- Email: [email protected]
Synthesis of hollow nickel oxide nanotubes by electrospinning with structurally enhanced lithium storage properties
Materials Letters 136 (2014) 74–77
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis of hollow nickel oxide nanotubes by electrospinning with structurally enhanced lithium storage properties Xiaoyan Yan a,n, Xili Tong b,n, Jian Wang a, Changwei Gong a, Mingang Zhang a, Liping Liang a a b
Institute of Materials Science and Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, PR China State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, CAS, Taiyuan 030001, China
art ic l e i nf o
a b s t r a c t
Article history: Received 1 July 2014 Accepted 31 July 2014 Available online 8 August 2014
We report a simple electrospinning synthesis method to prepare hollow NiO nanotubes composed of interconnected nanoparticles. The synthesized NiO nanotubes show diameters of 250–300 nm. As an anode material for lithium-ion batteries, the resultant hollow NiO nanotubes exhibit high capacity and good cycle stability (726 mA hg 1 at 0.2C up to 150 cycles), as well as good rate capability. The hollow nanotube structure possesses the following features: high NiO-electrolyte contact area, fast Li ion diffusion and better accommodation of volume change. It suggests that the hollow NiO nanotube is a promising anode material for high energy density lithium-ion batteries. & 2014 Elsevier B.V. All rights reserved.
Keywords: Porous materials Deposition Nanotube Nickel oxide Lithium ion battery Energy storage and conversion
1. Introduction Nowadays, great efforts have been devoted to exploring new electrode materials to meet the ever-growing demand for lithiumion batteries (LIBs) with higher power and energy densities. Transition metal oxides have attracted great attention since Poizot et al. reported the transition metal oxides were a new type of anode materials of LIBs in 2000 [1]. Compared with commercial graphite materials (a theoretical capacity of 374 mAh g 1), transition metal oxides can deliver much higher reversible lithium storage capacity of two to three times higher than that of carbon materials. Among the explored transition metal oxides, NiO is believed to be a promising candidate because of its high theoretical capacity of 718 mAh g 1 (according to the reaction NiO þ2Li þ þ2e 2NiþLi2O), low cost and excellent chemical stability [2]. However, the commercial application of NiO as LIBs anodes is still hampered by its poor capacity retention and cycling stability arising from large specific volume variation, which inherently accompanies the conversion reaction process and causes pulverization, aggregations and deterioration of active materials during cycling [3]. Until now, two main strategies (coupling with a conductive matrix [4], and construction of various nanostructures [5–7]) have been developed for alleviating the adverse mechanical effects to improve the overall electrochemical
n
Corresponding authors. Tel.: þ 86 351 4605282. E-mail addresses: [email protected] (X. Yan), [email protected] (X. Tong). http://dx.doi.org/10.1016/j.matlet.2014.07.183 0167-577X/& 2014 Elsevier B.V. All rights reserved.
performance of NiO anodes. The strategy of nanostructure design has been demonstrated to be an effective approach to improve the electrochemical performance of NiO because of the sufficient contact of active material/electrolyte, large surface area, and short diffusion length of Li þ in the nanostructures. In well-designed nanostructures, not only the Li þ diffusion is much easier, but also the strain associated with volume change is often much better accommodated. And these merits can lead to significantly improved electrochemical performance. Particularly, nanotube architecture is accepted as a favorable morphology for advanced electrode materials. Recently, NiO nanotubes were prepared via AAO template or biotemplate and enhanced performances were proven in these systems [8,9]. In this work, high-quality hollow NiO nanotubes have been prepared by a facile and scalable electrospinning method without template. The NiO nanotubes consist of interconnected nanoparticles of 30–50 nm. Impressively, the as-prepared NiO nanotubes exhibit superior performance with high specific capacity and excellent capacity retention. The hollow nanotube structure is responsible for the enhanced electrochemical properties.
2. Experimental In a typical synthesis, 0.8 g of polyacrylonitrile (PAN) was added into 20 mL N,N-dimethylformamide (DMF) and stirred at 60 1C for 8 h. After that, 2.4 g of Nickel(II) acetylacetonate (Ni (AcAc)2) was added into the above solution and stirred continuously for 8 h to yield homogeneous solution. A high-voltage power
X. Yan et al. / Materials Letters 136 (2014) 74–77
of 20 kV was applied to the needle tip. The flow rate of liquid was set to 1 mL h 1. The humidity level inside the electrospinning chamber was 5575%. The prepared samples were collected on an aluminum foil collector. The electrospun Ni(AcAc)2/PAN fiber precursor composites were firstly stabilized in the air atmosphere at 300 1C for 1 h. Then the samples were annealed at 500 1C for 3 h in air to obtain the NiO nanotubes. The morphology and microstructure of the products were characterized by X-ray diffractometer (XRD, Rigaku D/max 2550 PC, Cu Kα), scanning electron microscopy (SEM, Hitachi S-4700 and FESEM, FEI Sirion-100), transmission electron microscopy (TEM, JEM 200CX at 160 kV, Tecnai G2 F30 at 300 kV). The electrochemical tests were carried out using a coin-type half cell (CR 2025) with pure lithium foil as both the counter and the reference electrodes. The working electrode was made of a mixture containing the active material, conducting acetylene black, and polyvinylidene fluoride (PVDF) in a weight ratio of 85:10:5. The load weight of NiO nanotube is 1.2 mg cm 2. Test
(311) (222)
(220)
40
3. Results and discussion Fig. 1a and b show the SEM images of the as-prepared hollow NiO nanotubes. The synthesized NiO sample exhibits uniform nanotube structure with diameter from 250 to 300 nm. Notice that the NiO nanotube is composed of closely packed nanoparticles of 30–50 nm as primary building blocks (Fig. 1(b)). The tubular morphology is clearly distinguished by TEM image as illustrated in
50
2θ (degree)
60
70
80
160 120
Pore volume(cm g )
Volume Adsorbeed (cm3g-1)
(200)
30
cells were assembled in an argon-filled glove box with the electrolyte of 1 M LiPF6 in ethylene carbonate (EC)-dimethyl carbonate (DME) (1: 1 in volume), and a polypropylene (PP) micro-porous film (Cellgard 2300) as the separator. Cyclic voltammetry (CV) was performed on theCHI660E electrochemical workstation. The galvanostatic charge/discharge tests were conducted on a LAND battery program-control test system at room temperature (25 71 1C).
200
(111)
Intensity (a.u.) 20
75
0.85 0.80 0.75 0.70 0.65 0.60
2
4
6
80
8 10 12 14 Pore size (nm)
16
18
40 0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0)
Fig. 1. (a) and (b) SEM and (c) and (d) TEM images of hollow NiO nanotube (SAED pattern in inset); (e) XRD patterns of the hollow NiO nanotubes and (f) BET measurement of NiO nanotube.
76
X. Yan et al. / Materials Letters 136 (2014) 74–77
3.0 +
)
0.6
Potential(V vs. Li/Li
Current (mA)
0.4 0.2 0.0 -0.2 -0.4 -0.6
2.5 2.0
1 st 2 nd 3 rd
1.5 1.0 0.5 0.0
-0.8
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0
200
+
)
0.90 0.85
600
0.80 0.75
400 0
20
40
60
80 100 120 140 160
800
0.70
-1
1000
1200
)
) -1
0.95 800
600
1200
Capacity (mAh g
1.00
1000
400
Capacity ( mAh g
Coulombic Efficiency
Specific capacity (mAh g-1)
Potential (V vs. Li/Li
900
0.2 C 0.2 C
0.5 C
600
1C 2C
300 0
0
10
20
30
40
50
Cycle Number
Cycle number
Fig. 2. Electrochemical performances of hollow NiO nanotubes: (a) Cyclic voltammograms (CVs) at a scanning rate of 0.1 mV s 1 between 0.01 and 3.0 V; (b) charge/ discharge profiles at the first three cycles; (c) cycling performance at the current density of 0.2C; and (d) rate capability at different current densities ranging from 0.2C to 2C.
Fig. 1(c) and (d). The TEM result verifies that the NiO nanotube consists of interconnected nanoparticles of 30 50 nm (Fig. 1(d)). And the thickness of the nanotube wall is estimated to be 30 40 nm. In addition, all diffraction rings in the selected area electronic diffraction (SAED) pattern of the nanotube can be indexed with cubic NiO phase (JCPDS 47-1049), indicating that the NiO nanotube is polycrystalline in nature, supported by the XRD pattern (shown in Fig. 1(e)). All the diffraction peaks in the XRD pattern can be assigned to cubic structural NiO (JCPDS 471049). Five diffraction peaks at 37.2, 43.2, 62.8, 75.3 and 79.3 degree correspond to the 111, 200, 220, 311 and 222 reflections of the cubic NiO phase, respectively. Meanwhile, no other peaks are identified, implying the NiO samples have good phase purity. In addition, the NiO nanotube shows a surface area of 91 m2 g 1 (Fig. 1(f)). The pore size distribution shows peaks at 7 and 11 nm, respectively. The lithium storage capability of hollow NiO nanotubes is fully characterized to demonstrate its potential application as anode for LIBs. Fig. 2(a) shows the first cycle of CV for the NiO nanotube electrode. There is a strong broad peak at 1.0 V in the cathodic process, which corresponds to the initial reduction from NiO to metallic Ni and the formation of a partially reversible solid electrolyte interface (SEI) film. In the anodic process, a weak broad peak at about 1.5 V and a strong peak at about 2.3 V can be observed, which corresponds to the decomposition of SEI film and Li2O, respectively [2,10]. Fig. 2(b) shows the first three galvanostatic charge/discharge curves of the hollow NiO nanotube electrode at 0.2C. The NiO nanotube electrode presents an extended voltage plateau at about 0.95 V, followed by a sloping curve down to the cut-off voltage of 0.01 V during the first discharge curves, which is consistent with the CV results. And the first discharge and charge capacity of the hollow NiO nanotubes electrode is 1014 and 744.6 mAh g 1, respectively. The initial irreversible capacity loss may be mainly ascribed to irreversible formation of the inevitable SEI film and the decomposition of electrolyte [11,12]. After the first cycle, the extended discharge potential plateau is replaced by long sloped curves from 1.5 to 0.8 V. And the electrode shows a similar
shape and is nearly overlapped in the following cycles, suggesting an excellent cycling stability. Fig. 2(c) shows the cyclability of the hollow NiO nanotube electrode at 0.2C. The reversible capacity of the electrode remains almost constant after the second cycle, with a coulombic efficiency of each cycle over 98.5%. After cycling for 150 cycles, the NiO nanotube still exhibits a specific reversible capacity of 726 mAh g 1. This value is higher than other NiO powder materials [13,14]. In addition to the good cycling performance, the hollow NiO nanotubes electrode also shows good high rate performance (Fig. 2(d)). With increasing the current density from 0.2C to 2C, the discharge capacities decrease gradually, indicating the diffusion-controlled kinetics process for the electrode reaction. When the current density is as high as 2C, the electrode can deliver a stable capacity of about 391.6 mAh g 1 after 30 cycles of variable discharging rate, which is still much higher than that of the commercial graphite. Upon decreasing the current density back to 0.2C, nearly 90.1% of the capacity of the second cycle at 0.2C can be recovered. The good Li þ storage capacity and stability of hollow NiO nanotubes electrode is mainly attributed to its hollow nanotube structure, which greatly facilitates Li þ diffusion and increases contact areas [15]. And the voids in the hollow NiO nanotubes can also provide additional space to store Li þ contributing to an extra specific capacity and buffering against the local volume change during the conversion process. The results above imply that the as-prepared NiO nanotube is a promising anode material for LIBs with high energy/power density.
4. Conclusion In summary, hollow NiO nanotubes are prepared by a simple electrospinning synthesis method. The hollow NiO nanotubes electrode exhibit high discharge capacity, good cycle stability, and enhanced rate capability due to the unique nanotube structure, which provides high interfacial contact area between the active materials and the electrolyte and good accommodation of
X. Yan et al. / Materials Letters 136 (2014) 74–77
volume change. The construction of a hollow nanotube structure is suggested as a favorable strategy toward the development of transition metal oxides as high performance electrode materials for LIBs.
[2] [3] [4] [5] [6] [7]
Acknowledgements
[8] [9]
This work is supported by the Foundation of State Key Laboratory of Coal Conversion (Grant No. J14-15-909).
[10] [11] [12] [13]
References [1] Poizot P, Laruelle S, Grugeon S, Dupont L, Tarascon JM. Nature 2000;407:496–9.
[14] [15]
77
Liu H, Wang GX, Liu J, Qiao SZ, Ahn HJ. J Mater Chem 2011;21:3046–52. Zhu YQ, Guo HZ, Wu Y, Cao CB, Tao S, Wu ZY. J Mater Chem A 2014;2:7904–11. Wang Q, Zhang C-Y, Shan W-F, Xing L-L, Xue X-Y. Mater Lett 2014;118:66–8. Xia X, Zhang Y, Chao D, Guan C, Zhang Y, Li L, et al. Nanoscale 2014;6:5008–48. Cheng MY, Ye YS, Chiu TM, Pan CJ, Hwang BJ. J Power Sources 2014;253:27–34. Yan XY, Tong XL, Wang J, Gong CW, Zhang MG, Liang LP. Mater Lett 2013;106:250–3. Needham SA, Wang GX, Liu HK. J Power Sources 2006;159:254–7. Liu LX, Guo YY, Wang YP, Yang XJ, Wang SX, Guo H. Electrochim Acta 2013;114:42–7. Xie D, Su QM, Dong ZM, Zhang J, Du GH. Cryst Eng Comm 2013;15:8314–9. Xia X, Chao D, Qi X, Xiong Q, Zhang Y, Tu J, et al. Nano Lett 2013;13:4562–8. Xia X, Xiong Q, Zhang Y, Tu J, Ng CF, Fan HJ. Small 2014;10:2419–28. Liu L, Li Y, Yuan SM, Ge M, Ren MM, Sun CS, et al. J Phys Chem C 2010;114:251–5. Mai YJ, Shi SJ, Zhang D, Lu Y, Gu CD, Tu JP. J Power Sources 2012;204:155–61. Xia X, Chao D, Fan Z, Guan C, Cao X, Zhang H, et al. Nano Lett 2014;14:1651–8.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis of hollow nickel oxide nanotubes by electrospinning with structurally enhanced lithium storage properties Xiaoyan Yan a,n, Xili Tong b,n, Jian Wang a, Changwei Gong a, Mingang Zhang a, Liping Liang a a b
Institute of Materials Science and Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, PR China State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, CAS, Taiyuan 030001, China
art ic l e i nf o
a b s t r a c t
Article history: Received 1 July 2014 Accepted 31 July 2014 Available online 8 August 2014
We report a simple electrospinning synthesis method to prepare hollow NiO nanotubes composed of interconnected nanoparticles. The synthesized NiO nanotubes show diameters of 250–300 nm. As an anode material for lithium-ion batteries, the resultant hollow NiO nanotubes exhibit high capacity and good cycle stability (726 mA hg 1 at 0.2C up to 150 cycles), as well as good rate capability. The hollow nanotube structure possesses the following features: high NiO-electrolyte contact area, fast Li ion diffusion and better accommodation of volume change. It suggests that the hollow NiO nanotube is a promising anode material for high energy density lithium-ion batteries. & 2014 Elsevier B.V. All rights reserved.
Keywords: Porous materials Deposition Nanotube Nickel oxide Lithium ion battery Energy storage and conversion
1. Introduction Nowadays, great efforts have been devoted to exploring new electrode materials to meet the ever-growing demand for lithiumion batteries (LIBs) with higher power and energy densities. Transition metal oxides have attracted great attention since Poizot et al. reported the transition metal oxides were a new type of anode materials of LIBs in 2000 [1]. Compared with commercial graphite materials (a theoretical capacity of 374 mAh g 1), transition metal oxides can deliver much higher reversible lithium storage capacity of two to three times higher than that of carbon materials. Among the explored transition metal oxides, NiO is believed to be a promising candidate because of its high theoretical capacity of 718 mAh g 1 (according to the reaction NiO þ2Li þ þ2e 2NiþLi2O), low cost and excellent chemical stability [2]. However, the commercial application of NiO as LIBs anodes is still hampered by its poor capacity retention and cycling stability arising from large specific volume variation, which inherently accompanies the conversion reaction process and causes pulverization, aggregations and deterioration of active materials during cycling [3]. Until now, two main strategies (coupling with a conductive matrix [4], and construction of various nanostructures [5–7]) have been developed for alleviating the adverse mechanical effects to improve the overall electrochemical
n
Corresponding authors. Tel.: þ 86 351 4605282. E-mail addresses: [email protected] (X. Yan), [email protected] (X. Tong). http://dx.doi.org/10.1016/j.matlet.2014.07.183 0167-577X/& 2014 Elsevier B.V. All rights reserved.
performance of NiO anodes. The strategy of nanostructure design has been demonstrated to be an effective approach to improve the electrochemical performance of NiO because of the sufficient contact of active material/electrolyte, large surface area, and short diffusion length of Li þ in the nanostructures. In well-designed nanostructures, not only the Li þ diffusion is much easier, but also the strain associated with volume change is often much better accommodated. And these merits can lead to significantly improved electrochemical performance. Particularly, nanotube architecture is accepted as a favorable morphology for advanced electrode materials. Recently, NiO nanotubes were prepared via AAO template or biotemplate and enhanced performances were proven in these systems [8,9]. In this work, high-quality hollow NiO nanotubes have been prepared by a facile and scalable electrospinning method without template. The NiO nanotubes consist of interconnected nanoparticles of 30–50 nm. Impressively, the as-prepared NiO nanotubes exhibit superior performance with high specific capacity and excellent capacity retention. The hollow nanotube structure is responsible for the enhanced electrochemical properties.
2. Experimental In a typical synthesis, 0.8 g of polyacrylonitrile (PAN) was added into 20 mL N,N-dimethylformamide (DMF) and stirred at 60 1C for 8 h. After that, 2.4 g of Nickel(II) acetylacetonate (Ni (AcAc)2) was added into the above solution and stirred continuously for 8 h to yield homogeneous solution. A high-voltage power
X. Yan et al. / Materials Letters 136 (2014) 74–77
of 20 kV was applied to the needle tip. The flow rate of liquid was set to 1 mL h 1. The humidity level inside the electrospinning chamber was 5575%. The prepared samples were collected on an aluminum foil collector. The electrospun Ni(AcAc)2/PAN fiber precursor composites were firstly stabilized in the air atmosphere at 300 1C for 1 h. Then the samples were annealed at 500 1C for 3 h in air to obtain the NiO nanotubes. The morphology and microstructure of the products were characterized by X-ray diffractometer (XRD, Rigaku D/max 2550 PC, Cu Kα), scanning electron microscopy (SEM, Hitachi S-4700 and FESEM, FEI Sirion-100), transmission electron microscopy (TEM, JEM 200CX at 160 kV, Tecnai G2 F30 at 300 kV). The electrochemical tests were carried out using a coin-type half cell (CR 2025) with pure lithium foil as both the counter and the reference electrodes. The working electrode was made of a mixture containing the active material, conducting acetylene black, and polyvinylidene fluoride (PVDF) in a weight ratio of 85:10:5. The load weight of NiO nanotube is 1.2 mg cm 2. Test
(311) (222)
(220)
40
3. Results and discussion Fig. 1a and b show the SEM images of the as-prepared hollow NiO nanotubes. The synthesized NiO sample exhibits uniform nanotube structure with diameter from 250 to 300 nm. Notice that the NiO nanotube is composed of closely packed nanoparticles of 30–50 nm as primary building blocks (Fig. 1(b)). The tubular morphology is clearly distinguished by TEM image as illustrated in
50
2θ (degree)
60
70
80
160 120
Pore volume(cm g )
Volume Adsorbeed (cm3g-1)
(200)
30
cells were assembled in an argon-filled glove box with the electrolyte of 1 M LiPF6 in ethylene carbonate (EC)-dimethyl carbonate (DME) (1: 1 in volume), and a polypropylene (PP) micro-porous film (Cellgard 2300) as the separator. Cyclic voltammetry (CV) was performed on theCHI660E electrochemical workstation. The galvanostatic charge/discharge tests were conducted on a LAND battery program-control test system at room temperature (25 71 1C).
200
(111)
Intensity (a.u.) 20
75
0.85 0.80 0.75 0.70 0.65 0.60
2
4
6
80
8 10 12 14 Pore size (nm)
16
18
40 0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0)
Fig. 1. (a) and (b) SEM and (c) and (d) TEM images of hollow NiO nanotube (SAED pattern in inset); (e) XRD patterns of the hollow NiO nanotubes and (f) BET measurement of NiO nanotube.
76
X. Yan et al. / Materials Letters 136 (2014) 74–77
3.0 +
)
0.6
Potential(V vs. Li/Li
Current (mA)
0.4 0.2 0.0 -0.2 -0.4 -0.6
2.5 2.0
1 st 2 nd 3 rd
1.5 1.0 0.5 0.0
-0.8
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0
200
+
)
0.90 0.85
600
0.80 0.75
400 0
20
40
60
80 100 120 140 160
800
0.70
-1
1000
1200
)
) -1
0.95 800
600
1200
Capacity (mAh g
1.00
1000
400
Capacity ( mAh g
Coulombic Efficiency
Specific capacity (mAh g-1)
Potential (V vs. Li/Li
900
0.2 C 0.2 C
0.5 C
600
1C 2C
300 0
0
10
20
30
40
50
Cycle Number
Cycle number
Fig. 2. Electrochemical performances of hollow NiO nanotubes: (a) Cyclic voltammograms (CVs) at a scanning rate of 0.1 mV s 1 between 0.01 and 3.0 V; (b) charge/ discharge profiles at the first three cycles; (c) cycling performance at the current density of 0.2C; and (d) rate capability at different current densities ranging from 0.2C to 2C.
Fig. 1(c) and (d). The TEM result verifies that the NiO nanotube consists of interconnected nanoparticles of 30 50 nm (Fig. 1(d)). And the thickness of the nanotube wall is estimated to be 30 40 nm. In addition, all diffraction rings in the selected area electronic diffraction (SAED) pattern of the nanotube can be indexed with cubic NiO phase (JCPDS 47-1049), indicating that the NiO nanotube is polycrystalline in nature, supported by the XRD pattern (shown in Fig. 1(e)). All the diffraction peaks in the XRD pattern can be assigned to cubic structural NiO (JCPDS 471049). Five diffraction peaks at 37.2, 43.2, 62.8, 75.3 and 79.3 degree correspond to the 111, 200, 220, 311 and 222 reflections of the cubic NiO phase, respectively. Meanwhile, no other peaks are identified, implying the NiO samples have good phase purity. In addition, the NiO nanotube shows a surface area of 91 m2 g 1 (Fig. 1(f)). The pore size distribution shows peaks at 7 and 11 nm, respectively. The lithium storage capability of hollow NiO nanotubes is fully characterized to demonstrate its potential application as anode for LIBs. Fig. 2(a) shows the first cycle of CV for the NiO nanotube electrode. There is a strong broad peak at 1.0 V in the cathodic process, which corresponds to the initial reduction from NiO to metallic Ni and the formation of a partially reversible solid electrolyte interface (SEI) film. In the anodic process, a weak broad peak at about 1.5 V and a strong peak at about 2.3 V can be observed, which corresponds to the decomposition of SEI film and Li2O, respectively [2,10]. Fig. 2(b) shows the first three galvanostatic charge/discharge curves of the hollow NiO nanotube electrode at 0.2C. The NiO nanotube electrode presents an extended voltage plateau at about 0.95 V, followed by a sloping curve down to the cut-off voltage of 0.01 V during the first discharge curves, which is consistent with the CV results. And the first discharge and charge capacity of the hollow NiO nanotubes electrode is 1014 and 744.6 mAh g 1, respectively. The initial irreversible capacity loss may be mainly ascribed to irreversible formation of the inevitable SEI film and the decomposition of electrolyte [11,12]. After the first cycle, the extended discharge potential plateau is replaced by long sloped curves from 1.5 to 0.8 V. And the electrode shows a similar
shape and is nearly overlapped in the following cycles, suggesting an excellent cycling stability. Fig. 2(c) shows the cyclability of the hollow NiO nanotube electrode at 0.2C. The reversible capacity of the electrode remains almost constant after the second cycle, with a coulombic efficiency of each cycle over 98.5%. After cycling for 150 cycles, the NiO nanotube still exhibits a specific reversible capacity of 726 mAh g 1. This value is higher than other NiO powder materials [13,14]. In addition to the good cycling performance, the hollow NiO nanotubes electrode also shows good high rate performance (Fig. 2(d)). With increasing the current density from 0.2C to 2C, the discharge capacities decrease gradually, indicating the diffusion-controlled kinetics process for the electrode reaction. When the current density is as high as 2C, the electrode can deliver a stable capacity of about 391.6 mAh g 1 after 30 cycles of variable discharging rate, which is still much higher than that of the commercial graphite. Upon decreasing the current density back to 0.2C, nearly 90.1% of the capacity of the second cycle at 0.2C can be recovered. The good Li þ storage capacity and stability of hollow NiO nanotubes electrode is mainly attributed to its hollow nanotube structure, which greatly facilitates Li þ diffusion and increases contact areas [15]. And the voids in the hollow NiO nanotubes can also provide additional space to store Li þ contributing to an extra specific capacity and buffering against the local volume change during the conversion process. The results above imply that the as-prepared NiO nanotube is a promising anode material for LIBs with high energy/power density.
4. Conclusion In summary, hollow NiO nanotubes are prepared by a simple electrospinning synthesis method. The hollow NiO nanotubes electrode exhibit high discharge capacity, good cycle stability, and enhanced rate capability due to the unique nanotube structure, which provides high interfacial contact area between the active materials and the electrolyte and good accommodation of
X. Yan et al. / Materials Letters 136 (2014) 74–77
volume change. The construction of a hollow nanotube structure is suggested as a favorable strategy toward the development of transition metal oxides as high performance electrode materials for LIBs.
[2] [3] [4] [5] [6] [7]
Acknowledgements
[8] [9]
This work is supported by the Foundation of State Key Laboratory of Coal Conversion (Grant No. J14-15-909).
[10] [11] [12] [13]
References [1] Poizot P, Laruelle S, Grugeon S, Dupont L, Tarascon JM. Nature 2000;407:496–9.
[14] [15]
77
Liu H, Wang GX, Liu J, Qiao SZ, Ahn HJ. J Mater Chem 2011;21:3046–52. Zhu YQ, Guo HZ, Wu Y, Cao CB, Tao S, Wu ZY. J Mater Chem A 2014;2:7904–11. Wang Q, Zhang C-Y, Shan W-F, Xing L-L, Xue X-Y. Mater Lett 2014;118:66–8. Xia X, Zhang Y, Chao D, Guan C, Zhang Y, Li L, et al. Nanoscale 2014;6:5008–48. Cheng MY, Ye YS, Chiu TM, Pan CJ, Hwang BJ. J Power Sources 2014;253:27–34. Yan XY, Tong XL, Wang J, Gong CW, Zhang MG, Liang LP. Mater Lett 2013;106:250–3. Needham SA, Wang GX, Liu HK. J Power Sources 2006;159:254–7. Liu LX, Guo YY, Wang YP, Yang XJ, Wang SX, Guo H. Electrochim Acta 2013;114:42–7. Xie D, Su QM, Dong ZM, Zhang J, Du GH. Cryst Eng Comm 2013;15:8314–9. Xia X, Chao D, Qi X, Xiong Q, Zhang Y, Tu J, et al. Nano Lett 2013;13:4562–8. Xia X, Xiong Q, Zhang Y, Tu J, Ng CF, Fan HJ. Small 2014;10:2419–28. Liu L, Li Y, Yuan SM, Ge M, Ren MM, Sun CS, et al. J Phys Chem C 2010;114:251–5. Mai YJ, Shi SJ, Zhang D, Lu Y, Gu CD, Tu JP. J Power Sources 2012;204:155–61. Xia X, Chao D, Fan Z, Guan C, Cao X, Zhang H, et al. Nano Lett 2014;14:1651–8.