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Carbon-encapsulated Mn-doped V2O5 nanorods with long span life for high-power rechargeable lithium batteries
Electrochimica Acta 192 (2016) 216–226
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Carbon-encapsulated Mn-doped V2O5 nanorods with long span life for high-power rechargeable lithium batteries Can Penga , Fang Xiaoa , Jie Yanga , Zhaohui Lia,1,* , Gangtie Leia , Qizhen Xiaoa , Yanhuai Dingb , Zhongliang Huc a Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, College of Chemistry, Xiangtan University, Hunan 411105, PR China b Institute of Fundamental Mechanics and Materials Engineering, Xiangtan University, Hunan 411105, PR China c College of Metallurgic Engineering, Hunan University of Technology, Hunan 412007, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 October 2015 Received in revised form 25 December 2015 Accepted 26 January 2016 Available online 29 January 2016
In order to enhance electronic/ionic conductivity and stabilize the crystal structure of the layered V2O5 material, carbon-encapsulated Mn-doped V2O5nanorods (Mn0.1V2O5@C) were prepared through hydrothermal treatment and followed by annealing at a high temperature. The as-prepared sample appears as quasi spheres with a diameter range of 2–5 mm, in which the Mn0.1V2O5 nanorods (20 nm in diameter and 200 nm in length) are embedded within the porous carbon matrix. These quasi spheres have a hierarchically porous structure and possess a specific surface area of 26.48 m2 g1. They can deliver the specific capacities of 265, 247, 236, 219, 186 and 164 mAh g1 at the rates of 0.1, 0.5, 2, 5, 10 and 20C over the potential range of 4.0–2.0 V (vs. Li+/Li), respectively. After cycled at 5C rate for 500 times at room temperature, they can retain 94% of the initial capacity due to both carbon encapsulation and Mn doping. The results indicate that the Mn0.1V2O5@C quasi spheres possess excellent rate capability and long span life, which are promising to be applied in high-power rechargeable lithium batteries. ã 2016 Elsevier Ltd. All rights reserved.
Keywords: vanadium oxide carbon encapsulation doping rechargeable lithium batteries
1. Introduction To meet the demand of high-power rechargeable lithium batteries, novel cathode materials with high-energy density are necessary to be exploited so that the electric vehicles (EVs) and hybrid-electric vehicles (HEVs) could be driven farther and faster [1–3]. Layered vanadium pentoxide (V2O5) has caught much attention because of its relatively high specific capacity of 294 mAh g1 over the potential range of 2.0–4.0 V (vs Li+/Li). Within this potential range, it could perform a two-electron reaction per unit formula undergoing three phase transformations from a to e (x < 0.5), e to d (0.5 < x < 1), and d to g (1 < x < 2) [4–6]. It is worth noticing that the phase change would happen three times during insertion/extraction of lithium ion thus the crystal structure of V2O5 might collapse. As a result, it would suffer from serious capacity deterioration during the cycling process. Besides this drawback, low electronic conductivity and slow lithium-ion diffusion limit its widespread application in high-power rechargeable lithium batteries [7,8].
* Corresponding author. Tel.: +86 731 58292206; fax: +86 731 58292251. E-mail address: [email protected] (Z. Li). ISE member
1
http://dx.doi.org/10.1016/j.electacta.2016.01.195 0013-4686/ ã 2016 Elsevier Ltd. All rights reserved.
It is known that nanomaterials possess shorter diffusion distance, larger specific surface area and less change of crystallites than bulk materials [9].To improve the electrochemical properties of V2O5 cathode, people have prepared a great variety of nanomaterials such as nanotube [10], nanofiber [11], nanoplatelet [12], nanosheet [13–15], nanospike [16] and nanorod [17] in the past decade. Unfortunately, huge specific surface area renders the nanomaterials easy to dissolve into the electrolyte. Therefore, surface coating has been adopted to modify the V2O5 nanomaterials to reduce their dissolution. The surfacecoated materials include AlPO4 [18], TiO2 [19], polyaniline [20], polypyrrole [21] and carbon materials [22–25]. In addition to reducing the dissolution, crystal structure stability is important for the V2O5 cathode, which could be stabilized by doping of metallic elements such as Al [26,27], Cu [28], Fe [29], Cr [30], Mn [31–34], Sn [35] and Ag [36]. Nevertheless, people pursue few works upon the element doping and the carbon coating simultaneously. Herein, carbon-encapsulated Mn-doped V2O5nanorods with hierarchical porosity were prepared by hydrothermal treatment in an autoclave followed by sintering at a high temperature. In the fabrication process, poly(acrylic acid) (PAA) is used as the template, b-cyclodextrin (b-CD) as the template’s adjusting agent and glucose as the carbon source. The as-prepared composite material
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Fig. 1. XRD patterns (a) and the enlarged view (b) of the V2O5, V2O5@C and Mn0.1V2O5@C samples.
was evaluated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charging-discharging measurements. The results suggest the product has high rate capability and stable cycling performance when used as cathode material for rechargeable lithium batteries.
2. Experimental 2.1. Synthesis and characterization At first, 0.50 g of PAA (Mw = 4.5 105 g mol1, Sigma–Aldrich), 0.10 g of b-CD and 5.00 g of glucose were dissolved in 50 ml ethanol/water (1:4 in volume) forming the solution A. 11.70 g of ammonium metavanadate (NH4VO3) and 13.50 g of oxalic acid (H2C2O4) were dissolved in 100 ml water yielding the blue solution
Fig. 2. V2p2/3 signal in the XPS curves of the V2O5 (a), V2O5@C (b), and Mn0.1V2O5@C (c) samples.
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Fig. 3. N2 adsorption/desorption isothermal plots and the pore size distribution (inset) of the Mn0.1V2O5@C quasi spheres.
B. Subsequently, the solution B was mixed with the solution A to forma mixed solution. 2.45 g of Mn(CH3COO)24H2O in 25 ml ethanol was then added to the mixed solution. After the pH value of the mixed solution was adjusted to 1 by HNO3, it was put into a 400-ml stainless steel autoclave with a Teflon liner and heated at 180 C for 10 h. After that, the autoclave was allowed to cool down to room temperature naturally. The reacted mixture was dried at 80 C in air overnight to form the precursor. The precursor was
transferred to a tubular furnace and heated at 200 C for 2 h in an air flowing. Afterwards, it was heated from 200 C to 450 C at a ramping rate of 2 C min1 under a nitrogen atmosphere and hold at 450 C for 5 h to produce the Mn-doped V2O5@C composite material finally, which was designated as the MVC. Its thermogravimetry (TG) curve was shown in Fig. S1 (Supplementary information).
Fig. 4. SEM (a) and TEM (b), and HRTEM images (c, d) of the Mn0.1V2O5@C quasi spheres, and the SAED spectra (e) and (f) from the selected areas of “” and “&” signed in the Figure d respectively.
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Fig. 5. Schematic illustration of the possible formation mechanism of the Mn0.1V2O5@C quasi spheres.
The un-doped V2O5@C was prepared by the same method except for no addition of Mn(CH3COO)2, designated as the VC. The amount of carbon was determined by weighing the mass difference before and after firing the MVC or VC at 450 C for 2 h in air, which is about 8% of the composite. The concentrations of Mn and V elements in the MVC sample were measured by inductively coupled plasma atomic emission spectrometry (ICP-AES, Leeman, Profile) after its dissolving into concentrated hydrochloric acid. The elemental analysis determined that the molar ratio of Mn to V was close to 1:20, so the chemical formula of the MVC could be designed as Mn0.1V2O5@C. The bare V2O5 sample was synthesized by this method without addition of glucose and Mn(CH3COO)24H2O, and designated as the BV. 2.2. Characterizations The morphologies were characterized by scanning electron microscope (SEM, JEOL JEM-6610LV) and transmission electron microscopy (TEM, JEOL JEM-2010). Powder X-ray diffraction (XRD) patterns were recorded on Bruker D8 Advanced Diffractomiter using a monochromatic Cu Ka X-ray source (l = 0.154 nm). X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI) measurement was conducted to determine the chemical states of V element. Nitrogen (N2) sorption isotherms were measurements at 77 K using Micromeritics ASAP2010. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method. 2.3. Elecrtochemical measurements Cathode was obtained by mixing the electrochemical active material (MVC, VC or BV), poly(vinylidene difluoride) (PVDF), and acetylene black at a mass ratio of 80:10:10, then added N-methyl pyrrolidinone forming a slurry, coated on an Al-foil, dried at 100 C in vacuum overnight, and pouched into discs with 12 mm diameters. The mass loading of the electrochemical active material was about 3.20 mg in one piece of disc. 2016-type coin cell was assembled by the cathode and lithium circular piece that separated by a porous polypropylene membrane (Celgard 2300), in which 1 M LiPF6 in ethylene carbonate (EC)/ dimethyl carbonate (DMC) (1:1 in volume) was used as the electrolyte. Galvanostatic charging-discharging tests were carried out using a Neware battery tester at different current densities within the
potential range of 4.0–2.0 V. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed using the EG&G 2273 electrochemical workstation at room temperature (25 C). The scanning rate in the CV test was 0.1 mV s1 whereas the frequency in the EIS measurement ranged from 100 kHz to 0.01 Hz. 3. Results and discussion 3.1. Crystal structure Fig. 1 shows the XRD patterns of the bare V2O5, V2O5@C and Mn0.1V2O5@C (a) and the enlarged view (b). It is found that for the BV sample the diffraction peaks at 15.3 , 20.3 , 21.7, 25.6 , 26.1, 31.0 , 32.4 , 33.3 , 34.3 , 36.0 , 37.4 , 41.3 , 42.0 , 45.5 , 47.3 , 47.9 , and 48.9 can be ascribed to the crystal planes of (2 0 0), (0 0 1), (1 0 1), (2 0 1), (11 0), (1 0 0), (0 11), (111), (3 1 0), (2 11), (4 0 1), (0 0 2), (1 0 2), (4 11), (6 0 0), (3 0 2), and (0 1 2), respectively. All the characteristic diffraction peaks can be indexed to the orthorhombic phase of V2O5 (space group: Pmmn) (JCPDS no. 41–1426). Its lattice constants are (a = 11.48 Å, b = 3.57 Å, and c = 4.36 Å). The VC exhibited the similar diffraction peaks to the BV besides an additional weak peak at 26.7, which is also present in the XRD pattern of the MVC but shifts to 26.6 . Such a diffraction peak is unclear to be ascribed to what crystal plane but its intensity seems to increase with Mn-doping. Both the VC and MVC displayed lower intensity of the corresponding diffraction peaks and increased full width at half maximum (FWHM) of the crystal plane (0 0 1), which suggests that carbon encapsulation hampered the growth of the crystallites. In the case of the MVC, some typical diffraction peaks shift to lower angles (Fig. 1b) and the lattice constants are a = 11.51 Å, b = 3.57 Å, and c = 4.48 Å, indicating an expansion of the lattice compared with the BV. The enlarged lamellar spacing would lead to this lattice expansion, which implies Mn2+ ions might intercalate into the [VO5] slabs [27,30,35]. No impurities were detected for all the samples. Therefore, doping of 0.1 mol of Mn2+ ions into one mole of V2O5 did not change the crystal structure, meaning that the MVC still possesses the orthorhombic phase [31,34]. Fig. 2 shows the V2p2/3 signal in XPS curves for all the samples. It can be seen that two binding energy peaks located at 516.73 eV and 515.31 eV, which are attributed to V5+ and V4+ ions, respectively [35]. The integral area of the peak at 515.31 eV can be used to evaluate the amount of V4+ ion in the sample. From Fig. 2a and b, one can see the VC revealed an integral area of V2p2/ 3 signal peak with respect to V4+ ion slightly larger than that of the
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BV, indicating the amount of V4+ ion in the VC is slightly higher than that in the BV. The reason is that a small number of V5+ ions is reduced to V4+ ones when the sample is annealed at a high temperature in nitrogen atmosphere. Clearly the MVC had the largest integral area of this peak (Fig. 2c) among three samples, indicating the amount of V4+ ions in the MVC is higher than those in other two samples. The results suggest that a few of Mn2+ ions were chemically inserted into the layered structure causing the valence reduction of V atoms.
3.2. Porous structures Fig. 3 shows the N2 adsorption-desorption isotherms and the corresponding pore-size-distribution curve of the MVC. The adsorption isotherm displays a type III curve, indicating a macroporous structure, whereas the type-H3 hysteresis loop suggests a molecular adsorption in the mesopores [37].The MVC exhibits the maximum pore size distribution at 3.7 nm and 35 nm, suggesting that the MVC possesses a hierarchically porous structure. The macropores facilitate penetration of the electrolyte
Fig. 6. Galvanostatic charging-discharging curves of the bare V2O5 (a), V2O5@C (c) and Mn0.1V2O5@C (e), and their corresponding cycling performances and coloumbic efficiency (b, d, f) at 0.1C rate (1C = 294 mA g1) for 50 cycles over the voltage range of 4.0–2.0 V at room temperature.
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Fig. 7. Rate capability of the V2O5, V2O5@C and Mn0.1V2O5@C (a) and their cycling performance at 5C rate (b) over the voltage range of 4.0–2.0 V at room temperature. (c) Exsitu XRD patterns of the V2O5, V2O5@C and Mn0.1V2O5@C charged to 4.0 V after the 5C-rate-cycling process.
into the bulk particles while the mesopores enlarge the contacting surface area between the particles and electrolyte [38].Much absorbed electrolyte could promise the electrochemically active materials’ enough Li+ ions while large contacting area provided a large number of intercalation sites. The measured BET specific surface area is 26.48 m2 g1, and the BJH volume is 0.136 cm3 g1. Such a porous structure would be very beneficial to the kinetics of (de) lithiation for the V2O5 material. The BET surface area is 21.34 m2 g1 for the VC and 13.65 m2 g1 for the BV. Their N2 adsorption-desorption isotherms and the corresponding poresize-distribution curves are shown in Fig. S2. (Supplementary information) 3.3. Morphologies of the Mn1V2O5@C SEM image of the MVC is shown in Fig. 4a. One can see that the MVC sample appeared as quasi spheres with an average diameter range of 2–5 mm. The quasi spheres are constructed with some nanorods that embed in a loose matrix(Fig. 4b). The nanorod displayed about 120 nm in length (Fig. 4b) and 20 nm in diameter (Fig. 4c). Mn element was detected in the MVC by energy dispersive spectrometry (EDS) analysis of the selected area in Fig. 4c (Fig. S3, supporting information). From the HRTEM image (Fig. 4d), the spacing between lamellas is found to be 0.45 nm, which is a little larger than that calculated by the Bragg equation
(0.437 nm) concerning (0 0 1) crystal plane. The result implied that a few of Mn2+ ions have intercalated the [VO5] interlayer and thus expand the lamellar spacing by the electrostatic interaction of Mn2 + ions with the anionic [VO5] slabs without destroying the crystal structure [27,39–41]. It is found that from the selected area electron diffraction (SAED) images (Fig. 4e & f), surrounding the nanorods revealed an amorphous phase while the nanorods exhibited a polycrystalline structure. A possible mechanism for the formation of such morphology is presented in Fig. 5. As is known, b-CD composed of seven a-1,4linked glucopyranose units has a hydrophilic external surface and a hydrophobic cavity. When PAA macromolecules mixed with b-CD in the ethanol/water, they could absorb onto the surface of b-CD cavity and aggregate to form spheres. After that, VO2+ ions that originated from the reaction of NH4VO3 with H2C2O4 are bound to the PAA macromolecules through an electrostatic interaction between the VO2+ cations and the anionic carbonyl groups. V2O5 nuclei originate from the absorbed VO2+ ions and deposit on the PAA macromolecular chains during hydrothermal treatment. Afterwards the PAA@V2O5 composite forms the nanorods after annealing at a high temperature. Meanwhile, glucose is carbonized to form a porous carbon matrix. Without encapsulated by the porous carbon matrix, the BV displayed the aggregated nanorods with smooth surface as shown in Fig. S4. (Supplementary information)
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Table 1 Comparison of the rate capability and cycling performance of the various kind of V2O5 materials. Morphology
Rate capability Discharge capacity (at x mA g1) (mAh g1)
Hierarchical V2O5 nanoflower
293 275 254 232 209
Ultrathin V2O5 nanosheet
Cycling performance Capacity retention (at x mA g1 after n cycles)
Ref.
74.2% (at 100 mA g1 after 250 cycles)
[14]
292 (59 mA g1) 266 (294 mA g1) 233 (1470 mA g1) 192 (2940 mA g1) 156 (5880 mA g1) 137 (8820 mA g1)
93.8% (at 59 mA g1 after 50 cycles)
[15]
V2O5/Mesoporous carbon Composite
291 (100 mA g1) 265 (250 mA g1) 247 (500 mA g1) 200 (1000 mA g1) 150 (2000 mA g1) 75 (5000 mA g1)
66% (at 500 mA g1 after 100 cycles)
[24]
Al0.16V2O5/RGO nanocomposite
290 (90 mA g1) 274 (300 mA g1) 122 (3000 mA g1)
90% (at 300 mA g1 after 50 cycles)
[27]
3D porous Fe0.1V2O5.15 thin film
278 (29.4 mA g1) 250 (88.2 mA g1) 245 (176.4 mA g1) 222 (441 mA g1) 202 (882 mA g1) 180 (1764 mA g1)
70.14% (at 58.8 mA g1 after 48 cycles)
[29]
Two-dimensional 5% Mn-V2O5 sheet network
165 (1000 mA g1)
Very stale (at 1000 mA g1 after 100 cycles)
[34]
Cu doped V2O5 flowers
229 (294 mA g1) 197 (882 mA g1) 182 (1470 mA g1) 126 (2940 mA g1) 97 (5880 mA g1)
85% (at 58.8 mA g1 after 50 cycles)
[41]
Ni–V2O5 hollow microspheres
294 (50 mA g1) 259 (300 mA g1) 228 (600 mA g1) 203 (1200 mA g1) 166 (2400 mA g1)
91% (at 300 mA g1 after 50 cycles)
[43]
Self-assembled hairy ball-like V2O5 nanostructures
278.3 (100 mA g1) 273 (500 mA g1)
92.89% (at 100 mA g1 after 500 cycles)
[44]
b-Na0.33V2O5 mesoporous flake
323 (20 mAg1) 271 (50 mAg1) 251 (100 mAg1) 226 (300 mA g1)
68.1% (at 300 mAg1 after 70 cycles)
[45]
Nanoflake assembled 3D hollow-porous microsphere
283 (100 mA g1) 225 (500 mA g1) 189 (1000 mA g1) 119 (2000 mA g1)
76.68% (at 100 mA g1 after 60 cycles)
[46]
2D Leaf-like V2O5nanosheets
303 (20 mA g1) 273 (200 mA g1) 251 (500 mA g1) 219 (1000 mA g1) 160 (2000 mA g1)
82.1% (at 500 mA g1 after 100 cycles)
[47]
V2Onanosphere/MWCNT layer-by-layer nano-architecture
292 (29.4 mA g1) 215 (294 mA g1) 170 (588 mA g1) 130 (1176 mA g1)
93% (at 294 after 100 cycles)
[48]
Mn0.1V2O5Nanorods embedded in carbon matrix
265 (29.4 mA g1) 247 (147 mA g1) 236 (588 mA g1) 219 (1470 mA g1) 186 (2940 mA g1) 164 (5880 mA g1).
94% (at 1470 mA g1 after 500 cycles)
This work
(30 mA g1) (60 mA g1) (150 mA g1) (300 mA g1) (600 mA g1)
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Fig. 8. Cyclic voltammogrames of the V2O5 (a), V2O5@C (b) and Mn0.1V2O5@C (c) at the 0.1 mV s1 scanning rate over the voltage range of 4.0–2.0 V at room temperature.
3.4. Electrochemical properties Fig. 6 shows the galvanostatic charging-discharging curves of the BV (a), VC (c) and MVC (e) samples and their cycling performances at 0.1C rate. It is found that the charging and discharging curves for all samples exhibited multiple potential plateaus within the 2.0–4.0 V range, indicating the multi-step
insertion of lithium into the interlayers of V2O5. However, the initial discharging curve is different from the subsequent ones for the BV and VC samples whereas the MVC sample revealed similar curves for the whole cycles. For the BV sample, the discharging potential plateaus located at 3.37, 3.19, 2.76, 2.54 and 2.33 V in the first cycle and displayed two additional plateaus at 3.60 and 3.45 V in the subsequent cycles (Fig. 6a). The VC sample showed the
Fig. 9. Nyquist curves at the discharge state of 3.0 V (a) and the Zre v1/2 plots in the low frequency range (b) of the V2O5, V2O5@C and Mn0.1V2O5@C samples.
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similar situation to the BV sample in the 3 V potential regions, but two additional plateaus at 3.60 and 3.45 V were presented in the following cycles (Fig. 6a). The VC sample revealed the similar situation to the BV sample in the 3 V potential regions except that two plateaus appeared at 2.13 and 2.01 V for the former (Fig. 6c). In the case of the MVC sample, the discharging potential plateaus looked the same in all cycles except for an additional plateau at 2.05 V in the first cycle (Fig. 6e). It is obvious that at the low potentials, e.g. 2.13 and 2.01 V for the VC, and 2.05 V for the MVC, the carbon encapsulated samples could display potential plateaus because the electrons might transport more quickly in these samples than in that not encapsulated. The discharging specific capacities are 272, 273 and 265 mAh g1 for the BV, VC and MVC in the first cycle, respectively, and retain 126 (Fig. 6b, after 30 cycles), 165 (Fig. 6 d, after 45 cycles) and 206 mAh g1 (Fig. 6f, after 45 cycles) in turn. To evaluate their rate capability, the samples were operated at various rates over the voltage range of 2.0–4.0 V at room temperature. Fig. 7a compares the rate capabilities of the MVC, VC and BV samples in the first cycle. It can be seen that the MVS quasi spheres could deliver the initial capacities of 265, 247, 236, 219, 186 and 164 mAh g1 at the rates of 0.1, 0.5, 2, 5, 10 and 20C, respectively. The discharging capacity at 20C rate is about 62% of that at 0.1C rate. The rate capability of the MVC is better than those of nanomaterials [14,15,44,46,47] and the doped materials [27,29,34,41,43,45], and is comparable to those of composites (Table 1) [24,27,48]. One can see that the discharging capacity at 20C rate is 126 and 68 mAh g1 for the VC and the BV, respectively, corresponding to 46% and 25% of their initial ones at 0.1C rate. Therefore, the MVC possessed the best rate capability among the samples. That the Mn-doping V2O5 nanorods are embedded uniformly in the porous carbon matrix is beneficial to the electrochemical kinetics of electrode materials. Meanwhile, the nanorods shorten diffusion distance of Li+ ion while the carbon matrix accelerates electrons’ transfer. Fig. 7b displays the cycling performances of the BV, VC and MVC samples at 5C rate over the potential range of 2.0–4.0 V. It is found that after 100 cycles, the BV retains a capacity of 93 mAh g1 (59% of the initial one) due to its unstable crystal structure, which can be proved by the ex-situ XRD pattern after cycling (Fig. 7c). One can see that the layered crystal structure has already collapsed. On the contrary, the MVC is found to possess some diffraction peaks belonging to the crystal planes of (2 0 0), (0 0 1), (11 0), (1 0 0), (0 11), and (3 1 0) after 500 cycles, which confirmed the pillar effect of Mn2+ ion between the VO5 slabs. Consequently, it can deliver the capacities of 206 mAh g1 at 5C rate after 500 cycles, retaining 94% of the initial one. So far, this cycle stability is the best among the reported as well [14–48].The results suggest that the MVC possesses the best cycling performance, which is attributed to both Mn-doping and carbon encapsulation. The former stabilizes the crystal structure of the V2O5nanorods while the latter decreases their dissolution into the electrolyte. Fig. 8 shows the cyclic voltammograms (CVs) of the BV, VC and MVC samples at the scan rates of 0.1 mV s1 within the potential range of 2.0–4.0 V (vs. Li+/Li). One can see that the MVC and VC samples exhibit higher peak currents than the BV sample. The MVC reveals a larger integral area between 2.0 V and 3.0 V and better overlayed CV for the first five cycles compared with the other two samples. The potential peaks at 3.35, 3.16, and 2.26 V (vs. Li+/Li) in the cathodic sweep correspond to the phase changes from a-V2O5 to e-Li0.5V2O5, e-Li0.5V2O5 to d-LiV2O5, and d-LiV2O5 to g-Li2V2O5, respectively [49]. The other two peaks locate at 3.54, 3.42 V (vs. Li+/ Li) whose ascriptions are not clear at this stage. These reduction (intercalation) potential peaks are consistent with the multi-step
Fig. 10. Schematic illustration of the carbon encapsulated V2O5 quasi sphere.
potential plateaus occurring in the discharge curve (Fig. 6e). In the anodic sweep, the CVs also appear multi-peaks for all of the samples. All the potentials at peaks are listed in Table S1 (Supplementary information). Fig. 9a displays the Nyquist curves of the BV, VC and MVC samples that have been activated for several times and discharged to the potential of 3.0 V (vs. Li+/Li). It is found that the Nyquist curves consist of a depressed semicircle in the high-frequency region and a sloped line in the low-frequency region. The straight line is relative to the diffusion control of Li+ ions in the bulk electrode, corresponding to the Warburg impedance. The fitting equivalent circuit was depicted and shown as Fig. 9a inset. The intercept at the real resistance axis with respect to the left terminal of the depressed semicircle represents the ohmic resistance of the electrolyte (Rs) as shown in the equivalent circuit (Fig. 8a inset) while the right one relates to the charge transfer resistance (Rct). The values of Rs and Rct are summarized in Table S2 (Supplementary information). CPE represents the non-ideal double layer capacitance for lithium-ion intercalation. W is the Warburg impedance of the solid-phase diffusion. When the diffusion of Li is a rate-controlling step in the electrochemical reaction, the lithium-diffusion coefficient (DLi) can be calculated according to Eq. (1) [50–54]. DLi ¼
R2 T 2 2A2 n4 F 4 C 2
sw
ð1Þ
where R is the gas constant (8.314 J mol1 K1), T the absolute temperature (298.15 K), A the surface area of the electrode (1.13 cm2), n the number of electron, F the Faraday’s constant (96485C mol1), C the concentration of lithium ion in the LiV2O5(1.32 102 mol cm3), 27 and s w is the Warburg factor, which is the slope of the Zre versus v1/2 plots (Fig. 9b) based on Eq. (2). Z re ¼ Rs þ Rct þ s w v1=2
ð2Þ
According to Eq. (1) and Eq. (2), the lithium diffusion coefficients of the BV, VC and MVC samples are 1.27 1011, 2.02 1011 and 6.81 1011 cm2 s1, respectively. It is clear that the BV had a lower DLi value than the VC and MVC because the BV exhibited a lower BET specific surface area than the other two samples, which led to less amount of intercalation sites. As shown in Fig. 10, owing to highly penetrating pores within the quasi sphere, Li+ ions in the absorbed electrolyte can migrate to the surface of the V2O5 or Mn0.1V2O5 nanorods fast. Moreover, intercalation of Li+ ions benefits from the enlarged lamellar spacing by doping of Mn2+ ions. As a result, the MVC exhibits the largest DLi value.
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4. Conclusions In summary, carbon encapsulated Mn-doped V2O5 quasi spheres (Mn0.1V2O5@C) have been successfully prepared by a hydrothermal treatment followed by high-temperature calcination. These quasi spheres were built of nanorods that embedded uniformly in a porous carbon matrix. The porous structure facilitated penetrations of electrolyte into the quasi spheres leading to forming 3D ion-conduct network while the porous carbon matrix resulting in the 3D electron-conduct network. Owing to such a unique morphology, the obtained composite material possessed an excellent rate capability over the potential rang of 4.0–2.0 V (vs. Li/Li+). It could deliver 164 mAh g1 at 20C rate, which is about 62% of the discharge capacity at 0.1C rate (265 mAh g1). Also they exhibited a long span life(retaining 94% of the initial capacity after 500 cycles at 5C rate) due to carbon encapsulation and Mn-doping. The results suggest that the Mn0.1V2O5@Cquasi spheres could be used as cathode material in rechargeable lithium batteries. Acknowledgements The authors greatly appreciate the financial support from the Key R&D Program of Hunan Province (2015JC3091), National Natural Science Foundation of China (No. 21174119, 21376069, 21576075), Hunan Natural Science Foundation (2015JJ3115), and the Program for Innovative Research Cultivation Team in University of Ministry of Education of China (1337304). References [1] B. Kang, G. Ceder, Battery materials for ultrafast charging and discharging, Nature 458 (2009) 190–193. [2] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7 (2008) 845–954. [3] L. Croguennec, M.R. Palacin, Recent achievements on inorganic electrode materials for lithium-ion batteries, J. Am. Chem. Soc. 137 (2015) 3140–3156. [4] Y.X. Tang, Y.Y. Zhang, W.L. Li, B. Ma, X.D. Chen, Rational material design for ultrafast rechargeable lithium-ion batteries, Chem. Soc. Rev. 44 (2015) 5926– 5940. [5] M.S. Whittingham, Lithium batteries and cathode materials, Chem. Rev. 104 (2004) 4271–4301. [6] Y. Wang, G. Cao, Developments in nanostructured cathode materials for highperformance lithium-ion batteries, Adv. Mater. 20 (2008) 2251–2269. [7] J. Liu, H. Xia, D. Xue, L. Lu, Double-shelled nanocapsules of V2O5-based composites as high-performance anode and cathode materials for Li ion batteries, J. Am. Chem. Soc. 131 (2009) 12086–12087. [8] E. Potiron, A.L.G. La Salle, A. Verbaere, Y. Piffard, D. Guyomard, Electrochemically synthesized vanadium oxides as lithium insertion hosts, Electrochim. Acta 45 (1999) 197–214. [9] A.S. Arico, P. Bruce, B. Scrosati, Nanostructured materials for advanced energy conversion and storage devices, Nat. Mater. 4 (2005) 366–377. [10] V.M. Mohan, B. Hu, W. Qiu, Synthesis structural, and electrochemical performance of V2O5 nanotubes as cathode material for lithium battery, J. Appl. Electrochem. 39 (2009) 2001–2006. [11] D. Yu, C. Chen, S. Xie, Y. Liu, G. Cao, Mesoporous vanadium pentoxide nanofibers with significantly enhanced li-ion storage properties by electrospinning, Energy Environ. Sci. 4 (2011) 858–861. [12] J. Yang, Z. Li, J. Wang, Q. Xiao, G. Lei, X. Zhou, An exfoliated vanadium pentoxide nanoplatelet and its electrochemical properties for lithium-ion batteries, Func. Mater. Lett. 5 (2012) 1250019. [13] R. Yu, C. Zhang, Q. Meng, Z. Chen, H. Liu, Z. Guo, Facile synthesis of hierarchical networks composed of highly interconnected V2O5 nanosheets assembled on carbon nanotubes and their superior lithium storage properties, ACS Appl. Mater. Interfaces 5 (2013) 12394–12399. [14] G.Z. Li, Y.C. Qiu, Y. Hou, H.F. Li, L.S. Zhou, H. Deng, Y.G. Zhang, Synthesis of V2O5 hierarchical structures for long cycle-life lithium-ion storage, J. Mater. Chem. A 3 (2015) 1103–1109. [15] X. Rui, Z. Lu, H. Yu, D. Yang, H.H. Hng, T.M. Lim, Q. Yan, Ultrathin V2O5 nanosheets cathodes: realizing ultrafast reversible lithium storage, Nanoscale 5 (2013) 556–560. [16] X.W. Zhou, C.J. Cui, G.M. Wu, A novel and facile way to synthesize vanadium pentoxide nanospike as cathode materials for high performance lithium ion batteries, J. Power Sources 238 (2013) 95–102.
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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Carbon-encapsulated Mn-doped V2O5 nanorods with long span life for high-power rechargeable lithium batteries Can Penga , Fang Xiaoa , Jie Yanga , Zhaohui Lia,1,* , Gangtie Leia , Qizhen Xiaoa , Yanhuai Dingb , Zhongliang Huc a Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, College of Chemistry, Xiangtan University, Hunan 411105, PR China b Institute of Fundamental Mechanics and Materials Engineering, Xiangtan University, Hunan 411105, PR China c College of Metallurgic Engineering, Hunan University of Technology, Hunan 412007, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 October 2015 Received in revised form 25 December 2015 Accepted 26 January 2016 Available online 29 January 2016
In order to enhance electronic/ionic conductivity and stabilize the crystal structure of the layered V2O5 material, carbon-encapsulated Mn-doped V2O5nanorods (Mn0.1V2O5@C) were prepared through hydrothermal treatment and followed by annealing at a high temperature. The as-prepared sample appears as quasi spheres with a diameter range of 2–5 mm, in which the Mn0.1V2O5 nanorods (20 nm in diameter and 200 nm in length) are embedded within the porous carbon matrix. These quasi spheres have a hierarchically porous structure and possess a specific surface area of 26.48 m2 g1. They can deliver the specific capacities of 265, 247, 236, 219, 186 and 164 mAh g1 at the rates of 0.1, 0.5, 2, 5, 10 and 20C over the potential range of 4.0–2.0 V (vs. Li+/Li), respectively. After cycled at 5C rate for 500 times at room temperature, they can retain 94% of the initial capacity due to both carbon encapsulation and Mn doping. The results indicate that the Mn0.1V2O5@C quasi spheres possess excellent rate capability and long span life, which are promising to be applied in high-power rechargeable lithium batteries. ã 2016 Elsevier Ltd. All rights reserved.
Keywords: vanadium oxide carbon encapsulation doping rechargeable lithium batteries
1. Introduction To meet the demand of high-power rechargeable lithium batteries, novel cathode materials with high-energy density are necessary to be exploited so that the electric vehicles (EVs) and hybrid-electric vehicles (HEVs) could be driven farther and faster [1–3]. Layered vanadium pentoxide (V2O5) has caught much attention because of its relatively high specific capacity of 294 mAh g1 over the potential range of 2.0–4.0 V (vs Li+/Li). Within this potential range, it could perform a two-electron reaction per unit formula undergoing three phase transformations from a to e (x < 0.5), e to d (0.5 < x < 1), and d to g (1 < x < 2) [4–6]. It is worth noticing that the phase change would happen three times during insertion/extraction of lithium ion thus the crystal structure of V2O5 might collapse. As a result, it would suffer from serious capacity deterioration during the cycling process. Besides this drawback, low electronic conductivity and slow lithium-ion diffusion limit its widespread application in high-power rechargeable lithium batteries [7,8].
* Corresponding author. Tel.: +86 731 58292206; fax: +86 731 58292251. E-mail address: [email protected] (Z. Li). ISE member
1
http://dx.doi.org/10.1016/j.electacta.2016.01.195 0013-4686/ ã 2016 Elsevier Ltd. All rights reserved.
It is known that nanomaterials possess shorter diffusion distance, larger specific surface area and less change of crystallites than bulk materials [9].To improve the electrochemical properties of V2O5 cathode, people have prepared a great variety of nanomaterials such as nanotube [10], nanofiber [11], nanoplatelet [12], nanosheet [13–15], nanospike [16] and nanorod [17] in the past decade. Unfortunately, huge specific surface area renders the nanomaterials easy to dissolve into the electrolyte. Therefore, surface coating has been adopted to modify the V2O5 nanomaterials to reduce their dissolution. The surfacecoated materials include AlPO4 [18], TiO2 [19], polyaniline [20], polypyrrole [21] and carbon materials [22–25]. In addition to reducing the dissolution, crystal structure stability is important for the V2O5 cathode, which could be stabilized by doping of metallic elements such as Al [26,27], Cu [28], Fe [29], Cr [30], Mn [31–34], Sn [35] and Ag [36]. Nevertheless, people pursue few works upon the element doping and the carbon coating simultaneously. Herein, carbon-encapsulated Mn-doped V2O5nanorods with hierarchical porosity were prepared by hydrothermal treatment in an autoclave followed by sintering at a high temperature. In the fabrication process, poly(acrylic acid) (PAA) is used as the template, b-cyclodextrin (b-CD) as the template’s adjusting agent and glucose as the carbon source. The as-prepared composite material
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Fig. 1. XRD patterns (a) and the enlarged view (b) of the V2O5, V2O5@C and Mn0.1V2O5@C samples.
was evaluated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charging-discharging measurements. The results suggest the product has high rate capability and stable cycling performance when used as cathode material for rechargeable lithium batteries.
2. Experimental 2.1. Synthesis and characterization At first, 0.50 g of PAA (Mw = 4.5 105 g mol1, Sigma–Aldrich), 0.10 g of b-CD and 5.00 g of glucose were dissolved in 50 ml ethanol/water (1:4 in volume) forming the solution A. 11.70 g of ammonium metavanadate (NH4VO3) and 13.50 g of oxalic acid (H2C2O4) were dissolved in 100 ml water yielding the blue solution
Fig. 2. V2p2/3 signal in the XPS curves of the V2O5 (a), V2O5@C (b), and Mn0.1V2O5@C (c) samples.
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Fig. 3. N2 adsorption/desorption isothermal plots and the pore size distribution (inset) of the Mn0.1V2O5@C quasi spheres.
B. Subsequently, the solution B was mixed with the solution A to forma mixed solution. 2.45 g of Mn(CH3COO)24H2O in 25 ml ethanol was then added to the mixed solution. After the pH value of the mixed solution was adjusted to 1 by HNO3, it was put into a 400-ml stainless steel autoclave with a Teflon liner and heated at 180 C for 10 h. After that, the autoclave was allowed to cool down to room temperature naturally. The reacted mixture was dried at 80 C in air overnight to form the precursor. The precursor was
transferred to a tubular furnace and heated at 200 C for 2 h in an air flowing. Afterwards, it was heated from 200 C to 450 C at a ramping rate of 2 C min1 under a nitrogen atmosphere and hold at 450 C for 5 h to produce the Mn-doped V2O5@C composite material finally, which was designated as the MVC. Its thermogravimetry (TG) curve was shown in Fig. S1 (Supplementary information).
Fig. 4. SEM (a) and TEM (b), and HRTEM images (c, d) of the Mn0.1V2O5@C quasi spheres, and the SAED spectra (e) and (f) from the selected areas of “” and “&” signed in the Figure d respectively.
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Fig. 5. Schematic illustration of the possible formation mechanism of the Mn0.1V2O5@C quasi spheres.
The un-doped V2O5@C was prepared by the same method except for no addition of Mn(CH3COO)2, designated as the VC. The amount of carbon was determined by weighing the mass difference before and after firing the MVC or VC at 450 C for 2 h in air, which is about 8% of the composite. The concentrations of Mn and V elements in the MVC sample were measured by inductively coupled plasma atomic emission spectrometry (ICP-AES, Leeman, Profile) after its dissolving into concentrated hydrochloric acid. The elemental analysis determined that the molar ratio of Mn to V was close to 1:20, so the chemical formula of the MVC could be designed as Mn0.1V2O5@C. The bare V2O5 sample was synthesized by this method without addition of glucose and Mn(CH3COO)24H2O, and designated as the BV. 2.2. Characterizations The morphologies were characterized by scanning electron microscope (SEM, JEOL JEM-6610LV) and transmission electron microscopy (TEM, JEOL JEM-2010). Powder X-ray diffraction (XRD) patterns were recorded on Bruker D8 Advanced Diffractomiter using a monochromatic Cu Ka X-ray source (l = 0.154 nm). X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI) measurement was conducted to determine the chemical states of V element. Nitrogen (N2) sorption isotherms were measurements at 77 K using Micromeritics ASAP2010. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method. 2.3. Elecrtochemical measurements Cathode was obtained by mixing the electrochemical active material (MVC, VC or BV), poly(vinylidene difluoride) (PVDF), and acetylene black at a mass ratio of 80:10:10, then added N-methyl pyrrolidinone forming a slurry, coated on an Al-foil, dried at 100 C in vacuum overnight, and pouched into discs with 12 mm diameters. The mass loading of the electrochemical active material was about 3.20 mg in one piece of disc. 2016-type coin cell was assembled by the cathode and lithium circular piece that separated by a porous polypropylene membrane (Celgard 2300), in which 1 M LiPF6 in ethylene carbonate (EC)/ dimethyl carbonate (DMC) (1:1 in volume) was used as the electrolyte. Galvanostatic charging-discharging tests were carried out using a Neware battery tester at different current densities within the
potential range of 4.0–2.0 V. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed using the EG&G 2273 electrochemical workstation at room temperature (25 C). The scanning rate in the CV test was 0.1 mV s1 whereas the frequency in the EIS measurement ranged from 100 kHz to 0.01 Hz. 3. Results and discussion 3.1. Crystal structure Fig. 1 shows the XRD patterns of the bare V2O5, V2O5@C and Mn0.1V2O5@C (a) and the enlarged view (b). It is found that for the BV sample the diffraction peaks at 15.3 , 20.3 , 21.7, 25.6 , 26.1, 31.0 , 32.4 , 33.3 , 34.3 , 36.0 , 37.4 , 41.3 , 42.0 , 45.5 , 47.3 , 47.9 , and 48.9 can be ascribed to the crystal planes of (2 0 0), (0 0 1), (1 0 1), (2 0 1), (11 0), (1 0 0), (0 11), (111), (3 1 0), (2 11), (4 0 1), (0 0 2), (1 0 2), (4 11), (6 0 0), (3 0 2), and (0 1 2), respectively. All the characteristic diffraction peaks can be indexed to the orthorhombic phase of V2O5 (space group: Pmmn) (JCPDS no. 41–1426). Its lattice constants are (a = 11.48 Å, b = 3.57 Å, and c = 4.36 Å). The VC exhibited the similar diffraction peaks to the BV besides an additional weak peak at 26.7, which is also present in the XRD pattern of the MVC but shifts to 26.6 . Such a diffraction peak is unclear to be ascribed to what crystal plane but its intensity seems to increase with Mn-doping. Both the VC and MVC displayed lower intensity of the corresponding diffraction peaks and increased full width at half maximum (FWHM) of the crystal plane (0 0 1), which suggests that carbon encapsulation hampered the growth of the crystallites. In the case of the MVC, some typical diffraction peaks shift to lower angles (Fig. 1b) and the lattice constants are a = 11.51 Å, b = 3.57 Å, and c = 4.48 Å, indicating an expansion of the lattice compared with the BV. The enlarged lamellar spacing would lead to this lattice expansion, which implies Mn2+ ions might intercalate into the [VO5] slabs [27,30,35]. No impurities were detected for all the samples. Therefore, doping of 0.1 mol of Mn2+ ions into one mole of V2O5 did not change the crystal structure, meaning that the MVC still possesses the orthorhombic phase [31,34]. Fig. 2 shows the V2p2/3 signal in XPS curves for all the samples. It can be seen that two binding energy peaks located at 516.73 eV and 515.31 eV, which are attributed to V5+ and V4+ ions, respectively [35]. The integral area of the peak at 515.31 eV can be used to evaluate the amount of V4+ ion in the sample. From Fig. 2a and b, one can see the VC revealed an integral area of V2p2/ 3 signal peak with respect to V4+ ion slightly larger than that of the
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BV, indicating the amount of V4+ ion in the VC is slightly higher than that in the BV. The reason is that a small number of V5+ ions is reduced to V4+ ones when the sample is annealed at a high temperature in nitrogen atmosphere. Clearly the MVC had the largest integral area of this peak (Fig. 2c) among three samples, indicating the amount of V4+ ions in the MVC is higher than those in other two samples. The results suggest that a few of Mn2+ ions were chemically inserted into the layered structure causing the valence reduction of V atoms.
3.2. Porous structures Fig. 3 shows the N2 adsorption-desorption isotherms and the corresponding pore-size-distribution curve of the MVC. The adsorption isotherm displays a type III curve, indicating a macroporous structure, whereas the type-H3 hysteresis loop suggests a molecular adsorption in the mesopores [37].The MVC exhibits the maximum pore size distribution at 3.7 nm and 35 nm, suggesting that the MVC possesses a hierarchically porous structure. The macropores facilitate penetration of the electrolyte
Fig. 6. Galvanostatic charging-discharging curves of the bare V2O5 (a), V2O5@C (c) and Mn0.1V2O5@C (e), and their corresponding cycling performances and coloumbic efficiency (b, d, f) at 0.1C rate (1C = 294 mA g1) for 50 cycles over the voltage range of 4.0–2.0 V at room temperature.
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Fig. 7. Rate capability of the V2O5, V2O5@C and Mn0.1V2O5@C (a) and their cycling performance at 5C rate (b) over the voltage range of 4.0–2.0 V at room temperature. (c) Exsitu XRD patterns of the V2O5, V2O5@C and Mn0.1V2O5@C charged to 4.0 V after the 5C-rate-cycling process.
into the bulk particles while the mesopores enlarge the contacting surface area between the particles and electrolyte [38].Much absorbed electrolyte could promise the electrochemically active materials’ enough Li+ ions while large contacting area provided a large number of intercalation sites. The measured BET specific surface area is 26.48 m2 g1, and the BJH volume is 0.136 cm3 g1. Such a porous structure would be very beneficial to the kinetics of (de) lithiation for the V2O5 material. The BET surface area is 21.34 m2 g1 for the VC and 13.65 m2 g1 for the BV. Their N2 adsorption-desorption isotherms and the corresponding poresize-distribution curves are shown in Fig. S2. (Supplementary information) 3.3. Morphologies of the Mn1V2O5@C SEM image of the MVC is shown in Fig. 4a. One can see that the MVC sample appeared as quasi spheres with an average diameter range of 2–5 mm. The quasi spheres are constructed with some nanorods that embed in a loose matrix(Fig. 4b). The nanorod displayed about 120 nm in length (Fig. 4b) and 20 nm in diameter (Fig. 4c). Mn element was detected in the MVC by energy dispersive spectrometry (EDS) analysis of the selected area in Fig. 4c (Fig. S3, supporting information). From the HRTEM image (Fig. 4d), the spacing between lamellas is found to be 0.45 nm, which is a little larger than that calculated by the Bragg equation
(0.437 nm) concerning (0 0 1) crystal plane. The result implied that a few of Mn2+ ions have intercalated the [VO5] interlayer and thus expand the lamellar spacing by the electrostatic interaction of Mn2 + ions with the anionic [VO5] slabs without destroying the crystal structure [27,39–41]. It is found that from the selected area electron diffraction (SAED) images (Fig. 4e & f), surrounding the nanorods revealed an amorphous phase while the nanorods exhibited a polycrystalline structure. A possible mechanism for the formation of such morphology is presented in Fig. 5. As is known, b-CD composed of seven a-1,4linked glucopyranose units has a hydrophilic external surface and a hydrophobic cavity. When PAA macromolecules mixed with b-CD in the ethanol/water, they could absorb onto the surface of b-CD cavity and aggregate to form spheres. After that, VO2+ ions that originated from the reaction of NH4VO3 with H2C2O4 are bound to the PAA macromolecules through an electrostatic interaction between the VO2+ cations and the anionic carbonyl groups. V2O5 nuclei originate from the absorbed VO2+ ions and deposit on the PAA macromolecular chains during hydrothermal treatment. Afterwards the PAA@V2O5 composite forms the nanorods after annealing at a high temperature. Meanwhile, glucose is carbonized to form a porous carbon matrix. Without encapsulated by the porous carbon matrix, the BV displayed the aggregated nanorods with smooth surface as shown in Fig. S4. (Supplementary information)
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Table 1 Comparison of the rate capability and cycling performance of the various kind of V2O5 materials. Morphology
Rate capability Discharge capacity (at x mA g1) (mAh g1)
Hierarchical V2O5 nanoflower
293 275 254 232 209
Ultrathin V2O5 nanosheet
Cycling performance Capacity retention (at x mA g1 after n cycles)
Ref.
74.2% (at 100 mA g1 after 250 cycles)
[14]
292 (59 mA g1) 266 (294 mA g1) 233 (1470 mA g1) 192 (2940 mA g1) 156 (5880 mA g1) 137 (8820 mA g1)
93.8% (at 59 mA g1 after 50 cycles)
[15]
V2O5/Mesoporous carbon Composite
291 (100 mA g1) 265 (250 mA g1) 247 (500 mA g1) 200 (1000 mA g1) 150 (2000 mA g1) 75 (5000 mA g1)
66% (at 500 mA g1 after 100 cycles)
[24]
Al0.16V2O5/RGO nanocomposite
290 (90 mA g1) 274 (300 mA g1) 122 (3000 mA g1)
90% (at 300 mA g1 after 50 cycles)
[27]
3D porous Fe0.1V2O5.15 thin film
278 (29.4 mA g1) 250 (88.2 mA g1) 245 (176.4 mA g1) 222 (441 mA g1) 202 (882 mA g1) 180 (1764 mA g1)
70.14% (at 58.8 mA g1 after 48 cycles)
[29]
Two-dimensional 5% Mn-V2O5 sheet network
165 (1000 mA g1)
Very stale (at 1000 mA g1 after 100 cycles)
[34]
Cu doped V2O5 flowers
229 (294 mA g1) 197 (882 mA g1) 182 (1470 mA g1) 126 (2940 mA g1) 97 (5880 mA g1)
85% (at 58.8 mA g1 after 50 cycles)
[41]
Ni–V2O5 hollow microspheres
294 (50 mA g1) 259 (300 mA g1) 228 (600 mA g1) 203 (1200 mA g1) 166 (2400 mA g1)
91% (at 300 mA g1 after 50 cycles)
[43]
Self-assembled hairy ball-like V2O5 nanostructures
278.3 (100 mA g1) 273 (500 mA g1)
92.89% (at 100 mA g1 after 500 cycles)
[44]
b-Na0.33V2O5 mesoporous flake
323 (20 mAg1) 271 (50 mAg1) 251 (100 mAg1) 226 (300 mA g1)
68.1% (at 300 mAg1 after 70 cycles)
[45]
Nanoflake assembled 3D hollow-porous microsphere
283 (100 mA g1) 225 (500 mA g1) 189 (1000 mA g1) 119 (2000 mA g1)
76.68% (at 100 mA g1 after 60 cycles)
[46]
2D Leaf-like V2O5nanosheets
303 (20 mA g1) 273 (200 mA g1) 251 (500 mA g1) 219 (1000 mA g1) 160 (2000 mA g1)
82.1% (at 500 mA g1 after 100 cycles)
[47]
V2Onanosphere/MWCNT layer-by-layer nano-architecture
292 (29.4 mA g1) 215 (294 mA g1) 170 (588 mA g1) 130 (1176 mA g1)
93% (at 294 after 100 cycles)
[48]
Mn0.1V2O5Nanorods embedded in carbon matrix
265 (29.4 mA g1) 247 (147 mA g1) 236 (588 mA g1) 219 (1470 mA g1) 186 (2940 mA g1) 164 (5880 mA g1).
94% (at 1470 mA g1 after 500 cycles)
This work
(30 mA g1) (60 mA g1) (150 mA g1) (300 mA g1) (600 mA g1)
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Fig. 8. Cyclic voltammogrames of the V2O5 (a), V2O5@C (b) and Mn0.1V2O5@C (c) at the 0.1 mV s1 scanning rate over the voltage range of 4.0–2.0 V at room temperature.
3.4. Electrochemical properties Fig. 6 shows the galvanostatic charging-discharging curves of the BV (a), VC (c) and MVC (e) samples and their cycling performances at 0.1C rate. It is found that the charging and discharging curves for all samples exhibited multiple potential plateaus within the 2.0–4.0 V range, indicating the multi-step
insertion of lithium into the interlayers of V2O5. However, the initial discharging curve is different from the subsequent ones for the BV and VC samples whereas the MVC sample revealed similar curves for the whole cycles. For the BV sample, the discharging potential plateaus located at 3.37, 3.19, 2.76, 2.54 and 2.33 V in the first cycle and displayed two additional plateaus at 3.60 and 3.45 V in the subsequent cycles (Fig. 6a). The VC sample showed the
Fig. 9. Nyquist curves at the discharge state of 3.0 V (a) and the Zre v1/2 plots in the low frequency range (b) of the V2O5, V2O5@C and Mn0.1V2O5@C samples.
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similar situation to the BV sample in the 3 V potential regions, but two additional plateaus at 3.60 and 3.45 V were presented in the following cycles (Fig. 6a). The VC sample revealed the similar situation to the BV sample in the 3 V potential regions except that two plateaus appeared at 2.13 and 2.01 V for the former (Fig. 6c). In the case of the MVC sample, the discharging potential plateaus looked the same in all cycles except for an additional plateau at 2.05 V in the first cycle (Fig. 6e). It is obvious that at the low potentials, e.g. 2.13 and 2.01 V for the VC, and 2.05 V for the MVC, the carbon encapsulated samples could display potential plateaus because the electrons might transport more quickly in these samples than in that not encapsulated. The discharging specific capacities are 272, 273 and 265 mAh g1 for the BV, VC and MVC in the first cycle, respectively, and retain 126 (Fig. 6b, after 30 cycles), 165 (Fig. 6 d, after 45 cycles) and 206 mAh g1 (Fig. 6f, after 45 cycles) in turn. To evaluate their rate capability, the samples were operated at various rates over the voltage range of 2.0–4.0 V at room temperature. Fig. 7a compares the rate capabilities of the MVC, VC and BV samples in the first cycle. It can be seen that the MVS quasi spheres could deliver the initial capacities of 265, 247, 236, 219, 186 and 164 mAh g1 at the rates of 0.1, 0.5, 2, 5, 10 and 20C, respectively. The discharging capacity at 20C rate is about 62% of that at 0.1C rate. The rate capability of the MVC is better than those of nanomaterials [14,15,44,46,47] and the doped materials [27,29,34,41,43,45], and is comparable to those of composites (Table 1) [24,27,48]. One can see that the discharging capacity at 20C rate is 126 and 68 mAh g1 for the VC and the BV, respectively, corresponding to 46% and 25% of their initial ones at 0.1C rate. Therefore, the MVC possessed the best rate capability among the samples. That the Mn-doping V2O5 nanorods are embedded uniformly in the porous carbon matrix is beneficial to the electrochemical kinetics of electrode materials. Meanwhile, the nanorods shorten diffusion distance of Li+ ion while the carbon matrix accelerates electrons’ transfer. Fig. 7b displays the cycling performances of the BV, VC and MVC samples at 5C rate over the potential range of 2.0–4.0 V. It is found that after 100 cycles, the BV retains a capacity of 93 mAh g1 (59% of the initial one) due to its unstable crystal structure, which can be proved by the ex-situ XRD pattern after cycling (Fig. 7c). One can see that the layered crystal structure has already collapsed. On the contrary, the MVC is found to possess some diffraction peaks belonging to the crystal planes of (2 0 0), (0 0 1), (11 0), (1 0 0), (0 11), and (3 1 0) after 500 cycles, which confirmed the pillar effect of Mn2+ ion between the VO5 slabs. Consequently, it can deliver the capacities of 206 mAh g1 at 5C rate after 500 cycles, retaining 94% of the initial one. So far, this cycle stability is the best among the reported as well [14–48].The results suggest that the MVC possesses the best cycling performance, which is attributed to both Mn-doping and carbon encapsulation. The former stabilizes the crystal structure of the V2O5nanorods while the latter decreases their dissolution into the electrolyte. Fig. 8 shows the cyclic voltammograms (CVs) of the BV, VC and MVC samples at the scan rates of 0.1 mV s1 within the potential range of 2.0–4.0 V (vs. Li+/Li). One can see that the MVC and VC samples exhibit higher peak currents than the BV sample. The MVC reveals a larger integral area between 2.0 V and 3.0 V and better overlayed CV for the first five cycles compared with the other two samples. The potential peaks at 3.35, 3.16, and 2.26 V (vs. Li+/Li) in the cathodic sweep correspond to the phase changes from a-V2O5 to e-Li0.5V2O5, e-Li0.5V2O5 to d-LiV2O5, and d-LiV2O5 to g-Li2V2O5, respectively [49]. The other two peaks locate at 3.54, 3.42 V (vs. Li+/ Li) whose ascriptions are not clear at this stage. These reduction (intercalation) potential peaks are consistent with the multi-step
Fig. 10. Schematic illustration of the carbon encapsulated V2O5 quasi sphere.
potential plateaus occurring in the discharge curve (Fig. 6e). In the anodic sweep, the CVs also appear multi-peaks for all of the samples. All the potentials at peaks are listed in Table S1 (Supplementary information). Fig. 9a displays the Nyquist curves of the BV, VC and MVC samples that have been activated for several times and discharged to the potential of 3.0 V (vs. Li+/Li). It is found that the Nyquist curves consist of a depressed semicircle in the high-frequency region and a sloped line in the low-frequency region. The straight line is relative to the diffusion control of Li+ ions in the bulk electrode, corresponding to the Warburg impedance. The fitting equivalent circuit was depicted and shown as Fig. 9a inset. The intercept at the real resistance axis with respect to the left terminal of the depressed semicircle represents the ohmic resistance of the electrolyte (Rs) as shown in the equivalent circuit (Fig. 8a inset) while the right one relates to the charge transfer resistance (Rct). The values of Rs and Rct are summarized in Table S2 (Supplementary information). CPE represents the non-ideal double layer capacitance for lithium-ion intercalation. W is the Warburg impedance of the solid-phase diffusion. When the diffusion of Li is a rate-controlling step in the electrochemical reaction, the lithium-diffusion coefficient (DLi) can be calculated according to Eq. (1) [50–54]. DLi ¼
R2 T 2 2A2 n4 F 4 C 2
sw
ð1Þ
where R is the gas constant (8.314 J mol1 K1), T the absolute temperature (298.15 K), A the surface area of the electrode (1.13 cm2), n the number of electron, F the Faraday’s constant (96485C mol1), C the concentration of lithium ion in the LiV2O5(1.32 102 mol cm3), 27 and s w is the Warburg factor, which is the slope of the Zre versus v1/2 plots (Fig. 9b) based on Eq. (2). Z re ¼ Rs þ Rct þ s w v1=2
ð2Þ
According to Eq. (1) and Eq. (2), the lithium diffusion coefficients of the BV, VC and MVC samples are 1.27 1011, 2.02 1011 and 6.81 1011 cm2 s1, respectively. It is clear that the BV had a lower DLi value than the VC and MVC because the BV exhibited a lower BET specific surface area than the other two samples, which led to less amount of intercalation sites. As shown in Fig. 10, owing to highly penetrating pores within the quasi sphere, Li+ ions in the absorbed electrolyte can migrate to the surface of the V2O5 or Mn0.1V2O5 nanorods fast. Moreover, intercalation of Li+ ions benefits from the enlarged lamellar spacing by doping of Mn2+ ions. As a result, the MVC exhibits the largest DLi value.
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4. Conclusions In summary, carbon encapsulated Mn-doped V2O5 quasi spheres (Mn0.1V2O5@C) have been successfully prepared by a hydrothermal treatment followed by high-temperature calcination. These quasi spheres were built of nanorods that embedded uniformly in a porous carbon matrix. The porous structure facilitated penetrations of electrolyte into the quasi spheres leading to forming 3D ion-conduct network while the porous carbon matrix resulting in the 3D electron-conduct network. Owing to such a unique morphology, the obtained composite material possessed an excellent rate capability over the potential rang of 4.0–2.0 V (vs. Li/Li+). It could deliver 164 mAh g1 at 20C rate, which is about 62% of the discharge capacity at 0.1C rate (265 mAh g1). Also they exhibited a long span life(retaining 94% of the initial capacity after 500 cycles at 5C rate) due to carbon encapsulation and Mn-doping. The results suggest that the Mn0.1V2O5@Cquasi spheres could be used as cathode material in rechargeable lithium batteries. Acknowledgements The authors greatly appreciate the financial support from the Key R&D Program of Hunan Province (2015JC3091), National Natural Science Foundation of China (No. 21174119, 21376069, 21576075), Hunan Natural Science Foundation (2015JJ3115), and the Program for Innovative Research Cultivation Team in University of Ministry of Education of China (1337304). References [1] B. Kang, G. Ceder, Battery materials for ultrafast charging and discharging, Nature 458 (2009) 190–193. [2] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7 (2008) 845–954. [3] L. Croguennec, M.R. Palacin, Recent achievements on inorganic electrode materials for lithium-ion batteries, J. Am. Chem. Soc. 137 (2015) 3140–3156. [4] Y.X. Tang, Y.Y. Zhang, W.L. Li, B. Ma, X.D. Chen, Rational material design for ultrafast rechargeable lithium-ion batteries, Chem. Soc. Rev. 44 (2015) 5926– 5940. [5] M.S. Whittingham, Lithium batteries and cathode materials, Chem. Rev. 104 (2004) 4271–4301. [6] Y. Wang, G. Cao, Developments in nanostructured cathode materials for highperformance lithium-ion batteries, Adv. Mater. 20 (2008) 2251–2269. [7] J. Liu, H. Xia, D. Xue, L. Lu, Double-shelled nanocapsules of V2O5-based composites as high-performance anode and cathode materials for Li ion batteries, J. Am. Chem. Soc. 131 (2009) 12086–12087. [8] E. Potiron, A.L.G. La Salle, A. Verbaere, Y. Piffard, D. Guyomard, Electrochemically synthesized vanadium oxides as lithium insertion hosts, Electrochim. Acta 45 (1999) 197–214. [9] A.S. Arico, P. Bruce, B. Scrosati, Nanostructured materials for advanced energy conversion and storage devices, Nat. Mater. 4 (2005) 366–377. [10] V.M. Mohan, B. Hu, W. Qiu, Synthesis structural, and electrochemical performance of V2O5 nanotubes as cathode material for lithium battery, J. Appl. Electrochem. 39 (2009) 2001–2006. [11] D. Yu, C. Chen, S. Xie, Y. Liu, G. Cao, Mesoporous vanadium pentoxide nanofibers with significantly enhanced li-ion storage properties by electrospinning, Energy Environ. Sci. 4 (2011) 858–861. [12] J. Yang, Z. Li, J. Wang, Q. Xiao, G. Lei, X. Zhou, An exfoliated vanadium pentoxide nanoplatelet and its electrochemical properties for lithium-ion batteries, Func. Mater. Lett. 5 (2012) 1250019. [13] R. Yu, C. Zhang, Q. Meng, Z. Chen, H. Liu, Z. Guo, Facile synthesis of hierarchical networks composed of highly interconnected V2O5 nanosheets assembled on carbon nanotubes and their superior lithium storage properties, ACS Appl. Mater. Interfaces 5 (2013) 12394–12399. [14] G.Z. Li, Y.C. Qiu, Y. Hou, H.F. Li, L.S. Zhou, H. Deng, Y.G. Zhang, Synthesis of V2O5 hierarchical structures for long cycle-life lithium-ion storage, J. Mater. Chem. A 3 (2015) 1103–1109. [15] X. Rui, Z. Lu, H. Yu, D. Yang, H.H. Hng, T.M. Lim, Q. Yan, Ultrathin V2O5 nanosheets cathodes: realizing ultrafast reversible lithium storage, Nanoscale 5 (2013) 556–560. [16] X.W. Zhou, C.J. Cui, G.M. Wu, A novel and facile way to synthesize vanadium pentoxide nanospike as cathode materials for high performance lithium ion batteries, J. Power Sources 238 (2013) 95–102.
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