High-Crystallinity Urchin-like VS4 Anode for High-Performance

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Energy, Environmental, and Catalysis Applications

High Crystallinity Urchin-like VS4 Anode for High-performance Lithium Ion Storage Guang Yang, Bowei Zhang, Jianyong Feng, Huanhuan Wang, Mingbo Ma, Kang Huang, Jilei Liu, Srinivasan Madhavi, Ze Xiang Shen, and Yizhong Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01876 • Publication Date (Web): 06 Apr 2018 Downloaded from http://pubs.acs.org on April 6, 2018

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ACS Applied Materials & Interfaces

High Crystallinity Urchin-like VS4 Anode for High-performance Lithium Ion Storage Guang Yang,a Bowei Zhang,b Jianyong Feng,a Huanhuan Wang,d Mingbo Ma,a Kang Huang,e Jilei Liu,f* Srinivasan Madhavi,a,b* Zexiang Shena,b,c and Yizhong Huanga* a

School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang

Avenue, Singapore 639798 b

Energy Research Institute @ NTU (ERI@N), Nanyang Technological University, Research

Techno Plaza, 50 Nanyang Drive, Singapore 637553 c

Division of Physics and Applied Physics, School of Physical and Mathematical Sciences,

Nanyang Technological University, 637371, Singapore d

CINTRA CNRS/NTU/Thales, UMI 3288, 50 Nanyang Drive, 637553, Singapore.

e

Institute of Advanced Materials and Technology, University of Science and Technology Beijing,

Beijing, 100083, China f

College of Materials Science and Engineering, Hunan University, Changsha, 410082, China

KEYWORDS:

urchin-like,

VS4, lithium

storage,

kinetic

characteristics

ACS Paragon Plus Environment

parameter,

electrochemical

ACS Applied Materials & Interfaces 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

ABSTRACT: VS4 anode materials with controllable morphologies from hierarchical microflower, octopus-like structure, sea grass-like structure, to urchin-like structure have been successfully synthesized by a facile solvothermal synthesis approach using different alcohols as solvents. Their structures and electrochemical properties with various morphologies are systematically investigated and the structure-property relationship is established. Experimental results reveal that Li+ ion storage behavior in VS4 significantly depends on physical features such as the morphology, crystallite size, and specific surface area. According to this study, electrochemical performance degrades in the order of urchin-like VS4 > octopus-like VS4 > sea grass-like VS4 > flower-like VS4. Amongst them, urchin-like VS4 demonstrates the best electrochemical performance benefiting from its peculiar structure which possesses large surface area that accommodates the volume change to a certain extent, and single-crystal thorns which provide fast electron transportation. Kinetic parameters derived from EIS spectra and sweep rate dependent CV curves, such as charge transfer resistances, Li+ ion apparent diffusion coefficients and stored charge ratio of capacitive and intercalation contributions, both support this claim well. In addition, EIS measurement was conducted during the first discharge-charge process to study the solid electrolyte interface (SEI) formation on urchin-like VS4 and kinetics behavior of Li+ ion diffusion. A better fundamental understanding on Li+ storage behavior in VS4 is promoted, which is applicable to other vanadium-based materials as well. This study also provides invaluable guidance for morphologycontrolled synthesis tailored for optimal electrochemical performance.


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1. Introduction Li-ion batteries are one of the critical energy storage devices because of the high energy density, lightweight, and long cycle life.1-2 However, for the application of electric vehicles (EV) and hybrid electric vehicles (HEV), the current commercially used anode material (graphite) is inadequate in satisfying the requirements due to the shortage of high specific capacity, safety, cyclability, and rate performance. Thus, growing efforts have been made to search for high capacity materials like Si, Sn, transitional metal oxide, etc.3-6 Recently, transition metal sulfides, such as MoS2, Sb2S3, Co9S8, etc. came into light owing to their high capacity and relatively high electronic conductivity, which translated to significant improvement in charge transportation and rate performance.7-11 In addition, the diverse crystal structures of these transition metal sulfides open up great opportunities for designing high-performance Li-ion batteries electrodes.12 Recently, VS4 has attracted much attention in virtue of the low cost and high theoretical capacity (1196 mAh g-1), different from its analogue VS2 (466 mAh g-1).13 The higher capacity has been attributed to the higher sulfur content since the sulfur acts as an active participant in the electrochemical reaction.14 In addition, the one-dimensional chain-like crystal structure of VS4 provides more potential storage sites for alkali ions. Furthermore, the charge-transfer kinetics is facilitated by the weak bonding between the chains.15 Although VS4 is quite attractive, the underlying Li+ ion storage mechanism in VS4 is still unclear. Initially, Cho et al.13 proposed that lithiation of VS4 becomes Li2S and vanadium metal. Subsequently, vanadium metal remains inert while the electrochemical reaction occurs between S and Li2S during subsequent cycles and proceeds similarly to Li-S battery. However, due to the amorphous nature of products/ intermediate products during the conversion reaction of VS4 and detection limitations of the XRD, the reaction mechanism could not be fully verified. To further clarify the Li storage



mechanism in VS4, more advanced tools (NMR, PDF, and XANES) were employed by Grey and coworkers.16

Unlike previous findings, they discovered that the metastable intermediate

Li3+xVS4 is formed during lithiation process. It translates back to VS4 upon charging. This is different from the previous findings, provoking the debate about the underlying mechanism. Further work has also shown the effect of compositing with different carbon or conductive polymer on the electrochemical performance of VS4, basically to enhance the electronic conductivity and suppress the dissolution of the produced polysulfide. For example, VS4 nanoparticles embedded carbon coated MWCNTs delivers a reversible capacity of 922 mAh g-1 at a current density of 0.5 A g-1 when used as an anode for Li-ion batteries and shows excellent performance in Na-ion batteries.12 3D VS4/graphene through the hydrothermal method delivers a reversible discharge capacity 1044 mAh g-1 at 0.2 A g-1 when used as anode material for Li-ion batteries.17 In addition, VS4 compositing with different substrates (graphene oxide, CNT, pyrene, graphite, TiO2, and Au nanoparticles) were also synthesized through a hydrothermal method.15 VS4/graphene delivers a 630 mAh g-1 at a high rate of 10 A g-1.16 Mai et al. also prepared VS4 on reduced graphene oxide layer for Na-ion batteries.18 VS4 coated with different conductive polymers such as PEDOT, PPY, and PANI, were also prepared and showed improved performance due to the enhanced electronic conductivity and lower dissolution of polysulfides.14 Although significant progress has been made, several limitations such as difficulty in controlled synthesis, and lack of in-depth understanding in the effect of morphologies on electrochemical performance, Li+ ion storage, and kinetic behaviors in VS4, greatly impede its wide applications. In this work, various VS4 with controlled morphologies from hierarchical microflower, octopus-like structure, sea grass-like structure, to urchin-like structure were developed by a facile solvothermal synthesis approach. The morphologies and structures of these four samples were


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examined and their effects on the battery performance were also investigated. The obtained results reveal that the Li+ storage behavior relies on the crystallite size and morphology of VS4. Specifically, the urchin structure with single-crystal long thorns shows the best cycling and rate performance. This improvement in electrochemical performance can be ascribed to 1) long single-crystal thorns that ensure the fast electron and ion transport; 2) peculiar urchin-like structure that provides a large surface area facilitating Li+ ion diffusion and the abundant porosity accommodating the volume change upon cycling. Moreover, the formation of solid electrolyte interface (SEI) film on urchin-like VS4 and the kinetics of Li+ ion diffusion were systematically investigated using EIS. 2. Experimental section 2.1 Urchin-like VS4 with single-crystal long thorns (denoted as urchin-VS4): 7.5 mmol thioacetamide (TAA) was dissolved in 30 mL methanol first, and then 1.5 mmol NH4VO3 was introduced into the above solution. The precursors were mixed well by stirring the solution and followed by the transfer of solution into a 45 mL Teflon-lined stainless-steel autoclave. The autoclave was sealed and heated at a temperature of 160 °C for 24 hrs. Finally, the obtained products were centrifuged and washed three times using water and ethanol, respectively. The obtained VS4 powder was vacuum dried at 80 °C overnight. 2.2 Hierarchical microflower VS4, octopus-like structure VS4, and sea grass-like structure VS4 (flower-VS4, octopus-VS4, and sea grass-VS4): they were prepared by changing of solvent from methanol to ethanol, propanol, and butanol, respectively.



2.3 Materials Characterization The morphological and structural information were obtained from XRD (Bruker D8 Advance Xray diffractometer (Cu Kα radiation, λ=1.5418 Å), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution TEM (JEOL JSM-7600, Japan and JEOL JEM-2100F). More structure information was collected from Fourier transform infrared spectroscopy (FTIR) (Perkin Elmer Spectrum GX), and Raman spectroscopy (WITecCRM200 Raman system with a laser wavelength of 532 nm). Micromeritics ASAP Tristar II 3020 was employed to measure the Brunauer-Emmett-Teller (BET) specific surface areas and pore size distribution. 2.4 Electrochemical Characterization 2016 coin cells were fabricated by coupling lithium metal with VS4. The weight ratio of active material, binder (PVDF Kynar) and Super P (Timcal) is 7:1:2 in N-methyl-pyrrolidinone (NMP, Sigma Aldrich). The mass loading of active material was ca. 0.8 mg cm-1. 1 M solution of LiPF6 in ethylene carbonate (EC) /diethylene carbonate (DEC) (1:1 v/v, Charslton Technologies Pte. Ltd) was used as electrolyte. Cyclic voltammetry (CVs) and electrochemical impedance spectroscopy (EIS) measurements were conducted on Solartron Instrument. Galavanostatic charge/discharge curves were collected form Neware Battery systems. Results and discussion VS4 materials with a variety of morphologies (Figure. 1 & Figure. S1) from hierarchical microflower, octopus-like structure, sea grass-like structure, to urchin-like structure were synthesized. Flower-VS4 consisting of a hierarchical porous fluffy flower spheres with a core size around 1~2 µm were clearly identified (Figure. 1a and 1a1). Octopus-VS4 (Figure. 1b and 1b1) delivers an octopus-like structure with a core size ~300 nm and tentacles with tapering


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diameter ~100 nm. In contrast, the sea grass-VS4 (Figure. 1c and 1c1) has a sea grass-like structure, in which the core is not obvious, with leaf length ~1 µm. Both tentacle/leaf lengths of octopus-VS4 and leaf length of sea grass-VS4 are shorter than the thorn lengths of urchin-VS4 (Figure. 1d1). The detailed morphology of urchin-VS4 (Figure. 1d1) reveals that the thorns have a diameter of 50~100 nm, and the thorns of one urchin-like hierarchical nanospheres bundle with other thorns from neighboring urchin nanospheres to form an interconnected network. Viewed under a higher magnification, flower-VS4 (Figure. 1a2) demonstrates a core surrounded by myriads of short nanowires or nanosheets on the surface. In contrast, octopus-VS4 ((Figure. 1b2)) demonstrates tentacles structure with a tapering diameter. Sea grass-VS4 ((Figure. 1c2)) shows a porous leaf-like structure with a diameter of approximately 100 nm. The thorns of urchin-VS4 are uniform and longer in length. More detailed morphologies properties were listed in Table 1. The structures of flower-VS4, octopus-VS4, sea grass-VS4, and urchin-VS4 (Figure. 1a3, 1b3, 1c3 and 1d3) were further characterized by HRTEM images, and different degree of crystallinity have been identified. The lattice fringes and FFT of all four products corroborate the pure phase of VS4. Flower-VS4 consists of both amorphous phase and crystalline phase. The crystallite size is around 10 nm in length and 5 nm in width. Similarly, sea grass-VS4 also has both an amorphous phase and a crystalline phase. The crystallite size is around 40 nm in length, which is much longer than that of flower-VS4. While octopus-VS4 and urchin-VS4 deliver an even higher crystallinity compared to these of flower-VS4 and sea grass-VS4 from the HRTEM images. In a further comparison, urchin-VS4 with much more uniform single-crystal thorns demonstrates the highest crystallinity among them. Uniform lattice fringes with the width of 0.52 nm of one thorn have been identified from SAED pattern, which is related to the d-spacing of the (020) plane of VS4. Moreover, its single-crystal feature is manifested by the well-defined periodic dots in



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SAED patterns. Therefore, crystallinity of the samples descends in the order of urchin-VS4 > octopus-VS4 > sea grass-VS4 > flower-VS4. The homogeneous distribution of element S and V across the urchin is corroborated by TEM and their respective EDX elemental mapping images (Figure. S1d-f). The stoichiometric S/V ratio is determined to be 4, giving the stoichiometric formula of VS4. XRD results show that all four samples are composed of pure VS4 phase (JCPDS NO. 72-1294) (Figure. 2a), agreeing well with HRTEM analysis. More structural and/or element information were collected from Raman (Figure. 2b) and FTIR characterizations (Figure. 2c). The peaks around 190 and 279 cm-1 in the Raman spectra are attributed to the stretching and bending modes of V-S (Figure. 2b).15, 17-19 In FTIR spectra, the bands at 550 and 980 cm-1 are characteristic of doubly bonded/bridged S2- (V-S-S) and terminal S stretching of VS4 (Figure. 2c).14-15, 20 Apart from the signals of VS4, a band of amorphous carbon is also observed at 1400 cm-1.21 Other bands around 1200 and 1631 cm-1 arise from the C-O-R vibrations and residual moisture present in the samples during solvothermal synthesis.14, 21 The electrochemical performance of flower-VS4, octopus-VS4, sea grass-VS4, and urchin-VS4 was evaluated and compared. Typical cyclic voltammogram (CV) curves of urchin-VS4 electrode for the first five cycles are illustrated in Figure. 3a. In the first cathodic scan, a peak at 1.61 V indicates the lithiation process from VS4 to Li3+xVS4.16 It is noted that there is another peak around 0.64 V which is attributed to the formation of solid-electrolyte interface (SEI), decomposition of the electrolyte, and the reduction of Li3+xVS4 to Li2S and V.13,

16

In the

delithiation process, two anodic peaks at 1.91 V and 2.38 V were identified to the delithiation of Li2S and the formation of VS4 or Li3+xVS4, respectively.16 Subsequent cycles reveal a shift in cathodic peaks toward the positive. Such could be attributed to the compositional change and


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structure rearrangements.12, 22-23 The second anodic curves were quite similar to the first cycle. After the first cycle, the curves are very close and almost identical, indicating the good cycling stability for the lithiation/delithiation processes. In contrast, curves become smaller with the increasing cycling numbers for samples flower-VS4, octopus-VS4, and sea grass-VS4 (Figure. S2). This implies poor cycling stability probably attributed to plausible structure degradation. These results further highlight the advantages of urchin-like structure of VS4. In good agreement with the CV curves (Figure. 3b), representative galvanostatic discharge/charge profiles show an obvious plateau at 1.85 V and two shallow plateaus at 1.5 V and 0.8 V. The first discharge capacity is 1305 mAh g-1 and the first charge capacity is 847 mAh g-1, giving rise to an initial coulombic efficiency of 65%. This value for flower-VS4, octopus-VS4, and sea grass-VS4 is 73%, 69%, and 74%, respectively, suggesting a serious side reaction.24-25 After the first cycles, specific capacities of flower-VS4, octopus-VS4, and sea grass-VS4 drop rapidly compared to that of urchin-VS4. The rate capability of flower-VS4, octopus-VS4, sea grass-VS4, and urchin-VS4 was evaluated by charge/discharge at a series of current densities from 0.1 A g-1 to 2 A g-1. UrchinVS4 delivers a high reversible specific capacity of 500 mAh g-1 at 0.1 A g-1. It decreases to 400, 250, 150 and 60 mAh g-1, as the current densities increase to 0.2, 0.5, 1 and 2 A g-1, respectively. As the current density goes back to 0.1 A g-1, the capacity reverts to 480 mAh g-1. Note that all values of specific capacities for urchin-VS4 are higher than those of flower-VS4, octopus-VS4, and sea grass-VS4, corroborating its good rate capability. Moreover, good cycling stability has been demonstrated for urchin-VS4 in which a specific capacity of 490 mAh g-1 is delivered even after 100 cycles. In contrast, the specific capacities of flower-VS4, octopus-VS4, and sea grassVS4 after 100 cycles are 81, 230, and 145 mAh g-1, respectively. It is noteworthy that the capacity degrades in the sequential order of urchin-VS4 > octopus-VS4 > sea grass-VS4 > flower-



VS4 (Figure. 3c and 3d), which is consistent with crystallinity mentioned previously (observed in Figure. 1). Higher crystallinity provides a larger capacity and stable cycling. This is in good agreement with previous reports which states that too small crystallite size may not result in good electrochemical performance. For instances, Shen et al.26 prepared Li4Ti5O12 with different nanocrystal sizes. They found that 6 nm size particles have a poor rate capability compared to the 9 nm size particles. They attributed the poor performance to the amorphous phase composed of disordered atoms for 6 nm particles which makes the lithium ions difficult to diffuse into the particle core. A similar study also reported that the low crystallinity sample TiO2 contains a considerable amount of defect sites hindering Li-ion diffusion.27 Furthermore, urchin-VS4 also has the second largest surface area and pore volume (Table 1). The large surface area might increase the interface of the active material and electrolyte, thus facilitating the Li+ ion transport. Moreover, its mesoporous structure is believed to favor ion diffusion and allow easy permeability of electrolyte.28-29 Urchin-VS4 demonstrates 3D structures with many long nanowires emanating from the core. The nanowires also provide shortened diffusion length and large interfacial area of active material and electrolyte. The large volume change upon charging and discharging can also be alleviated by the porosity.30-43 Thus, the best cycling and capacity retention performance of urchin-VS4 among all the samples is accredited to its peculiar nanostructure with the large surface area and high crystallinity, which facilitate the Li+ ion diffusion and electron transport. EIS measurements were performed to evaluate the impedances of all four samples after 5 CV cycles and the equivalent circuit model was used to fit the Nyquist plots (Figure. 3e). The simulated parameters are listed in Table 2. As seen, the charge transfer resistance (Rct) of flowerVS4, octopus-VS4, sea grass-VS4, and urchin-VS4 is found to be 273, 181, 213 and 327 Ω,


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respectively. The higher Rct value for urchin-VS4 could be attributed to the side reactions between electrode and electrolytes since it has a relatively higher surface area.44-45 However, urchin-VS4 displays the highest diffusion coefficient (DLi+) of 3.15246 x 10-17 (Figure. 3f and Table 2). This could be ascribed to the high crystallinity of urchin-VS4 that facilitates fast electron and ionic transport. The good kinetic properties for urchin-VS4 are further backed by CV testing at a set of sweep rates (Figure. S5). The reduction peaks move to a lower potential range and oxidation peaks shift to the higher potential range. This is owing to the high polarization with increasing sweep rates.46 Figure. 4a shows the relationship between reduction peak current Ip and the square root of the scan root ν1/2. The linear relation indicates that reaction is a Li-ion diffusion controlled. Based on the Randles-sevcik equation:47 Ip = (2.69 x 105)n3/2AD01/2C0ν1/2

(1)

where Ip is peak current, D0 is the diffusion coefficient, A is the apparent surface area, C0 is the maximum Li+ concentration, and ν is the CV scanning rate. The diffusion coefficient for urchinVS4 was calculated to be 1.57853 x 10-13 cm2 s-1, which is higher compared to those of octopusVS4 (1.82601 x 10-14 cm2 s-1), sea grass-VS4 (1.48008 x 10-14 cm2 s-1), and flower-VS4 (7.68579 x 10-15 cm2 s-1) (Table 2). The enhanced kinetics may explain the better performance of urchin-VS4. To further explore the influence of different morphologies on the performance, the total charge storage at one sweep rate was quantified by separating each contribution from the diffusioncontrolled and capacitive charge.48 The results (Figure. b) reveal that urchin-VS4 exhibited an intercalation effects contribution ratio (∼96%), which is equal or higher than that of octopus-VS4 (96%), sea grass-VS4 (88%), and flower-VS4 (89%). This can be ascribed to its high crystallinity and enhanced ion diffusion.



To gain more insight into lithium storage behaviors, including formation of SEI and kinetics of lithium-ion diffusion during the initial discharging and charging process, EIS profiles at different discharge/charge potential were recorded to evaluate the tendencies of electronic conductivity (Rs), charge transfer resistance (Rct), and apparent Li diffusion coefficients (Dapp) (Figure. 5 and S7). The changing in Rs during the initial discharging and charging is shown in Figure. S7, and no much difference is observed for the value of Rs during the first charging/charging process. However, in comparison to Rs, the value of Rct is almost two order of magnitude higher and the variation tendency of Rct is shown in Figure. 5b. During the discharge process, the Rct decreases rapidly and becomes stable at around 1.6 V due to initial intercalation of Li+ ions.49 In the charging process, Rct increases slightly because of Li+ ion deinsertion. The Daap values (Figure. 5c) derived from EIS analysis ranges from 5.5888 x 10-18 cm2 s-1 to 6.88601 x 10-17 cm2 s-1 upon discharging and charging, respectively, over the potential range of 0.01-2.25 V. The relative larger Dapp values during discharging than charging below 1.5 V imply that insertion process is more favorable than extraction process. All these, together, demonstrate the practical feasibility of VS4 anode. 3. Conclusion VS4 with controlled morphologies from hierarchical microflower, octopus-like structure, sea grass-like structure, to urchin-like structure were synthesized through a facile solvothermal approach in different alcohols. Systematically investigation of the effect of morphologies, surface area, and crystallite size on the electrochemical performance was conducted. The urchinlike VS4 with long single-crystal thorns was found to be beneficial due to its peculiar structure that provides a large surface area for Li+ ion transportation, high crystallinity that ensures fast electron transportation, and porosity that accommodates the volume change upon cycling.


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Kinetic parameters derived from EIS spectra and sweep rate dependent CV curves such as charge transfer resistances and Li+ ion apparent diffusion coefficients both support this claim well. In addition, EIS measurement was performed upon the first discharge-charge process to study the SEI formation on urchin-like VS4 and the kinetics of Li+ ion diffusion. These findings from this work deepen the fundamental understanding of lithium ion storage behavior in VS4 as well as other vanadium-based materials. It also provides invaluable guidance for morphology-controlled synthesis to tailor optimal electrochemical performance. ASSOCIATED CONTENT Supporting Information Other magnifications of SEM images, TEM EDX elemental mapping and spectrum for VS4; FTIR peaks and their assignments; First five CV curves and GDC profiles of flower-VS4, octopus-VS4, sea grass-VS4 and urchin-VS4; BET results of flower-VS4, octopus-VS4, sea grassVS4 and urchin-VS4; capacitive and intercalation contribution for flower-VS4, octopus-VS4, sea grass-VS4 and urchin-VS4 at a sweep rate of 0.3 mV/s. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] /[email protected]. *E-mail: [email protected]. *E-mail: [email protected] (Y. Huang). Notes The authors declare no competing financial interest.



ACKNOWLEDGMENTS The authors acknowledge the support from Tier 1 (AcRF grant MOE Singapore M4011528), Tier 2 (MOE2015-T2-1-148 & AcRF grant MOE Singapore M4020159) and the Chinese Natural Science Foundation (Grant No. 51271031), J Liu thanks financial support from Chinese Young 1000 Talents Program. REFERENCES 1. Armand, M.; Tarascon, J. M. M., Building better batteries. Nature 2008, 451, 652-657. 2. Winter, M.; Besenhard, J. O.; Spahr, M. E.; Novák, P., Insertion electrode materials for rechargeable lithium batteries. Adv. Mater. 1998, 10, 725-763. 3. Casimir, A.; Zhang, H.; Ogoke, O.; Amine, J. C.; Lu, J.; Wu, G., Silicon-based anodes for lithium-ion batteries: Effectiveness of materials synthesis and electrode preparation. Nano Energy 2016, 27, 359-376. 4. Derrien, G.; Hassoun, J.; Panero, S., Nanostructured Sn-C composite as an advanced anode material in high‐performance Lithium‐ion batteries. Adv. Mater. 2007, 19, 2336–2340. 5. Sun, X.; Hao, G. P.; Lu, X.; Xi, L.; Liu, B.; Si, W.; Ma, C., High-defect hydrophilic carbon cuboids anchored with Co/CoO nanoparticles as highly efficient and ultra-stable lithium-ion battery anodes. J. Mater. Chem. A 2016, 4, 10166-10173. 6. Liu, J.; Zhou, W.; Lai, L.; Yang, H.; Lim, S.; Zhen, Y.; Yu, T.; Shen, Z.; Lin, J., Three dimensionals -Fe2O3/polypyrrole (Ppy) nanoarray as anode for micro lithium ion batteries. Nano Energy 2013, 2, 726-732. 7. Bindumadhavan, K.; Srivastava, S. K., MoS2–MWCNT hybrids as a superior anode in lithium-ion batteries. Chem. Commun. 2013, 49, 1823-1825


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34. Shu, J.; Ma, R.; Shao, L.; Shui, M.; Hou, L.; Wu, K.; Chen, Y., Facile preparation of nanomicro structure PbSbO2Cl as a novel anode material for lithium-ion batteries. RSC Adv. 2013, 3, 372-376 35. Zhang, T.; Yue, H.; Qiu, H.; Wei, Y.; Wang, C.; Chen, G., Nano-particle assembled porous core–shell

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Figure. 1 SEM, TEM and HRTEM images of (a, a1, a2, and a3) flower-VS4; (b, b1, b2, and b3) octopus-VS4; (c, c1, c2, and c3); sea grass-VS4, and (d, d1, d2, and d3) urchin-VS4.



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Figure. 2 (a) XRD patterns, (b) Raman spectra and (c) FTIR spectra of flower-VS4, octopus-VS4, sea grass-VS4, and urchin-VS4. Table 1. VS4 morphologies, BET surface area, and pore volume.

Sample

Morphologies

SBET(m2g-1)

Pore volume (cm3g-1)

Flower-VS4

Sheet or small nanowires

6.644

0.0614

Octopus-VS4

Tentacles length ~1 µm

3.717

0.0231

Sea grass-VS4

Leaves length ~800 nm

5.596

0.0262

Urchin-VS4

Thorns length~2 µm

5.885

0.0280


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Figure. 3 (a) First five CV cycles of urchin-VS4 at sweep rate of 0.1 mV s-1; (b) Galvanostatic discharge/charge profiles of urchin-VS4 for the first four cycles at a current rate of 0.1 A g-1 in the voltage range of 0.01-3.00 V vs Li+/Li; (c) Rate performance of flower-VS4, octopus-VS4, sea grass-VS4, and urchin-VS4 at varied current densities from 0.1 to 2 A g-1 in the voltage range of 0.01-3.00 V vs Li+/Li; (d) Cycling performance of flower-VS4, octopus-VS4, sea grass-VS4,



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and urchin-VS4 at 0.1 A g-1; (e) EIS spectra of electrodes after 5 CV cycles with a scan rate of 0.1 mV s-1. The inset shows the equivalent circuit model and the resistance data from fitting EIS; (f) Linear fitting of z' vs ω-1/2 relationship.

Table 2. Equivalent circuit parameters and apparent Li ion diffusion coefficient calculated from EIS and CV rates. Sample

R1 (Ω)

Rct (Ω)

DLi+ (cm2s-1) from EIS

DLi+ (cm2s-1) from CV

Flower-VS4

5.279

272.9

6.59443 x 10-19

7.68579 x 10-15

Octopus-VS4

7.884

181.3

2.56739 x 10-18

1.82601 x 10-14

Sea grass-VS4

7.43

213.1

1.34254 x 10-17

1.48008 x 10-14

Urchin-VS4

3.317

327.6

3.15246 x 10-17

1.57853 x 10-13

Figure. 4 (a) Dependence of reduction peak current on the square root of scan rate. (b) Bar chart showing total stored charge with percentage contribution from capacitive and intercalation at 0.3


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mV/s for all samples with stored charge calculated by integrating the CV area according to the ௜

equation: Q = ‫ ׬‬ቀ ቁ ܸ݀.50 ௩

Figure. 5 (a) EIS profiles of urchin-VS4 at the first discharge and charge. (b) Rct during the first discharge and charge. (c) Diffusion coefficients of Li calculated from EIS vs potential.



VS4 with four controlled novel morphologies were comprehensively characterized and compared from kinetic point of view

370x163mm (150 x 150 DPI)


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High-Crystallinity Urchin-like VS4 Anode for High-Performance

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