In Situ Topology Synthesis of Orthorhombic NaV2O5 with High

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In Situ Topology Synthesis of Orthorhombic NaV2O5 with High Pseudocapacitive Contribution for Lithium-Ion Battery Anode Wenbin Li,† Jianfeng Huang,*,† Liyun Cao,† Yijun Liu,‡ Limin Pan,‡ and Liangliang Feng*,† †

School of Materials Science & Engineering, Shaanxi University of Science and Technology, Weiyang University Campus, Xi’an, Shaanxi 710021, P.R. China ‡ Monalisa Group Co., Ltd., Taiping Industrial Zone, Xiqiao Town, Nanhai District, Foshan, Guangdong 528200, P.R. China ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 10/19/18. For personal use only.

S Supporting Information *

ABSTRACT: Based on the in situ topology transition of monoclinic NaVO3, a submicrobelts-assembled orthorhombic NaV2O5 microbundle is synthesized by a facile hydrothermal method. When applied as the anode for lithium-ion batteries, orthorhombic NaV2O5 with a high pseudocapacitive contribution delivers a high reversible capacity of 470 mA h g−1 at 200 mA g−1 after 400 cycles and that of 385 and 302 mA h g−1 even at 500 and 1000 mA g−1, respectively.

KEYWORDS: In situ topology synthesis, Orthorhombic NaV2O5, Pseudocapacitive contribution, Anode, Lithium-ion battery

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INTRODUCTION As a family of typical graphite-like layered materials, vanadiumbased oxides have attracted research interest as alternative high-performing anode materials for lithium-ion batteries (LIBs).1−3 Among various vanadium-based oxides, orthorhombic NaV2O5 (o-NaV2O5) is believed to be a very promising candidate because of its stable crystal structure with large interlayer spacing (4.80 Å), excellent transport properties, and unique physical and electrochemical properties (a high theoretical specific capacity of 261.5 mAhg−1 for Li2NaV2O5).4−6 However, when applied to sodium-ion batteries, o-NaV2O5 shows poor cycling performance.7 Meanwhile, o-NaV2O5 employed as anode of LIBs has been little reported.7,8 Recently, the development of three-dimensional (3D) assembled architectures that can provide a prominent physical entrapment for the expansion−shrinkage of building blocks during Li+ insertion−extraction gives us a great inspiration to improve the cycling performance of o-NaV2O5.9 Nevertheless, most of the reported o-NaV2O5 samples exhibit low-dimensional structures (such as needle-like,10 flake-like,11 or platelike12 morphology) and usually require relatively complex and high-cost synthetic processes. Thus, a simple and feasible method to synthesize 3D nanomicro architectured o-NaV2O5 for Li+ storage is highly desirable. Monoclinic NaVO3 (m-NaVO3) is well-known to exist commonly with the unique 3D microbundle structure (Figure S1a,b), which contributes to the reduction of volume change and the improvement of cycling performance. Moreover, the © XXXX American Chemical Society

crystal of m-NaVO3 features the linear VO3 chain structure formed by VO4 tetrahedra sharing two corners with adjacent tetrahedrons (Figure S2a) and which can be regulated to form other configurations.13 On the basis of such promising characteristics, we herein successfully prepare a 3D oNaV2O5 microbundle through a S2−-assisted in situ topology transition approach. This synthetic protocol involves the phase transition from m-NaVO3 to o-NaVO3 (orthorhombic NaVO3, Figure S2b) promoted by the reaction temperature and pressure under the premise of maintaining the microstructure, simultaneously proceeding the reaction of V5+ reduced toV4.5+ for inducing the rearrangement of VO5 pyramid in o-NaVO3 to form o-NaV2O5 (Figure S2c) by addition of Na2S (i.e., oNaVO3 → o-NaV2O5), eventually leading to the successful in situ topology formation of 3D microbundle-structured oNaV2O5 from original m-NaVO3 for LIBs (further details are provided below).

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RESULTS AND DISCUSSION As shown in Figure 1a, the XRD pattern of original m-NaVO3 exhibits typical monoclinic crystal structure with I2/a symmetry (PDF no. 78-2265). For the as-obtained material, all of the diffraction peaks can be indexed to orthorhombic NaV2O5 with Pmmn symmetry (PDF no. 89-8040), indicating successful formation of orthorhombic NaV2O5 with a wellReceived: May 25, 2018 Revised: September 18, 2018 Published: October 17, 2018 A

DOI: 10.1021/acssuschemeng.8b02403 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 1. (a) XRD patterns, (b) Raman spectra, (c) FTIR spectra, and (d) high-resolution V 2p-O 1s XPS spectra of m-NaVO3 and o-NaV2O5.

Figure 2. SEM images, TEM images, and SAED patterns of (a−c) m-NaVO3 and (d−f) o-NaV2O5.

NaV2O5 indicates the reduction of the types of V−O bands,16 which suggests that the reconsitution of vanadium oxide polyhedron appears in the topology transition.7 In addition, inductively coupled plasma−atomic emission spectroscopy (ICP-AES) and survey X-ray photoelectron spectroscopy (XPS) spectra (Figure S3a) analysis confirm that the atomic ratio of Na/V is ∼1:1 for m-NaVO3 and ∼1:2 for o-NaV2O5. Furthermore, the high-resolution V 2p XPS spectra of oNaV2O5 (Figure 1d) shows that V 2p3/2 and V 2p1/2 peaks split into two peaks; the peaks at 516.0/523.2 eV are ascribed to

crystallized structure. As presented in Figure 1b, m-NaVO3 shows 17 Raman peaks and o-NaV2O5 exhibits 10 Raman peaks between 100 and 1000 cm−1, which result from the deformation, symmetric and antisymmetric, of V−O band13 and the V−O bending, stretching, or rocking vibrations of NaV2O5 (Table S1).7,8,10 As depicted in Figure 1c, these Fourier transfrom infrared (FTIR) peaks between 400 and 1200 cm−1 are assigned to the bending and stretching vibration of V−O−V and V=O bonds of m-NaVO314 and o-NaV2O5.15 The decrease of the number of Raman and FTIR peaks of oB

DOI: 10.1021/acssuschemeng.8b02403 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 3. Schematic diagram illustrating the phase transformation process from monoclinic NaVO3 to orthorhombic NaV2O5: structural model of (a) monoclinic NaVO3 viewed along b direction, (b) orthorhombic NaVO3 viewed along c direction, and (c) orthorhombic NaV2O5 viewed along c direction.

V4+, and the peaks at 517.2/524.6 eV are assigned to V5+.17 Meanwhile, the signal of V−O−V can also be probed in the high-resolution O 1s XPS spectra (Figure 1d).18 The energy difference of 12.7 and 13.9 eV between V 2p3/2 and O 1s corresponds to the characteristic of 4+ and 5+ oxidation state, respectively,19 further confirming the simultaneous existence of V4+ and V5+. Nevertheless, the high-resolution V 2p XPS spectrum of m-NaVO3 indicates that the valence of V is mainly +5. In addition, V4+/V5+ ratio is estimated to be ∼1:1, suggesting the average oxidation state of +4.5. Thus, on the basis of the molar ratios of Na/V and V4+/V5+, combined with the peak at 1071.3 eV in Figure S3b that can be assigned to Na+, we conclude that the V5+ is reduced to V4.5+ with the release of a Na atom and an O atom during the topology transition process. As displayed in Figures 2a,d and S1a,c, o-NaV2O5 shows a similar morphology with m-NaVO3, which are composed of microbundles. Closer observation in Figure S1b,d finds that the microbundles consist of some narrow submicrobelts, further confirmed by the as-observed submicrobelts in Figure 2b,e. Notably, all submicrobelts in an individual bunch align along the same orientation and combine with each other through the width direction (Figure S4a,c). The ordered 3D assembled structure can dramatically restrain the expansion− shrinkage of submicrobelts favoring the excellent cycling performance. Moreover, the observed lattice fringes with interplanar distance of 3.48 and 4.80 Å (Figure S4b,d) are associated with the (220) and (001) planes of m-NaVO3 and o-NaV2O5, respectively. The corresponding SAED patterns of m-NaVO3 and o-NaV2O5 in Figure 2c,f clearly demonstrate discrete and patterned spots, suggesting their intrinsic singlecrystal characteristics. Meanwhile, the diffraction lattices of panels f and c of Figure 2 indexed by orthorhombic system with Pmmn symmetry and monoclinic system with I2/a symmetry, respectively, lead to the conclusion that the in situ topology transition from monoclinic NaVO3 to orthorhombic NaV2O5 is successfully realized by us.

To provide insight into the in situ topology transition mechanism, additional control experiments were performed, and the following results were obtained. (i) No precipitation can be collected without the addition of Na2S·9H2O. (ii) The o-NaVO3 microbundles are formed (Figure S5) when the deionized water is substituted by ethanol, without Na2S·9H2O addition. These results, coupled with previous studies,13,20 reveal that the phase transition effect of temperature and pressure and the reduction effect of S2− synergistically determine the in situ topology synthesis of o-NaV2O5, as illustratesd in Figure 3. Specifically, the phase transition effect tends to prompt the transition of a VO4 tetrahedron sharing two corners with adjacent tetrahedra (Figure 3a) to a VO5 pyramid sharing two edges with adjacent pyramids (i.e., mNaVO3 → o-NaVO3) (Figure 3b). Meanwhile, the reduction effect of S2− drives the valence transition of V from +5 to +4.5, inducing the rearrangement of VO5 pyramid in o-NaVO3 to form o-NaV2O5, eventually leading to the successful in situ topology transition from m-NaVO3 to o-NaV2O5 (eq S1 and Figure 3c). When applied as anode for LIBs, the o-NaV2O5 electrode shows excellent rate and cycling performances, as shown in Figure 4a,b. Specifically, the o-NaV2O5 electrode yields a specific capacity of 515, 463, 385, 302, and 228 mA h g−1 at a rate of 100, 200, 500, 1000, and 2000 mA g−1, respectively. After testing at high rates, a reversible capacity of 579 mA h g−1 can still be achieved at 100 mA g−1. Meanwhile, the charge/ discharge profiles of the o-NaV2O5 electrode (Figure S6) exhibit more obvious potential plateaus and smaller potential hysteresis with increasing current densities, revealing a better kinetic feature and higher-efficiency lithiation/delithiation processes at different rates. Furthermore, o-NaV2O5 electrode delivers the initial discharge and charge capacities of 956 and 774 mA h g−1 with the first Coulombic efficiency of 81% at 200 mA g−1. The irreversible capacity loss is possibly attributed to the irreversible intercalation of Li+ and the formation of solid electrolyte interphase. After 400 cycles, the reversible C

DOI: 10.1021/acssuschemeng.8b02403 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 4. (a) Rate and (b) cycling performance of o-NaV2O5 and m-NaVO3 electrode, (c) CV curves at different scan rates from 0.1 to 10 mVs−1 within a potential range of 0.01−3.00 V, (d) fitted lines and log (peak current) vs log (scan rate) plots at different oxidation and reduction states, and (e) capacitive contribution shown by the shaded region at 0.5 mVs−1 for o-NaV2O5 electrode.

capacities are observed to be as large as 470 mA h g−1, which is comparable to or even outperforms the previously reported other layered niobate and titanate.21−23 The increase of capacity after 200 cycles is possibly attributed to the gradually expanded and exfoliated interlayers.24 As detected in Figure S7, there are two pairs of cathodic/ anodic peaks at 1.97/2.55 and 0.49/0.64 V in the following sweeps of o-NaV2O5 electrode, corresponding to the multistep electrochemical reaction related to the complex phase transitions of LixNaV2O5 as a function of Li content, whereas there is only a pair of cathodic/anodic peaks at 0.56/0.75 V for the m-NaVO3 electrode, demonstrating more abundant Li+ storage sites of the o-NaV2O5 electrode.7,25 To elaborate the lithium storage mechanism of o-NaV2O5 electrode, ex situ XRD measurements after discharging and charging to different potential states at 100 mA g−1 were performed. As shown in Figure S8a, the typical XRD pattern in different potential states are well-preserved, demonstrating its very stable layered structure and excellent electrochemical reversibility, which mainly results from the Na+ bridging effect between V2O5 layers during lithiation/delithiation processes.26 As shown in

Figure S8b, the enlarged (001) peak gradually decreases and shifts left with decreasing the discharge voltages from 3.00 to 0.01 V, resulting from the increase of the interlayer spacing caused by the insertion of Li+ during the discharge process. In contrast, the enlarged (001) peak gradually increases and shifts right, indicating that the recovering NaV2O5 structure is induced by the extraction of Li+ during the charge process. Notably, when charged to 3.00 V, the (001) peak shows the left shift and the obvious decrease of intensity compared with that of the initial state, confirming the gradually expanded interlayers during cycling. In summary, the reaction mechanism of the orthorhombic NaV2O5 is mainly the intercalation reaction. The cathodic/anodic peaks in Figure 4c are well-preserved, but the cathodic peak at ∼0.56 V in Figure S9a becomes invisible with the increase of scan rate. Thus, the o-NaV2O5 electrode is capable of sustaining a quicker CV response to a fast potential scan than the m-NaVO3 electrode, demonstrating a faster charge mobility and improved Li+ diffusion, which can be confirmed by the smaller charge-transfer resistance and diffusion resistance of o-NaV2O5 electrode obtained by the EIS D

DOI: 10.1021/acssuschemeng.8b02403 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Notes

analysis in Figure S10. As displayed in Figures 4d and S9b, the b-value (calculated by eq S2) of the o-NaV2O5 electrode is higher than that of the m-NaVO3 electrode as a whole, showing its enhanced capacitive behavior, which can be further proved by the more vertical line of the o-NaV2O5 electrode (76° vs 47°) at low frequency in the Nyquist plots in Figure S10a. As shown in Figures 4e and S9c, the capacitive area determined by eq S3 presents clear redox peaks, suggesting that the capacitive process belongs to a pseudocapacitive behavior. For the oNaV2O5 and m-NaVO3 electrodes, 63% and 50% of the total charge comes from pseudocapacitive contribution at 0.5 mVs−1, respectively. Thus, we conclude that the excellent rate and cycling performances of o-NaV2O5 electrode is mainly attributed to the pseudocapacitive contribution, which is chiefly induced by the synergistic effect of unique orthorhombic structure and submicrobelts-assembled microbundle structure. On the one hand, the unique orthorhombic structure with less Na+ between the interlayer, larger interlayer spacing (4.80 Å), and two different tunnel cavities in the lattice can provide not only unimpeded paths for fast Li+ insertion/ extraction but also more active sites for Li+ storage,12 contributing to enhancing the rate capability and specific capacity. On the other hand, the unique 3D assembled structure with high crystallinity contributes to enhancing the structure stability and then improving the cycling performance, which is certified by the well-retained submicrobelts-assembled microbundle-like morphologies of o-NaV2O5 during cycling (Figure S11)

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC (Nos. 51472152, 51672165, and 21701107), Doctoral Scientific Research Startup Foundation of SUST (2016QNBT-07), Platform Construction Fund for Imported Talent of SUST (134080038).

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CONCLUSIONS We successfully realize the in situ topology transition from monoclinic NaVO3 to orthorhombic NaV2O5 by a simple onestep hydrothermal method, and the synergistic effects of temperature, pressure, and S2− are revealed. When applied as anode for LIBs, the 3D o-NaV2O5 microbundle delivers a high reversible capacity of 470 mA h g−1 at 200 mA g−1 after 400 cycles and that of 385 and 302 mA h g−1 even at 500 and 1000 mA g−1, respectively. The controllability and simplicity of the in situ topology transition strategy provide an example for the development of inexpensive and versatile synthesis techniques.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b02403.

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Experimental Section, SEM images, crystal structure model, table of Raman frequencies and corresponding vibration modes, XPS spectra, TEM and HRTEM images, reaction equation, structural characterization of orthorhombic NaVO3, charge/discharge profiles, CV characterization, ex situ XRD characterization, pseudocapacitance analysis, EIS characterization, SEM images after cycling (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Liangliang Feng: 0000-0001-5596-9853 E

DOI: 10.1021/acssuschemeng.8b02403 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acssuschemeng.8b02403 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX