3D-Structured Polyoxometalate Microcrystals with Enhanced Rate

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3D Structured Polyoxometalate Microcrystals with Enhanced Rate Capability and Cycle Stability for Lithium Ion Storage Kang Sun, Hongqin LI, Haijun Ye, Fangqing Jiang, Hui Zhu, and Jiao Yin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03071 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 14, 2018

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ACS Applied Materials & Interfaces

3D Structured Polyoxometalate Microcrystals with Enhanced Rate Capability and Cycle Stability for Lithium Ion Storage Kang Sun†, Hongqin Li‡, Haijun Ye‡, Fangqing Jiang‡, Hui Zhu‡*, Jiao Yin§*

† Institute of Chemical Industry of Forest Products, CAF, National Engineering Lab for Biomass Chemical Utilization, Nanjing 210042, China ‡ College of Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China § Key Laboratory of Functional Materials and Devices for Special Environments, Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Sciences, 40-1 South Beijing Road, Urumqi, Xinjiang 830011, China *

Corresponding author: E-mail: [email protected] (Hui Zhu), [email protected] (Jiao Yin)

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ABSTRACT The unsatisfactory rate capability and poor cycle stability are two major obstacles for polyoxometalates (POM) in lithium ion storage. On the other hand, how to endow POM with 3D macrostructures for further practice is a challenge. To this end, a facile hydrothermal strategy was practiced to fabricate Co8W12O42(OH)4(H2O)8 microcrystals or CoWO4 aggregates onto the foamed substrate (denoted as CoW-POM and CoW-Salt, respectively). Integrating the extraordinary redox stability and lattice deformability of POM with the excellent volume accommodation, the as prepared CoW-POM presents an extraordinary better electrochemical performance (specific capacity, rate capability and cycle life) than that of CoW-Salt. In detail, the CoW-POM can deliver a reversible capacity of 737.8 mAh g-1 at the current density of 0.1 A g-1 and provide a capacity retention of 90.1% even after 100 cycles. This work not only promotes the application of POM in energy storage and conversion, but also guides an effect methodology to endow POMs with 3D structures.

Keywords: Polyoxometalate, Microcrystals, Metal oxide, Li-ion battery, 3D structure


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1. Introduction It is widely considered that electrode materials play key role in improving overall performance in lithium ion batteries (LIBs).1-5 Hence, more and more recent researches have focused on developing a wide variety of materials for anodes to enlarge the capacity, improve the rate performance, extend the cycling lifespan and solve the safety issue.6-8 For example, a variety of transition metal oxides (TMOs) with nanostructures (nonawires, nanosheets, core-shell structures and etc.) have been screened. Comparing to these traditional TMOs with monometallic counterparts, the polyoxometalates (POMs) have attracted much attention due to their enhanced electrical conductivities and larger theoretical capacity, potentially enabling further improvements in electrochemical properties.9-12 To date, a series of POM-containing salts including [CoMo8O26], [Ni2.5(Hpen)4(PW9O34)] and [SiW11O39] with various structures have been widely researched as anode materials.13-15 Besides, POMs-based hybrids, including α-Keggin-type POM3- clusters containing TBA3[PMo12O40] and PMo12-PPy/RGO have also been reported as cathode materials for LIBs to further achieve an important advance in theoretical and practical aspects.16,17 However, most of these POMs suffered from poor capacity retention, inferior rate capability and unsatisfactory cycle lifetime. So how to overcome these shortcomings of POMs for LIBs is still a great challenge. On the other hand, several groups have successfully explored a serious of Co/W based POMs for efficient oxygen evolution,18-19 attributing to the synergistic effects of strong binding energy between W and Co atoms and the redox active Co II. Whether the Co/W based POMs can be explored for the lithium ion storage is still a concern. More recently, electrodes with 3D configuration have presented several advantageous characteristics over the traditional powdered materials, including exceptional electron transfer, easy access to electrolytes, good strain accommodation and sufficient spare volumes, endowing the correlated LIBs with extended cycle lifetime and improved rate capability. Even though various methodologies have been proposed for the preparation of POM-based materials,20-27 most of them are powders, making the



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electrode fabrication difficult and complex. Hence, it is high desired to develop an easy and effective route to make 3D-structured POM-based active material for lithium ion storages. Herein, we chose Co/W base POMs as models. Co8W12O42(OH)4(H2O)8 microcrystals or CoWO4 particulates were selectively decorated on 3D copper foams (named as CoW-POM and CoW-Salt, respectively) via the control of reaction temperature with a one-pot hydrothermal method. When they were adopted as anodes for LIBs, the 3D structured CoW-POM presented more satisfactory electrochemical performance than the correlated CoW-Salt counterparts and other reported POMs. In detail, the as-prepared CoW-POM presented a high reversible capacity of 737.8 mAh g-1, enhanced rate capability and an unexpected cycling stability (100 cycles, 90.1% retention). The significant improvement in rate performance and cycling stability is ascribed to the cooperation of stable POM microcrystal and the 3D structures of the substrate. This work opens up a facile avenue for the fabrication of 3D structured POMs for other applications. 2 Experimental Sections 2.1 Preparation of materials All chemicals that used in this work were all bought from Sigma Aldrich Co. Ltd without further purification. The CoW-POM was decorated on copper foam by a straightforward hydrothermal method. First, the Cu foam (1 cm x 6 cm) was ultrasonically washed with acetone, ethyl alcohol and deionized (DI) water respectively for 30 min to clean the impurities. Subsequently, the cobalt nitride hexahydrate (Co(NO3)2·6H2O, 2 mmol) and ammonium metatungstate hydrate ((NH4)6H2W12O40·XH2O, 1:1 by mole of Co:W) were dissolved in water forming a clear solution. After stirring for 30 min, the above solution and the Cu foam were transferred into a 60 ml Teflon-lined autoclave and kept at 120oC for 8 h. After cooling to room temperature naturally, the as-prepared foams were washed with ethanol and water, and dried under vacuum at 50 oC for 6 h. In control, the CoW-Salt was prepared with the same chemical route at 170 oC for 8 h. Furthermore, another M8W12O42(OH)4(H2O)8 (M=Ni, Cu, and Mn)


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polyoxometalate mircorcrystals grown on Cu foam (denoted as NiW-POM, Cu-POM and MnW-POM) were also synthesized and compared (Supporting Information, supplementary experiment 1.1). 2.2Characterization The crystalline phase of CoW-POM and CoW-Salt was characterized on a XD-3 X-ray powder diffractometer using Cu-Ka radiation at 40 kV in the range of 10-90o. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) were recorded on a JSM 6701F field emission scanning electron microscope with an acceleration voltage of 5.0 kV. X-ray photoelectron spectroscopy (XPS) analysis was measured on ESCALAB250xi (Therm-Fisher Scientific) X-ray photoelectron spectrometer with an Al-Ka achromatic X-ray source. A Renishaw Raman RE01 scope was employed by using argon laser to obtain Raman spectra. Fourier transform infrared (FTIR) spectra of the sample were implemented with a Nicolet5700 FTIR spectrometer in the range of 500-2000 cm-1. 2.3 Electrochemical measurements Standard CR 2032-coin-type half cells were assembled to discuss their electrochemical performance. CoW-POM with mass loading around 3.5 mg cm-2was applied as working electrode, lithium foil as counter and reference electrodes. Besides, the electrolyte was composed of 1 M LiPF6 dissolving in dimethyl carbonate (DMC)/ethylene carbonate (EC) (1:1, v/v). In addition, the separator was a Celgard 2502 membrane. Remarkably, only the mass of active materials was applied to calculate the specific capacity. Galvanostatic charge-discharge measurements were carried out via NEWARE Battery Testing System (CT3008W, Shenzhen, China) at a voltage window of 0.01-2.8 V. Furthermore, cyclic voltammetry (CV) tests were executed by using electrochemical work station (CHI 630D) with scanning rate at 0.1 mV s-1. The electrochemical work station (PARSTAT 2273) was used to explore the electrochemical behavior by electrochemical impedance spectroscopy (EIS) with a AC amplitude of 5 mV in the frequency range from 10-4-105Hz. The apparent Li+ diffusion coefficient (DLi+) can be calculated as following equation: R2T2 + DLi = 2 4 4 2 2 (1) 2S n F C σ ACS Paragon Plus Environment


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where the value of R, T and F is constants (8.314 J K-1, 298K, 96500 C mol-1 respectively), S is the active surface area of the electrode which is 0.36 cm2, n is the electronic transfer number, C is the lithium ion concentration in cathode electrode. In this work, it is 6.24 x 10-3 mol cm-3. σ (Warburg factor) is the slope of the linear fitting of Z~ω−1/2.28,29 3 Results and discussion 3.1Composition and morphology To confirm the crystal structure of the synthesized samples, the XRD patterns were recorded. As shown in Figure 1a, five characteristic peaks located at 9.7o, 16.9o, 21.8o, 25.9o and 31.1o are indexed to the (110), (211), (013), (123) and (420) lattice plane of [M8W12O42(OH)4(H2O)8] (JCPDS no. 80-1525, Im-3(204) group) with a calculated lattice parameter of 12.9 A.19 Meanwhile, the diffraction peaks at 43.3o, 50.4o and 74.1ocoincide with the (111), (200) and (220) planes of Cu substrate (JCPDS no. 04-0836).30 As for the CoW-Salt sample, peaks at 19.0o, 23.8o, 30.6o and 36.3o are assigned as (001), (-110), (-111) and (200) planes of CoWO4 phase (JCPDS card no. 15-0867, P2/a(13) group), where the calculated lattice parameters are a=c=4.9 A and b=5.9 A (Figure 1b).31 Hence, it is deduced that the reaction temperature plays a critical role in determining the crystal phase of the products. Besides, the XRD patterns of MnW-POM, CuW-POM and NiW-POM were also recorded in Figure S-1. These data also match well with the standard card of Cu (JCPDS no. 04-0836) and [M8W12O42(OH)4(H2O)8] (JCPDS no. 80-1525). Furthermore, the representative morphologies of the as-obtained CoW-POM were systemically characterized by SEM and HRTEM technologies (Figure 2). It is observed that the surface of the copper foam is coated by a large number of microcrystals (Figure 2a). Enlarged image reveals that these particles have well-proportioned polyhedral morphologies with the sizes of 5-6 um (Figure 2b and c). Furthermore, the EDX spectra of CoW-POM within a selected area unambiguously prove that three elements of Co, W, O are uniformly dispersed and give a Co/W ratio of 8.1:11.9, which is coincident with the composition of [Co8W12O42(OH)4(H2O)8] (Figure S-2a and Table S-1).


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Additionally, the HRTEM and the Selected area diffraction (SAD) technologies further demonstrate the polyhedral microcrystals exist two lattice distance of 0.29 nm and 0.52 nm, corresponding to the lattice fringes in (211) and (420) planes respectively (Figure 2d and inset). Comparatively, the morphologies of the CoW-Salt were presented in Figure S-2b, 2c. It is observed that as a reaction at high temperature, the CoWO4 micro-particulates aggregate randomly with a coarse surface. The HRTEM image (Figure S-2d) of the CoWO4 particles manifests the lattice fringes of 0.47 and 0.29 nm in the grain, which are corresponded to the (001) and (-111) lattice planes in CoWO4, respectively. As shown in Figure S-2e, 2f, 2g, the morphologies of NiW-POM and MnW-POM are similar to CoW-POM with well-proportioned polyhedral morphologies, while CuW-POM presents a flowerlike structure. What’s more, as displayed in Figure S-2h, 2i, the CoW-POM exhibits satisfactory mechanical strength, comparing with the fragile CoW-Salt. In all, these discrepancies in morphologies and mechanical characteristics between CoW-POM and CoW-Salt might hint the different behaviors in lithium ion storage. 3.2 Electrochemical characterization To evaluate the electrochemical performance of CoW-POM and CoW-Salt, the galvanostatic charge-discharge measurements were conducted at a current density of 0.1 A g-1. As shown in Figure 3a, CoW-POM delivers discharging capacities of 959.4, 737.8, 733.5 and 732.8 mAh g-1in the 1st, 2nd, 5th and 10th cycle, respectively. Moreover, the correlated charging capacities of CoW-POM in the 1st, 2nd, 5th and 10th cycle are 693.5, 748.4, 715.9 and 739.6 mAh g-1 respectively, informing the coulombic efficiencies (CEs) of 72, 101, 97.7 and 100.9%. It has to be noted that these practical capacities are even larger than the theoretical data (690 mAh g-1, Supporting Information, supplementary experiment), due to a certain contribution of the pseudo-capacitance mechanism.32 On the contrary, at a same current density, CoW-Salt contributes discharging/charging capacities of 918.5/569.2, 567.2/495.2, 382.8/351.5 and 244.5/233.3 mAh g-1 with the correlated CEs of 61.9, 87.3, 91.8 and 95.4% in the 1st, 2nd, 5th and 10th cycle, respectively (Figure 3b). The initial discharging ACS Paragon Plus Environment


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capacity losses are about 23.1% and 38.3% for CoW-POM and CoW-Salt respectively, informing the formation of solid electrolyte interphase (SEI) and the irreversible electrolyte dissolution.33 Moreover, the other metal based POMs are also test. The 2nd cycle discharge capacities are 454.6, 666.2 and 352.5 mAh g-1 for MnW-POM, Cu-POM and NiW-POM respectively, which are a little lower than that of CoW-POM (Figure S-3a, 3b, 3c).Such discrepancies in capacity might result from the integrated differences in microstructures (Figure S-2) and the inner coordination environments. Hence, we will choose CoW-POM as our objective model in our research. To discover the rate capabilities of CoW-POM and CoW-Salt, the galvanostatic charge-discharge profiles at different current densities were measured (Figure S-4a and Figure 3c). As reflected in Figure S-4a, the CoW-POM exhibits excellent rate capabilities within the current density range from 0.1 to 0.6 A g-1. Along with the increase of the current density, the discharge plateaus maintain relatively stable, while the charge plateaus decrease obviously. This is owing to the advanced resistance and kinetic overpotentials at higher current densities. Furthermore, the CoW-POM electrodes can deliver stable capacities of 737.8, 621.9, 462.6, 388.3 and 303.7 mAh g-1 when current densities are setted as 0.1, 0.2, 0.4, 0.8 and 1.6 A g-1 respectively. However, at the identical rate intervals, the as-prepared CoW-Salt experiences a server capacity loss with 559.7, 291.6, 206.3, 141.2 and 87.3 mAh g-1. When the current density returns to 0.1 A g-1, the discharge capacities of CoW-POM and CoW-Salt reach back to 648.4 or 358.1 mAh g-1 respectively. In addition, the rate capabilities of MnW-POM, CuW-POM and NiW-POM are also displayed with capacity retentions of 43.2%, 36.9% and 39.9%respectively (Figure S-3d), when current densities are set from 0.1 to 1.6 A g-1. In all, the POM-based materials present an enhanced capacity with excellent rate capability. For a further verification, the cycling stability and CEs at 0.1 A g-1 are also illuminated in Figure 3d. Different from the frail performance of CoW-Salt electrode (capacity fades to 156.7 mAh g-1 (ca. 28.1% after 50 cycles), the as-prepared CoW-POM shows a stable cycling performance with a prolonged lifespan (the capacity remains at 664. 8 mAh g-1 after 100 cycles (90.1%, retention)), which is higher ACS Paragon Plus Environment

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than that of isolated Co8W12O42(OH)4(H2O)8 powder with a capacity of 429.4 mAh g-1 after 100 cycles (retention rate: 60.1%, Figure S-4c).In addition, a reversible charge/discharge behavior with a CE near 100% is observed for CoW-POW. Moreover, the overall electrochemical performance of the CoW-POM was summarized and compared with other POM based, W-based and Co-based materials (Table S-2). Remarkably, considering the contribution of MOF/graphene11 or organic ligands,13 our work presents a high capacity with a facile procedure. It is concluded that the performances of CoW-POW are better than most of others in terms of initial CEs, reversible capacities, cycling life and capacity retention. Therefore, it is solidly concluded that the CoW-POM possesses better electrochemical performances (capacity, rate capability and cycle life) than CoW-Salt. The cyclic voltammetry was also employed to track the Li+ insertion/de-insertion process (Figure 3e, 3f). In the first cycle, three reduction peaks at~1.77, ~1.25, and ~0.25 V are obviously observed for CoW-POM, which are possibly corresponded to the reduction of Co8W12O42(OH)4(H2O)8 to W and Co, and the formation of the amorphous Li2O respectively according to the following equation (Eq. (2)):34 Co8W12O42(OH)4(H2O)8 + 88Li+ + 88e− → Co + W + 44Li2O (2) After the first cycle, the appearance of the reduction peak of W at around 0.45V, the disappearance of the reduction peak of W at around 1.77 V and the shift of the reduction peak of Co from around 1.25 V to a broad shape at around 1.28 V imply the formation of SEI on Co8W12O42(OH)4(H2O)8 and the partial break of Co8W12O42(OH)4(H2O)8 structure.35 Furthermore, two broad oxidation peaks in the charging process appear at around 1.1-1.4 V and 2.3-2.5 V, corresponding to the reversible conversion of W intoWO3 and Co into CoO according to the Eq. (3): Co +W+4 Li2O↔CoO + WO3 + 8 Li+ + 8e-

(3)

However, as for CoW-Salt shown in Figure 3f, the reduction peaks at ~2.29, ~1.51 and 0.60 V are ascribed to the reduction of CoWO4 to W(0) and Co(0) in the first cycle according to the Eq. (4): CoWO4 + 8 Li+ + 8 e-→Co + W + 4 Li2O


(4)


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The distinction of reduction peaks at 0.60 and 2.29 V in the following cycles indicates that CoW-Salt possesses serious capacity loss due to the unrecoverable phase transformation and the formation of solid electrolyte interphase (SEI). In conclusion, CoW-POM demonstrated more satisfied reversibility compared with CoW-Salt. 3.3 Mechanistic discussion To deepen the understanding of the impressive reversibility and stability of CoW-POM, SEM and XRD technologies were performed after cycling. As depicted in Figure 4a, the regular polyhedron morphologies are retained for CoW-POM, different from the pulverization behavior of CoW-Salt aggregates (Figure S-5). In addition, as revealed in Figure S-6, the insertion of lithium ions diminishes the peak intensities of CoW-POM with a low-angle shift phenomenon, which indicates crystallization weakening (partial amorphization) and lattice deformation with the increase in lattice distance. After de-insertion of lithium ions, the crystal structure of CoW-POM begins to recovery to a certain extent. XRD pattern also certifies that the initial crystal phase of CoW-POM is preserved after 20 cycles (Figure 4b).All these observations indicate that the CoW-POM can endure the volume variation with structural integrity during the repeating intercalation and removal of lithium ions, further endowing them with satisfactory capacity retention and cycle stability. To discover the interfacial information of CoW-POW during cycling, the spectrum technologies were also investigated. As for the FTIR spectra of the original CoW-POM, peaks at 920 cm-1, 853 cm-1 and 769 cm-1 can be assigned the stretching of W=O, the stretching of O-W-O and the bending of W-O-W bridges, respectively (Figure 4c).36,37 In addition, the signals at 683 cm-1and 561 cm-1 reveal the formation of new bonds due to the mechanical activation.36 Differently, the adsorptions peaks are broadened due to the increase of the interplanar distance after the continuous insertion/extraction of lithium ions. In addition, a new peak appears at 1434 cm-1, which is originated from the defect vibration in the host lattice.38 Furthermore, Raman spectra are also recorded in Figure 4d. A strong band at 961 cm-1 and a weak band at 889 cm-1 are attributed to symmetric and asymmetric stretching ACS Paragon Plus Environment

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vibrations of teriminal W=O bond.39 The weak peaks at 730, 616 and 518 cm-1 are owning to the asymmetric and symmetric stretching of O-W-O bringing bond. Two bands at 408 and 370 cm-1 are assigned as the in-plane deformation and rotation of W-O bond (terminal and bridging), respectively.38 Besides, the peaks at 265 and 189 cm-1 arise from Co-O bond stretching vibration and out-of-plane vibration.40,41 After lithiation, the O-W-O (730, 616 and 518 cm-1), W-O (408 and 370 cm-1) and Co-O (265 cm-1) vibration for CoW-POM shift to the low-wave side, suggesting the transformation from W6+ and Co2+ to Wx+ and Cox+.42 In addition, a new peak appears at 1168 cm-1, which is probably corresponded to the lithium atoms vibration against the host lattice.38 All these evolution strongly evidenced that the successful interaction between CoW-POM and lithium ions. More detailed information about surface functionalities were also provided by XPS Survey (Figure 5 and Figure S-7). The Co 2p signal of the original CoW-POM, can be deconvoluted into 2 pairs of spin-orbit peaks

(Co(II) 2p3/2 and Co(II) 2p1/2 at 780.3 and 796.6 eV, Co(II) 2p3/2 (sat) and Co(II) 2p1/2

(sat) at 786.4 and 802.9 eV, Figure 5a).19,43 After lithium ion cycling, the double peaks located at 774.65 and 790.14 eV, informing that Co2+ is partially reduced to Cox+ (Figure 5d).44 Furthermore, the XPS signal of Co 2p becomes quite low than that of Co 2p before discharging, indicating the loss of Co after discharging, which may be due to the Co2+ ions replaced by Li+ ions. Similarly, different from the original W(VI) oxidation state in CoW-POM (Figure 5b, W 4f5/2 and W 4f7/2 at 37.4 and 35.4 eV), partially reduced intermediate Wx+ is also observed for lithium ion insertion (Figure 5e, peaks at 33.98 and 35.79 eV).10 In addition, a new peak with a binding energy at 531.3 eV can be identified as Li2O in the O1s narrow scan (Figure 5f), suggesting the oxidation process of lithium.45 Therefore, it is evidently deduced that CoW-POM is transformed into low valued metal and Li2O during lithium ion intercalation. To gain insight into the electrochemical mechanics of CoW-POM and CoW-Salt, EIS analysis was implemented (Figure 6a). In principle, the X-intercept of the semicircle corresponds to the resistance of the solution (Rs) and the diameter of semicircle represents the charge-transfer resistance (Rct) in the ACS Paragon Plus Environment


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high-frequency.46 The Rct is mainly internal resistance of electrode materials and contact resistance of electrode-electrolyte interface.47 The straight line in the low-frequency region is owing to the diffusion of Li+ into the electrode materials (Warburg diffusion, Zw). The Rs, Rct and Zw (Table S-3) for CoW-POM is smaller than those for CoW-Salt, demonstrating the enhanced electronic conductivity and shortened Li+ diffusion length. Moreover, the apparent Li+ diffusion coefficient DLi+ (Figure 6b) of CoW-POM material is calculated to be 5.85×10-14 cm2 s-1, which is bigger than that of the CoW-Salt (5.32×10-15 cm2 s-1). This may be due to the smaller Li/transition-metal exchange ratio in the lattice structure of CoW-POM.48 Therefore, the higher ion diffusion coefficient and the smaller charge-transfer resistance may lead to the better performance of CoW-POM than CoW-Salt. 4. Conclusions In summary, this work demonstrates a facile hydrothermal strategy to successfully fabricate Co8W12O42(OH)4(H2O)8 microcrystals or CoWO4 aggregates onto the 3D foamed substrate. Combining the extraordinary redox stability and excellent lattice deformability of POM with the volume accommodation abilityof 3D dispersed substrate, the as prepared CoW-POM was endowed with an extraordinary better electrochemical performance (capacity, rate capability and cycle life) than that of CoW-Salt. In detail, the CoW-POM can deliver a reversible capacity of 737.8 mAh g-1 at the current density of 0.1 A g-1 and provide a capacity retention of 90.1% even after 100 cycles. This work not only promotes the application of POM in energy storage and conversion, but also guides an effect methodology to endow POMs with 3D structures. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications. Calculation of the theoretical capacities (CoW-POM and CoW-Salt), additional characterizations of POMs and other metal salts (SEM, TEM, XPS, EDX and electrochemical studies) and tabled electrochemical comparisons of POMs (PDF). ACS Paragon Plus Environment

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AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (H. Zhu) *E-mail: [email protected] (J. Yin) ORCID Hui Zhu: xxx Jiao Yin: xxx Author Contributions K. Sun and H. Li contributed equally to this work. Notes The authors declare no competing financial interest. Acknowledgements This work was supported by the Natural Science Foundation of China (51663016 and 21403295), the Natural Science Foundation of Jiangxi Province (2016BAB203075, 20162BCB23015), the National Key Technology Research and Development Program of the Ministry of Science and Technology of China (2015BAD14B06).



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Figure Captions: Figure 1. XRD patterns of CoW-POM (a) and CoW-Salt (b). Figure 2. (a-c) SEM images of CoW-POM in different magnification; (d) HRTEM image and the correlated SAD pattern of CoW-POM. Figure 3. Electrochemical charge/discharge profiles of CoW-POM (a) and CoW-Salt (b) in the potential range of 0.01-2.8 V (vs. Li/Li+) at a current density of 0.1 A g-1; The rate capability behaviors of CoW-POM and CoW-Salt at current density intervals from 0.1 to 1.6 A g-1 (c); Cycling performance and corresponding Coulombic efficiency of CoW-POM and CoW-Salt at 0.1 A g-1 for 100 cycles (d); The CV curves of CoW-POM (e) and CoW-Salt (f) in the potential range of 0-2.8 V (vs. Li/Li+) at a scan rate of 0.1 mV s-1. Figure 4. SEM image (a) and XRD pattern (b) of CoW-POM after charge/discharge cycling 20 times, FTIR spectra (c) and Raman spectra (d) of CoW-POM before and after cycling. Figure 5. XPS spectra of CoW-POM before (a-c) and after discharging to 0.01 V (d-f): Co 2p (a, d); W 4f (b, e); O 1s (c, f). Figure 6. (a) Electrochemical impedance spectra (EIS) of CoW-POM and CoW-Salt; (b) the relationship plot between Zre and ω-1/2 at low-frequency region.


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Figure 1. XRD patterns of CoW-POM (a) and CoW-Salt (b).



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Figure 2. (a-c) SEM images of CoW-POM in different magnification; (d) HRTEM image and the correlated SAD pattern of CoW-POM.


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Figure 3. Electrochemical charge/discharge profiles of CoW-POM (a) and CoW-Salt (b) in the potential range of 0.01-2.8 V (vs. Li/Li+) at a current density of 0.1 A g-1; The rate capability behaviors of CoW-POM and CoW-Salt at current density intervals from 0.1 to 1.6 A g-1 (c); Cycling performance and corresponding Coulombic efficiency of CoW-POM and CoW-Salt at 0.1 A g-1 for 100 cycles (d); The CV curves of CoW-POM (e) and CoW-Salt (f) in the potential range of 0-2.8 V (vs. Li/Li+) at a scan rate of 0.1 mV s-1.



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Figure 4. SEM image (a) and XRD pattern (b) of CoW-POM after charge/dischargecycling 20 times, FTIR spectra (c) and Raman spectra (d) of CoW-POM before and after cycling.


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Figure 5. XPS spectra of CoW-POM before (a-c) and after discharging to 0.01 V (d-f): Co 2p (a, d); W 4f (b, e); O 1s (c, f).



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Figure 6. (a) Electrochemical impedance spectra (EIS) of CoW-POM and CoW-Salt; (b) the relationship plot between Zre and ω-1/2 at low-frequency region.


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The table of contents entry


3D-Structured Polyoxometalate Microcrystals with Enhanced Rate

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