Porous-Nickel-Scaffolded TinâAntimony Anodes with Enhanced

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Porous Nickel Scaffolded Tin-Antimony Anodes with Enhanced Electrochemical Properties for Li/Na-Ion Batteries Li Jiachen, Jun Pu, Ziqiang Liu, Jian Wang, Wenlu Wu, Huigang Zhang, and Haixia Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b04635 • Publication Date (Web): 10 Jul 2017 Downloaded from http://pubs.acs.org on July 15, 2017

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Porous Nickel Scaffolded Tin-Antimony Anodes with Enhanced Electrochemical Properties for Li/Na-Ion Batteries Jiachen Li,1,2 Jun Pu,2 Ziqiang Liu,2 Jian Wang,2 Wenlu Wu,2 Huigang Zhang,2* Haixia Ma1* 1. School of Chemical Engineering, Northwest University, Xi’an, Shaanxi, 710069, China 2. National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, Collaborative Innovation Center of Advanced Microstructures, Institute of Materials Engineering, Nanjing University, Nanjing, 210093, China KEYWORDS: Porous nickel scaffold, Li-ion batteries, Na-ion batteries, electrodeposition, full cells ABSTRACT: The energy and power densities of rechargeable batteries need to be increased urgently to meet the ever-increasing demands of consumer electronics and electric vehicles. Alloy anodes are among the most promising candidates for next generation high-capacity battery materials. However, the high capacities of alloy anodes usually suffer from some serious difficulties related to the volume changes of active materials. Porous supports and nanostructured alloy materials have been explored to address these issues. However, these approaches seemingly increase the active material based properties and actually decrease the electrode based capacity because of the oversize pores and heavy mass of mechanical supports. In this study, we developed an ultralight porous nickel to scaffold high capacity SnSb alloy anodes. The porous nickel supported SnSb alloy demonstrates a high specific

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capacity and good cyclability for both Li-ion and Na-ion batteries. Its capacity retains 580 mAh g−1 at 2 A g−1 after 100 cycles in Li-ion batteries. For a Na-ion battery, the composite electrode can even deliver a capacity of 275 mAh g−1 at 1 A g−1 after 1000 cycles. This study demonstrates that combining the scaffolding function of ultralight porous nickel and the high capacity of SnSb alloy can significantly enhance the electrochemical performances of Li/Na-ion batteries. 1. INTRODUCTION Lithium-ion (Li-ion) batteries become indispensable to everyday lives.1 However, current Li-ion battery technologies are unable to meet the increasing demands on the energy and power densities.2−5 Due to the limited availability of lithium resources, research interests of rechargeable batteries have turned to sodium ion (Na-ion) batteries. Developing high capacity Li/Na-ion anodes has gradually become an important approach to addressing the issues facing rechargeable batteries.6,7 Due to the high specific capacities, alloy anodes have received increasing attention for both Li-ion and Na-ion batteries. For examples, silicon, tin, antimony, bismuth, aluminum, and zinc have been investigated for increasing the energy density of Li/Na-ion batteries.8−11 However, there are some scientific and technical challenges in developing practical Li/Na-ion batteries. High capacity alloys could accommodate a large number of Li/Na-ions that even exceed the number of host ions.12 The insertion of many Li/Na-ions into the host materials lead to several serious issues: 1) large volume change that may destroy the electrode structure, 2) repeated cycling that results in pulverization of alloy particles, 3) the formation of solid electrolyte interface (SEI) on the cracked surface that consumes cyclable Li/Naions and leads to low Coulombic efficiency.13−16 Mechanical scaffolds are usually employed to support the high capacity materials and accommodate the volume change. Metallic foam, carbon network, textile cloth, and graphene foam have been studied to act as the mechanical supports.17−19 Although previous research has shown the advantages of using

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mechanical supports to scaffold active materials, the oversized pores of foam-based scaffolds and relatively low electron conductivities of carbon-based scaffolds limit the application of mechanical supports in the high capacity anodes. Conventional metal foam has a pore size of 50~500 µm (Figure 1a-c). When it is used and its pore filled with active materials, the diffusion length of electrons and ions in the active materials is too long and the induced strains are too high.20 As illustrated in Figure 1b and c, high strains lead to cracks and poor cyclability. For alloy anodes, there is a critical size which is usually on the order of a hundred nanometers. Below the critical size, alloy particles will not crack and but exhibit superior cycling stability.21–24 Nanostructuring alloy particles has become an important approach to reducing the active material size and stabilizing the anode performance. To assemble or deposit the nano-sized alloy particles onto a mechanical scaffold seems to be able to address the issues related to the volume change. However, because of the oversized pores, conventional foam is unable to concurrently increase the active/inactive materials ratio and effectively wire all alloy nanoparticles.25,26 There are some other approaches to improve the performance of alloy anodes. Heterogeneous doping could form a buffer matrix to accommodate the strains induced by the volume change. For example, Ni, Co, Cu, and C can act as the inactive buffer matrix and show improved cyclability.27−29 However, these inactive components do not contribute to capacity and decrease the specific capacity. Among the previously studied alloy materials, antimony, with a slightly higher potential (0.8 V) plateau than tin (0.4 V), could deliver a specific capacity of 660 mAh g−1. Especially, it demonstrates much high cyclability during Na-ion insertion and extraction.30,31 In this work, we developed a nanostructured SnSb alloy electrode with a mechanically stable threedimensional (3D) scaffold via a templated electrodeposition route (Figure 1d-g). In such a hierarchical anode, an ultralight porous nickel scaffold (PNS) is fabricated to accommodate the alloy strains induced by the volume change and concurrently conduct electrons as a current collector. Nanostructuring the SnSb alloy layer on the surface of 3D scaffold decreases the probability of alloy 3



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pulverization. The antimony components are introduced to buffer the volume change during the tin alloying process. The hierarchical PNS-supported SnSb alloy (PNS@SnSb) electrode is able to significantly enhance the capacity retention for both Li-ion and Na-ion batteries. More importantly, the overall electrode-based capacity is significantly increased because of using the ultralight current collector instead of heavy foil counterparts.

Figure 1. Schematic illustration of (a) conventional metal foam with an active material coating, (b, c) the cycled conventional electrode with the formation of cracks, and (d-g) the fabrication procedure for PNS@SnSb. (Note the characteristic pore sizes between PNS and conventional foam) 2. EXPERIMENTAL SECTION 2.1 Fabrication of PNS: All chemicals were analytical grade and used without additional purification. Copper particles (Aladdin Corp. China) were dispersed within isopropanol to form a slurry which was then cast on a graphite plate by a doctor blade. The film and plate were annealed at 900 °C in forming gas (5% H2 and 95% Ar) for 2 h. The porous Cu foil obtained was detached from the graphite plate and placed in a Ni plating solution (SN-10, Transene Corp.). A thin Ni layer was electroplated onto the surface of porous Cu foil. The sample was removed out of the solution after 4


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electrodeposition and rinsed with de-ionized water. The Cu template was dissolved by an etchant solution of 0.6 M Na2S2O8, 1.9 M (NH4)2SO4, and 3.5 M NaOH under stirring for ~5 h. Finally, the resulting PNS was rinsed with de-ionized water and dried in a vacuum oven. 2.2 Electrodeposition of PNS@SnSb: The obtained PNS as the working electrode was immersed in a plating solution of 0.035 M SbCl3, 0.088 M SnCl2·2H2O, 0.9 M K4P2O7, 0.1 M glycine, 0.028 M C4O6H4KNa. A piece of Sn sheet was used as the counter electrode. The plating current pulses were adjusted to -15 mA cm−2 (1 s on, 10 s off). The SnSb loading could be tuned by the deposition time (pulse number × pulse duration). Typically, about 1 min was able to coat about 20 nm of SnSb alloy. The SnSb loading was controlled ~2 mg cm−2. The resulting samples were rinsed with de-ionized water and dried for further characterization. 2.3 Materials and Electrochemical Characterizations: X-ray diffraction (XRD) patterns were collected on a D/MAX-3C diffractometer at 40 kV. The morphology and elemental mappings of PNS@SnSb were observed by a scanning electron microscope (SEM, Zeiss Ultra 55). PNS@SnSb alloy electrodes were assembled with Li or Na foils into coin cells in an Ar-filled glove box, respectively. The electrolyte for Na-ion batteries consisted of 1 M NaClO4 in a 1:1 volume ratio mixture of ethylene carbonate (EC)/propylene carbonate (PC) with 5% fluoroethylene carbonate (FEC). Celgard (2400) and Whatman glass fibers were used as the separator for Li-ion and Na-ion cells. The electrolyte for Li-ion batteries was 1 M LiPF6 within a 1:1 volume ratio mixture of ethylene carbonate and dimethyl carbonate. Galvanostatic cycling measurements were conducted by a battery test system (LANHE-CT2001A) between 0.01 and 2.0 V. The commercial LiCoO2 (LCO) together with 10 wt% of acetylene black and 10 wt% of polyvinylidene fluoride (PVDF) were used as the cathode for Li-ion full cell characterizations. The home-made Na3V2(PO4)3 (NVP, fabricated according to the reference32) together with 10 wt% acetylene black and 10 wt% PVDF were assembled with PNS@SnSb for Na-ion

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full cell characterizations. Cyclic voltammograms (CVs) and electrochemical impedance spectroscopy (EIS) measurements were recorded by a VSP potentiostat (Bio-Logic, France). 3. RESULTS AND DISCUSSIONS Figure 1d-g illustrate the typical fabrication procedure of PNS@SnSb composite anodes. Copper powders were slurry-cast on a graphite plate to form a porous foil consisting of loosely-contacted Cu particles. After annealing in forming gas, the interparticle connections were enhanced due to the formation of sintering necks. The Cu powder in the green body foil basically formed a continuous network with interpenetrating void spaces (Figure 1d). When immersed in a Ni plating solution, the void space of the porous Cu foil was filled with the plating solution. A negative current was applied between the Cu template and a Ni foil. A thin layer of Ni was plated on the Cu surface (Figure 1e). The resulting Ni shell replicates the accessible surface of the Cu template. After removing the Cu template, a PNS was obtained as shown in Figure 2a. The shell thickness is about 50 nm, which can be tuned by the nickel plating time. The average pore size of the Ni scaffold is around 5 µm, which can also be adjusted by the Cu powder size. The surface area was estimated to be 10 m2 g−1 (see the supporting information for details). The fabrication technique provides some tunability of conductive scaffold parameters and shows the promise for the ideal microstructure configuration of Li-ion and Na-ion batteries current collectors. SnSb alloy was electrodeposited onto the PNS surface. Figure 2b presents the SEM image of the PNS@SnSb. The relative loading ratio was adjusted by varying the plating time and current according to the demanded energy or power density of batteries. For comparison, SnSb was electroplated on the conventional nickel foam (NF) as shown in Figure S2. To simply evaluate the mechanical properties, PNS@SnSb was compressed under 5 MPa pressures. As shown in Figure S3, the compressed PNS@SnSb retained the scaffolding structures and the SnSb layer adhered to PNS. The good mechanical properties are favorable to accommodate the SnSb volume changes during battery cycling. 6


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The elemental mapping in Figure 2c clearly indicates that the Sn and Sb elements are uniformly distributed on the PNS@SnSb electrode. A rough quantification according to the energy dispersive Xray (EDX) spectrum shows the existence of Sn and Sb with the ratio of about 5:3 (Figure S4). The XRD pattern in Figure 2d identifies the electrodeposited alloy layer as a mixture of metallic Sn and SnSb alloy.33

Figure 2. SEM images of (a) PNS and (b) PNS@SnSb, (c) SEM image and element mapping, and (d) XRD patterns of PNS@SnSb. The PNS@SnSb electrode is first assembled with lithium into a coin cell. Its CV curves are recorded from 0.01 to 2.0 V at a scan rate of 0.1 mV s−1. Figure 3a shows that there are two peaks around 0.71 and 0.55 V in the first reduction scan. The peak at 0.71 V is attributed to the lithiation of antimony. The shoulder peak at 0.55 V indicates the lithiation of tin, which usually involves the multistep alloying reaction according to the literature.34,35 In the reverse scan, there is a significant oxidation peak at 1.17 7



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V, corresponding to the delithiation of the alloy anode. The main redox peaks basically agree well with previous reports.35 There are barely distinguishable humps along the baseline of CV curves, which are possibly related to the multistep alloying/de-alloying reactions. Because the scan rates are relatively high, the humps do not form peaks but merge into the baseline. The multistep reactions manifest themselves in terms of small plateaus on the galvanostatic charge/discharge curves in Figure 3b. The main plateaus at 0.75 V during lithiation and 1.1 V during delithiation are in agreement with the sharp peaks of CV curves in Figure 3a. Besides the large two-phase reaction regions, there are several minor plateaus such as 1.24 V during lithiation and 0.72 V during delithiation. In Figure 3b, the PNS@SnSb electrode shows an initial capacity of 1070 mAh g−1 (active material basis), which decreases to 906 mAh g−1 in the next few cycles. The large initial capacity and the capacity gap between the first two cycles may be attributed to the SEI formation. Interestingly, the delithiation capacity increases with cycling, indicating the gradually increased accessibility of active materials. The PNS@SnSb electrode is galvanostatically charged/discharged at 2 A g−1 for 100 cycles. Its specific capacity decreases from 1070 to 734 mAh g−1 in the first 10 cycles and its Coulombic efficiency increases from the initial 62.2% to 97.1%. The capacity decay rate is about 3.1% per cycle. Until the 100th cycle, the PNS@SnSb could still deliver a capacity of 580 mAh g−1. A conventional NF is also studied as the scaffold because NF is widely used for high capacity anodes in the literature.18,28 It is loaded with the same amount of SnSb alloy by electrodeposition and denoted as NF@SnSb. Figure 3c indicates that the NF@SnSb sample shows a capacity of 909 mAh g−1 in the first cycle, indicating a lower utilization of active materials as compared to the PNS@SnSb electrode. There appears a rapid capacity decay from 604 to 300 mAh g−1 between the 20th and 40th cycles. The sudden capacity drop may imply the mechanical detachment of alloy materials instead of electrochemical decay. The rate capabilities of two samples are characterized at varied current densities. As shown in Figure 3d, the two samples deliver a similar initial capacity around 1000 mAh g−1 at 60 mA g−1, indicating the similar 8


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utilization of active materials at low rates. With the increase of current densities, the deliverable capacities of the two samples decreased. When the current density increases over 0.6 A g−1, the capacity difference between the two samples became larger. The NF@SnSb sample demonstrates the lower rate capability than the PNS@SnSb. The active material based electrochemical properties in Figure 3 show that the PNS is able to significantly improve the utilization of SnSb alloy.

Figure 3. (a) CV and (b) galvanostatic charge/discharge curves of PNS@SnSb anodes. Comparison of PNS@SnSb and NF@SnSb anodes: (c) cycling properties at 2 A g−1, (d) rate properties at varied current densities, and (e) Nyquist plots after 10 charge/discharge cycles. 9



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EIS spectra are collected for PNS and NF scaffolded SnSb alloy anodes in Figure 3e. PNS@SnSb has a lower interface reaction impedance than conventional NF@SnSb. The apparent diffusion coefficient of Li+ in PNS@SnSb and NF@SnSb are calculated to be 4.05×10−12 and 8.46×10−13 cm2 s−1, respectively, according to the previously reported method.36−43 It should be noted that the increase of diffusion coefficient may not be due to a change of intrinsic Li+ chemical diffusivity in SnSb deposit, but can be attributed to the relatively easy accessibility of the PNS@SnSb electrode due to the thin deposit.44

By convention, the mass of current collector, binder, and conductive additives is not included in the calculation of the specific capacity for studies on active materials. Whether an improvement on current collectors increase the cell properties or not should be evaluated on the electrode basis. The electrodebased characterization is more important for practical applications. Because of the ultralight mass and large surface area of PNS, the PNS@SnSb Li-ion anode shows the electrode-based capacity of 665.2 −1 mAh g−1 el , which is much higher than 59.1 mAh gel for NF@SnSb.

The good electrochemical properties of PNS@SnSb inspired us to explore its Na-ion battery applications. The high capacity anode materials are highly demanded by high energy Na-ion batteries. The PNS@SnSb electrode is tested as a Na-ion anode as shown in Figure 4a. There is a sharp peak at 0.36 V and another minor peak at 0.2 V in the first reduction scan, which may be attributed to the NaSb and Na-Sn alloying and activation reactions.31,45 A weak hump around 1.0 V may be due to the SEI formation on the surface of PNS@SnSb. In the second scan, three peaks at 0.66, 0.38, and 0.22 V are related to the alloying reactions. The anodic scans show two obvious peaks at approximately 0.82 and 0.93 V, which are attributed to the de-alloying processes. The charge/discharge curves of PNS@SnSb in Figure 4b show the two plateaus at about 0.4 and 0.75 V, which are in agreement with the CV curves. The two plateaus correspond to the multistep alloying/dealloying reactions during the

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charge/discharge processes.45 The PNS@SnSb anode shows an initial capacity of 794 mAh g−1 with a Coulombic efficiency of 53%, which is increased to 95% in next two cycles.

Figure 4. (a) CV curves and (b) galvanostatic charge/discharge curves of PNS@SnSb as the Na-ion anode. (c) Rate capability and (d) Nyquist plots of the PNS@SnSb and NF@SnSb electrodes in Na-ion batteries after 10 cycles. (e) Cycling properties of PNS@SnSb and NF@SnSb at 1 A g−1. The rate capability of PNS@SnSb is measured and compared with NF@SnSb in Figure 4c. At low current densities (

Porous-Nickel-Scaffolded TinâAntimony Anodes with Enhanced

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