Facile Tailoring of Multidimensional Nanostructured Sb for Sodium

Aug 8, 2019 - Electrical and Electronic Engineering, Huazhong University of Science and ... State Key Laboratory of Materials Processing and Die & Mou...
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Facile Tailoring of Multi-Dimensional NanoStructured Sb for Sodium Storage Applications Haomiao Li, Kangli Wang, Min Zhou, Wei Li, Hongwei Tao, Ruxing Wang, Shijie Cheng, and Kai Jiang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b04520 • Publication Date (Web): 08 Aug 2019 Downloaded from pubs.acs.org on August 8, 2019

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Facile Tailoring of Multi-Dimensional NanoStructured Sb for Sodium Storage Applications Haomiao Li1,2, Kangli Wang*1, Min Zhou1, Wei Li1,2, Hongwei Tao1, Ruxing Wang1, Shijie Cheng1 and Kai Jiang*1 1

State Key Laboratory of Advanced Electromagnetic Engineering and Technology, School of

Electrical and Electronic Engineering, Huazhong University of Science and Technology Wuhan, Hubei, China 430074. E-mail: [email protected]; [email protected] 2

State Key Laboratory of Materials Processing and Die & Mould Technology, College of

Materials Science and Engineering, Huazhong University of Science and Technology Wuhan, Hubei, China 430074.

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ABSTRACT: Nanoengineering of metal electrodes are of great importance for improving the energy density of alkali-ion batteries, which have been deemed one of most effective tools for addressing the poor cycle stability of metallic anodes. However, the practical application of nanostructured electrodes in batteries is still challenged by a lack of efficient, low-cost and scalable preparation methods. Herein, we propose a facile chemical dealloying approach to the tunable preparation of multidimensional Sb nanostructures. Depending on dealloying reaction kinetics regulated by different solvents, zero-dimensional Sb nanoparticles (Sb-NP), twodimensional Sb nanosheets (Sb-NS) and three-dimensional nanoporous Sb are controllably prepared via etching Li-Sb alloys in H2O, H2O-EtOH, and EtOH, respectively. Morphological evolution mechanisms of the various Sb nanostructures are analyzed by SEM, TEM and XRD measurement. When applied as anodes for sodium ion batteries (SIBs), the as-prepared Sb-NS electrodes without any chemical modifications exhibit high reversible capacity of 620 mAh g-1 and retain 90.2% of capacity after 100 cycles at 100 mA g-1. The excellent Na+ storage performance observed is attributable to the 2D nanostructure, which ensures high degrees of Na+ accessibility, robust structural integrity, and rapid electrode transport. This facile and tunable approach can broaden ways of developing high performance metal electrodes with designed nanostructures for electrochemical energy storage and conversion applications. KEYWORDS: nanostructured antimony, chemical dealloying, tailored synthesis, nanosheets, sodium ion batteries

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Sodium ion batteries (SIBs) have been considered one of most promising alternatives to lithium ion batteries (LIBs) as large-scale energy storage applications due to an abundance of sodium resources.1-5 However, since the radius of Na+ is 35% larger than that of Li+, the development of SIBs is severely restricted by a lack of appropriate electrode materials to accommodate Na+. Challenges are particularly serious with regard to anode materials because it is difficult to insert Na+ into the commercial graphite anode used in LIBs.6 Hence, the development of highperformance anodes is urgently required for practical applications of SIBs. With their high capacities, good conductivity and suitable potential (vs. Na/Na+), metallic anodes7-9 are more competitive than other anode candidates including hard carbon,10-12 titanates13,14 and phosphides.15,16 Among the reported metallic anodes, Sb is considered one of the most promising anodes of SIBs owing to its high theoretical capacity (660 mAh g-1), suitable potential (0.6 V vs. Na/Na+) and low cost.17-20 However, the main issue impeding the practical application of Sb anodes relates to its poor cyclability, which can be attributed to the following reasons. First, slow kinetics and the loss of electrical contact results from the aggregation or/and pulverization of Sb particles induced by strong volume changes (~293% during sodiation). Additionally, the subsequent formation of SEI layers on the newly exposed surfaces of electrodes consumes active materials and retards electrode kinetics, resulting in a decline in capacity.7,21,22 Over the past years, tremendous efforts have been devoted to addressing capacity decline in Sb electrodes and significant progress has been achieved. Effective strategies mainly focus on designing nanostructures (Sb nanoparticles,23 Sb nanosheets,24,25 Sb hollow nanospheres,26 porous Sb27 and leaf-like Sb28) and on introducing conductive carbon networks,26,29 which could promote reaction kinetics and improve cycling stability. In addition, the engineering of Sb-based alloy electrodes (e.g., FeSb2,30 Mo3Sb7,31 Ni-Sb,32 SnSb,33 Zn-Sb34 and Bi-Sb35) and the

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development of suitable binders36 and electrolytes/additives37 have proven to be effective means of enhancing the Na+ storage performance of Sb electrodes. Among those strategies, nanostructured Sb anodes have been demonstrated to be some of most effective routes for relieving volumetric changes in Sb anodes during cycling.21,38,39 For example, He et al.23 reported on monodispersed Sb nanocrystals with 10-30 nm synthesized by a colloidal method, showing enhanced levels of rate-capability and cycling stability relative to that of bulk Sb used as SIB anodes. Gu et al.40 designed free-standing metallic Sb nanosheets@graphene electrodes using a liquid-phase exfoliation method and observed a high volumetric capacity of 1226 mAh cm-3 and good cycling performance in SIBs. Yuan et al.22 examined a nanoporous Sb@C anode for SIBs using a bottom-up template method and demonstrated high Na+ storage capacity (480 mAh g-1 at 0.1 A g-1) and excellent cycling stability outcomes. Although the nanoarchitectures of Sb anodes exhibit promising Na+ storage performance, the preparation processes of high quality nanostructured Sb generally require the use of complicated procedures such as hydrothermal methods for nanoparticles,41 liquid-phase exfoliation42 and chemical vapor deposition (CVD)43 for nanosheets, and template methods for 3D porous electrodes,44,45 rendering preparation costly, uncontrollable, low-yield, and unscalable for industrial manufacture. Therefore, the development of low cost, controllable, efficient, and scalable approaches to preparing high-quality Sb-based nanostructured electrodes is critical to their practical application in SIBs. Dealloying is a common corrosion process that occurs when an alloy is selectively etched of its most electrochemically active elements in specific solvents.46 This process has been widely used to prepare 3D nanoporous sponge metals/alloys.35,27,47,48 Nevertheless, to the best of our knowledge, there has been no report on the dealloying preparation of low-dimensional nanostructures such as 2D nanosheets. Herein, we propose a facile method for controllably

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synthesizing multidimensional Sb nanostructures. Zero-dimensional Sb nanoparticles (0D SbNP), three-dimensional (3D) porous Sb, and two-dimensional Sb nanosheets (2D Sb-NS) can be tunably prepared through the chemical dealloying of Li-Sb alloys in deionized water, ethanol (EtOH), and binary mixture solvents, respectively. Moreover, morphological evolution mechanisms are further demonstrated by investigating the dealloying processes of different proportions of Li-Sb alloy precursors. The as-prepared Sb nanostructure anodes exhibit greatly enhanced Na+ storage performance compared to bulk Sb electrodes. Among multidimensional Sb nanostructures, Sb-NS with a high aspect ratio (the thickness and diameter of a single layer are ~4.2 nm and ~2 μm, respectively) enables the accommodation of volumetric changes in Sb-NS electrodes with rapid kinetics. In turn, the tailored synthesized Sb-NS electrodes exhibit superior Na+ storage capacity (620 mAh g-1 at 100 mA g-1), long-term cycle stability (90.2% capacity retention after 100 cycles) and excellent rate performance (212 mAh g-1 at 6.4 A g-1). It is noteworthy that the facile dealloying approach is also applicable to other nanomaterial preparations (e.g., Pb and Bi), providing a means for the facile preparation of high performance nanostructured electrodes. RESULTS AND DISCUSSION A schematic of the synthesis of Sb nanomaterials with different morphologies is illustrated in Scheme 1. First, Li-Sb alloy precursors (Scheme 1(ii)) were fabricated through the electric smelting of bulk Sb and Li on the basis of an Li-Sb phase diagram (Figure S1, Supporting Information), which was performed in a glove box filled with Ar. Deionized water and EtOH were selected as etching solvents to adjust dealloying kinetic rates in this study due to their miscibility, obvious activity difference to Li and inertness to metallic Sb. A comparison of reaction rates in water, EtOH and the binary mixture is presented in Video S1-S3 included in the

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Supporting Information. The dealloying of Li-Sb precursors in deionized water is the most violent process among those observed in the three solvents. When the precursor is immersed into the deionized water as shown in Scheme 1(iii-1), Li atoms on the alloy surface are fleetly dissolved, leaving many terrace vacancies behind and simultaneously producing large H2 bubbles. Meanwhile, Sb atoms coordinating with those vacancies diffuse and start to agglomerate into clusters on the surfaces of alloy precursors. These Sb clusters are rapidly peeled off from the surfaces of precursors with the aid of the vigorous H2 bubbles, dispersing into the solution as is shown in Scheme 1(iv-1). The dealloying kinetics of Li-Sb alloys slow with the addition of EtOH into deionized water, as EtOH exhibits less oxidizability than H2O. As is shown in Scheme 1(iii-2), during moderate etching, the left Sb atoms agglomerate into 2D clusters at the solid-liquid interface. The 2D nanocrystals can then further grow into nanosheets on the surfaces of precursors, which are peeled off from the interface by H2 bubbles as is shown in Scheme 1(iii2). Predictably, etching rates should be further decreased when pure EtOH is used as an etching solvent. As is shown in Scheme 1(iii-3) and Video S3, during mild dealloying in EtOH, Sb nanosheets cannot be peeled off from the interface by small bubbles. Therefore, 2D nanocrystals further agglomerate into islands and bind together. Finally, the 3D porous structure is left after Li atoms in the interspaces of Sb islands are etched away as is shown in Scheme 1(iv-3).

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Scheme 1. Schematic illustration of the synthesis routes of different morphologies of Sb nanomaterials: 0D nanoparticles, 2D nanosheets and 3D porous Sb. Figure 1 displays the morphologies of metallic Sb nanomaterials obtained from different etching solvents, including deionized water, EtOH and their mixed solvents (H2O: EtOH = 3:1, 1:1, v/v). As noted above, the rapidly dealloying of Li95Sb5 alloy (~1.5 g, XRD patterns shown in Figure S2, Supporting Information) in deionized water was completed within 1 minute and Sb-NPs dispersed in LiOH solution were obtained. After several rounds of rinsing with deionized water and alcohol, the uniformly dispersed Sb-NPs were obtained as is shown in Figures 1-a1 and a2. The single Sb particle diameter is less than 30 nm, as shown by a high resolution transmission electron microscopy (HRTEM) image (Figure 1-a3). Moreover, Figures 1b and c present the stacked 2D Sb-NS obtained from H2O-EtOH mixed etching solvents. After ultrasonic treatment,

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aqueous solutions with single or several layers of Sb-NS (Figure 1-b2) were easily obtained, resulting in a colorless dispersion and a Faraday-Tyndall effect (Figure S3a, Supporting Information). The thickness of single layer Sb-NS was investigated through an atomic force microscope (AFM) (Figure S3b, Supporting Information), revealing a diameter of ~2 μm and a thickness of less than 5 nm, indicating that a high aspect ratio of 2D Sb-NS was obtained. As is shown in Figures 1-b3 and c3, HRTEM images and selected area electron diffraction (SAED) patterns are well assigned to the (012) planes of hexagonal Sb, indicating the high quality of SbNS obtained in this work. As is shown in Figures 1-c1 and c2, the Sb-NS derived from high concentration EtOH solvents (H2O-EtOH 1:1) are thicker than those derived from H2O-EtOH (3:1), suggesting that slow dealloying kinetics are helpful for the growth of nanosheets. In addition, the mild dealloying processes of Li-Sb alloys in pure EtOH are similar to those observed from the sacrificial template method where Li atoms act as a sacrificial element. As is shown in Figure 1d, 3D nanoporous Sb of dozens of micrometers in diameter is composed of several cohering nanoparticles and numerous nanopores (Figure 1-d2). To draw comparisons, the chemical dealloying of Li-Sb alloys in various alcoholic solvents such as methyl alcohol (MeOH), n-butyl alcohol (N-BAI) and ethylene glycol (EG) was further investigated. As is presented in Figure S4 (Supporting Information), needle-like Sb (Figure S4d) and porous Sb (Figures S4e and f) were obtained by adjusting etching rates in different solvents, further demonstrating advantages of the method’s universality.

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Figure 1. SEM, (HR-) TEM images and SAED pattern of as-prepared Sb nanostructures by Indicated magnification: 300kx

etching Li-Sb alloys in different solvents: H2O (a1-a3), H2O-EtOH v/v=3:1 (b1-b3), H2O-EtOH v/v=1:1 (c1-c3), and EtOH (d1-d3). A phase structure analysis of the as-prepared Sb nanostructures, including Sb-NP, Sb-NS and nanoporous Sb, was performed by XRD. As is shown in Figure 2a, all peaks observed in the patterns are well assigned to hexagonal Sb (PDF Card#35-0732), suggesting that the as-prepared Sb nanostructures are of a pure phase. Compared to those of bulk Sb, the peak intensities of the as-prepared samples gradually decreased from porous Sb to Sb-NP, suggesting the grain refinement of metallic Sb under different etching kinetics. Surface states of the as-prepared and cycled Sb-NP, Sb-NS and porous Sb electrodes were further investigated by X-ray photoelectron spectroscopy (XPS). Figure 2b presents XPS spectra of the pristine Sb nanostructures, from which signals of Sb, C and O can be easily detected. High-resolution spectra of Sb 3d were collected for the careful analysis of surfaces states of the

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Sb-NS as is shown in Figure 2c, and the spectra of other products are shown in Figure S5 b-c. As can be observed, symmetric signals of all of the pristine samples with binding energy levels of ~537.6 and ~540.2 eV correspond to Sb metal and Sb oxides, respectively. The presence of Sb oxides is mainly a product of the surface oxidation of Sb during the synthesis of Sb nanostructures or/and the following treatment processes. To compare and analyze the surface states of Sb-NS after cycling in SIBs, the XPS spectra of nanostructured electrodes after 50 cycles are displayed in Figure 2c. As is shown in Figure 2c, signals of roughly ~540.2 eV in Sb oxides disappear after 50 cycles, indicating that the thin oxide surface layer on the pristine SbNS was reduced during cycling. An N2 adsorption-desorption isotherm analysis was conducted to evaluate the surface areas and pore structures of the as-obtained Sb nanostructures. Figure 2d compares N2 adsorptiondesorption isotherms of bulk Sb and the as-prepared nanoscale Sb. The BET surface areas of bulk Sb, Sb-NP, Sb-NS and porous Sb are measured as 0.568, 67.2, 22.2 and 12.2 m2 g-1, respectively. Relative to that of the bulk Sb (~ 200 mesh), the surface area of Sb-NP is increased by more than two magnitudes as a result of the dealloying process. The stability of metallic nanostructures with high specific area in the air is of significance for their commercial application. Therefore, the phase structures of as-prepared Sb nanostructures were examined by XRD after exposed them to the air for 7 days as shown in Figure S6 (Supporting Information). Slight oxidation in the Sb-NP is observed and is mainly attributed to it large specific surface area and ultrafine particles. No detectable peaks of Sb2O3 are detected from the XRD patterns of Sb-NS and nanoporous Sb, suggesting their good stability in the air, which is consistent with the above listed XPS results.

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003 012

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Figure 2. XRD patterns of bulk Sb and nanoscale Sb products derived from different etching solvents (a). XPS spectra of as-prepared Sb nanostructures (b), and high-resolution XPS spectra of Sb 3d of Sb-NS fresh and after 50 cycles in SIBs (c). The N2 adsorption-desorption isotherms of bulk Sb, Sb-NP, Sb-NS and 3D porous Sb products (d). Moreover, series of Li-Sb alloys with different Sb contents (Li95Sb5, Li90Sb10, Li66Sb34, and Li50Sb50) were designed to regulate the activity of alloys, and their dealloying processes in deionized water were also investigated to further demonstrate the structural evolution of Sb nanostructures with different dealloying kinetics. Comparisons of the morphologies and phase structures of samples drawn from precursors of Li95Sb5, Li90Sb10, Li66Sb34, and Li50Sb50 are displayed in Figures 3 and S7 (Supporting Information). With Li content levels decreasing in LiSb alloys, the morphologies of nanoscale Sb products transform from particles into nanosheets, which is attributed to the slowing of dealloying reaction kinetics. As is shown in Figure 3d,

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when Li content in the Li-Sb alloy precursor was decreased further, Sb-NS could not be fully exfoliated from the solid-liquid interface during moderate etching, after which clusters composed of close packed Sb-NS were obtained.

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Figure 3. SEM images of the as-prepared Sb nanostructures by etching Li-Sb alloys with different levels of Sb in deionized water: (a) Li95Sb5, (b) Li90Sb10, (c) Li66Sb34, and (d) Li50Sb50. To demonstrate the applicability of this facile approach for the preparation of other metallic nanostructures, the dealloying processes of Li-Pb and Li-Bi alloys were investigated using the same procedures. Figure S8-11 (Supporting Information) presents the morphologies and XRD results of the as-prepared Pb and Bi nanostructures. As is shown in Figure S8a-b, Pb nanosheets of different thicknesses (50-100 nm) were obtained through the chemical dealloying of Li90Pb10 alloy in deionized water. Coral- (Figure S8c) and rod-like (Figure S8d) Pb nanostructures can be achieved from H2O-EtOH and EtOH etching solvents, respectively. Similarly, Bi nanostructures with different morphologies, including nanosheets (Figure S10a), nanocrystals (Figure S10b)

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and porous Bi (Figure S10d), can be easily obtained by adjusting the dealloying rates in different etching solvents. Figures S9 and S11 display the XRD patterns of the as-prepared Pb and Bi nanomaterials, in which all peaks are well assigned to the cubic (PDF Card#04-0486) and hexagonal (PDF Card#44-1246) phases, respectively. The controllable synthesis of metallic Pb and Bi nanostructures with tailored morphologies further demonstrates the broad applicability of this facile approach. As an attractive SIB anode candidate, the Na+ storage performances of the as-prepared Sb electrodes were investigated in coin-type cells with Na counter electrodes. Figure 4a presents the typical cyclic voltammetry curves (CVs) of Sb-NS electrodes at a scan rate of 0.1 mV s-1 for 0 to 2.5 V (vs. Na/Na+). During the first cathodic scan, the peak measured at 0.25 V is attributed to the sodiation of Sb into NaxSb alloys accompanied with the formation of a solid electrolyte interface (SEI).31 This peak shifts positively and splits into two peaks (0.5 and 0.75 V) in subsequent cathodic scans corresponding to the multistep alloying processes of Na+ into metallic Sb until Na3Sb intermetallic forms.19 The broadened peak observed at 0.8 V during the anodic process is attributed to the desodiation of Na3Sb back into Sb. The nonrepeatability observed between initial and subsequent cycles may be ascribed to the decomposition of electrolytes and to the formation of an SEI layer.49,50 An ex situ XRD analysis of Sb-NS electrodes of different charge-discharge states was carried out to further understand the mechanisms of sodium storage. Figure 4b shows the charge/discharge curves of Sb nanosheets and the XRD patterns at selected potentials (points marked in the figures). In the early discharge (sodiation) stages (from 1 to 2), two peaks of 23.7° and 28.7° corresponding to hexagonal Sb are observed. In the later discharge stage (from 3 to 5), the intensities of Sb peaks gradually weaken and almost disappear when the cell is fully discharged to 0.01 V. Meanwhile, new peaks of 33.5° and 34.3° appear and

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correspond to the formation of the hexagonal Na3Sb phase, denoting the conversion reaction of Sb with Na to form Na3Sb. During the subsequent charge (desodiation) process (from 7 to 11), Na3Sb peaks gradually disappear while the intensities of Sb peaks increase, suggesting the transformation of Na3Sb into Sb. Above all, the conversion reaction mechanism of Sb-NS in SIB is clearly revealed by the ex situ XRD results, which is consistent with the CVs shown in Figure 4a.

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Figure 4. Mechanism of the electrochemical Na+ storage of Sb-NS electrodes. (a) Cyclic voltammetry (CV) curves of Sb-NS electrodes investigated in a coin-type cell at a 0.1 mV s-1 scan rate. (b) Ex situ XRD patterns of Sb-NS electrodes at selected charge-discharge states (marked points). Figure 5a displays the charge-discharge curves of Sb-NS electrodes at 100 mA g-1 and shows that voltage plateaus occurring during charge and discharge processes are consistent with the peaks recorded by CVs. As is shown in Figure 5a, the Sb-NS electrode delivers 626 mAh g-1 of initial charge capacity with 62.4% of initial coulombic efficiency. The irreversible capacity observed in initial cycles is mainly a result of the formation of the SEI layer on the electrode

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surface.19,51 Figure 5b compares initial charge-discharge curves of the bulk and nanostructured Sb electrodes. Although the nanostructured Sb electrodes possess high specific surface areas, they still achieve higher initial coulombic efficiencies (53.8-62.4%) than the bulk Sb electrodes (48.9%), which is attributed to the more stable reaction interfaces and stronger sodiation/desodiation reversibility of nanostructured Sb electrodes relative to bulk Sb electrodes. Figure 5c presents the cyclic performance of Sb-NS and bulk Sb electrodes. The Sb-NS electrode delivers 620 mAh g-1 of reversible capacity corresponding to the 94% theoretical capacity of Sb and maintains 559 mAh g-1 (90.2% capacity retention) after 100 cycles, which is far superior to that of bulk Sb electrodes in terms of cyclic performance. We must note that the stated cyclic performance was derived from the as-prepared Sb-NS electrodes without making any sophisticated chemical modifications. Even so, as is shown in Table S1 (Supporting Information), the Sb-NS electrode reported here still exhibits creditable Na+ storage performance compared to the previously reported Sb-based anodes. Additionally, the Na+ storage performance of Sb-NP and porous Sb electrodes was investigated as is shown in Figure S12. The Sb-NP electrode exhibited over 660 mAh g-1 of reversible capacity and still maintains 618 mAh g-1 after 20 cycles. The subsequent pattern of rapid decline is mainly a result of the aggregation of nanoparticles and of their peeling off from current collectors.26 Figure 5d compares rate capabilities of the Sb-NS and bulk Sb electrodes. As can be observed, compared to that of the bulk Sb electrode, the rate performance of the Sb-NS electrode is clearly superior. The Sb-NS electrode delivers capacities of 617, 590, 573, 537, 455, 359 and 212 mAh g-1 at current densities of 0.1, 0.2, 0.4, 0.8, 1.6, 3.2 and 6.4 mA g-1, respectively, indicating the rapid Na+ transfer kinetics and high electronic conductivity of this electrode. When the current

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density is changed back to 100 mA g-1, the capacity can recover to ~600 mAh g-1, reflecting the strong rate capabilities of Sb-NS electrodes.

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Figure 5. Electrochemical performance comparisons of the as-prepared nanostructured Sb electrodes and bulk Sb electrode. (a) Initial 3-cycle charge-discharge curves for the cell with SbNS electrodes at a current density of 100 mA g-1. (b) Comparison of first charge-discharge curves of the nanostructured and bulk Sb electrodes. (c) Cycling performance of Sb-NS electrodes at 100 mA g-1. (d) Comparisons of Sb-NS and bulk Sb electrode rate performance. Electrochemical impedance was carried out to further analyze the different electrode kinetics of the as-prepared nanostructured and bulk Sb electrodes. Figures 6a-b present Nyquist plots of the Sb-NS and bulk Sb electrodes at fresh and cycled states (after 3, 30, and 50 cycles). The EIS data

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for Sb-NP and porous Sb and the fitting equivalent electrical circuit are shown in Figure S13 (Supporting Information). The derived impedance parameters are given in Table S1 (Supporting Information). As is shown in Figure 6a-b, the spectra form a semicircle in the high-frequency region and follow a slanted line in the low-frequency region corresponding to the charge transfer resistance (Rct) of electrodes and Na+ diffusion, respectively.52 As is shown in Figures 6a and b, the electrolyte resistance (Re) of both electrodes remains similar over 0 to 50 cycles, and Rct values are similar under fresh states. However, after the first 3 cycles, the Rct values of bulk and Sb-NS electrodes rapidly decrease to 297.7 and 68.0 Ω, respectively, due to the impregnation of electrolyte in the electrodes.51,53 However, the Rct of the bulk Sb electrode increases from 297.7 to 927.5 Ω during subsequent cycling due to the pulverization of active materials and the reconstruction of the SEI layer. The consequent consumption of active materials and slowed electrode kinetics lead to the rapid capacity decline of bulk Sb electrodes. By contrast, the Sb-NS electrode exhibits stable Rct values, revealing its robust nanostructure and the formation of a stable SEI layer, contributing to the cycling stability of Sb nanosheet electrodes in SIBs. To glean further insight into the excellent cyclic performance of Sb-NS electrodes, comparisons of the morphology evolution patterns of bulk Sb and Sb-NS electrodes after 100 cycles were drawn and SEM images are presented in Figures 6c-f. As is shown in Figure 6c, the bulk Sb electrodes are cracked and pulverized after 100 cycles, which directly results in rapid capacity decline. As is shown in Figure 6c-d, smooth Sb-NS surfaces become wrinkled and twisted after 100 cycles mainly due to volume changes occurring in radial directions during sodiation/desodiation. However, the plicated Sb-NS retain an integrated structure without any cracks, exhibiting its strong tolerance of volume changes, which is consistent with the above listed EIS results.

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Figure 6. Nyquist plots of (a) bulk Sb and (b) Sb-NS electrodes in a fully charged state from 1 MHz to 10 mHz. SEM images of pristine bulk Sb (c) and Sb-NS electrodes (e) and a comparison of the morphologies of bulk Sb electrodes (d) and Sb-NS electrodes (f) after 100 cycles at 0.1 A g-1. As is clearly demonstrated in the above results, the Sb-NS electrodes with 2D structures exhibit strong Na+ storage performance, which can be interpreted as follows. First, the robust 2D nanosheet structure undergoes minor volume changes in the axial direction during sodiation/desodiation, maintaining the integrity of the Sb-NS electrode during cycling. Second, the random stacking of Sb-NS with high-aspect-ratio 2D nanosheets can provide enough space, not only facilitating electrolyte penetration and facilitating mass transfer but also relieving electrode volume changes during cycling. Third, ultrathin nanosheet structures shorten the Na+ transport path in the axial direction, endowing Sb-NS anodes with superior rate capabilities. Furthermore, both metallic Sb and NaxSb alloy products exhibit high levels of electronic

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conductivity, enabling the Sb-NS electrode to act as a high-performance Na+ storage material with no need for chemical modification. CONCLUSIONS In summary, a facile chemical dealloying approach is demonstrated as a means to tunably prepare different morphologies of Sb nanostructures, which could promote their Na+ storage performance as anodes of SIBs. By regulating dealloying reaction kinetics with different etching solvents, 0D Sb-NP, 2D Sb-NS and 3D nanoporous Sb were controllably synthesized, and the morphological evolution mechanisms of Sb nanostructures were investigated via SEM, TEM, AFM and XRD measurements. The Na+ storage performances of the as-prepared Sb nanostructures are systematically evaluated. Among the different nanostructures examined, the electrodes with Sb-NS exhibit a high capacity of 620 mAh g-1 at 100 mA g-1 with long-term cycle stability and excellent rate performance. An EIS and postmortem analysis of electrode morphologies demonstrates that Sb-NS with specific 2D nanostructure can ensure high levels of Na+ accessibility, robust structural integrity, and rapid electron transport, thus showing the excellent Na+ storage performance. Moreover, the universality of the facile route is further demonstrated for the preparation of Pb and Bi nanostructures. Therefore, this strategy should allow for the mass production of nanostructured materials with tailored morphologies, facilitating the nanomanufacturing of electrode materials for practical applications of electrochemical energy storage. EXPERIMENTAL SECTION Preparation of Li-X alloys (X= Sb, Bi, and Pb): metal granules of Li, Sb, Bi, and Sb used in this work are very pure (> 99%). Typically, a given mass of Li and Sb metallic granules was placed

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into a tungsten crucible that was heated to 600 °C over 2 h in a furnace in an Ar atmosphere. The bulk Li-Sb alloys were obtained after the liquid alloys rapidly cooled to room temperature in an Ar atmosphere. Preparation of Sb-NP, Sb-NS, porous Sb, and Pb/Bi nanostructures: The series of metallic Sb, Bi and Pb nanostructures were obtained using a facile dealloying strategy by etching Li-Sb, LiBi and Li-Pb alloys with different solvents. Li-Sb alloy blocks were slowly added to different solvents: deionized water, water-EtOH 75%-25%V, water-EtOH 50%-50%V and EtOH (400 ml, analytically pure, Sinopharm Chemical Reagent Co. Ltd.) in ambient conditions. After the reaction was terminated, the remaining Sb powder was filtered out and rinsed with deionized water to neutral conditions. Sb powder was then filtered out and dried at 40 °C in a vacuum for 300 min. Characterization: Phase structures of the precursors and as-prepared Sb nanostructures were investigated with PANalytical X’Pert PRO (CuK generator, 2 theta: 20-80°). X-ray photoelectron spectroscopy (XPS) of the as-prepared samples was performed at AXIS-ULTRA DLD-600 W (Kratos, Shimadzu). BET surface and pore structures of the samples were examined on an N2 adsorption-desorption unit (Micromeritics, TriStar II 3020). SEM images of the products were taken using FEI Nova NanoSEM 450. TEM, SAED and HRTEM observations were executed using FEI Tecnai G2 F30. The thickness of the nanosheets was measured by AFM (SPM9700, Shimadzu). Electrochemical Measurements: The Na+ storage performance of the samples was evaluated in CR2025 coin-type cells. First, the active materials, conductive additive (Ketjen black) and binder (poly acrylic acid/carboxyl methyl cellulose (1:1) dissolved in deionized water) were mixed with

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a mass ratio of 7:2:1 with stirring to form a suspension slurry using a mortar. The slurry was coated on a Cu film and dried in a vacuum and then cut into slices as electrodes (diameter, ~11 mm). Approximately 2.0 mg of active material was loaded onto each electrode. As electrolytes, 1 M of NaPF6 dissolved in a solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (v/v =1:1) with 10 wt% fluoroethylene carbonate (FEC) additives was used. Coin-type cells were assembled in a glove box with metal Na counter electrodes and a glass fiber membrane (Whatman). CVs of the electrodes were recorded on a potentiostat/galvanostat (AUTOLABPGSTAT302N, Netherlands), and galvanostatic charge-discharge was carried using on a battery tester (LANHE-LAND2001A, China). ASSOCIATED CONTENT Supporting Information. Supporting information files are available free of charge from the ACS Publications website at DOI: ××××××. Binary metal phase diagram; XRD patterns of Li-Sb alloys, videos of the synthesis of the nanostructured Sb samples, additional structure and morphology characterization of the asprepared samples, electrochemical characterization of the samples, and supporting references (PDF). AUTHOR INFORMATION Corresponding Authors * Kai Jiang, [email protected]; * Kangli Wang, [email protected] The authors have no competing financial interests to declare.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (Grants: 51774148, 51622703,

U1766216,

51804128)

and

the

China

Postdoctoral

Science

Foundation

(2017M622430). The authors acknowledge the Analytical and Testing Center of HUST for providing XRD, XPS, AFM, SEM and FETEM measurements. REFERENCES 1.

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Table of contents

Facile Tailoring of Multi-Dimensional Nano-Structured Sb for Sodium Storage Applications

Sb nanoparticles

Li-Sb alloy

Nanoporous Sb

Sb nanosheets

A facile chemical dealloying strategy for tailoring the synthesis of multidimensional Sb nanostructures is reported. The obtained Sb nanosheets exhibit excellent Na+ storage performance due to their high levels of Na+ accessibility, robust structural integrity, and rapid electron transport capacities. This facile, scalable and tunable approach can facilitate the nanomanufacturing of metals for electrochemical energy storage applications.

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