Ultrathin Zn2(OH)3VO3 Nanosheets: First Synthesis, Excellent Lithium

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Ultrathin Zn2(OH)3VO3 Nanosheets: First Synthesis, Excellent LithiumStorage Properties and Investigation of Electrochemical Mechanism Gongzheng Yang, Mingmei Wu, and Chengxin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08048 • Publication Date (Web): 25 Aug 2016 Downloaded from http://pubs.acs.org on September 1, 2016

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ACS Applied Materials & Interfaces

Ultrathin Zn2(OH)3VO3 Nanosheets: First Synthesis, Excellent Lithium-Storage Properties and Investigation of Electrochemical Mechanism Gongzheng Yang, Mingmei Wu and Chengxin Wang* State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials Science and Engineering, Sun Yat-sen (Zhongshan) University, Guangzhou 510275, People’s Republic of China Abstract Nowadays, exploiting novel electrode materials is widely accepted as a key for meeting the growing demands of high-performance lithium ion batteries. Several transition metal vanadates, which can in situ form an elastic buffer to adapt the volume expansion during lithium uptake/removal, have recently attracted much attention as anode materials since their high capacity and superior cycling stability. Herein, Zn2(OH)3VO3 nanostructures are successfully fabricated for the first time by a facile hydrothermal method and also firstly studied as lithium ion anode material. The ultrathin Zn2(OH)3VO3 nanosheets deliver a high reversible capacity close to 900 mAh g-1 at a current density of 1 A g-1 over 100 cycles. Even at a high current rate of 5 A g-1, capacity retention as high as 83% (by compared with the 2nd discharge capacity) is still obtained after 500 cycles, showing a high-rate capability. Moreover, we have also carefully investigated the lithium-storage mechanism of Zn2(OH)3VO3, and corresponding results reveal that the Zn2(OH)3VO3 nanosheets has in situ transformed into ZnO nanoparticles anchoring on lithiated vanadium oxides matrix. The synergistic effect of zinc and vanadium oxides upon lithium ions intercalation, and the stable conductive skeleton of amorphous lithiated vanadium oxides matrix, both contribute to the excellent battery performance of Zn2(OH)3VO3 nanosheets. Finally, a full cell composed of lithiated Zn2(OH)3VO3/LiFePO4 with a high energy density of 293 Wh kg-1 (vs. total mass of active materials) at the current density of 100 mA g-1 was successfully assembled, which could cycle well over 100 cycles with 79% capacity retention and also exhibit good rate stability. Thus, we believe that our research demonstrates a promising anode material for lithium ion batteries. Keywords: Zn2(OH)3VO3, nanosheets, anode, lithium-storage mechanism, full cell *Correspondence and requests for materials should be addressed to C. X. Wang. Tel & Fax: +86-20-84113901, E-mail: [email protected] 1

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Introduction Starting from the discovering of grapheme1 – 3, two-dimensional (2D) graphenebased nanomaterials have gained increasing interest in the application for lithium ion batteries due to their many attractive merits, majorly including the large exposed surface offering ample reaction sites and the shortened paths for fast ions/electrons transport.4 – 14 In such structures, the ultrathin graphene nanosheets will not only act as conductive carrier for Li+ ions diffusion and electrons transport, but also work as the effective buffers to inhibit the volume expansion and also impede the subsequent agglomeration of pulverized electrode materials upon lithium ions insertion/extraction, both contributing to the enhanced lithium-storage properties.10, 14, 15 However, it is known that construction of such hybrids suffers from the high cost and tedious fabrication procedures. So naturally, researchers worldwide are working hard to exploit alternative 2D nanomaterials with similar architecture to realize the simple and low-cost preparation of high-performance lithium ion anode materials.16, 17 As the typical layered compounds, numerous 2D organic-inorganic intercalation vanadium oxides based products have been widely studied as possible active materials for lithium ion batteries systems owing to their abundant source, easy synthesis, low cost, as well as high chemical stability.18 – 20 However, despite of their high specific capacities, most of the vanadium oxide-based compounds demonstrate poor electrochemical performances particularly the weak cycling stability when introduced as lithium ion anode materials, which is usually ascribed to their low intrinsic conductivity and dramatic volume changes upon lithium uptake/removal.21 – 23 Fortunately, several vanadium oxides-based electrode materials that can in situ form surface-binding 2D nanostructures or possess the similar architecture with 2D graphene-based nanomaterials recently are exploited and have achieved significantly improved lithium-storage performances.24

– 27

For instance, our group reported the

first application of Co3V2O8 multi-layered nanosheets for lithium storage and found that Co nanoclusters homogenously anchored on amorphous LixV2O5 matrix, which served as an elastic medium and exhibited an excellent battery performance.25 Subsequently, hierarchical Co2V2O7 nanosheets consisted of interconnected nanoparticles28 and Co3V2O8 hollow microspheres29 with stable high capacities and robust rate capability were developed by Luo and co-workers. Thoroughly 2


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investigations of the electrochemical reaction mechanism clearly proved the crucial role of the formation of Li-V-O matrix in enhancing the battery properties. However, cobalt is toxic and expensive and hence it is necessary to find cheaper and more ecofriendly metal vanadates. Zinc-based compounds commonly possess high theoretical capacity, widespread sources and environmentally benignity30 and zinc can form a variety of structures with vanadium oxide. To date, considerable amounts of zinc vanadates were fabricated and introduced as lithium ion anode materials.31

– 39

Unfortunately, some problems in

particular focus on the fast capacity fading, which is a must to overcome to implement these materials in practical lithium ion batteries. In this work, ultrathin Zn2(OH)3VO3 nanosheets are fabricated via a facile hydrothermal method and to our best this is the first report on the successfully preparation of the nanostructure of this compound. Then, we examined in detail the lithium-storage properties of the ultrathin Zn2(OH)3VO3 nanosheets when employed as negative electrodes for lithium ion batteries. The products exhibit high capacity, long life span (898 mAh g-1 after 100 cycles) and good rate capability (384 mAh g-1 at 10 A g-1). More importantly, the electrochemical mechanism during the battery operation was carefully investigated. Corresponding results confirm that the Zn2(OH)3VO3 nanosheets demonstrate an identical electrochemical behaviour with Co3V2O8, namely, the initial Zn2(OH)3VO3 transformed into ZnO and lithiated vanadium oxides while the following reactions primarily happened between the ZnO and LiZn on the lithiated vanadium oxides matrix. This is meaningful for enriching the metal vanadates-based lithium ion batteries. Furthermore, a full cell is assembled with a commercially available LiFePO4 cathode under suitable mass loading. The LiFePO4/Zn2(OH)3VO3 nanosheets cell delivered a high energy density of 293 Wh kg1

(vs. total mass of active materials) at the current density of 100 mA g-1, which

further suggests the possible applications of the ultrathin Zn2(OH)3VO3 nanosheets.

Experimental Section Synthesis and characterization.

Ultrathin Zn2(OH)3VO3 nanosheets were

prepared by a simple hydrothermal method. Initially, 32 mL deionized water was heated to 80 oC and 37 mg ammonium metavanadate (NH4VO3, Sigma-Aldrich, 99%) 3



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was dissolved into the hot water to form a light yellow solution with stirring. Then we added 95 mg zinc nitrate hexahydrate (Zn(NO3)2·2H2O, Sigma-Aldrich, 98%) into the solution that will further be stirred for several minutes. The mixing turbid solution was subsequently poured into a hydrothermal reaction vessel and heated to 180 oC for 12 hours. Finally, the precipitate in the bottom of vessel was collected as cooling to room temperature following with repeated washing by water and alcohol, and dried in air at 80 oC for 6h. The crystal structure of the samples was confirmed by X-ray diffraction measurements (Rigaku Co., Japan). Transition electron microscopy (TEM, 300 kV, FEI, F30) was used to analyses the morphology and microstructure of the products. Atomic force microscopy (AFM, MikroMash, Wislsonville, OR, USA) tests were performed to verify the thickness of the samples. Electrochemical characterization. Firstly, the electrodes consisted of 70wt% Zn2(OH)3VO3 nanosheets, 20wt% carbon black, and 10wt% sodium alginate (SA) using deionized water as solvent. Subsequently, the slurry was brushed evenly on the copper foil (thickness of the current collector: 18µm), dried and cut into circular discs. A CR2032-type cell that consists of a lithium disk and composite electrodes was fabricated to evaluate the lithium-storage properties of the products. The typical mass of the active materials was 0.5 mg cm-2. A solution composed of 1 mol L-1 LiPF6 in 1:1 (V: V) ethylene carbonate (EC) and diethyl carbonate (DEC) was selected as the electrolyte. The electrochemical behaviours and galvanostatically charge-discharge properites of the batteries were tested in the same potential window of 0.01 – 2.5 V using an electrochemical workstation (IM6e-X) and a NEWARE battery tester, respectively. The working cathode was prepared by mixing of commercial LiFePO4 powders, Super-P, and SA in a weight ratio of 80:10:10 following by coating on an aluminium foil with a proper thickness. For the assembly of a full cell, the mass ratio of cathode/anode materials (6:1) was optimised based upon the practical specific capacities of both electrodes. Results and Discussion Figure 1a shows the XRD spectrum of the obtained products, where all the diffraction peaks can be well indexed to Zn2(OH)3VO3 with a rhombohedral phase 4


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(JCPDS No. 50-0232). No diffractions other than Zn2(OH)3VO3 can be detected, demonstrating the high purity of the products. Besides, the strongest Bragg peak collected from the as-synthesized samples is found locating at 2θ = 34.21o that are unlike with the standard data, which can be attributed to the (011) plane, indicating the preferential growth direction of the products. TEM experiments were performed to reveal the morphologies and microstructure. From the low-magnification TEM images (Figure S1, supporting information), the samples are mainly consist of irregular sheetlike nanoparticles with dimensions ranging from 20 to 200 nm. A large quantity of rodlike nanoparticles with narrow widths of less than 10 nm, but broad lengths of ca. 20 – 200 nm were also observed (Figure 1b). Energy dispersive X-ray test was further carried out to detect the chemical composition of the Zn2(OH)3VO3 nanoparticles, and the results confirm the composition of the products (Figure S2). High-resolution

TEM

(HRTEM)

observations

will

help

us

in-depth

understanding the crystal structures of these nanoparticles. As exhibited in Figure 1c, the clearly visible lattice fringes in the magnification of a part of a sheetlike nanoparticle confirm the high crystallinity of these nanoparticles, while corresponding Fast Fourier transform (FFT) analysis suggests the exposed (100) facet of these sheetlike nanoparticles. Figure 1d gives the HRTEM image of a rodlike nanoparticle. The basal spacing which parallels to the axis of the rod is calculated to be 0.266 nm that is in good accordance with the (100) lattice fringe of crystal Zn2(OH)3VO3. Moreover, the FFT pattern shown in the inset of Figure 1d implies the [011] growth direction of the rodlike nanoparicle. Therefore, the rodlike nanoparticles are actually nanosheets with the lateral view and the width indicates the thickness of the nanosheets. More convincing evidences were proposed by AFM studies (Figure 2), and corresponding results demonstrate the average height of the nanosheets is 3.5 – 6.5 nm that further confirms with the results from the TEM observations (Figure S3). To our best this is the first report on the successful synthesis of a Zn2(OH)3VO3 nanostructure. In our previous work, 2D vanadium-based mixed metal oxide (Co3V2O8) had been proved an excellent lithium-storage electrode material.24 The ultrathin nanoarchitecture and in particular the surface-to-surface construction that arisen along with the lithium ions insertion had demonstrated the crucial role in enhancing the electrochemical performances. Herein, the Zn2(OH)3VO3 nanosheets 5



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have displayed the ultrathin nature, which innovates us to investigate their lithiumstorage performances. The electrochemical performances of the electrodes made from

the

Zn2(OH)3VO3 nanosheets were evaluated by cycling voltammetry (CV) and galvanostatic charge-discharge cycling. A sweep rate of 0.2 mV s-1 was set up, with the voltage of 0.01 – 2.5 V, to examine the electrochemical behaviour of Zn2(OH)3VO3 nanosheets. As shown in Figure 3a, the CV discharge curve of the first cycle exhibits a multi-step insertion of lithium ions into the Zn2(OH)3VO3 nanosheets. During the subsequent extraction of Li+, there is only a broad anodic peak locating between 1.0 – 1.6 V can be observed. In the next two cycles, the similar broad peak with a little left shift for anodic processes is clearly visible. Two stable cathodic peaks can be seen centred at 0.45 and 0.80 V, respectively, suggesting irreversible electrochemical reactions had happened in the first cycle. The discharge/charge profiles in Figure 3b further illustrate the CV results. In the first cycle, the Zn2(OH)3VO3 nanosheets electrodes deliver delivers a specific discharge capacity of 1450 mAh g-1 and charge capacity of 835 mAh g-1 with initial Coulombic efficiency of 57.5% (Figure 3c). The large initial capacity loss is usually attributed to the formation of a SEI membrane in the first discharge step. It is worth noting that a reversible capacity as high as 898 mAh g-1 could be well preserved after 100 cycles, which is equal to an intercalation of approximately 9.4 Li per formula unit that is much higher than those obtained in other widely-studied metal oxides (e.g., ~2.0 Li for CoO40, ~4.4 Li for SnO225, 41). The rate performances collected at different current densities of the electrodes are shown in Figure 3f. The results reveal that the electrodes present outstanding rate capabilities. When the current density was increased from 0.1 to 0.2, 0.5, 1, and 5 A g-1, the average discharge capacity gradually decreased from 1016 to 844, 802, 728, and 530 mAh g-1, respectively. At a high rate of 10 A g-1, a capacity of ca. 390 mAh g-1 was still achieved that is higher than the theoretical specific capacity of the graphite electrode. Figure 3d shows the dischargecharge profiles associated with various currents of the electrodes. The most attractive property of the Zn2(OH)3VO3 nanosheets is their splendid long-term high-rate cyclic stability. As shown in Figure 3e, the Zn2(OH)3VO3 electrodes exhibit ca. 500 cycles with a capacity retention (545 mAh g-1) of 83% 6


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compared with the 2nd discharge capacity at a high current rate of 5 A g-1. Thus, we can see that the ultrathin Zn2(OH)3VO3 nanosheets are a promising anode material for high-performance lithium ion batteries. However, now that this is the first report on the synthesis and application of Zn2(OH)3VO3 nanostructures, it is necessary to make clear the electrochemical mechanism of the Zn2(OH)3VO3 nanosheets. Firstly, ex situ XRD tests were introduced to examine the structural evolutions of Zn2(OH)3VO3 upon lithium intercalation. Figure 4b exhibits XRD patterns collected at different stages (Figure 4a). Upon discharging to 1.4 V (first cycle) all the diffraction peaks remain constant but decrease in intensity (red line in Figure 4b), indicating the gradually destroyed crystalline structure of Zn2(OH)3VO3. As lithiation proceeding, further lithium insertion cause a new phase emerge (Figure 4b, blue line). After careful comparison the new diffraction peaks can be indexed to cubic ZnO, namely, upon lithium insertion the pristine Zn2(OH)3VO3 has transformed to the cubic ZnO. Subsequently, the crystalline ZnO gradually changed into amorphous state as full insertion of lithium into the electrodes (Figure 4b, olive line). For the extraction of lithium from the electrodes, cubic ZnO with weak crystallization was detected again, while the diffraction peaks were found right-shift that might because of the incomplete extraction of lithium ions. And hence it is known that the reaction between ZnO and Li+ is the main electrochemical process in the following cycles. TEM analyses can help us to directly observe the morphological and structural changes of the Zn2(OH)3VO3 nanosheets electrodes. Figure 5a gives a TEM images of electrodes collected at initial stage (red line in Figure 4b), while no obvious morphological changes can be observed. The clear lattice fringes (Figure 5b) of a rodlike nanoparticle illustrate the structural integrity of the Zn2(OH)3VO3 nanosheets. However, the distorted lattice fringes of an ultrathin nanosheet shown in Figure 5c suggests the possible collapse of intact nanosheets in the continuing lithium ions insertion, which matches well with the XRD results. As expected, the electrodes are disrupted and transform into a surface-to-surface architecture that is consist of many nanoparticles uniformly anchoring on the amorphous matrix when discharging to 0.7 V (Figure 5d, f). The FFT pattern collected from a locally enlarged HRTEM image (Figure 5e) suggests that these quantum dots are actually cubic ZnO nanograins. Element mapping images in Figure 5f show the homogenous distribution of the 7



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elements V, Zn, and O. It was mentioned above that the ZnO had completely changed to amorphous state as the full insertion of lithium ions. However, many crystalline nanograins can be seen in the TEM images (Figure 6a and b) that seem to be contrary to the XRD results. According to the previous studies, the discharge process of ZnO towards Li involves two steps: ZnO + 2Li → Zn + Li2O; Zn + Li → LiZn.30, 42 Thus we speculate that these crystalline nanograins are probably Zn or LiZn, which possesses high activity and is probably been oxidized as exposing in air. Figure 6c exhibits a representative TEM image of the fully oxidized electrode. HRTEM image (Figure 6d) attached with the SAED pattern (Figure 6e) implies that the electrodes is reoxidized to ZnO during the delithiation process, while STEM image and element mapping images in Figure 6f shows the uniformly distributed ZnO nanoparticles, as well as the tightly bonding of the ZnO nanoparticles with the lithiated vanadium oxides matrixes. Moreover, TEM observations of the long-cycled electrodes were carried out and corresponding results are shown in Figure S4, in which one can see that the morphological changes of Zn2(OH)3VO3 nanosheets upon cycling are completely reversible that is crucial for achieving a long cycle life. So far, we have known that the Zn2(OH)3VO3 nanosheets transform into Zn-Li alloy nanoparticles and amorphous LixV2O5 in the initial discharge process, while the following reaction mainly occurs between ZnO and Li-Zn alloys, as well as the lithiation/delithiation of the lithium vanadate. More importantly, the lithiated vanadate matrix had always serves as a stable skeleton during the subsequent lithium insertion/extraction, which not only can effectively accommodate the volume expansion, but also enlarge the space that is in favour of the electrolyte transport. However, based on the fully reduced transformation of ZnO to LiZn, the theoretical specific capacity of Zn2(OH)3VO3 is calculated merely 572.9 mAh g-1 according to the Faraday equation that is much lower than the measured delivered capacities, which further demonstrates the vanadium oxides matrices must have participated in the electrochemical reactions. To verify the electrochemical processes, ex situ XPS obtained at different stages were performed, as shown in Figure 7. The V 2p spectrum of Zn2(OH)3VO3 displays two peaks at 517.7 and 525.3 eV (Figure 7a) that can be ascribed to the spin-orbit splitting of the components, V 2p3/2 and V 2p1/2, which 8


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matches well with previous literatures.43 As the continuous insertion of lithium ions, the bonding energies of vanadium gradually decrease (Figure 7b – c). In Figure 7d, the binding energies of 515.3 and 522.6 eV confirm the formation of V3+ after the electrode was fully reduced. Meanwhile, the characteristic peaks of V2+ 2p3/2 and V 2p1/2 located at 513.6 and 521.0 eV implies that the reduction of V5+ to mixed valence of V3+ and V 2+. Due to the strong sensitivity to the electronic configuration, the intensity ratio of V 3p3/2 (L3) and V 3p1/2 (L2) excitations is regarded as a reliable indicator for the determination of the oxidation state of vanadium. The EELS value for the L3/L2 intensity ratio in Figure 7f equals 1.57 that is the signature of mixed valence of V3+ and V2+, further confirming the XPS results.44 For the electrode charged back to 2.5 V (Figure 7e), the vanadium has not been fully oxidized to V5+, with the existence of considerable amount of V4+, which suggests that there are still many lithium storing in the vanadium oxides. Hence the theoretical capacity of Zn2(OH)3VO3 is recalculated to 859.3 mAh g-1 by resuming the fully reduction of V5+ to V2+ which is close to the practical values. The schematic illustration of the possible electrochemical behaviours of Zn2(OH)3VO3 towards lithium insertion/extraction is shown in Figure 8, which is verified by all the above experimental results. By comparison with the previous zinc vanadates,

the

performances.

Zn2(OH)3VO3

31 – 39

nanosheets demonstrate the

superior

battery

The high surface area derived from the ultrathin characteristic of

the nanosheets is a main merit inherent to the superior battery performance, which is conductive to shorten the diffusion paths and enhance the surface electron/ion storage. Moreover, the greatly improved cycling performance suggests that the amorphous lithiated vanadium oxides locating with ZnO nanoparticles plays as a reliable carrier to inhibit the agglomeration of cracked particles. As illustrated in inset of Figure 8, the skeleton of Zn2(OH)3VO3 is made up the lamellar stack of [ZnO] and [VO] polyhedra chains by sharing the edges or corners. A phase separation firstly happens upon lithium ions insertion, resulting the precipitation of ZnO nanoparticles that are firmly tightened by the remaining amorphous lithiated vanadium oxides. Undoubtedly, this two-dimensional architecture enables a great enhancement in increasing the capacity and cycling performance of Zn2(OH)3VO3 in the subsequent cycles. The excellent performance of Zn2(OH)3VO3 half cell encourages us to further 9



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evaluate its practicability in a lithium-ion full cell. In consideration of the relatively low initial Coulombic efficiency of Zn2(OH)3VO3, one discharge-charge process of the Zn2(OH)3VO3 electrodes were performed before the assembly of the full cell. Subsequently, the pre-cycled Zn2(OH)3VO3 anode was paired with commercial LiFePO4 cathode with an optimized weight ratio according to both the delivered capacities collected in half cell tests (Figure 9) and potential window is set up to 1.1 – 3.6 V. The two slopes ranging from 3.4 – 2.5 V and 2.3 – 1.8 V can be regarded as the reversible redox reactions on both electrodes. The full cell delivered a reversible capacity of 114 mAh g-1 (calculated on the basis of total mass of active materials) at 100 mA g-1 with a Coulombic efficiency of 87.7%. The lesser Coulombic efficiency is expected since an identical amount of irreversible capacity loss of Zn2(OH)3VO3 electrode is noted in half-cell configuration in the initial several cycles. A high discharge capacity of approximately 90 mAh g-1 (~79% of the initial capacity) can be obtained up to the 100 cycles tested. By gradually increasing the current rate from 10, 20, 50, 100, 200 to 400 mA g-1, the full cell could deliver average capacities of 123, 118, 116, 106, 95 and 80 mAh g-1 (Figure 10b), respectively, and finally recovered to 95 mAh g-1 at 100 mA g-1 successively, suggesting a good rate capability. The Ragone chart in Figure 10c compares the power- and energy-densities of the Zn2(OH)3VO3based full cell with commercial graphite-based lithium-ion full cells, in which one can see that the Zn2(OH)3VO3-based full cell demonstrates the better battery performances. This indicates that Zn2(OH)3VO3 nanosheets have potential for applications in lithium ion batteries.

Conclusion In summary, Zn2(OH)3VO3 nanostructures are successfully synthesized for the first time and the application of this compound for lithium ion anode materials are also firstly studied. The ultrathin Zn2(OH)3VO3 nanosheets electrodes have demonstrated high delivered specific capacity, superior rate and cycling performances. Detailed investigations of the morphological and structural evolutions during battery operation enable us to clearly understand the electrochemical mechanism of Zn2(OH)3VO3 towards lithium ions intercalations. Finally, the ultrathin Zn2(OH)3VO3 nanosheets are employed as anode in a full-cell configuration with a LiFePO4 cathode. The full cell 10


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exhibits a reversible capacity of 114 mAh g-1 at 100 mA g-1 and retained 79% of its reversible capacity after 100 cycles. Thus, the novel Zn2(OH)3VO3 nanosheets show great promise in applying as negative materials of lithium ion batteries.

Supporting Information Available: Low-magnification TEM image, EDS spectrum, AFM image and statistic of average thickness of the Zn2(OH)3VO3 nanosheets. TEM images of the cycled Zn2(OH)3VO3 nanosheets electrodes. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (11274392, U1401241).

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13. Zhou, X.; Wang, L.; Guo, Y. Binding SnO2 Nanocrystals in Nitrogen-Doped Graphene Sheets as Anode Materials for Lithium-Ion Batteries. Adv. Mater. 2013, 25, 2152 – 2157. 14. Wang, H.; Dai, H. Strongly Coupled Inorganic-Nano-Carbon Hybrid Materials for Energy Storage. Chem. Soc. Rev. 2013, 42, 3088 – 3113. 15. Liu, J.; Liu, X. Two-Dimensional Nanoarchitectures for Lithium Storage. Adv. Mater. 2012, 24, 4097 – 4111. 16. Hwang, H.; Kim, H.; Cho, J. MoS2 Nanoplates Consisting of Disordered Graphene-like Layers for High Rate Lithium Battery Anode Materials. Nano Lett. 2011, 11, 4826 – 4830. 17. Wang, H.; Feng, H.; Li, J. Graphene and Graphene-like Layered Transition Metal Dichalcogenides in Energy Conversion and Storage. Small. 2014, 10, 2165 – 2181. 18. Chernova, N. A.; Roppolo, M.; Dillon, A. C.; Whittingham, M. S. Layered Vanadium and Molybdenum Oxides: Batteries and Electrochromics. J. Mater. Chem. 2009, 19, 2526 – 2552. 19. Wang, Y.; Cao, G. Synthesis and Enhanced Intercalation Properties of Nanostructured Vanadium Oxides. Chem. Mater. 2006, 18, 2787 – 2804. 20. Wu, C.; Xie, Y. Promising Vanadium Oxide and Hydroxide Nanostructures: from Energy Storage to Energy Saving. Energy Environ. Sci. 2010, 3, 1191 – 1206. 21. Poizot, P.; Baudrin, E.; Laruelle, S.; Dupont, L.; Touboul, M.; Tarascon, J. M. Low Temperature Synthesis and Electrochemical Performance of Crystallized FeVO4·1.1H2O. Solid State Ionics. 2000, 138, 31 – 40. 22. Denis, S.; Baudrin, E.; Orsini, F.; Ouvrard, G.; Touboul, M.; Tarascon, J. M. Synthesis and Electrochemical Properties of Numerous Classes of Vanadates. J. Power Sources. 1999, 81 – 82, 79 – 84. 23. Sim, D. H.; Rui, X.; Chen, J.; Tan, H.; Lim, T. M.; Yazami, R.; Hng, H. H.; Yan, Q. Direct Growth of FeVO4 Nanosheet Arrays on Stainless Steel Foil as HighPerformance Binder-Free Li Ion Battery Anode. RSC Adv. 2012, 2, 3630 – 3633. 24. Yang, G. Z.; Cui, H.; Yang, G. W.; Wang, C. X. Self-Assembly of Co3V2O8 Multilayered Nanosheets: Controllable Synthesis, Excellent Li-Storage Properties, and Investigation of Electrochemical Mechanism. ACS Nano. 2014, 8, 4474 – 4487. 13



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25. Yang, G.; Song, H.; Wu, M.; Wang, C. SnO2 Nanoparticles Anchored on 2D V2O5 Nanosheets with Enhanced Lithium-Storage Performances. Electrochim. Acta. 2016, 205, 153 – 160. 26. Yin, Z.; Xiao, Y.; Wang, X.; Wang, W.; Zhao, D.; Cao, M. MoV2O8 Nanostructures: Controlled Synthesis and Lithium Storage Mechanism. Nanoscale. 2016, 8, 508 – 516. 27. Yu, Y.; Niu, C.; Han, C.; Zhao, K.; Meng, J.; Xu, X.; Zhang, P.; Wang, L.; Wu, Y.; Mai, L. Zinc Pyrovanadate Nanoplates Embedded in Graphene Networks with Enhanced Electrochemical Performance. Ind. Eng. Chem. Res. 2016, 55, 2992 – 2999. 28. Luo, Y.; Xu, X.; Zhang, Y.; Chen, C.; Zhou, L.; Yan, M.; Wei, Q.; Tian, X.; Mai, L. Graphene Oxide Templated Growth and Superior Lithium Storage Performance of Novel Hierarchical Co2V2O7 Nanosheets. ACS Appl. Mater. Interfaces. 2016, 8, 2812 – 2818. 29. Luo, Y.; Xu, X.; Tian, X.; Wei, Q.; Yan, M.; Zhao, K.; Xu, X.; Mai, L. Facile Synthesis of a Co3V2O8 Interconnected Hollow Microsphere Anode with Superior High-Rate Capability for Li-Ion Batteries. J. Mater. Chem. A. 2016, 4, 5075 – 5080. 30. Yang, G.; Song, H.; Cui, H.; Liu, Y.; Wang, C. Ultrafast Li-Ion Battery Anode with Superlong Life and Excellent Cycling Stability from Strongly Coupled ZnO Nanoparticle/Conductive Nanocarbon Skeleton Hybrid Materials. Nano Energy. 2013, 2, 579 – 585. 31. Xiao, L.; Zhao, Y.; Yin, J.; Zhang, L. Clewlike ZnV2O4 Hollow Spheres: Nonaqueous Sol-Gel Synthesis, Formation Mechanism, and Lithium Storage Properties. Chem. Eur. J. 2009, 15, 9442 – 9450. 32. Zheng, C.; Zeng, L.; Wang, M.; Zheng, H.; Wei, M. Synthesis of Hierarchical ZnV2O4 Microspheres and Its Electrochemical Properties. CrystEngComm 2014, 16, 10309 – 10313. 33. Zhu, X.; Jiang, X.; Xiao, L.; Ai, X.; Yang, H.; Cao, Y. Nanophase ZnV2O4 as Stable and High Capacity Li Insertion Electrode for Li-Ion Battery. Curr. Appl. Phys. 2015, 15, 435 – 440. 34. Zeng, L.; Xiao, F.; Wang, J.; Gao, S.; Ding, X.; Wei, M. ZnV2O4-CMK 14


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Nanocomposite as an Anode Material for Rechargeable Lithium-Ion Batteries. J. Mater. Chem. 2012, 22, 14284 – 14288. 35. Sun, Y.; Li, C.; Wang, L.; Wang, Y.; Ma, X.; Ma, P.; Song, M. Ultralong Monoclinic ZnV2O6 Nanowires: Their Shape-Controlled Synthesis, New Growth Mechanism, and Highly Reversible Lithium Storage in Lithium-Ion Batteries. RSC Adv. 2012, 2, 8110 – 8115. 36. Luo, L.; Fei, Y.; Chen, K.; Li, D.; Wang, X.; Wang, Q.; Wei, Q.; Qiao, H. Facile Synthesis of One-Dimensional Zinc Vanadate Nanofibers for High Lithium Storage Anode Materal. J. Alloys Compd. 2015, 649, 1019 – 1024. 37. Zhang, S.; Xiao, X.; Lu, M.; Li, Z. Zn3V2O7(OH)2·2H2O and Zn3(VO4)2 3D Microspheres as Anode Materials for Lithium-Ion Batteries. J. Mater. Sci. 2013, 48, 3679 – 3685. 38. Vijayakumar, S.; Lee, S.; Ryu, K. Synthesis and Zn3V2O8 Nanoplatelets for Lithium-Ion Battery and Supercapacitor Applications. RSC Adv. 2015, 5, 91822. 39. Ni, S.; Zhou, G.; Lin, S.; Wang, X.; Pan, Q.; Yang, F.; He, D. Hydrothermal Synthesis of Zn3V2O7(OH)2·nH2O Nanosheets and Its Application in Lithium Ion Battery. Mater. Lett. 2009, 63, 2459 – 2461. 40. Peng, C. X.; Chen, B. D.; Qin, Y.; Yang, S. H.; Li, C. Z.; Zuo, Y. H.; Liu, S. Y.; Yang, J. H. Facile Ultrasonic Synthesis of CoO Quantum Dot/Graphene Nanosheet Composites with High Lithium Storage Capacity. ACS Nano. 2012, 6, 1074 – 1081. 41. Wang, X.; Cao, X. Q.; Bourgeois, H.; Guan, H.; Chen, S. M.; Zhong, Y. T.; Tang, D. M.; Li, H. Q.; Zhai, T. Y.; Li, L.; Bando, Y.; Golberg, D. N-Doped GrapheneSnO2 Sandwich Paper for High-Performance Lithium-Ion Batteries. Adv. Funct. Mater. 2012, 22, 2682 – 2690. 42. Huang, X. H.; Xia, X. H.; Yuan, Y. F.; Zhou, F. Porous ZnO Nanosheets Grown on Copper Substrates as Anodes for Lithium Ion Batteries. Electrochim. Acta. 2011, 56, 4960 – 4965. 43. Silversimit, G.; Depla, D.; Poelman, H.; Marin, G. B.; Gryse, R. D. Determination of the V2p XPS Binding Energies for Different Vanadium Oxidation States (V5+ to V0+). J. Raman Spectrosc. 2004, 135, 167 – 175. 44. Djerdj, I.; Sheptyakov, D.; Gozzo, F.; Arcon, D.; Nesper, R.; Niederberger, M. 15



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Oxygen Self-Doping in Hollandite-Type Vanadium Oxyhydroxide Nanorods. J. Am. Chem. Soc. 2008, 130, 11364 – 121375.

16


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Figure caption Figure 1. Structural and morphological characterizations of the Zn2(OH)2VO3 nanosheets. (a) XRD pattern; (b) Low-magnification TEM image; (c, d) HRTEM images of a single nanosheet and a rod-like nanosheet. Figure 2. AFM image of the Zn2(OH)2VO3 nanosheets and the right “height-width” profile corresponds to lines in the AFM image. Figure 3. (a) CV curves and (b) charge/discharge profiles of the Zn2(OH)2VO3 nanosheets. (c, d) Cycling performances obtained at different current densities. (e) Charge/discharge profiles obtained at various current densities. (f) Rate performance of the Zn2(OH)2VO3 nanosheets. Figure 4. Ex-situ XRD patterns collected at different stages (from A to E) marked in the left charge/discharge profiles. Figure 5. (a – c) Low-magnification TEM and HRTEM images of the reduced electrodes obtained as discharging to 1.4 V. (d, e) Low-magnification TEM and HRTEM images of the reduced electrodes obtained as discharging to 0.7 V. Inset in (e) show the corresponding FFT pattern. (f) Element mapping images. Figure 6. (a, b) Low-magnification TEM and HRTEM images of the reduced electrodes obtained at the cutoff voltage of 0.01 V. (c, d, e) Low-magnification TEM, HRTEM images and SAED pattern of the oxidized electrodes obtained when charging to 2.5 V. (f) Element mapping images. Figure 7. XPS spectra of (a) the fresh-prepared electrode, (b) reduced electrode discharging to 1.4 V, (c) discharging to 0.7 V, (d) discharging to 0.01 V and (e) reoxidized electrode charging to 2.5 V. (f) EELS spectrum of Vanadium white lines obtained at the cutoff voltage of 0.01 V. Figure 8. The possible electrochemical reactions between lithium ions and Zn2(OH)2VO3 nanosheets electrodes. The inset illustrates the lithium-storage mechanism in Zn2(OH)2VO3 nanosheets electrode. Figure 9. Charge/discharge profiles of the half cells of LiFePO4 cathode and Zn2(OH)2VO3 nanosheets anode and the full cell of LiFePO4/Zn2(OH)2VO3. Figure 10. (a) Charge/discharge profiles of the full cell obtained at different current densities. (b) Rate performance, (c) Ragone plots and (d) cycling performance of the full cell. 17



b 011

a PDF# 50-0232

017 110 018

014 015

003

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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50 nm 10

20

30

40

50

60

2θ (degree)

c

d

014 012

006

7.3 nm [100]

d=0.235 nm

d=0.266 nm

(006)

(100)

011 111

d=0.25 nm 2 nm

100

5 nm [011]

18

Figure 1


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3

1

Line 1

4

d = 4.5 nm

Height (nm)

5

3 2 1 0 0

50

100

250

300

d = 6 nm

Height (nm)

200

Line 2

6

2

150

Width (nm)

7

5 4 3 2 1 0 0

50

100

150

200

250

Width (nm)

7 6

Line 3

5

d = 6.2 nm

Height (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4 3 2 1 0

-1 0

50

100

150

200

Width (nm)

19

Figure 2


250

300

350


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

20

Figure 3


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Page 21 of 28

3

B 1

D C 0

Intensity (a.u.)

2

0

E D C

ZnOcubic (111)

ZnOcubic (200)

B A

400

Cu

Ex-situ XRD

E

A = Pristine Voltage (vs. Li/Li+)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


800

1200 -1

30

35

Zn2(OH)3VO3

Zn2(OH)3VO3

(011)

(014)

40

2θ (degree)

Capacity (mAh g )

21

Figure 4


45


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a

b

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c d = 0.266nm Amorphous

d = 0.262nm

50 nm

5 nm

d

e

5 nm (222) (220) (111)

f

Zn

50 nm

5 nm

O

22

Figure 5


V

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a

b

0.22 nm (220)LiZn

50 nm

c

5 nm

f

d

2 nm

Zn

(222)

(220) (111)

e

20 nm

23

Figure 6


O

V


5+

b

V 2p3/2 517.7 eV

Intensity (a.u.)

Intensity (a.u.)

a

5+

V 2p1/2 515.2 eV

528

524

520

516

512

5+

V 2p3/2 517.2 eV

5+

V 2p1/2

528

508

524

520

V 2p3/2 516.3 eV 3+

5+

V 2p1/2 522.6 eV

523.7 eV

528

V 2p3/2 515.3 eV

4+

524

520

516

512

508

515.3 eV

3+

522.6 eV

2+

V 2p1/2 521.0 eV

528

524

520

2+

V 2p3/2 513.6 eV

516

512

508

Binding energy (eV)

f 5+

517.5 eV 4+

4+

V 2p3/2 516.3 eV

OK

V L3 V L2

Intensity (a.u.)

V 2p3/2

V 2p1/2 523.6 eV

508

V 2p1/2

e V 2p1/2 524.9 eV

512

V 2p3/2

Binding energy (eV)

5+

516

3+

d

4+

V 2p1/2

V 2p3/2 516.0 eV

Binding energy (eV)

Intensity (a.u.)

Intensity (a.u.)

c

4+

4+

V 2p1/2 523.4 eV

524.7 eV

Binding energy (eV)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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L3/L2 = 1.57 528

524

520

516

512

508

510

520

530

540

Binding energy (eV)

Binding energy (eV)

24

Figure 7


550

560

Page 25 of 28

5 Li+ insertion

4

Li+ insertion Reversible

3

Li+ extraction 1

+

Voltage (V vs. Li /Li)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5

2 2

ZnO nanoparticles /amorphous LiyV2O5

LiZn alloys /amorphous LixV2O5

ZVO nanosheets Zn2(OH)3VO 3 + xLi+ +xe- → LixZn2(OH)3VO3 (x < 1)

1 3

0

4

LixZn2(OH)3VO3 + yLi+ +ye- → 2ZnO + Lix+yH3VO4 (crystal) (amorphous) (x + y ≤ 2)

LiδH3VO4 → LiωH3VO4 + (δ-ω)Li+ + (δ-ω)e(δ-ω < 3) LiZn → Zn + Li+ + eZn + Li2O → ZnO + 2Li+ + 2e-

ZnO + 2Li+ + 2e- → Zn + Li2O Zn + Li+ + e- → LiZn Lix+yH3VO4 + zLi+ + ze- → LiδH3VO4 (δ = x + y +z < 3)

-1

-2 0

400

800

1200

1600

Capacity (mA h/g)

25

Figure 8


2000

2400

2800


+

Voltage (vs. Li/Li )

4

Cathode: LiFePO4 Anode: Zn2(OH)3VO3

3 2 1 0 0

100

200

300

400

500

600

700

800

900

Capacity (mAh/g) 3.5 +


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 28

3.0 2.5 2.0

LFP ZVO 1.5

1.1 - 3.6 V 100 mA/g

1.0 0

20

40

60

80

Capacity (mAh/g)

26

Figure 9


100

120

200

2.0

0.01 A/g 0.02 A/g 0.05 A/g 0.1 A/g 0.2 A/g 0.4 A/g

1.5

1.0

g

g m A/ 0 10

40 0

m

m

A/ g

0

A/ g

m A/ g

g 10

120

20 0

2.5

20

3.0

Capacity (mAh/g)

+

10

160


Discharge Charge

m A/

m

3.5

m A/

b

4.0

A/ g

a

80

Full cell: 1.1 - 3.6 V 40

0 0

20

40

60

80

100

120

140

160

0

10

20

c

30

40

Cycle number

Capacity (mAh/g)

d

5

10

180

3.6 ms 1.0 150

36 s

3

Capacity (mAh/g)

Specific power (W/kg)

4

10

1h

10

2

10

0.8 120 0.6 90

Discharge Charge

60

0.4

100 mA/g

1

10

10 h

30

0

10 -2 10

0 -1

10

0

10

1

10

2

10

0.0 0

3

10

0.2

20

40

60

Cycle number

Specific energy (Wh/kg)

Figure 10

27


80

100

Coulombic efficiency

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50

Page 27 of 28


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328x329mm (72 x 72 DPI)


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Ultrathin Zn2(OH)3VO3 Nanosheets: First Synthesis, Excellent Lithium

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