Synthesis of Hierarchical Sisal-Like V2O5 with Exposed Stable {001

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Synthesis of Hierarchical Sisal-like V2O5 with Exposed Stable {001} Facets as Long Life Cathode Materials for Advanced Lithium-ion Batteries Naiteng Wu, Wuzhou Du, Guilong Liu, Zhan Zhou, HongRu Fu, Qianqian Tang, Xianming Liu, and Yan-Bing He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13944 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 19, 2017

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ACS Applied Materials & Interfaces

Synthesis of Hierarchical Sisal-like V2O5 with Exposed Stable {001} Facets as Long Life Cathode Materials for Advanced Lithium-ion Batteries Naiteng Wu,† Wuzhou Du,† Guilong Liu,† Zhan Zhou,† Hong-Ru Fu,† Qianqian Tang,† Xianming Liu†* and Yan-Bing He‡*

†

Key Laboratory of Function-oriented Porous Materials, College of Chemistry and Chemical

Engineering, Luoyang Normal University, Luoyang, 471934, P. R. China. ‡

Engineering Laboratory for the Next Generation Power and Energy Storage Batteries, Graduate

School at Shenzhen, Tsinghua University, Shenzhen 518055, PR China, *Corresponding authors. E-mail: [email protected] and [email protected]

ABSTRACT Vanadium pentoxide (V2O5) is considered a promising cathode material for advanced lithium-ion batteries due to its high specific capacity and low cost. However, the application of V2O5-based electrodes has been hindered by their inferior conductivity, cycling stability and power performance. Herein, hierarchical sisal-like V2O5 microstructures consisting of primary one-dimension (1D) nanobelt with [001] facets orientation growth and rich oxygen vacancies are synthesized through a facile hydrothermal process using polyoxyethylene-20-cetyl-ether as surface control agent, and followed by calcination. The primary 1D nanobelt shortens the transfer path of electrons and ions, and the stable {001} facets could reduce the side reaction at the interface of electrode/electrolyte, simultaneously. Moreover, the formation of low valence state vanadium would generate the oxygen vacancies to facilitate the lithium

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ion diffusion. As a result, the sisal-like V2O5 manifests excellent electrochemical performances, including high specific capacity (297 mAh g-1 at a current of 0.1 C) and robust cycling performance (capacity fading 0.06 % per cycle). This work develops a controllable method to craft the hierarchical sisal-like V2O5 microstructures with excellent high rate and long-term cyclic stability.

KEYWORDS: V2O5, sisal-like morphology, hierarchical structure, cathode materials, lithium-ion batteries

1 INTRODUCTION Lithium-ion batteries (LIBs) have been the critical issue for supporting the green and sustainable society.1-3 High energy capacities, long cyclic lifespan and low cost are the essential characteristics for next-generation LIBs to meet the increasing demands of the hybrid electric vehicles (HEVs) and electric vehicles (EVs) applications.4-7 The common cathode materials such as LiCoO2,8 LiMn2O4,9 LiFePO4,10 LiCoxMnyNi1-x-yO211 and LiNi0.8Co0.15Al0.05O212 deliver the specific capacity within a range of 120-180 mAh g-1. Unsatisfied capacities of these cathode materials restrict further application of energy storage and conversion. Besides, long & high temperature sintering (750-950 °C for 10-25 h) in the synthesized processes and the strict sintering atmospheres (e. g. inert atmosphere for LiFePO4 or oxygenenriched atmosphere for LiNi0.8Co0.15Al0.05O2 and other Ni-rich layered cathodes etc.) are raising the cost of these cathode materials. V2O5 as a typical layered oxide is considered as a potential cathode material for advanced LIBs due to the high capacity (294 mAh g-1 in theoretical), low cost and facile preparation.13 However, V2O5 electrodes have been limited by their inferior


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electronic conductivity and poor cycling stability.14-16 Construction of nanostructure and preparation of carbon-V2O5 composite materials14, 17, 18 are generally considered as two effective approaches to shorten the transfer path for electrons and ions to increase the electrode conductivity, respectively. In view of the characteristics of nanostructure, various V2O5-based nanostructures such as nanowire,19, 20 nanobelt,16, 21 nanosheet,22-24 hollow nanosphere25-28 and nanoflower29 had been synthesized to improve the electrochemical performances V2O5-based electrode. Designing the suitable nanostructures indeed improved the rate performance of V2O5, while which also introduced other negative issues such as undesirable side reactions and low volumetric capacity density etc. One-dimension (1D) architectures have been proved to enhance the transportation of electrons and ions.30 Since these 1D architectures were self-assembled to form a hierarchical structure, the primary particles would maintain the features of nanostructure

and

possess

the

accommodative

expansion/constriction. In addition, the

ability of

the

volumetric

microsized secondary agglomerates

synchronously achieve high volumetric energy density and long cycling lifespan.31-33 Furthermore, stable crystal structure is an essential role for cycling stability. For V 2O5, {001} facets have the lowest surface energy of 0.22 J m-2, which is known to be the most stable facets.16 On the other hand, oxygen vacancies which induced by cation doping, such as Ta,34 Cu,35 Ni,36 and Mn,37 could effectively enhance the conductivity and cycling stability of V2O5-based materials. However, the reduction of specific capacity is inevitable due to the doping of inactive cations. The formation of V4+ had been also reported to generate the oxygen vacancies on the surface of V2O5-based materials.38 Encouraged by the benefits of 1D architectures, oxygen vacancies and the exposure of suitable facets such as {001}, it is urgently necessary to design



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hierarchical architectures of V2O5 cathode materials with oxygen vacancies and stable crystal facets, i.e. {001} to achieve highly efficient transportation of Li+ and simultaneously improve the rate and cycling stability. In this work, a facile hydrothermal method to synthesis hierarchical sisal-like V2O5 assembled by 1D nanobelt with [001] orientation growth and rich oxygen vacancies (Figure 1) using NH4VO3 powders as the vanadium sources and polyoxyethylene-20cetyl-ether (Brij®58, a typical long chain block copolymer) as the surfactant has been presented. The sisal-like V2O5 delivered excellent electrochemical properties, including longer cycling lifespan, higher reversible capacity and higher rate capability. The primary 1D nanobelt promotes the transportation ions, and the stable {001} facets could reduce the side reaction at electrode/electrolyte interface. The oxygen vacancies generated by the low valence state vanadium can facilitate the diffusion of lithium ions.

Figure 1. Schematic illustration of the preparation process of sisal-like V2O5

2 EXPERIMENTAL 2.1 Synthesis of sisal-like V2O5 microstructure The sisal-like V2O5 were prepared by a facile hydrothermal treatment followed by calcination approach. In a typical synthesis, 5 mmol NH4VO3 as vanadium sources and 0.01 g polyoxyethylene-20-cetyl-ether (Brij®58) as the surfactant were firstly 4 ACS Paragon Plus Environment

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dissolved in 40 mL deionized water (DI water) at 60 °C to form a yellow clear solution. Then, the yellow solution was sealed in 50 mL autoclave followed by kept at 180 °C for 20 h. After centrifugation, washing by DI water and dried at 80 °C, the cyan precipitate was collected. Sisal-like V2O5 were obtained by further calcination in quartz ampoule (an inclosed calcination container provided a finite oxygen environment) at 500 °C for 5 h with a heating rate of 1.5 °C min-1 in air (denoted as SV). The bulk V2O5 counterpart (denoted as BV) was synthesized by direct pyrolysis of NH4VO3 under the same sintering processes. 2.2 Materials Characterization The X-ray diffraction (XRD) analyses were obtained on a Bruker D8 with Cu Kα radiation at 2θ ranging from 10 to 70°to determine the crystal structure of materials. The detail morphology and structure of the samples had been investigated by the scanning electron microscope (SEM, Hitachi SU8200) and the transmission electron microscope (TEM, JEOL JEM2100). The valence states of the products were determined on an X-ray photoelectron spectroscope (XPS, EscaLab 250Xi). The Brunauer-Emmett-Teller (BET) specific surface areas were measured with Quadrasorb SI analyzer. 2.3 Electrochemical Measurements Coin cells are used to measure the lithium storage properties of the as-prepared cathode materials, and the fabricated process of the working cells is similar to our previous works.8, 12 Notably, the mass ratio of the V2O5 powders, carbon black and binder is 7: 2: 1. The Neware CT-3008W tester was carried out to evaluate the electrochemical performances of the as-prepared cells in the potential range of 2.0-4.0 V vs. Li/Li+ from 0.1 to 10 C (1 C = 300 mA g-1). The Parstat 4000+ electrochemical workstation (Princeton, USA) was used to measure the electrochemical impedance



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spectra (EIS) in the frequency range from 100 KHz to 0.01 Hz with AC amplitude of 5 mV and the cyclic voltammetry (CV) curves in the potential range from 2.0 to 4.0 V vs. Li/Li+ with a scanning rate of 0.1 mV s-1, respectively. 3 RESULTS AND DISCUSSIONS The XRD pattern of the precursor presents that all the diffraction peaks of precursor correspond well to the standard NH4V4O10 (JCPDS: 31-0075) (Figure S1). As shown in Figure S2, the solution without added Brij®58 is clear and yellow, indicating that there were no chemical reactions during the hydrothermal process. This result demonstrated that Brij®58 not only acts as the surfactant to induct the morphology formation, but also as a reactor to reduce the NH4VO3. After calcination, the diffraction peaks corresponding to the orthorhombic V2O5 phase with Pmmn space group (JCPDS: 41-1426) can be seen in the resultant SV and BV (Figure 2a), and no other impurities phase can be detected. Furthermore, the intensity of (001) peak is much higher than the (110) peak, which exhibited the obvious [001] orientation of the sisal-like V2O5 crystal growth.23 Moreover, the high resolution XPS spectra of vanadium acquired from SV (Figure 2b) displays three prominent peaks corresponding to V5+ (524.8 eV), V5+ (517.3 eV) and V4+ (516.3 eV), respectively.29 The presence of low valence state V4+ would improve the conductivity of the V2O5,35, 36

and meanwhile generate the oxygen vacancies to facilitate the lithium ion diffusion

during the intercalation/deintercalation process.38 Figure 2 (c-f) presents SEM images of the precursor and the sisal-like V2O5 annealed at 500 °C for 5 h in air. It is seen from Figure 2c that the precursor consists of 1D micrometer-sized belts with about 50 μm in length, 2-3 μm in width and 200-500 nm in thickness.


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Figure 2. (a) XRD patterns of as-prepared two type V2O5; (b) XPS spectra of V 2p of the sisallike V2O5; SEM images of (c) as-prepared precursor, (d-f) sisal-like V2O5 with different scale bars

Figure 2d displays the panoramic SEM images of as-prepared V2O5. After calcination, the morphology of the as-prepared sample inherited the precursor's 1D belt-like feature. It's worth noting that the primary belts were assembled together to form a sisal-like architecture during the annealing process. As shown in Figure 2e and 7 ACS Paragon Plus Environment


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2f (the magnified area marked by the red square in Figure 2e), the high resolution SEM image clearly manifest the features of submicron primary belt consisting by direction growth by primary nanoparticles, which benefit for the lithium ion diffusions. For comparison, the morphology of bulk V2O5 displays micro-sized monodisperse irregular particles with sharp edges (Figure S3). Besides, the BET specific surface area of SV sample is estimated to be 30.51 m2 g-1 (Figure S4), which is much higher than that of BV (9.38 m2 g-1).

Figure 3. (a)TEM image of sisal-like V2O5; (b-c) HRTEM images corresponding to the red region in (a), respectively; the inset in (c) shows the SAED pattern from the selected square region ii; (d) model diagram of V2O5 (001) facets

The detailed structures of SV are investigated by the TEM, HRTEM and selected area electron diffraction (SAED). The SV exhibits the feature of 1D belt-like structure with about 120 nm in width, which is in agreement with the morphology of the little fragment exfoliated from the sisal-like structure (Figure 3a). Figure 3b-c display the HRTEM images taken from the red square region marked in Figure 3a. The same 8 ACS Paragon Plus Environment

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interplanar distance of the clear lattice fringes is 0.576 nm, which assigned to the dspacing of (200) plane of the orthorhombic V2O5 phase. Notably, the uniform lattice fringes and the rectangle symmetry plots (inset of Figure 3c) proves that these belts are single-crystalline and capped by the {001} facets.39 Figure 3d presents the scheme of the layered V2O5 (001) facet. In every pyramid, a vanadium atom is embedded by the five oxygen atoms; every oxygen atom is bonded by two or three vanadium atoms. A series of VO5 pyramids connect the stable {001} facets. Such stable facets would improve the V2O5 structural stability and cyclic stability effectively.

Figure 4. (a) Rate performance of BV and SV; (b) Cyclic performance and coulombic efficiency of BV and SV at 1 C; (c, d) Charge-discharge curves of SV and BV at different cycles; (e) Cyclic performance and coulombic efficiency of sisal-like V2O5 at 5 C

The lithium storage performances of as-prepared sisal-like V2O5 and its counterpart are evaluated as shown in Figure 4. The rate performances of as-prepared V2O5 at 9 ACS Paragon Plus Environment


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different current densities are depicted in Figure 4a. It can be clearly discerned that the capacities of SV are much higher than that of BV, especially under high current density. The SV delivers the stable discharge capacities of 297, 261, 235, 211, 190, 160 and 105 mAh g-1 at 0.1, 0.2, 0.5, 1, 2, 5, 10 C (1 C = 300 mA g-1), respectively. Whereas, the corresponding capacity of BV is only 269, 244, 225, 175, 143, 92 and 38 mAh g-1, respectively. Therefore, the SV presents much high rate performance than BV. Figure S5a and S5b present the charge-discharge curves of BV and SV electrodes at different current densities. A stable voltage plateau corresponding to the redox peaks is observed under 0.1 C. With the increase of the charge-discharge rate, the voltage plateau of SV exhibits the much lower degree of polarization than that of BV, especially at high current densities. The improved rate performance of sisal-like V2O5 would be attributed to the 1D belt-like structure and the oxygen vacancies, which not only shorten the transfer path of both lithium ions and electrons, but also enhance the conductivity of electrode materials. The cyclic performances of SV and BV under the current density of 1 C are displayed in Figure 4b. The SV electrode delivers excellent cyclic capacity retention. A reversible capacity of about 234 mAh g-1 (93.6 % retention) at the end of 100 cycles and a high retention of 74.8 % (187 mAh g-1) at the end of 400 cycles, corresponding capacity fading rates only 0.06% per cycle, is retained during cycling at 1C. Therefore, SV exhibits robust long lifespan capability. However, the BV deliver an inferior cyclic capability, only about 150 mAh g-1 can be retained at the end of 400 cycles (60 % retention). In Figure 4c and 4d, the SV exhibits the longer charge-discharge plateau at about 3.5 and 2.5 V, respectively, than that of SV. The differences of chargedischarge curves between two electrodes would attribute to the V4+ doping. As shown in Figure 4b, the SV exhibits near 100% CE without the obviously fluctuation, which


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is obviously higher than that of BV electrode. The more stable and near 100 % columbic efficiency demonstrate that the SV presents a better reversible lithium storage property with low side reaction and the lower degree polarization during the cycles. Moreover, the sisal-like V2O5 also delivers the stable cycling performance at 5 C (Figure 4e), corresponding only 0.1% capacity fading per cycle at the tedious high rate charge-discharge processes. Owing to the exposure of stable {001} facets and the accommodative ability of the volumetric expansion/constriction attributed to the hierarchical structure, the sisal-like V2O5 manifests excellent cyclic property compared to the most of previous literatures on various morphologies and cation doped V2O5-based cathode materials (detail data as listed in Table S1).

Figure 5. (a and b) Cyclic voltammetry of the SV and BV electrode at a scanning rate of 0.1 mV s-1 between 2.0-4.0 V (vs. Li/Li+); (c) Nyqusit plots of the BV and SV electrodes after 400 cycles (inset showing the equivalent circuit for the Nyquist plots; (d) XRD patterns of BV and SV electrodes after 400 cycles at 1C.



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CV measurements at a scanning rate of 0.1 mV s-1 in the range of 2.0-4.0 V (vs. Li/Li+) were carried out to investigated the kinetic behavior of sisal-like V2O5. As shown in Figure 5a, the CV curves of SV exhibit several obvious pairs of redox peaks. The peaks between 3.0 and 4.0 V are attributed to the reversible lithium ion intercalation/deintercalation into α-V2O5 and δ-LiV2O5 phase;22 and the other peaks located at the range between 2.0 and 3.0 V represent the reversible phase transformation of δ-LiV2O5 and γ-Li2V2O5.40 Moreover, the similar lithiation behavior and better reversibility of the charge-discharge process can be observed from the symmetrical and overlapping CV curves from 1st to 3rd CV cycle in sisal-like V2O5 cells. Whereas, it is seen that the potential gap of redox peaks of BV in the 1st CV is much larger than that of 2nd and 3rd CV, suggesting a much larger activation polarization for BV than SV. EIS measurements were carried out to reveal the robust cyclic performance of SV electrodes. As shown in Figure 5c, the Nyquist plots of SV and BV after 400 cycles consist of the intercept at Z'-axis at high frequency region, a semicircle in high-middle frequency region and an oblique line in the low frequency region, corresponding to the resistance of electrolyte and cell components (R s), charge-transfer resistance (Rct) and Warburg impendence (W), respectively.31, 41 The Rct value of SV is 246.1 Ω (Table S2), which is much lower than that of BV (494.8 Ω). The remarkably decreased charge-transfer resistances indicate reduction of the unacceptable side reactions at the interface of electrode/electrolyte due to the exposure of the stable {001} facets. Besides, the lithium ion diffusion coefficient of BV and SV after the cycles has been calculated according to the previous literatures.42, 43

Figure S6 presents the relationship between Z’ and ω-0.5 in the low frequency region.

Clearly, Clearly, the calculated DLi for the SV is approximately 2 times larger than that for the BV, which demonstrates that the SV with a sisal-like structure can shorten


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the pathways for Li+ ion diffusion as compared with the SV with an irregular morphology, thus a remarkable improved electrochemical performances are achieved.

Figure 6. TEM images of the (a) bulk V2O5 and (b) sisal-like V2O5 after 400 cycles

Furthermore, XRD measurements of the cycled cathode materials were carried out to examine the structural stability. As shown in Figure 5d, after 400 cycles, the XRD diffraction peaks of SV can be indexed to the LiV2O5 (JCPDS: 18-0756), due to the intercalation of lithium ions during the repeated cycles. However, the obvious amorphous state of cycled BV material implies that the BV would undergo a structural degradation during the cycles. TEM has been carried out to characterize the structural integrity of the cycled materials. The structure and morphology of bulk V2O5 has changed (Figure 6a) and the broken particles and amorphous structure are clearly found in the cycled electrode. Whereas, the TEM image of cycled sisal-like V2O5 (Figure 6b) presents the typical 1D belt-like structure similar to the primary morphology. Furthermore, the HRTEM image exhibits the clear lattice fringes with the interplanar distances of 0.44nm, corresponding to the d-spacings of the (201) facets of LiV2O5. This observation is consistent with the XRD analysis. The TEM analysis of cycled materials proves that the sisal-like morphology endow the V2O5 electrode with more structural accommodative ability than that of bulk counterpart.



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4 CONCLUSIONS In conclusion, we have developed a facile approach to synthesize hierarchical sisallike V2O5 by using Brij®58 as the surfactant. The unique hierarchical structure assembled by 1Dnanobelt, the exposure of stable {001} facets and the oxygen vacancies on the surface generated by the low valence state vanadium endow the sisal-like V2O5 with excellent electrochemical performances, including high specific capacity (297 mAh g-1 at 0.1 C), robust cyclic performance at 1 C (capacity fading 0.06 % per cycle) and high rate (5 C, capacity fading 0.1 % per cycle). All the results reflect the opportunities of sisal-like V2O5 as advanced cathode materials for LIBs. ACKNOWLEDGEMENTS This work is financially supporting by the National Natural Science Foundation of China (No. 21373107), Colleges and Universities in Henan Province Key Science and Research Project (No. 16A530007) and Henan Natural Science Foundation of China (No. 162300410200), Guangdong special support program (2015TQ01N401), and Production-Study-Research Cooperation Project Of Guangdong Province (No. 2014B090901021). REFERENCES (1) Sheng, T.; Xu, Y.; Jiang, Y.; Huang, L.; Tian, N.; Zhou, Z.; Broadwell, I.; Sun, S. Structure Design and Performance Tuning of Nanomaterials for Electrochemical Energy Conversion and Storage. Accounts Chem. Res. 2016, 49, 2569-2577. (2) Konarov, A.; Myung, S.; Sun, Y. Cathode Materials for Future Electric Vehicles and Energy Storage Systems. ACS Energy Lett. 2017, 2, 703-708. (3) Chu, S.; Cui, Y.; Liu, N. The Path Towards Sustainable Energy. Nat. Mater. 2016, 16, 16-22. (4) Liu, W.; Oh, P.; Liu, X.; Lee, M.; Cho, W.; Chae, S.; Kim, Y.; Cho, J. Nickel-Rich Layered Lithium Transitional-Metal Oxide for High-Energy Lithium-Ion Batteries. Angew. Chem. 2015, 127, 4518-4536.


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Graphical abstract


Synthesis of Hierarchical Sisal-Like V2O5 with Exposed Stable {001

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