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Letter Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Hierarchical Porous Sb Films on 3D Cu Substrate Have Promise for Stable Sodium Storage Xiao-Yong Fan,† Jiaxing Han,† Yu Jiang,‡ Jiangfeng Ni,*,‡ Lei Gou,† Dong-Lin Li,*,† and Liang Li*,‡ †

School of Materials Science and Engineering, Chang’an University, Xi’an 710061, China School of Physical Science and Technology, Center for Energy Conversion Materials & Physics (CECMP), Jiangsu Key Laboratory of Thin Films, Soochow University, Suzhou 215006, China

‡

ACS Appl. Energy Mater. Downloaded from pubs.acs.org by 79.133.107.197 on 08/22/18. For personal use only.

S Supporting Information *

ABSTRACT: We report a hierarchical porous Sb film deposited on a threedimensional (3D) Cu substrate as an efficient anode for sodium storage. Fabrication of a porous Sb film involves co-electrodeposition of Zn−Sb alloys on a 3D Cu substrate and sequential dealloying of Zn, giving rise to a mesoporous Sb structure. The hierarchically porous Sb film manifests a superior electrochemical feature when serving as a self-supported electrode for sodium storage. It affords a high reversible capacity of 624 mAh g−1, a retention of 575 mAh g−1 over 200 cycles at 330 mA g−1, and a rate capability of 514 mAh g−1 at 3300 mA g−1, outperfoming most recently reported Sb based materials. Moreover, the full cell consisting of a hierarchical Sb anode and Na3V2(PO4)3 cathode affords a specific energy of 149 mWh g−1, suggesting that this hierarchical design may open new perspectives for high-performance battery materials. KEYWORDS: sodium-ion battery, electrodeposition, hierarchical porosity, alloying, antimony

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struction of Sb/carbon nanocomposite,11,16−18 formation of intermetallic alloys,19 and design of specific architectures.20−24 However, these methods are often tedious and hardly controllable, and thus less appealing for practical application. Herein, we present a simple yet extremely efficient approach toward highly active and stable Sb electrodes by engineering a hierarchical porous Sb film on three-dimensional (3D) Cu substrate (denoted as 3D p-Sb@Cu). Synthesis of 3D p-Sb@ Cu is a two-step process, as schematically illustrated in Scheme 1. First, a Zn−Sb alloy film is potentiostatically electro-

he ever-increasing energy consumption in modern society accelerates the depletion of fossil fuels and the emission of greenhouse gases. Therefore, the search for renewable and sustainable energies (such as wind and solar power) has become an urgent demand in recent years. The effective utilization of renewable energy calls for the development of highly efficient energy storage systems.1−3 Among possible systems, the sodium-ion battery (SIB) is one of the most promising technologies because of its abundance, low materials cost, and chemistry of sodium (Na) that is identical to that of lithium (Li).4,5 As a graphite anode only exhibits a limited amount of Na, one major challenge for the current SIBs is to find an anode material that can reversibly store a significant amount of Na+ ions at a suitable voltage.6,7 To date, cheap yet efficient carbon materials with tuned structure and properties have been extensively explored in SIBs.8−11 Elementals such as Si, Sn, and Sb are capable of storing a large amount of Na+ ions through a distinct alloying mechanism. Among these substances, Sb has been considered one of the most attractive alloy anodes due to its large capacity of 660 mAh g−1 and suitable Na alloying potential of 0.6 V (vs Na/Na+, unless otherwise stated).12,13 However, Sb often suffers from loss of structural integrity due to a huge volume swelling of 390% during electrochemical sodiation, which results in pulverization of Sb particles, electrical disconnection between the active material and current collector,14 and continuous evolution of solid−electrolyte interphase (SEI).15 To effectively solve these issues related to the Sb anode, there are several materials engineering strategies, such as con© XXXX American Chemical Society

Scheme 1. Schematic Illustration of Synthesis of 3D p-Sb@ Cu

Received: May 31, 2018 Accepted: August 8, 2018 Published: August 8, 2018 A

DOI: 10.1021/acsaem.8b00872 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Energy Materials

similar morphology is also observed on 3D Sb@Cu (Figure S4). A TEM image in Figure 1e reveals that 3D p-Sb@Cu has a pore size of 5−10 nm. The clear lattice fringes with spacing distances of 0.23 and 0.31 nm might be attributed to the (104) and (012) planes of rhombohedral Sb, respectively. The crystalline structure of the products is examined by XRD (Figure 1f). The pattern of 3D Zn−Sb@Cu reveals only two major peaks at 50.4° and 74.1°, which are characteristic for cubic Cu substrate (JCPDS 70-3039), suggesting that the asdeposited Zn−Sb alloy is still amorphous. After removal of Zn, three prominent peaks at 28.8°, 40.0°, and 41.9° appear in the pattern of 3D p-Sb@Cu, which could be unambiguously indexed to the (104), (110), and (211) planes of rhombohedral Sb (JCPDS 35-0732). The 3D p-Sb@Cu product can be directly adapted as binder-free, self-supported electrodes, taking full advantage of the hierarchical porous architecture that enables a free infiltration of electrolyte and a buffering of volume swelling upon sodiation. Electrochemical Na storage in the 3D p-Sb@ Cu electrodes is investigated by coin-type half cells. Figure 2a shows the galvanostatic curves at a rate of 66 mA g−1 (0.1 C, 1 C = 660 mA g−1) in the voltage range 0.01−1.5 V upon the initial cycles. This affords an initial sodiation capacity of 785 mAh g−1, out of which a capacity of 624 mAh g−1 is reversibly recharged (desodiated). The initial Coulombic efficiency of about 80% reveals irreversible capacity loss due to the formation of SEI and side reactions. In the following cycles, the reversible (desodiation) capacity retains a stable value of 622 mAh g−1, indicating a high reversibility.27 To probe the sodiation and desodiation process, the corresponding differential capacity curves are drawn and presented in Figure 2b and Figure S5. The peak at 0.76 V disappears from the second cycle, corresponding to SEI film formation caused by electrolyte decomposition.28 There are three other peaks at 0.55, 0.41, and 0.23 V, which represent the formation processes of amorphous NaxSb, then Na3Sb(hexagonal)/Na3Sb(cubic), and finally Na3Sb(hexagonal). After the second discharge, these cathodic peaks shifted to 0.71, 0.53, and 0.41 V, respectively, signifying a possible activation ascribed to the decrease of electrochemical polarization that resulted from larger pores generated after 1 cycle.12,29 From the second discharge, the curves are quite stable and reproducible. In addition, the huge difference between curves from the first discharge to the following one may also be caused by the structural rearrangement of Sb conversion from a crystalline phase with hexagonal structure into an amorphous phase with a certain fraction of remaining Na.30,31 As for charging, the different curves in the first cycle and the following ones show two anodic peaks at 0.78 and 0.88 V, corresponding to the transformation from Na3Sb(hexagonal) into amorphous Sb and the partial crystallization of Sb.14,29,32 To illustrate such a unique architecture design, we present the electrochemical Na performance of Sb deposited on Cu foil (Sb film) or on 3D Cu (3D Sb@Cu) in Figure S6. Although they exhibit charge and discharge profiles similar to that of 3D p-Sb@Cu, their activity toward Na and reversible capacity are inferior. More importantly, when tested at a constant rate of 330 mA g−1 for 200 cycles, they show much worse cycling stability. As Figure 2c shows, 3D p-Sb@Cu shows somewhat of a capacity gain upon initial cycles, and at the 200th cycle it retains a capacity of 575 mAh g−1, which is ∼100% of the highest value upon cycling. This stability is comparable to or even better than those for recently reported Sb based materials

deposited on a 3D Cu substrate (denoted as 3D Zn−Sb@Cu), which is then dealloyed in NaOH solution to remove Zn, leaving a porous Sb film behind. The hierarchical pores in 3D p-Sb@Cu not only allow free permeation and diffusion of electrolyte ions but also accommodate the volume changes during electrochemical Na cycling.25 As a result, 3D p-Sb@Cu manifests a superior electrochemical feature which serves as a binder-free, self-supported electrode for sodium storage, affording a high reversible capacity of 624 mAh g−1 and a remarkable stability (retaining 575 mAh g−1 over 200 cycles), therefore demonstrating its great potential in practical battery application. To make maximum usage of space, 3D porous Cu rather than Cu foil is used as substrate for the deposition of Zn−Sb alloys.26 The deposition potential of Zn−Sb alloys is determined by linear sweep voltammetry (Figure S1). In order to electrodeposit Zn and Sb simultaneously, a deposition potential lower than −1.2 V (vs SCE) is necessary. We have conducted the deposition of the Zn−Sb alloy at various potentials and finally chose the potential of −1.4 V (vs SCE) for deposition. Elemental mapping of the deposited Zn−Sb alloys by energy dispersive spectroscopy reveals an identical growth of Zn and Sb on 3D Cu substrates (Figure S2). Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) are used to characterize the obtained samples. As shown in Figure 1a,b, a

Figure 1. Characterization of 3D p-Sb@Cu products. SEM images of (a, b) 3D Zn−Sb@Cu and (c, d) 3D p-Sb@Cu. (e) TEM image of 3D p-Sb@Cu. (f) XRD patterns of 3D Zn−Sb@Cu and 3D p-Sb@ Cu.

porous structure is well recognized for 3D Zn−Sb@Cu, resembling the Cu substrate. The 3D Zn−Sb@Cu is then immersed in NaOH solution. After removal of Zn, a mesoporous Sb film is left, and the deposition layer becomes thinner and rougher (Figure 1c,d). Energy dispersive spectroscopy indicates there is only pure Sb element after the treatment of 3D Zn−Sb@Cu in NaOH solution (Figure S3). A B

DOI: 10.1021/acsaem.8b00872 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

Figure 2. Electrochemical Na storage in 3D p-Sb@Cu. (a) Charge and discharge curves and (b) corresponding differential capacity curves at a rate of 66 mA g−1 between 0.01 and 1.5 V. (c) Comparison of cycling of 3D p-Sb@Cu with Sb film and 3D Sb@Cu at a rate of 330 mA g−1. (d) Rate charge and discharge curves of 3D p-Sb@Cu at various current rates.

Figure 3. Electrochemical performance of full cells of 3D p-Sb@Cu//Na3V2(PO4)3. (a) The first and second galvanostatic charge−discharge curves of 3D p-Sb@Cu, Na3V2(PO4)3, and the full cell (the inset). (b) A “CHD” image powered by the full cell. (c) Typical galvanostatic charge− discharge profiles of the full cell at a rate of 120 mA g−1 (based on the cathode).

(Figure S7). More importantly, when the current density goes back to 100 mA g−1, a reversible capacity of 617 mAh g−1 is restored accordingly, demonstrating an amazing high-rate capability of 3D p-Sb@Cu. Such excellent electrochemical activity and stability of 3D pSb@Cu might be attributed to its hierarchical porous architecture. The hierarchical porosity serves as multiple levels of an electrolyte reservoir, greatly reducing the ion diffusion

(Table S1). In addition, 3D p-Sb@Cu features a remarkable rate capability, as presented in Figure 2d. The galvanostatic curves of 3D p-Sb@Cu remain almost unchanged when the current rate increases from 100 to 1320 mA g−1, indicating a negligible current polarization. The voltage profiles only exhibit a little shift at a higher rate of 3300 mA g−1, and the corresponding desodiation capacity still reaches 514 mAh g−1, which is much higher than that of Sb film and 3D Sb@Cu C

DOI: 10.1021/acsaem.8b00872 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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length and avoiding the depletion of Na+ ions. In addition, the hierarchical porosity efficiently accommodates local volume swelling of Sb upon sodiation, guaranteeing the structural integrity of electrode upon Na cycling (Figure S8). Moreover, self-supported 3D architecture ensures a direct and robust electrical connection with the current collector, thus minimizing the charge transfer resistance.33,34 In contrast, an Sb film without pores cannot endure a huge volume change, while 3D Sb@Cu without mesopores only accommodate partial volume swelling. As a result, their stability is more or less inferior to 3D p-Sb@Cu. The outstanding Na performance of 3D p-Sb@Cu in a half cell warrants further evaluation of its potential in full cells. In this regard, a 3D p-Sb@Cu anode is coupled with a Na3V2(PO4)3 cathode in a rough ratio of 1:5 to construct full cells. Electrochemical performance of Na3V2(PO4)3 in a half cell is given in Figure S9. It delivers a capacity of about 110 mAh g−1 at 20 mA g−1, and 96 mAh g−1 at 200 mA g−1. The assembled full cell exhibits a flat charge voltage at about 3.1 V and a discharge voltage plateau at about 2.6 V (Figure 3a). The specific energy of the full cell is calculated to be 149 mWh g−1 based on the mass of both anode and cathode, sufficient for powering most electronic devices such as LED bulbs. More interestingly, full cells in series work well when powering a “CHD” image consisting of 18 LED bulbs, as shown in Figure 3b. We have analyzed the structural evolution of 3D p-Sb@Cu upon electrochemical cycling by SEM imaging, as shown in Figure S10. After 1 cycle, the porous architecture remains but some large particles appear, possibly due to electrochemical agglomeration upon sodiation. After 100 cycles, particles become smaller again upon continuous volume expansion and shrinking, but their good connection with the current collector and the porous structure are unchanged. This morphological evolution can be ascribed to the volumetric changes of Sb during electrochemical cycling. After 1 cycle, 3D p-Sb@Cu undergoes volumetric expansion during sodiation and partial shrinkage during desodiation, leading to enlarged mesopores. After 100 cycles, the 3D p-Sb@Cu is gradually pulverized to smaller nanoparticles after repeated volumetric expansion and shrinkage.29,33 In summary, we have designed and fabricated hierarchical porous Sb on 3D Cu substrate via an electrodeposition and dealloying approach. The 3D p-Sb@Cu electrode reveals a superior electrochemical performance for Na storage. It shows a reversible capacity of 624 mAh g−1, a capacity retention of ∼100% over 200 cycles at 330 mA g−1, and a superior rate capability of 514 mAh g−1 at a rate of 3300 mA g−1. The excellent Na performance of 3D p-Sb@Cu might be attributed to its hierarchical porous architecture, which enables fast and reversible uptake and release of Na+ ions without compromising its structural integrity. When a 3D p-Sb@Cu is coupled with a Na3V2(PO4)3 cathode, the assembled full cells afford a specific energy of 149 mWh g−1 based on the mass of both anode and cathode with an average working voltage of 2.6 V, sufficient to light up LED bulbs. Due to simplicity and efficiency, this proposed material design offers a new possibility to construct electrode architectures with improved electrochemical performance.

Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00872. Experimental details, linear sweep voltammetry, further materials and electrochemical characterization of Sb film and 3D Sb@Cu, and structure evolution of 3D p-Sb@ Cu (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: jeff[email protected] (J.N.). *E-mail: [email protected]. (D.-L.L.) *E-mail: [email protected]. (L.L.) ORCID

Jiangfeng Ni: 0000-0002-1649-4282 Liang Li: 0000-0003-0708-7762 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of the National Natural Science Foundation of China (Grants 51672182 and 51772197), the Thousand Young Talents Plan, the China Postdoctoral Science Foundation (2016M590908), the Natural Science Foundation of Shaanxi Province of China (2016JM2024), the Special Fund for Basic Scientific Research of Central Colleges, Chang’an University (Grant No. 310831153505), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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DOI: 10.1021/acsaem.8b00872 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX