Journal of Power Sources 288 (2015) 145e149
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Free-standing three-dimensional continuous multilayer V2O5 hollow sphere arrays as high-performance cathode for lithium batteries Minghua Chen a, e, **, Xinhui Xia b, c, d, *, Jiefu Yuan a, Jinghua Yin a, e, Qingguo Chen e a
School of Applied Science, Harbin University of Science and Technology, Harbin 150080, China Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore, Singapore c Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China d State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, CAS, Taiyuan 030001, China e Key Laboratory of Engineering Dielectric and Applications (Ministry of Education), Harbin University of Science and Technology, Harbin 150080, China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Construct self-supported porous multilayer V2O5 hollow sphere arrays. Multilayer V2O5 hollow sphere arrays show high Li ion storage properties. Hollow sphere arrays is favorable for keeping electrode structure stable.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 February 2015 Received in revised form 4 April 2015 Accepted 20 April 2015 Available online
Construction of advanced cathode materials is highly important for the development of highperformance lithium batteries. Herein, we report a facile polystyrene sphere template-assisted electrodeposition method for the fabrication of porous multilayer V2O5 hollow sphere arrays on graphite paper substrates. The obtained V2O5 arrays consist of interconnected hollow spheres with diameters of ~500 nm as well as thin walls of 20e30 nm. All pores in the hollow sphere arrays are connected with each other and reach out in all directions forming 3D porous networks. As cathode of lithium batteries, the multilayer V2O5 hollow sphere arrays show a high speciﬁc capacity of 293 mAh g1 at 0.5C and good high-rate cycling stability with a speciﬁc capacity of 202 mAh g1 at 5C after 300 cycles. The enhancement of the electrochemical properties is mainly due to the unique hollow sphere arrays architecture, which provides fast ion/electron transfer path and sufﬁcient contact between active materials and electrolyte, and alleviates the structure degradation during the cycling process. © 2015 Elsevier B.V. All rights reserved.
Keywords: Vanadium pentoxide Li battery Hollow spheres Cathode Arrays
1. Introduction * Corresponding author. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore, Singapore. ** Corresponding author. School of Applied Science, Harbin University of Science and Technology, Harbin 150080, China. E-mail addresses: [email protected]
(M. Chen), [email protected]
(X. Xia). http://dx.doi.org/10.1016/j.jpowsour.2015.04.130 0378-7753/© 2015 Elsevier B.V. All rights reserved.
In the past decades, lithium batteries (LBs) have become a primary focus of scientiﬁc community. There has been great interest in developing and reﬁning more efﬁcient LBs with high capacity and good high-rate capability . The energy/power densities of LBs are largely dependent on the physical and chemical properties of the
M. Chen et al. / Journal of Power Sources 288 (2015) 145e149
electrode materials. Though LBs consist of four parts (such as cathode, anode, electrolyte and separator), the performance of LBs is mainly controlled by the cathode that needs to have large capacity and/or high working voltage. Currently, the cathode materials involved one-electron insertion/extraction reaction usually deliver relative low capacities and could not meet the growing needs of high energy/power densities [2,3]. Therefore, researchers are turning their attention to cathode materials with multi-electron insertion and extraction reactions, which can produce/store much higher capacity leading to enhanced energy/power densities. Of the explored cathode candidates, vanadium pentoxide (V2O5) is a promising cathode material due to the fact that it can capture multi-electrons because of its layered crystal structure along the c-axis and serve as a good host for the reversible Liþ insertion/extraction according to the simpliﬁed reaction (V2O5 þ xLiþ þ xe 4 LixV2O5) [4e7]. Accordingly, V2O5 can deliver a high capacity of 294 mAh g1 in the voltage range of 4.0e2.0 V (vs. Li/Liþ). However, its practical application is restrained by the poor rate capability and cycling stability due to its low diffusion coefﬁcient of Li ions, low electrical conductivity, and large speciﬁc volume change causing pulverization and deterioration of active materials during cycling . An effective way to enhance rate-capability and cycling stability is to fabricate highly porous nanostructured V2O5 electrodes. Recent researches have demonstrated that porous structures could effectively buffer the volume expansion in the V2O5 cathode avoiding rapid capacity fading, resulting in improved cycling performance, especially at high charge/discharge rates [8e11]. Additionally, these porous structures could provide a very short diffusion pathway for lithium ion as well as large active surface, which leads to enhanced electrochemical properties. Several V2O5 nanostructures (e.g., nanobelts , nanosheets , nanoﬂowers , and nanospheres [15,16]) have been prepared and improved electrochemical performance has been demonstrated in these systems when they are applied as cathode of LBs. In recent years, hollow-sphere materials have been intensively investigated as building components in LBs due to their novel interior geometry and surface functionality [17e20]. Up to now, several V2O5 micro/nano spheres have been prepared and enhanced Li ion storage properties are proven [15,21e27]. However, all these researches are focused on the powder materials, and there is no report about the fabrication of free-standing 3D multilayer interconnected V2O5 hollow sphere arrays and their application for LBs. In this work, we report self-supported 3D porous multilayer V2O5 hollow sphere arrays prepared by a templateassisted electro-deposition (ED) method. As cathode of LBs, the multilayer V2O5 hollow sphere arrays show good electrochemical performances with high capacitance and good high-rate capability. 2. Experimental The multilayer V2O5 hollow sphere arrays were prepared by a polystyrene sphere (PS) template-assisted ED method as shown in Fig. 1a. The multilayer PS (particle size of ~500 nm) template was assembled on the graphite paper substrate via a vertical deposition process described in detail in the previous work . The ED of V2O5 was performed in a three-compartment system at 25 C, the above template electrode as the working electrode, saturated calomel electrode (SCE) as the reference electrode and a Pt foil as the counter electrode. The electrolyte consisted of 1 M aqueous vanadium sulfate oxide hydrate solution containing 50% wt. ethanol with a pH value of 1.5. Anodic ED was carried out at a constant current of 3 mA cm2 for 1800 s. Then, the sample was immersed in toluene for 12 h to remove the PS spheres template. Finally, the sample was annealed at 350 C in air for 1 h to form
porous multilayer V2O5 hollow sphere arrays. The load weight of V2O5 is about 3.0 mg cm2. The samples were characterized by X-ray diffraction (XRD, RIGAKU D/Max-2550 with Cu Ka radiation), ﬁeld emission scanning electron microscopy (FESEM, FEI SIRION), high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010F), Raman spectroscopy (WITec-CRM200 Raman system with a laser wavelength of 532 nm). Electrochemical performances of the samples were investigated in a coin-type cell assembled in an argon-ﬁlled glove box with the as-fabricated multilayer V2O5 hollow sphere arrays as the working electrode, the metallic lithium foil as the counter electrode, 1 M LiPF6 in ethylene carbonate (EC)edimethyl carbonate (DME) (1:1 in volume) as the electrolyte, and a polypropylene (PP) ﬁlm (Cellgard 2400) as the separator. The CV measurements were carried out using a Parstat-2273 electrochemical station at a scanning rate of 0.25 mV s1 in the voltage range of 2e4 V. Galvanostatic charge/ discharge cycles were tested by Neware battery tester between 2 V and 4 V (vs. Li/Liþ) at room temperature. 3. Results and discussion Fig. 1b shows the assembled PS template on the graphite paper substrate. Notice that the PS is well organized and close-packed with each other forming a regular multilayer alignment perpendicular to the substrate. The individual PS has an average size of ~500 nm and the formed multilayer PS template shows a thickness of ~10 mm. After ED and etching the PS template, highly porous multilayer V2O5 hollow sphere arrays are observed (Fig. 1c). Note the fact that a 3D porous hollow sphere structure is well formed and the individual hollow sphere exhibits a size of ~500 nm (Fig. 1c and d). Moreover, all pores in the multilayer V2O5 hollow sphere arrays, whether on the surface or in the inner space, are connected with each other and reach out in all directions forming 3D porous networks, clearly conﬁrmed by the cross-sectional SEM image (inset in Fig. 1d). The wall of hollow spheres are about 20e30 nm (inset in Fig. 1d). The multilayer V2O5 hollow sphere arrays exhibit a thickness of ~10 mm. The hollow sphere architecture is further demonstrated by the TEM image (Fig. 1e). Obviously, the interconnected hollow spheres with size of ~500 nm are noticed. Additionally, the lattice fringes with a lattice spacing of ~0.57 nm correspond to the (200) planes of the orthogonal V2O5 phase (JCPDS 41e1426). Based on the SEM and TEM analysis, it is reasonable that the obtained porous multilayer V2O5 hollow sphere arrays possess impressive porous system, which would be beneﬁcial for the fast ion/electron transfer leading to fast reaction kinetics. This is especially important for the high-rate LBs application. The phase and composition of the samples were also monitored and conﬁrmed by the XRD pattern and Raman spectrum. From the XRD pattern (Fig. 2a), it is seen that except for the peak of graphite paper, the other diffraction peaks are indexed to the orthogonal V2O5 phase (JCPDS 41e1426) and no other purity peaks are observed. Meanwhile, the Raman peaks in the range of 100e1100 cm1 can be well indexed to crystalline V2O5 (Fig. 2b) . Thus, it is justiﬁed that the porous multilayer V2O5 hollow sphere arrays have been successfully prepared via the PS templateassisted ED method. Electrochemical tests were conducted to investigate the lithium ion storage properties of the multilayer V2O5 hollow sphere arrays. Fig. 3a shows the typical cyclic voltammograms (CV) curve at a scanning rate of 0.25 mV s1 at the 2nd cycle in the voltage range from 4 to 2 V (vs. Li/Liþ). Apparently, three redox peaks are noticed in the CV curve. The ﬁrst redox couple A1/C1 (3.46 V/3.35 V) is attributed to the conversion between V2O5 and ε-Li0.5V2O5. This reaction at this stage can deliver a theoretical capacity of
M. Chen et al. / Journal of Power Sources 288 (2015) 145e149
Fig. 1. (a) Schematic of the fabrication process of multilayer V2O5 hollow sphere arrays. SEM images of (b) multiplayer PS template, and (c, d) multilayer V2O5 hollow sphere arrays (cross sectional SEM images in inset). (e) HRTEM-TEM image of V2O5 hollow spheres (low magniﬁcation TEM image in inset).
73.5 mAh g1. The second redox couple A2/C2 (3.29 V/3.15 V) is due to the change between ε-Li0.5V2O5 and d-LiV2O5, which also can release a theoretical capacity of 73.5 mAh g1. The third redox couple A3/C3 (2.58 V/2.26 V) corresponds to the reaction between d-LiV2O5 and g-Li2V2O5, which can exhibit a theoretical capacity of 147 mAh g1. Hence, the total theoretical capacity of V2O5 in the voltage range from 4 to 2 V is 294 mAh g1. The simpliﬁed electrochemical reactions are shown as follows [5,6,30]. V2O5 þ 0.5 Liþ þ 0.5 e 4 ε-Li0.5V2O5
ε-Li0.5V2O5 þ 0.5 Liþ þ 0.5 e 4 d-LiV2O5
d-LiV2O5 þ Liþ þ e 4 g-Li2V2O5
The initial charge/discharge curves of the multilayer V2O5 hollow sphere arrays in the voltage range between 2.0 and 4.0 V at 0.5C rate (1C ¼ 294 mAh g1) are shown in Fig. 3b. Three main charge/ discharge plateaus are noticed, which is in agreement with the CV result above. Impressively, the multilayer V2O5 hollow sphere arrays deliver a high speciﬁc capacity of 293 mAh g1 (the graphite paper substrate does not have capacity in the voltage range between 2 and 4 V), close to the theoretical capacity. This value is higher than those of V2O5 microsphere powder electrodes (260e288 m Ah g1) in the literature [15,21e27]. Excellent highrate capability is essential and highly desirable for high-power
Fig. 2. (a) XRD pattern and (b) Raman spectrum of multilayer V2O5 hollow sphere arrays.
M. Chen et al. / Journal of Power Sources 288 (2015) 145e149
Fig. 3. Electrochemical characterizations of multilayer V2O5 hollow sphere arrays: (a) CV curve at a scanning rate of 0.25 mV s1 at the 2nd cycle; (b) Charge/discharge curves at 0.5C; (c) Rate capability; (d) Cycling performance. (e) SEM image of V2O5 hollow sphere arrays after cycling for 300 cycles at 0.5C (large SEM image in inset).
LBs applications. Notably, the multilayer V2O5 hollow sphere arrays exhibit good high-rate capability with a speciﬁc capacity of 293 mAh g1 at 0.5C, 263 mAh g1 at 2.5C, 231 mAh g1 at 5C, 199 mAh g1 at 10C, 152 mAh g1 at 20C, respectively (Fig. 3c). When the current density is recovered to 0.5C, a high speciﬁc capacity of 291 mAh g1 is regained. These values are about 10e30 % higher than those of other V2O5 sphere powder electrodes [15,21e27], and comparable to ultrathin V2O5 nanosheet . In addition, the multilayer V2O5 hollow sphere arrays show impressive cycling stability with a speciﬁc capacity of 285 mAh g1 at 0.5C and 202 mAh g1 at 5C after 300th cycle, respectively (Fig. 3d), much better and more stable than the V2O5 sphere powders (<250 mAh g1 at 0.5C and 160 mAh g1 at 5 after 100 cycles) [23e26]. The unique hollow sphere arrays architecture plays several positive roles in the enhancement of LBs performance. First, the thin wall and hollow sphere feature allow fast diffusion of Li ions/electron, thus leading to fast kinetics. The 3D connected porous inner space favors the efﬁcient contact between active material and electrolyte, providing more active sites for electrochemical reaction . Second, the direct growth of hollow sphere arrays on current collectors ensures good mechanical adhesion and electric connection of the active material to the current collector. Also dead mass could be avoided as the polymer binders and
conductive additives are not used. Third, this 3D porous structure is believed to alleviate structure degradation caused by volume expansion during the cycling process resulting in good cycling performance. As shown in Fig. 3e, the whole structure of the multilayer V2O5 hollow sphere arrays is well preserved after cycling for 300 cycles at 0.5C. 4. Conclusion In summary, we have demonstrated a facile PS templateassisted ED method for the fabrication of high-quality multilayer V2O5 hollow sphere arrays. The 3D porous hollow sphere arrays provide a fast transfer path for ion/electron and a stable framework for high-rate cycling life. A high capacity of 202 mAh g1 at 5C is obtained after 300th cycle. Our approach creates a new way for preparation of high-performance LBs cathode. This method can also be extended to fabricate other metal oxide hollow arrays for energy and environment applications. Acknowledgments This work is supported by the Foundation of State Key Laboratory of Coal Conversion (Grant No. J14-15-909) and Postdoctoral
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Science Foundation of Heilongjiang Province of China (Grant No. LBH-Z14120). References              
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