Self-assembled 3D Hierarchical Porous Bi2MoO6 Microspheres

and/or sulfides exhibit better cycling stability and larger reversible capacities .... filled glove box by using Li metal as the counter electrode, an...
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Self-Assembled 3D Hierarchical Porous Bi2MoO6 Microspheres toward High Capacity and Ultra-Long-Life Anode Material for Li-Ion Batteries Shuang Yuan,†,‡ Yue Zhao,† Weibin Chen,† Chun Wu,† Xiaoyang Wang,† Lina Zhang,† and Qiang Wang*,†,‡ †

Key Laboratory of Electromagnetic Processing of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China ‡ Department of New Energy Science & Engineering, School of Metallurgy, Northeastern University, Shenyang 110819, China S Supporting Information *

ABSTRACT: Three-dimensional (3D) hierarchical porous Bi2MoO6 microspheres (HPBMs) were successfully prepared and used as the anode material in Li-ion batteries (LIBs) for the first time. The HPBMs showed a high capacity (>830 mAh· g−1, 734.5 mAh·cm−2), high rate capability (20 A·g−1, 177.7 mAh·g−1), and superior long cycle life (>2700 cycles) in the temperature range 5−55 °C without adding any other conductive carbon materials, such as graphene and carbon nanotubes. This can be reasonably attributed to their substantially high surface area, 3D hierarchical porous structure, and homogeneous conductive matrix composed of metallic nanoparticles. HPBMs surprisingly showed a high reversible discharge capacity of 537.2 mAh·g−1 (475.4 mAh·cm−2) and an average discharge voltage >3.0 V even when coupled with LiCoO2 in a full cell. The results highlight the feasibility of HPBMs as anode material for LIBs. KEYWORDS: Bi2MoO6, anode, high capacity, ultralong-life, lithium-ion batteries

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INTRODUCTION There is a surge in the development of advanced rechargeable LIBs with a high capacity and long cycle life to fulfill the critical demands for electric vehicle supply equipment and communications equipment. It is essential to develop electrode materials to improve the electrochemical properties of LIBs.1−5 Most alloying anodes, such as Sn and Sb, have higher specific capacities than commercial graphite electrode material and good electrical conductivity, but a short battery life. Because they undergo a large volume change during the electrochemical reactions, the active materials are detached from the substrate.6−9 However, metallic oxides and sulfides exhibit better cycling stability and larger reversible capacities compared to alloy anodes.10,11 Conversion-type transition-metal oxides (TMOs) have also attracted increasing attention in the field of LIBs in recent decades compared to traditional intercalation materials because of their high specific capacities. They are attractive replacements to increase the energy densities of LIBs.12−15 However, TMOs often suffer from discrimination due to their low electrical conductivity and fragile electrode structure, resulting in poor cycling stability, as well as difficulties associated with applications in LIBs.16−18 Therefore, it is beneficial to prepare high-performance LIB anode materials by introducing metals with a high conductivity and high capacity into their oxides or sulfides. This not only improves the conductivity of oxides and sulfides but also prevents the volume © 2017 American Chemical Society

expansion during the electrochemical reactions. Hence, it is essential to develop suitable alloys and conversion-type TMOs as the anode materials with high capacities and excellent cycling stabilities for practical applications. Various approaches have been developed to improve the battery performance, as well as the structural stability of alloys and conversion-type electrode materials for LIB anode. One effective strategy is to reduce the size of the materials to the nanoscale. Nanostructured materials have a high rate performance and shorten the diffusion path for Li ions. Nanostructures reduce the strain during electrochemical reactions and suppress the pulverization during a significant volume change.19−22 Another promising approach is the use of hierarchically porous structures to improve the battery performance by enhancing the buffer space and electrochemical reaction sites.23−27 Moreover, hybrid active materials combined with carbon can effectively improve the cycle stability and rate capability of batteries.28−31 Although a high content of conductive carbon or graphene additives could significantly enhance the battery performances, this might affect the energy density of batteries. Therefore, it is possible and essential to develop nanostructureReceived: March 21, 2017 Accepted: June 6, 2017 Published: June 6, 2017 21781

DOI: 10.1021/acsami.7b04045 ACS Appl. Mater. Interfaces 2017, 9, 21781−21790

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a, b) XRD pattern and SEM image of prepared Bi2MoO6. Inset images in panels a and b are simulated Bi2MoO6 crystal structure and high-resolution SEM image of as-prepared HPBMs, respectively. (c, d) Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of as-prepared HPBMs. (f−h) Elemental mapping of HPBMs shown in panel e.

electrochemical performances when they were used as the anode material for LIBs. They exhibited a high reversible capacity of >830 mAh·g−1 (734.5 mAh·cm−2), high rate capability (20 A·g−1, 177.7 mAh·g−1), and high Coulombic efficiency. Surprisingly, they showed a superior long cycle life, even over 2700 times between 5−55 °C, and a high capacity of 625 mAh·g−1 (553 mAh·cm−2) was maintained at a current density of 0.5 A·g−1. Furthermore, such HPBMs showed a high average discharge voltage (>3.0 V) and high reversible discharge capacity of 537.2 mAh·g−1 (475.4 mAh·cm−2) in a full cell (LiCoO2 as the cathode). The above results show the feasible way of 3D HPBMs as LIBs anode materials.

based active materials with hierarchically porous structures to improve the rate capability and cycling stability of batteries. Recently, metal molybdates have attracted significant interest in supercapacitors, photocatalysts, and battery electrodes owing to their stable crystal structure and near-ideal band gaps.32−34 They are characterized by a large surface area, fast electron transfer capability, and facile strain relaxation during electrochemical reactions and are often introduced into nanostructures. These characteristics result in nanostructured metal molybdates potentially useful in various fields such as in LIBs.34−37 In this context, many nanostructured metal molybdates have been successfully synthesized and used as the anode materials in LIBs, especially NiMoO4 and CoMoO4.38−41 To the best of our knowledge, Bi2MoO6, a layered structural metal molybdate, has not been reported as the anode material for LIBs, as well as their electrochemical reaction mechanism and three-dimensional (3D) hierarchical porous microsphere structure. Thus, the development of a facile method to synthesize 3D hierarchical porous Bi2MoO6 microspheres (HPBMs) and elucidation of their electrochemical reaction mechanism for LIBs are of great importance and necessity. Herein, 3D HPBMs were successfully synthesized by using a method of the low-temperature and facility. Moreover, Bi2MoO6 was introduced into LIBs for the first time. Because of structural advantages, 3D HPBMs exhibited superior

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MATERIALS AND METHODS

Synthesis of 3D HPBMs. Bi(NO3)3·5H2O (0.84 g) and 0.2 g of Na2MoO4·7H2O were separately dissolved in 5 mL of ethylene glycol under magnetic stirring at 60 °C. The Na2MoO4·7H2O solution was added to the Bi(NO3)3·5H2O solution under magnetic stirring. Then, 25 mL of absolute ethanol was slowly added into the solution with continuous stirring for 10 min. After that the solution (35 mL) was transferred into 100 mL hydrothermal reactor and heated at 160 °C for 6 h and then cooled to room temperature spontaneous. The obtained precipitates were filtered and washed with absolute ethanol and deionized water at least six times to obtain pure HPBMs. Finally, the HPBMs were dried at 80 °C for 12 h. Bi2MoO6 powder was obtained by grinding HPBMs in a mortar for 3 h. Bi2MoO6 balls were obtained by heating the hydrothermal reactor at 140 °C for 6 h. 21782

DOI: 10.1021/acsami.7b04045 ACS Appl. Mater. Interfaces 2017, 9, 21781−21790

Research Article

ACS Applied Materials & Interfaces

Figure 2. (a, e) XRD patterns of prepared samples at different temperatures in 6 h and different reaction times under 160 °C, respectively. (b−d and f−h) SEM images of prepared samples in panels a and e, respectively. (i−k) Schematic mechanism for the preparation of HPBMs. Material Characterization. Powder X-ray diffraction (XRD) measurement was performed using an X’Pert Pro Powder X-ray diffractometer using Cu Kα (λ = 0.15405 nm) radiation (40 kV, 40 mA). FullProf program were used to analysis the phase compositions of as-prepared HPBMs via Rietveld method. The morphology and crystalline phase of the as-prepared samples were characterized by scanning electron microscopy (SEM, SU8010N) and transmission electron microscope (JEM-2100). X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB 250 spectrometer equipped with a monochromatized Al Kα source. Before the ex situ XRD test, the electrodes were washed with ethanol and vacuum-dried at 60 °C for 5 h. Electrochemical Measurement. Half cells (electrodes) were prepared by mixing active materials (80 wt %), carboxyl methyl cellulose (CMC, 10 wt %), and acetylene black (10 wt %) in deionized water. These electrodes were dried at 80 °C in vacuum for 12 h after the slurries spread onto a Cu or Al foil. Then, they were pressed and cut into disks (diameter = 12 mm, area = 1.13 cm2) before transferring them into an Ar-filled glovebox. Coin cells (CR2025) were assembled in Ar-filled glovebox by using Li metal as the counter electrode, and LiPF6 (1 mol/L) in dimethyl-carbonate/ethylene carbonate/methyl 2ethoxyethyl carbonate (DMC/EC/EMC = 1:1:1, LBC305-1) as the electrolyte, Celgard 2400 membrane as the separator. Galvanostatic charge−discharge tests were carried out by a Land CT2001A. Cyclic voltammetery (CV) and electrochemical impedance spectral (EIS) measurements were performed on a VMP3 Electrochemical Workstation. For Li-metal free full cells, both of HPBMs and LiCoO2 electrodes were prepared by mixing the active material (80 wt %), carboxyl methyl cellulose (CMC, 10 wt %) and acetylene black (10 wt %) in deionized water. After slurries coated onto a Cu/Al foil, the electrodes were dried at 80 °C in vacuum for 12 h. Then, they were pressed and cut into disks (diameter 12 mm, area 1.13 cm2) before transferring them into an Ar-filled glovebox. The electrolyte solution of LIB was LBC305-1. Both the HPBMs and LiCoO2 electrodes were activated in half cells for two cycles before assembling them as a full cell electrode (coin cell, CR2025). The active materials were loaded on the cathode and anode with a ratio of 8:1. All the data of full cells were calculated based on the mass of anode. Supercapacitor Measurements. The working electrodes were prepared by mixing the HPBMs with PVDF and acetylene black in a weight ratio of 80:10:10. The mixture was coated onto a Ni foam substrate using a spatula, then dried at 60 °C for 8 h. Electrochemical measurements were carried out using a beaker cell containing an

aqueous solution of 6 M KOH. The sample was tested using a conventional three-electrode cell with Hg/HgO as the reference electrode and Pt wire as the counter electrode. The cutoff voltage was between −1.0 and 0 V vs Hg/HgO. All the tests were carried out using a VMP3 electrochemical workstation at room temperature.

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RESULTS AND DISCUSSION XRD patterns of Bi2MoO6 is showed in Figure 1a. Inset image in Figure 1a is simulated Bi2MoO6 crystal structure with Rietveld refinement. The XRD patterns confirm that the phase structure is orthorhombic Bi2MoO6 in the Pna21 space group (JCPDF no. 21-0102); no other diffraction peak was observed. The XPS spectrum shown in Figure S1 further confirms the purity of the obtained Bi2MoO6. Bi2MoO6 shows perovskitelike slabs of (MoO4)2− as an important aurivillius oxide located between the (Bi2O2)2+ layers, as shown in the inset of Figure 1a. The (MoO4)2− slabs are composed of irregular MoO6 octahedra, sharing the four common corners with the neighboring MoO6 octahedra. This makes the slabs parallel to (010). In the (Bi2O2)2+ layers, the Bi3+ ions have a pyramidal coordination and stereochemically active lone pair of electrons; these are directed into the sandwiched (MoO4)2− slabs in an asymmetric manner.42 According to previous reports, open and stable channels could be offered by the layer structure for facile Li+ insertion/extraction during the discharge/charge.43−45 Morphology of the as-synthesized Bi2MoO6 was characterized using SEM. The SEM image (Figure 1b) clearly shows many spheres with a diameter of ∼1 μm. The highmagnification SEM image (inset in Figure 1b) shows that the as-prepared Bi2MoO6 has a hierarchical nanosheet-constructed porous structure. This porous structure was further verified from the nitrogen (N2) absorption−desorption isotherms (Figure S2a). This indicating a mesoporous morphology of Bi2MoO6 microspheres. The pore diameter is 1000 mAh·g−1 (885 mAh·cm−2) after 50 cycles.56 However, the capacity of Bi2O3 is only 140 mAh·g−1 (124 mAh·cm−2) after 50 cycles (Figure S12). Subsequently, the electrochemical mechanisms of MoO3 and Bi2O3 were investigated by ex situ XRD analysis (Figure S13). During the MoO3 discharging, the peak for lithium molybdate appeared. Some peaks were gradually distinguished when recharged to 3.0 V (Figure S13a). When Bi2O3 electrode was discharged to 1.0 V, most of these peaks belonged to Bi metal. However, they disappeared after discharging to 0 V. When charged to 3.0 V, the remaining peaks were indexed to Bi2O3 (Figure S13b). The results are consistent with eq 4, but they are different from the results of HPBMs. This is probably because the Mo or MoOx and Li2O limit the aggregation of Bi2O3.57 The SEM images of MoO3 and Bi2O3 electrodes after charging to 3.0 V are shown in Figures S14 and 15. After discharging to 0 V, MoO3 electrode was coated with a layer of particles and belt-like materials (Figure S14a). When charged to 3.0 V, the original rod-like morphology of MoO3 almost remained unchanged (Figure S14b). This result is consistent with our previous report.56 However, lots of balls were generated in the Bi2O3 electrode after discharging to 0 V (Figure S15a). When charged to 3.0 V, some changes occurred on the surface of the balls. However, it is significantly different from the original morphology of the electrode (Figure S15b). Therefore, Bi2O3 electrode shows a poor performance. Figure S16 shows the electrode morphology of HPBMs prior to the electrochemical test when discharged/ charged to 0/3.0 V at different magnifications. Prior to the electrochemical reactions (Figure S16a), some HPBMs were distributed in the electrode. After discharging to 0 V, the morphology changed a lot (Figure S16b). Even after charging to 3.0 V, their spherical morphologies are still maintained (Figure S16c). The results show their excellent structural stability. This indicates that the HPBM electrode with a porous and nanosheet structure is stable during the charging/ discharging. Furthermore, Li2O and Mo/MoOx matrixes play an important role in improving the battery performance.57−59 To pave the way for practical applications, full cells were assembled using HPBMs and LiCoO2 as the anode and cathode materials because of the superior half-cell performance. The charge and discharge profiles of LiCoO2 are shown in Figure S17. These full cells show a high average discharge voltage of >3.0 V and a high initial discharge capacity of 466.6 mAh·g−1 (413 mAh·cm−2) at a current of 50 mA·g−1 (Figure S18). In addition, still 537.7 and 513.9 mAh·g−1 (476 and 366 mAh· cm−2) capacities were retained after the second and fifth cycles (Figure 6). Surprisingly, such 3D HPBMs can also be used as supercapacitor electrode materials and show excellent cycling stability (Figure S19). We suggest that the porous Bi2MoO6 nanosheets structured microspheres are strong candidates among anode materials of high-performance LIBs on account of the outstanding cyclic performance, high capacity, and superhigh rate capability. The excellent performance of porous Bi2MoO6 nanosheet-structured microspheres is to the result of specific characteristics of the unique 3D porous microsphere structure. First, the crosslinked 3D open porous structure assembled by Bi2MoO6 nanosheets not only increases the mechanical robustness and enables highly conductive paths for electron transfer in all the directions, but also provides space to accommodate electrolytes

(2) (3)

Some other electrochemical reactions occur during the charging and discharging after the first cycle as follows: 2Bi + 3Li 2O ↔ Bi 2O3 + 6Li+ + 6e−

(4)

Mo + x Li 2O ↔ MoOx + 2x Li+ + 2x e− (x = 2, 3)

(5)

The Li insertion/extraction mechanism for HPBMs is schematically shown in Figure 5e. According to the reaction mechanism, the theoretical capacity of Bi2MoO6 can reach up to 791 mAh·g−1 (700 mAh·cm−2). To further determine the reason for the superior electrochemical properties of HPBMs, the morphologies and electrochemical mechanisms of Bi2MoO6, MoO3, and Bi2O3 were compared via ex situ XRD and SEM analyses. The synthesis methods for MoO3 nanobelts and Bi2O3 nanorods are shown in Supporting Information. Figures S9 and S10 show the XRD and SEM images of MoO3 nanobelts and Bi2O3 nanorods coated on Cu foils. Their charge/discharge profiles are shown in Figure S11a and b. The charge/discharge profiles of Bi2MoO6 and MoO3 are significantly different, whereas the 21787

DOI: 10.1021/acsami.7b04045 ACS Appl. Mater. Interfaces 2017, 9, 21781−21790

Research Article

ACS Applied Materials & Interfaces

capacity up 830 mAh·g−1 (735 mAh·cm−2), good cycling stability over 2700 cycles, and high rate capability (20 A·g−1, 177.7 mAh·g−1) when they are applied as novel anode materials for LIBs. This might be attributed to the porous and 3D hierarchical nanosheets structure of Bi2MoO6 microspheres. Ex situ XRD and XPS methods were employed to discuss the phase transformation combine with the process of discharging/ charging. Furthermore, the HPBMs show a high discharge voltage platform and high reversible discharge capacity in Lifree full cells even without optimization. The results obtained definitively certify the great potential of 3D HPBMs as excellent anode materials for LIBs. It might take the LIBs go to a step further to a sustainable electrical energy storage system with large-scale.

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Figure 6. Galvanostatic discharge/charge profiles of HPBM/LiCoO2 full cell at 50 mA·g−1.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04045. XPS of the original HPBMs, N2 sorption isotherms and pore size distributions of as-formed HPBMs, TEM image of HPBMs, GITT profiles of HPBMs versus Li, SEM images of Bi2MoO6 particles powder and Bi2MoO6 balls, cycling stability of HPBMs, Bi2MoO6 particles powder, and Bi2MoO6 balls, preparation method of MoO3 nanobelts and Bi2O3 nanorods, ex situ XRD patterns of HPBMs, MoO3, and Bi2O3 at different electrochemical reaction states, charge and discharge profiles of LiCoO2, MoO3, and Bi2O3 in half cells, cycling stability of Bi2O3 electrode, SEM images of HPBMs, MoO3, and Bi2O3 before and after electrochemical reactions, TEM images of HPBMs after five cycles, and Nyquist plots for HPBMs, Bi2MoO6 particles powder, and Bi2MoO6 balls (PDF)

for efficient ion access, thus significantly improving the battery performance.19−21,57−59 Second, metal nanoparticles act as a desirable conductive substrate dispersed in the electrode to promote the batteries performance.27,58,60 Figure S20 shows the TEM images of HPBMs after charging for five cycles to 3.0 V. After recharging to 3.0 V, several nanoparticles were distributed in the electrode. These nanoparticles contain Li2O, Bi2O3, MoO3, and/or Mo.56 The in-situ-formed matrix of Li2O effectively confines the diffusion and agglomeration of Bi/Mo, resulting in smaller Bi2O3 or Mo and Li−Bi alloy nanoparticles. This can improve the reversibility of active materials and prevent the volume change during the Li ion insertion/ extraction.57 The TEM images of MoO3 and Bi2O3 electrodes after charging to 3.0 V are shown in Figures S21 and 22. After charging to 3.0 V, its belt-like structure is maintained very well (Figure S21a and b) even though the crystal structure of MoO3 changed considerably. This is one of the reasons why MoO3 nanorods show a stable cycling stability.56 The change in crystal structure is consistent with the XRD results shown in Figures S9 and S13. The morphology of Bi2O3 particles (Figures S22a and b) was severely damaged, and they were significantly larger than the HPBM electrode (Figure S20) even though it is still Bi2O3 after charging to 3.0 V. This reasonably explains why the performance of Bi2O3 electrode decreased dramatically and shows that Mo significantly affected the performance of the battery. Third, the metal Bi/Mo conducting matrixes have critical importance for enabling HPBMs as feasible anode materials for LIBs. The electrochemical impedance spectra of three types of electrodes (Figure S23) show that all the electrochemical impedances decreased after the electrochemical reactions. The HPBMs have the lowest electrochemical impedance. Additionally, the hierarchically porous structures provide ample space for Li storage and accommodate the strain of volume change during the lithiation and delithiation.12−14,58,59 On the basis of the above results, we can deduce that the synergetic influence between nanosheets and 3D hierarchical porous structure is necessary and responsible for the superior electrochemical performances by take advantage of electrochemically active Bi2MoO6 maximum.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Qiang Wang: 0000-0002-9248-9186 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51425401, 51690161), the Fundamental Research Funds for the Central Universities (Grant Nos. N160903001, 160907001) and the Project Funded by China Postdoctoral Science Foundation (Grant No. 2016M591441).

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CONCLUSIONS In summary, 3D hierarchical nanosheet-constructed Bi2MoO6 microspheres with a porous structure were successfully synthesized. Surprisingly, the acquired HPBMs indicate outstanding electrochemical performances with high reversible 21788

DOI: 10.1021/acsami.7b04045 ACS Appl. Mater. Interfaces 2017, 9, 21781−21790

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DOI: 10.1021/acsami.7b04045 ACS Appl. Mater. Interfaces 2017, 9, 21781−21790