Low-cost synthesis of hierarchical Co3V2O8 microspheres as high-performance anode materials for lithium-ion batteries

Low-cost synthesis of hierarchical Co3V2O8 microspheres as high-performance anode materials for lithium-ion batteries

Accepted Manuscript Low-cost synthesis of hierarchical Co3V2O8 microspheres as high-performance anode materials for lithium-ion batteries Hui Chai, Yu...

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Accepted Manuscript Low-cost synthesis of hierarchical Co3V2O8 microspheres as high-performance anode materials for lithium-ion batteries Hui Chai, Yucheng Wang, Yingchun Fang, Yan Lv, Hong Dong, Dianzeng Jia, Wanyong Zhou PII: DOI: Reference:

S1385-8947(17)30931-2 http://dx.doi.org/10.1016/j.cej.2017.05.162 CEJ 17061

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

22 March 2017 24 May 2017 27 May 2017

Please cite this article as: H. Chai, Y. Wang, Y. Fang, Y. Lv, H. Dong, D. Jia, W. Zhou, Low-cost synthesis of hierarchical Co3V2O8 microspheres as high-performance anode materials for lithium-ion batteries, Chemical Engineering Journal (2017), doi: http://dx.doi.org/10.1016/j.cej.2017.05.162

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Low-cost synthesis of hierarchical Co3V2O8 microspheres as high-performance anode materials for lithium-ion batteries Hui Chaia *, Yucheng Wanga, Yingchun Fanga, Yan Lva, Hong Donga, Dianzeng Jiaa, Wanyong Zhoub a

Key Laboratory of Energy Materials Chemistry, Ministry of Education, Key Laboratory of

Advanced Functional Materials, Institute of Applied Chemistry, Xinjiang University, Urumqi 830046, Xinjiang, P. R. China b

College of Chemistry & Chemical Engineering, Xinjiang University, Urumqi, Xinjiang, P.R.

China 

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Abstract: Mixed-valence cobalt vanadates have attracted special attention for their perfect electrochemical performance as electrode material of lithium-ion batteries, yet their synthetic method is often beset with severe challenges. In this study, we report a new strategy to synthesize hierarchical Co3V2O8 mesoporous microspheres comprised of stacked nanoparticles via calcining the precursor obtained by a controllable co-precipitation of microspheres at room temperature. When evaluated as an anode material for lithium-ion batteries, the as-prepared samples manifest a high initial discharge capacity of 1099.0 mA h g-1 at 500 mA g−1, outstanding cycling retention rate of 114.3% after 200 cycles, and excellent rate capability with an average discharge capacity of 545.5 mA h g−1 at 2000 mA g−1. More importantly, the reported synthetic procedure is energy-saving, inexpensive, and straightforward rather than the complicated and energy-consuming synthesis approach reported by previous references. This new and low-cost route to fabricate hierarchical Co3V2O8 mesoporous microspheres with outstanding cycling stability is inspired for the application of lithium-ion batteries.

Keywords: Cobalt vanadate; Microspheres; Cycling stability; Lithium-ion batteries.

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1. Introduction To meet serious energy crisis requirements, the combination of structure and component of the electrode material has often been regarded as the most effective way for high performance anode materials of lithium-ion batteries (LIBs). Spherical multiporous micro-/nanostructures of mixed-metal oxides have attracted considerable attention in LIBs because of their high surface area, short diffusion path, and accommodation of the volume expansion [1-3]. As a result, controllable preparation of new spherical materials with a complex composition and superior electrochemical performance is an essential task to meet the tremendous growing requirements of LIBs [4-6]. Up to now, numerous efforts have been concentrated on developing methods to synthesize various spherical structure materials and great progress has been achieved. For examples, Wang and coworkers invented ZnMn2O4 ball-in-ball hollow microspheres exhibiting high performance in LIBs applications [7]. Hu and coworkers successfully developed a facile solvothermal method to synthesize mesoporous ZnCo2O4 microspheres, which also revealed perfect electrochemical performance of LIBs [8]. Li and coworkers presented a general strategy to prepare various hollow structures of MnCo2O4 submicrospheres for high-performance anode materials of LIBs [9]. Recently, porous mixed-valence cobalt vanadates have drawn special research and discussion due to their low cost, higher theoretical capacity, and the bimetallic synergic effects [10-13]. The mesoporous Co2V2O7 hexagonal microplatelets were prepared by Wu and coworkers, which displayed a high capacity and good cyclic capacity retention [14]. As another important cobalt vanadate, Co3V2O8 with multilayered nanosheets structure was synthesized and the reaction mechanism of it was investigated as anode materials of LIBs for the first time by Yang and coworkers [15]. Although the few previous references show promising results, cobalt vanadates appear to still face severe challenges. There lacks suitable synthetic methods due to the complicated valence state of V element during synthesis process. The synthetic routes reported by previous papers suffer from requiring trenchant synthesis condition, 2 / 18

wasting energy, and hard to synthesis large-scalely. The cycling stability of cobalt vanadates still need to be improved by controlling the structure of it. It should be noted that suitable synthetic methods of cobalt vanadates with high electrochemical performance are still on the road, especially the low-cost and low-temperature solution synthesis route. Herein, we have proposed an energy-saving, inexpensive, and straightforward co-precipitation method coupled with a post annealing treatment to prepare Co3V2O8 hierarchical mesoporous microspheres (HMMSs). Quite expectedly, the analysis results show that the as-synthesized Co3V2O8 HMMSs as lithium-ion anode materials manifest high capacity and excellent rate capability. The most important is that the outstanding cycling stability of Co3V2O8 HMMSs prepared by such a low-cost method is inspired for the application of lithium-ion batteries. The excellent electrochemical performance could attribute to the unique mesoporous spherical structure of Co3V2O8 and the synergy of vanadium ions and cobalt ions to a large extent. It is believed that the Co3V2O8 HMMSs show great prospects in the application of LIBs.

2. Experimental 2.1. Material synthesis All chemicals employed in the experiments were analytical grade and without further purification. Briefly, 0.5 g of PVP was firstly dissolved in 10 mL of distilled water at room temperature. Then 2.4 mmol of Co(NO3)2·6H2O were added into the above solution and stirred continually to form a transparent pink solution. Next, 4.8 mmol of NH4VO3 were dissolved in another 37 mL of distilled water at 80 ℃ and maintained at this temperature with vigorous magnetic stirring till a transparent yellow solution was formed. Under continuous stirring, the transparent yellow solution was added dropwise to the above transparent pink solution. After keeping stirring for an hour at room temperature, the resulting precursor suspension was not disturbed for several hours. Finally, the resultant precipitates were collected by centrifugation, rinsed with distilled water and ethanol several times, and dried at 80 ℃ for 12 h. In order to obtain the porous and highly crystalline Co3V2O8 HMMSs, the 3 / 18

as-prepared precursors were annealed in air at 450 ℃ for 8 h with a heating rate of 2 ℃ min-1. 2.2. Material characterization The crystallite structures were performed on X-ray diffraction (XRD) using a Bruker D8 diffractometer with Cu-Kα radiation (λ = 1.5406 Å). Thermogravimetric analysis (TGA) of the sample was carried out in an air flow with a heating rate of 10 ℃ min-1 on a Hitachi-STA7300 from room temperature to 1000 ℃. The scanning electron microscopy photos of the sample were obtained on a scanning electron microscopy (SEM, S-4800, Japan) equipped with an electron dispersive spectroscopy (EDS). The transmission electron microscopy (TEM, JEM-2100F, Japan) was used to acquired transmission electron microscopy images. The chemical compositions of the products were acquired by X-ray photoelectron spectroscopy (XPS) performing on an ESCALAB 250Xi electron spectrometer (Thermo Fisher Scientific) with an Al-Kα radiation. The Brunauer–Emmett–Teller (BET) surface area were evaluated by using a Quantachrome instrument. The pore size distribution is calculated by Density Functional Theory method (DFT). 2.3. Electrochemical measurements The electrochemical property tests were performed using 2032 type coin cells assembled in an Ar-filled dry glovebox, wherein lithium discs were used as the counter/reference electrode and Celgard 2400 was used as separator. The anode slurry was prepared by mixing the active material, acetylene black, and poly (vinylidene fluoride) at the weight ratio of 70:20:10 in N-methylpyrrolidinone solvent. The obtained mixture was then pasted onto Cu foil substrate and dried in a vacuum oven at 110 ℃ for 12 h. The loading density of the active materials on the electrodes is about 1.0-1.3 mg cm2. The electrolyte was 1 M solution of LiPF6 in ethylene carbonate:diethyl carbonate (1:1 vol). The galvanostatic charge/discharge and cycling measurements were conducted on a battery test system (CT2001A, Land, China) at different current densities. Cyclic voltammogram (CHI660D, Chenhua, China) test was carried out in the voltage from 0.01 to 2.5 V at 0.1 mV s-1. 4 / 18

3. Results and discussion

Figure 1. Schematic illustration of the synthetic process of Co3V2O8 HMMSs. The schematic illustration of the synthesis procedure of Co3V2O8 HMMSs anode material is shown in Figure 1. Interestingly, the presence of poly(vinylpyrrolidone) (PVP) is crucial to the morphology of precursors. In the initial stage, PVP molecules coordinate with cobalt ions into metal-polymer chelates. As the ammonium metavanadate solution is added dropwise, the small primary nanoparticles spontaneously aggregate, nuclear, recrystallization, and co-precipitation to reduce the energy of the entire system [16,17]. During the recrystallization process, PVP inductively regulates the size of particles growth and prevents particles agglomeration simultaneously, which can be confirmed by the scanning electron microscopy (SEM) result of the different precursors in the following discussion.

Figure 2. SEM images of the Co3V2O8 precursors prepared with PVP (a-c) and without PVP (d-f). It is worth noting that the presence of PVP has an essential influence on the morphology of the synthesized precursors seen from Figure 2. In the presence of PVP (a-c), the extremely inerratic and hierarchical microspheres are mainly composed of stacked nanoparticles. However, significant changes occurred in the morphology of 5 / 18

the synthesized precursors without PVP (d-f). The microspheres composed of the

same stacked nanoparticles turned into irregular and interconnected together like sugar-coated haws and the size of the nanoparticles on the surface of microspheres became larger. The result of SEM likely suggests that the PVP both play the role of chelating agent and dispersant [18-21]. Figure 3. (a) XRD patterns of the precursors and the Co3V2O8 HMMSs. (b) TGA curve of the precursors. The crystal phase of the product is characterized by X-ray powder diffraction (XRD). Acquired from XRD pattern of the precursor (A) shown in Figure 3a, there are no clear diffraction peaks are observed as the synthesis temperature is too low, which means that the synthesized precursors are amorphous phase. In order to achieve the crystallized Co3V2O8 HMMSs product, the thermogravimetric analyses (TGA) is used to identify the thermal stability and decomposition profile. As can be seen, there is no obvious weight loss observed after 450 ℃ from Figure 3b, so the precursors are annealed in air at 450 ℃ with a heating rate of 2 ℃ min-1 for 8 h to obtain the crystallized product. After calcination, all of reflection peaks of the crystallized Co3V2O8 HMMSs (B) can be indexed to the orthorhombic phase of Co3V2O8 (PDF # 16-0832). Although the peaks in the XRD pattern are not very sharp, we can also refer to the previous references to distinguish the crystal phase of the product. After the careful comparison with similar literatures, our experiment result in line with the previous reports which also have observed the similar weak reflection peaks [22,23].

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Figure 4. (a-b) shows the SEM images of the Co3V2O8 HMMSs. (c-e) Its corresponding elemental mapping images: (c) Co, (d) V and (e) O elements. Figure 4a-b shows the SEM images of the Co3V2O8 HMMSs, from which the hierarchical

microsphere-shaped morphology can be clearly observed. The

microspheres are composed of numerous nanoparticles whose sizes are approximately 20-50 nm. Moreover, it can be seen that there is no obvious structural collapse after calcination of the precursors suggesting such spherical structure is stable. The surface of the microspheres also becomes rather rough and a lot of mesoporous can be observed after calcination, which spontaneously increase the specific surface area and offer more lithium ion transport channel. The major weight loss, which occurred below 450 ℃, is about 12% and is ascribed to the elimination of free water and coordinated water according to the TGA data in Figure 3b. It is believed that the mesoporous is derived mainly from the release of water molecules during the calcination process [24,25], which plays a significant role in improving the electrochemical performance of LIBs. In addition, the EDS-Mapping results reveal that the Co3V2O8 HMMSs are mainly composed of the Co, V, and O elements, and all of the compositional elements (Co, V, and O) are homogeneously distributed in the 7 / 18

detected region from Figure 4c-e. The atomic ratio of Co to V is 26.9:18.5 determined by the EDS measurement, which matches well with the formula of Co3V2O8 (Figure S1). These results also can match very well with the previous XRD result.

Figure 5. X-ray photoelectron spectra of the Co3V2O8 HMMSs: (a) survey spectrum, (b) Co 2p, (c) V 2p, and (d) O 1s To further identify the purity and surface oxidation state of the Co3V2O8 HMMSs, X-ray photoelectron spectroscopy (XPS) (see Figure 5) analysis is carried out and the Co, V and O elements can be detected without any other impurities. By using the Gaussian fitting method, the fitting results verify the coexistence of Co2+, Co3+, V5+, V4+, and O2- in the Co3V2O8 HMMSs [14,26]. According to the above combined results, the Co3V2O8 HMMSs are successfully synthesized.

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Figure 6. (a) TEM images, (b) HRTEM images and inset is the SAED pattern of the Co3V2O8 HMMSs. The detailed microstructure of the as-prepared Co3V2O8 HMMSs is further investigated by transmission electron microscopy (TEM). The presence of mesoporous structures can be clearly confirmed by TEM image shown in Figure 6a, which consistent with the result of SEM images. The high resolution transmission electron microscopy (HRTEM) image extracted from the edge of a microsphere shown in Figure 6b. The distinctly visible lattice fringes with a spacing of 0.20 nm, 0.22 nm, and 0.30 nm are in good agreement with the lattice plane of (042), (132) and (131) respectively. These distinctly visible lattice fringes also further verify the XRD result. The inset is the selected area electron diffraction (SAED) pattern of the Co3V2O8 HMMSs, which exhibits regular concentric circles demonstrating a polycrystalline character of the Co3V2O8 HMMSs.

Figure 7. N2 adsorption/desorption isotherms of the Co3V2O8 HMMSs. Inset is the corresponding pore size distribution. Moreover, the specific surface area and pore structure of the Co3V2O8 HMMSs were further characterized by the N2 adsorption/desorption measurements as shown in Figure 7. The Brunauer–Emmett–Teller (BET) surface area calculation of the Co3V2O8 HMMSs shows a high specific surface of 48.7 m2 g-1 and the pore size distribution ranges around 13.3 nm on the basis of the Density Functional Theory method (DFT). However, the pore size distribution of the precursors before calcination ranges around 63.4 nm indicating the character of macroporous, which is 9 / 18

not beneficial for the stability of maintaining the hierarchical microspheres structure (Figure S2). It is noticeable that the high specific surface and mesoporous structure make the Co3V2O8 HMMSs permit the electrolyte to easily penetrate through the mesoporous and make close contact with the inner-outer surfaces of numerous primary particles. Hence the electrochemical performance of the Co3V2O8 HMMSs as the promising anode materials of LIBs is improved.

Figure 8. (a) CV curves of the Co3V2O8 HMMSs at a scan rate of 0.1 mV s-1 between 0.01 and 2.5 V. (b) Charge/discharge curves of the Co3V2O8 HMMSs at different cycles. (c) Cycling performance of the Co3V2O8 HMMSs at a current density of 500 mA g−1. (d) Rate capability of the Co3V2O8 HMMSs at current density ranging from 100 to 2000 mA g−1. (e-f) SEM images of the Co3V2O8 HMMSs after 200 cycles. Cyclic voltammogram (CV) was used to study structural transformations and redox 10 / 18

couples of the Co3V2O8 HMMSs during the charge/discharge process shown in Figure 8a. It can be clearly watched that two reduction peaks (0.69 V and 0.24 V) and two oxidation peaks (1.34 V and 2.33 V) are observed in the first cycle. The two reduction peaks can be assigned to the phase transformations process (Co3V2O8 → CoO + V2O5), the lithiation formation of LixV2O5, and the formation of a solid electrolyte interface (SEI). On the other hand, the peaks at 1.34 V and 2.33 V in the charge process correspond to the reverse extraction of Li ions from the LixV2O5 (LixV2O5 → Lix-yV2O5) and the reverse oxidation (Lix/x-yV2O5 + CoO → Co3V2O8) [10,17]. CV curves for the subsequent cycles are apparently different from the first cycle, but no obvious difference occurred from the second cycle onward. The two oxidation peaks are retained from the second cycle to the fourth cycle except for a slight shifting. However, the two initial reduction peaks disappear in the subsequent cycles and three new peaks (1.88 V, 1.01 V and 0.62 V) are observed, which can be ascribed the further insertion of Li ions into LixV2O5 (LixV2O5 → Lix+yV2O5) and the further reverse reduction (CoO → Co). During this process, LixV2O5 not only acted as the host for Li+ also was moderately reduced from V5+ to V3+ [15]. The fact is that the CV curves from the second cycle onward overlap very well indicating the structural integrity and good reversibility of the Co3V2O8 HMMSs anode materials. Cycling performance of the Co3V2O8 HMMSs at a current density of 500 mA g−1 and the corresponding representative charge/discharge profiles of the 1st, 2nd, 41st, 120th and 200th cycles are shown in Figure 8b-c. Notably, the initial discharge and charge capacity are as high as 1099.0 and 981.8 mA h g-1 respectively, corresponding to a high initial coulombic efficiency (CE) of 89.3%. In addition, the remarkable reversible discharge capacity of 845.9 mA h g-1 can be obtained for the 2nd cycle, demonstrating fewer side-reactions and ideal cycle stabilities [15]. Interestingly, there is a low discharge capacity at 41st cycle and enhanced performance at 200th cycle. The main reason for the decrease of capacity during cycling in the first 41 cycles is that the formation of SEI combining with original Co3V2O8 HMMSs deconstruction converts into nanocrystalline CoO during the electrochemical process, leading to a lower conductivity of active materials. This phenomenon is similar with other 11 / 18

Co3V2O8-based anodes reported previously [10,17,26,27]. The capacity increase of Co3V2O8 HMMSs during cycling is also like with many transition metal oxides usually reported elsewhere [7,27,28,29]. The major reasons can be summarized as the following two points according to previous references: (i) enhanced Li+ diffusion kinetics with continuous activation process, (ii) reversible reaction between electrolytes and inner metal particles. As expected, the Co3V2O8 HMMSs reveal outstanding cycling stability that a discharge capacity of 967.4 mA h g−1 can be obtained after 200 cycles with the discharge capacity retention of 114.3% compared with the second discharge capacity. From the SEM images of the Co3V2O8 HMMSs anode materials after 200 cycles (see Figure 8e-f), we found that the spherical structures still can be observed. However the surface of the microspheres becomes increasingly rough resulting in more active sites, which is beneficial for the discharge capacity of the Co3V2O8 HMMSs during the cycling. Besides high capacity and outstanding cycling performance, the excellent rate performance is also a vital parameter for LIBs. As revealed in Figure 8d, the average discharge capacities of the Co3V2O8 HMMSs are 823.2, 683.4, 622.9, 595.5, and 545.5 mA h g−1 with the current densities increased from 100 to 2000 mA g−1, respectively. Moreover, when the current density is reversed to 200 mA g−1, the average discharge capacity can still reach to 739.3 mA h g−1. The excellent rate performance of the Co3V2O8 HMMSs could be contributed to their mesoporous microspheres structures, which can promote the kinetics of redox reactions. Besides, we also compared the electrochemical performance of Co3V2O8 with the references reported up to now [10,15,17,23,26,27] (Table S1.). By comparison, our Co3V2O8 has a superior or comparable electrochemical performance over other previous reports using a milder and energy-saving synthesis condition, especially in cycling stability. It is worth noting that our synthetic method is facile, low-cost, and easy to synthesis rather than the multi-step and energy-consuming synthesis route reported by previous references.

4. Conclusions In summary, the Co3V2O8 HMMSs were successfully prepared by a low-cost 12 / 18

co-precipitation method coupled with a post annealing treatment. The room temperature solution route to fabricate the unique mesoporous microspheres is facile, low-cost, and could be scaled up easily. When assessed as a promising anode material for LIBs, the Co3V2O8 HMMSs deliver high reversible capacity, outstanding cycleability, and excellent rate capability. All these results clarify that the hierarchical mesoporous microspheres structures of Co3V2O8 HMMSs possibly make it turn into a promising material for energy storage devices with high-performance.

Acknowledgments This work was supported by National Natural Science Foundation of China (No. 21461024, 21271151), Natural Science Foundation of Xinjiang Uygur Autonomous Region (No. 2015211c250), Scientific Research Program of the Higher Education Institution of Xinjiang (No. XJEDU2014I008).

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Highlights • The hierarchical Co3V2O8 microspheres are synthesized by a low-cost method. • The Co3V2O8 microspheres manifest outstanding cycling retention rate for 114.3%. • This low-cost route to fabricate Co3V2O8 is inspired for the application of LIBs.

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Graphical abstract

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