, S. Ramakrishna, J. Chen, Multi-functional electrospun nanofibres for advances in tissue regeneration, energy conversion & storage, and water treatment, Chem. Soc. Rev. 45 (2016‐ 1225-1241.
TE D
[39] X. Liang, G. Gao, G. Wu, H. Yang, Synthesis and characterization of novel hierarchical starfish-like vanadium oxide and their electrochemical performance, Electrochim. Acta 188 (2016‐ 625-635.
EP
[40] Y. Wu, G. Gao, G. Wu, Self-assembled three-dimensional hierarchical porous V 2 O
AC C
5/graphene hybrid aerogels for supercapacitors with high energy density and long cycle life, J. Mater. Chem. 3 (2015‐ 1828-1832. [41] S.J. Azhari, M.A. Diab, Thermal degradation and stability of poly(4-vinylpyridine‐ homopolymer and copolymers of 4-vinylpyridine with methyl acrylate, Polym. Degrad. Stabil. 60 (1998‐ 253-256.
16
ACCEPTED MANUSCRIPT [42] D. Yu, C. Chen, S. Xie, Y. Liu, K. Park, X. Zhou, Q. Zhang, J. Li, G. Cao, Mesoporous vanadium pentoxide nanofibers with significantly enhanced Li-ion storage properties by electrospinning Energy Environ. Sci. 4 (2011‐ 858-861.
RI PT
[43] H.G. Wang, D.L. Ma, Y. Huang, X.B. Zhang, Electrospun V2O5 nanostructures with controllable morphology as high-performance cathode materials for lithium-ion batteries, Chem. Eur. J. 18 (2012‐ 8987-8993.
SC
[44] Z. Wang, X. Liu, M. Lv, P. Chai, Y. Liu, J. Meng, Preparation of Ferrite MFe2O4 (M = Co,
M AN U
Ni‐ Ribbons with Nanoporous Structure and Their Magnetic Properties, J. Phys. Chem. B 112 (2008‐ 11292-11297.
[45] Y.L. Cheah, R. von Hagen, V. Aravindan, R. Fiz, S. Mathur, S. Madhavi, High-rate and elevated temperature performance of electrospun V2O5 nanofibers carbon-coated by plasma
TE D
enhanced chemical vapour deposition, Nano Energy 2 (2013‐ 57-64. [46] R. Ostermann, D. Li, Y. Yin, J.T. McCann, Y. Xia, V2O5 Nanorods on TiO2 Nanofibers: A New Class of Hierarchical Nanostructures Enabled by Electrospinning and Calcination, Nano
EP
Lett. 6 (2006‐ 1297-1302.
AC C
[47] X. Chen, K.M. Unruh, C. Ni, B. Ali, Z. Sun, Q. Lu, J. Deitzel, J.Q. Xiao, Fabrication, Formation Mechanism, and Magnetic Properties of Metal Oxide Nanotubes via Electrospinning and Thermal Treatment, J. Phys. Chem. C 115 (2011‐ 373-378. [48] K. Dewangan, N.N. Sinha, P.G. Chavan, P.K. Sharma, A.C. Pandey, M.A. More, D.S. Joag, N. Munichandraiah, N.S. Gajbhiye, Synthesis and characterization of self-assembled
17
ACCEPTED MANUSCRIPT nanofiber-bundles of V2O5: their electrochemical and field emission properties, Nanoscale 4 (2012‐ 645-651. [49] M. Sathiya, A.S. Prakash, K. Ramesha, J.M. Tarascon, A.K. Shukla, V2O5-anchored
RI PT
carbon nanotubes for enhanced electrochemical energy storage, J. Am. Chem. Soc. 133 (2011‐ 16291-16299.
[50] Y.L. Cheah, V. Aravindan, S. Madhavi, Improved Elevated Temperature Performance of
SC
Al-Intercalated V2O5 Electrospun Nanofibers for Lithium-Ion Batteries, ACS Appl. Mater.
M AN U
Interfaces 4 (2012‐ 3270-3277.
[51] J.J. Yu, J. Yang, W.B. Nie, Z.H. Li, E.H. Liu, G.T. Lei, Q.Z. Xiao, A porous vanadium pentoxide nanomaterial as cathode material for rechargeable lithium batteries, Electrochim. Acta 89 (2013‐ 292-299.
TE D
[52] L. Mai, L. Xu, C. Han, X. Xu, Y. Luo, S. Zhao, Y. Zhao, Electrospun Ultralong Hierarchical Vanadium Oxide Nanowires with High Performance for Lithium Ion Batteries, Nano Lett. 10 (2010‐ 4750-4755.
EP
[53] D. Chao, X. Xia, J. Liu, Z. Fan, C.F. Ng, J. Lin, H. Zhang, Z.X. Shen, H.J. Fan, A
AC C
V2O5/conductive-polymer core/shell nanobelt array on three-dimensional graphite foam: a high-rate, ultrastable, and freestanding cathode for lithium-ion batteries, Adv. Mater. 26 (2014‐ 5794-5800.
[54] Y. Liu, X. Zhou, Y. Guo, Effects of fluorine doping on the electrochemical properties of LiV3O8 cathode material, Electrochim. Acta 54 (2009‐ 3184-3190.
18
ACCEPTED MANUSCRIPT [55] C.J. Zhang, S.H. Park, S.E. O'Brien, A. Seral-Ascaso, M. Liang, D. Hanlon, D. Krishnan, A. Crossley, N. McEvoy, J.N. Coleman, Liquid Exfoliation of Interlayer Spacing-Tunable 2D Vanadium Oxide Nanosheets: High Capacity and Rate Handling Li-ion Battery Cathodes,
AC C
EP
TE D
M AN U
SC
RI PT
Nano Energy 39 (2017‐ 151-161.
19
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Fig. 1 Schematic illustration the process of electrospinning and the SEM image of the as-spun NFs. Fig.2 The thermos-gravimetric analyses (TGA) of (a) the as-spun nanofibers and (b) the sintered samples: 1/3-V2O5, 1-V2O5, 2-V2O5. Fig.3 FE-SEM images of sintered (a) 1/3-V2O5, (b) 1-V2O5, (c) 2-V2O5. Fig.4 TEM images of sintered (a-b) 1/3-V2O5, (c-d) 1-V2O5, (e-f) 2-V2O5, And (g) VNTs formation process through NFs post-calcianted at 400°C in air for 1/3 hour, 1hour, and 2 hours. Fig.5 N2 adsorption/desorption isotherms of the V2O5 nanomaterials with different sintering time. Inset is the corresponding pore size distribution. Fig.6 X-ray diffraction (XRD) patterns of the 1/3-V2O5, 1-V2O5, and 2-V2O5 were obtained at 400 °C for various holding time. Fig.7 Cyclic voltammograms of first five cycles of (a) 1/3-V2O5, (b) 1-V2O5 and (c) 2-V2O5 cells at a scan rate of 0.1 mV s-1 between 2 ~ 4 V vs. Li at 100 mA g-1, respectively. Fig.8 Galvanostatic charge-discharge profiles of first five cycles of (a) 1/3-V2O5, (b) 1-V2O5 and (c) 2-V2O5 cells between 2 ~ 4 V at 100 mA g-1, respectively. Fig.9 (a) Cycling performance of 2-V2O5, 1-V2O5 and 1/3-V2O5 cells at a current density of 100 mA g-1 between 2.0-4.0 V; (b) cycling of 2-V2O5, 1-V2O5 and 1/3-V2O5 cells at various C rates. All voltages are reported vs. Li/Li+. Fig.10 FE-SEM images of three VNTs samples ((a) 1/3-V2O5, (b) 1-V2O5, and (c) 2-V2O5) before and after electrochemical tests. Fig.11 Nyquist plots of all the three samples after 100 cycles at a current density of 1 A g-1.
1
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Fig.1
2
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Fig.2
3
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Fig.3
4
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Fig.4
5
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Fig.5
6
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Fig.6
7
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Fig.7
8
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Fig.8
9
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Fig.9
10
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Fig.10
11
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Fig.11
12
ACCEPTED MANUSCRIPT
Enhanced electrochemical performance of electrospun V2O5 nanotubes as cathodes for lithium ion batteries Yindan Liu, Dayong Guan, Guohua Gao*, Xing Liang, Wei Sun, Kun Zhang, Wenchao Bi and
RI PT
Guangming Wu*
Shanghai Key Laboratory of Special Artificial Microstructure materials and technology, School of
University, Shanghai, 200092, China.
M AN U
E-mail: [email protected]; wugm@ tongji.edu.cn
SC
Physics Science and Engineering, College of Electronics and Information Engineering, Tongji
Highlights
Hollow porous V2O5 nanotubes are prepared by electrospinning and sintering process.
V2O5 products with various morphology are obtained by different sintering duration.
The obtained V2O5 nanotubes display improved electrochemical performance.
AC C
EP
TE D