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3D hierarchical porous Cu-based composite current collector with enhanced ligaments for notably improved cycle stability of Sn anode in Li-ion batteries Zheng Luo, Jincheng Xu, Bin Yuan, Renzong Hu, Lichun Yang, Yan Gao, and Min Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04049 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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ACS Applied Materials & Interfaces

3D hierarchical porous Cu-based composite current collector with enhanced ligaments for notably improved cycle stability of Sn anode in Li-ion batteries Zheng Luo1, Jincheng Xu1, Bin Yuan1,2*, Renzong Hu1,2, Lichun Yang1,2, Yan Gao1,2, and Min Zhu1,2 1

School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, P. R. China 2 Key Laboratory of Advanced Energy Storage Materials of Guangdong Province

Correspondence to this article please direct to Dr. Bin Yuan School of Materials Science and Engineering South China University of Technology 510640 Guangzhou [email protected] Tel: +86-20-87113091, Fax: +86-20-87112762

*

Corresponding authors: [email protected] (Bin Yuan) 1

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Abstract 3D porous Cu current collector used in Li ion batteries can improve the cycling performance of Sn anodes with high specific capacity due to the accommodation of large volume expansion by the pores. However, pure Cu ligament is too soft to endure enough stress from volume expansion, and then it leads to the fast fade of capacity due to the formation of cracks or the collapse of 3D porous structure. In this study, a novel micro-nano 3D hierarchical porous Cu-based composite current collector with enhanced ligaments has been fabricated by one-step dealloying of Cu-34Zn-6Al (wt.%) precursor and subsequent heat treatment. The pore and microstructure evolutions during dealloying and heat treatment have been studied by XRD, SEM and TEM. To confirm the validity of the 3D porous Cu/β/γ composite current collector, Sn has been directly electroless plated on it in comparison with the porous pure Cu and the common Cu foil. It is found that the Sn-coated 3D hierarchical porous Cu/β/γ composite current collector with higher hardness shows significantly improved cycling stability comparing with the Sn-coated 3D porous Cu current collector and the planar copper foil. Keywords: Dealloying; Hierarchical pores; Current collector; Li ion battery; Sn anode

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1. Introduction With the increasing market demands for those lithium ion batteries (LIBs) with high energy density and long cycle life in the consumer electrical apparatuses and electric vehicles, many researches have been focused on the high-capacity electrode materials in LIBs1-3. Sn anode has greatly attracted researchers’ attentions for its high specific capacity (994 mAh g-1) compared with commercialized graphite anode (370 mAh g-1)4. However, until now, Sn anode still cannot be widely used in LIBs for its large volume change (about 260%) during the lithiation and delithiation process5, which would lead to the pulverization of Sn particles and continuous formation of solid-electrolyte interface (SEI)6-7. Though the pulverization has been relieved by decreasing the size of Sn particles to nano-scale, long cycle life is still difficult to be achieved because the nanosized Sn particles would aggregate to large scale ones during cycling8-9. To prevent the aggregation of Sn nano-particles, dispersing nano-sized Sn particles into the secondary phase matrix has been demonstrated to be an effective method to improve the cycling performance of Sn anode. However, the limited matrix space isn’t enough to totally relieve the stress induced by the volume expansion of Sn during the lithiation process, and the cracks would inevitably form in the secondary phase matrix, which would lead to sharply decrease in capacity 10-11. To further relieve the stress induced by the volume expansion of Sn anode, 3D porous metals12-15, especially 3D nanoporous Cu15, were conducted as a current collector in LIBs. A large number of cavities in these 3D nanoporous metals can not only absorb the strain energy of Sn anode during cycling, but also greatly improve the surface area, which could load more active anode materials16-17. There are many methods to fabricate nanoporous metals, such as

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template technique18 and organic deposition19. However, these methods are costly and time consuming. In contrast, the dealloying method is thought to be a more efficient way to fabricate nanoporous metals by selectively etching one or more active elements from the precursor while noble metal atoms would diffuse and agglomerate into nanoporous structure20. Also, the morphology of the pores could be effectively controlled by adjusting the dealloying conditions, which can form a single-scale porous structure21 or even hierarchical porous structure22. The cycling performance of Sn anode has been greatly improved by conducting these kinds of nanoporous metals as current collector. For example, Yu23 prepared a nanoporous Au current collector in size of 50 nm by dealloying, and Sn nano-particles were then electroless plated on it. Compared with Sn anode electroless plated on planar copper foil, the Sn anode electroless plated on nanoporous Au shows a more stable cycling performance. To further accommodate the stress induced by the expansion of Sn anode, a hierarchical porous current collector with the nano-pores uniformly distributed in the micro-channels is fabricated. More strains can be relieved by the micro-channels, as well as a fast mass transportation could be achieved. For example, Zhang24 fabricated a Sn-coated composite electrode using hierarchical porous Cu current collector prepared by the dealloying of Al-Cu precursors, and demonstrated that this hierarchical porous structure can further improve the cycling performance. Although the nanoporous or the hierarchical porous structure can relieve the volume expansion of Sn anode to a certain extent, the ligaments of these nanoporous pure metals mentioned above are too soft to endure the large volume expansion of Sn anode, which would lead to the formation of cracks and even collapse of the pore structure. Recently, Yu25 reported that adopting

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Cu-based alloy with high hardness on the planar copper foil as a composite current collector can obviously improve the cycling stability of Si anode (Si anodes also suffer from huge volume expansion, maximum to 320%), which has engaged our great attentions. Therefore, we can expect a significantly improved cycling stability of Sn anode if the ligaments of the hierarchical porous structure could be enhanced by introducing alloy phases with higher hardness. Cu-Zn-Al ternary alloy is one of the most widely used Cu-based alloys with a higher hardness than pure Cu and a satisfied conductivity. According to the relative works26, the Cu-Zn-Al ternary alloy in β or γ phase shows a higher hardness than pure Cu. Also, Zn and Al atoms possess a much lower corrosion potential than Cu in acid solution, which would be preferentially etched away to form a nano or hierarchical porous structure during dealloying27,28. Moreover, the fast diffusion rate of Zn and Al atoms into Cu matrix would be of great help for forming alloy layer during heat-treatment, while the pore structure could be well preserved29. So in this work, we fabricated a novel 3D hierarchical porous Cu/β/γ composite current collector by one-step dealloying from a dual-phase Cu-Zn-Al ternary alloy precursor and subsequent heat treatment. It is found that the introduced β and γ phases with higher hardness in the ligaments have been confirmed to greatly improve the cycling stability of Sn anodes, comparing with hierarchical porous pure Cu current collector. 2. Experimental 2.1 Processing of 3D Sn-coated hierarchical porous anode electrode In order to obtain 3D hierarchical porous structure by dealloying, the designed precursor Cu-34Zn-6Al (wt.%) alloy with a dual-phase was prepared from pure Cu (99.9%), pure Zn (99.9%) and Al (99.9%) according to its ternary phase diagram, as shown in Fig. S130.

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Vacuum induction smelting was employed to melt the raw materials in a quartz tube under Ar atmosphere. The ingots were then homogenized at 1023K for 24h under the protection of Ar gas and cooled down in the furnace. These precursor ingots were remelted by high-frequency induction heating in a quartz tube and then melt-spun onto a copper roller at a speed of 3500 rpm in a controlled argon atmosphere. The melt-spun precursor ribbons obtained are typically 20-30 µm in thickness, 4-5 mm in width and several centimeters in length, as shown in Fig. S2. 3D hierarchical porous Cu current collectors with micro-channels and nano-pores were prepared by dealloying for different times (30, 90 and 180 min) under a free corrosion condition using 5 wt.% HCl-FeCl3 solution (5g FeCl3 per 100g 5 wt.% HCl solution). Heat treatment was then conducted in a tube furnace (CVD(G)-07/50/2, Risine Inc.) at 1123K for 3h under Ar gas protection to obtain 3D hierarchical porous Cu/β/γ composite current collectors, as illustrated in Fig.1. Then, the as-obtained porous current collectors were immersed into the electroless Sn plating solution (Jemply Electronic Technology Co. Ltd.) for 1 min at room temperature. In order to confirm the validity of the hierarchical porous Cu/β/γ composite current collector, Sn-coated hierarchical porous Cu before heat-treatment and planar Cu foil were also prepared in the same plating solution for the same time. The as-prepared anode electrodes without binder were then cleaned by distilled water and dried at 60 oC in a vacuum drying oven for 8h. Finally, the as-prepared electrodes were directly used in the coin-type half-cells after being compressed under the pressure of 6 MPa.

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Fig. 1 Schematic diagram showing the fabricating processing of 3D hierarchical porous Cu-based alloy composite current collector.

2. 2 Microstructure characterization and electrochemical measurements The phase constitution and microstructure of the 3D hierarchical porous current collectors or the anode electrodes were characterized by an X-ray diffractometer (XRD, MiniFlex 600, Rigaku) with Cu Kα radiation, and a scanning electron microscopy (SEM, Super 40, Zeiss). A focused ion beam (FIB, Crossbeam 540, Zeiss) was used to prepare TEM samples, and the fine porous structure and microstructure were observed by a transmission electron microscopy (TEM, JEM-2100, JEOL) at 200KV. Nitrogen adsorption and desorption isotherms were measured at 77 K with a Quadrachrome Adsorption Instrument. The conductivity of the porous Cu-based composite current collector was tested by a physical property measurement system (PPMS, PPMS-9, Quantum Design Inc.) at room temperature. A nanoindenter (NHTX, CSM) was used to measure the hardness of the ligament of porous current collectors using a Berkovich indenter, maximum load of 5 mN at a rate of 10 mN/min and maintained for 10s. The electrochemical performances at room temperature of the 3D hierarchical porous composite electrodes were studied with CR2016 coin-type half-cells assembled in an argon filled glove box (H2O＜0.1ppm; O2＜0.1ppm). The cell was assembled with the Sn-coated hierarchical porous composite electrode (The area of the electrode is about

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0.39 cm2), lithium metal slice, and polyethylene membrane (Teklon@Gold LP) as separator. The electrolyte is LiPF6 (1 mol/L) in mixture of EC+DEC+EMC (1:1:1 by volume). The cells were galvanostatically discharged and charged in a CT2001A (LAND, China) battery test system at a constant current density of 0.1 (or 0.5) mA·cm-2 between 0.01 V and 1.5 V. In order to better reflect the high loading capacity of the 3D hierarchical porous structure comparing with the planar foil, we choose to report the capacitance of these electrodes in mAh·cm-2, area being footprint area of the electrodes. The electrochemical impedance spectroscopy (EIS) test was performed by using a Gamry Interface 3000 Electrochemical System over the frequency ranging from 100 kHz to 0.01 Hz. 3. Results and discussion Figure 2 shows the microstructure of the ribbons after dealloying for 90min. A sandwich structure which consists of two porous layers of 5 µm thickness on each outer surface and a layer of non-corroded substrate is formed, as presented in Fig. 2(a). In order to observe porous structure more clearly, the magnification SEM was given in the inset image of Fig. 2(a). It can be seen that the porous structure is composed of many inter-connected micro-channels. Except for the micro-sized channels, a great deal of nano-sized pores distributed in the matrix ligaments as the white arrow shown in Fig. 2(b). It can be concluded that the porous layer is 3D, open and bicontinuous with hierarchical porous structure, which is composed of interconnected micro-sized channels and ligaments in the scale of 0.5-1 microns with nano-pores in the scale of tens of nanometers distributed in the ligaments. The thickness of porous layer would increase with prolonging the dealloying time, as shown in Fig.S3. For the sample dealloyed for 30 min, a thin layer of porous structure in 2 µm thickness is firstly

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formed on the outer surface of the ribbon, as shown in Fig. S3(a). Moreover, only some single-scale pores in size of 200-300 nm can be observed in the porous layer, as shown in Fig. S3(b). As the dealloying time is 180 min, the whole ribbon with 20 µm thickness becomes a porous structure, as shown in Fig.S3(c), and the hierarchical porous structure is the same with that in the sample dealloyed for 90 min, as shown in Fig. S3(d). It is indicated that the dealloying starts on the surface and gradually penetrates into the interior region. It can be seen from the XRD results in Fig. S4, the precursor ribbon is composed of primary γ phase and minor β phase, while the samples gradually convert to pure Cu with the increasing of dealloying time. Thus, the ligaments are mainly composed of pure Cu when most Zn and Al atoms have been corroded away after dealloying.

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Fig. 2 The microstructure of the as-dealloyed samples in 5 wt.% HCl-FeCl3 solution at RT for 90min: (a)cross-sectional view of SEM image; (b)top view of SEM image; (c) cross-sectional FIB SEM image showing the 3D porous layer; (d) TEM bright field image; (e) its SAED pattern of region C in Fig. (d); and (f) HR-TEM image corresponding to the selected region D in Fig. (d).

Thus, the sample dealloyed for 90 min was further studied because the sample can form a 3D hierarchical porous structure and the remaining substrate may be the source of Zn and Al atoms that can diffuse to the outer hierarchical porous Cu layer to form alloy phase layer. Fig.2(c) shows its cross-sectional FIB SEM image, it is clear that the irregular pores in size of 0.5-1 µm are interconnected, and some pores with several hundred nanometers are dispersed on the ligaments. And, the fine microstructure of the porous structure was studied by TEM and HR-TEM, as shown in Fig. 2(d)-2(f). The TEM image of pore structure is presented in Fig. 2(d), it can be seen that it still exists many irregular pores with 30-80 nm in the ligament. In order to determine the microstructure of the porous ligament, the SAED pattern from region C in Fig. 2(d), as given in Fig. 2(e). It consists of discontinuous polycrystalline diffraction rings, corresponding to the reflections of planes (111)Cu, (002)Cu, (022)Cu, and

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(113)Cu, respectively. This indicates that the ligaments are composed of pure Cu. Furthermore, Fig. 2(f) shows an HR-TEM image of the surface layer on these nano-sized pores (region D in Fig. 2(d)), it is clear that the surface of the ligament is a layer of dominating amorphous and minor nanocrystalline Cu. It can be concluded that the fine structure of the ligament is mainly composed of amorphous Cu with some nanocrystalline Cu. Moreover, it can be clearly seen from the EDS result of region C that the ligaments are mainly composed of Cu with few Zn and Al atoms, as shown in Fig. S5(a). In addition, the BET results show that this porous sample after dealloying has a specific surface area of 2.99 m2·g-1 (Fig. S6(a)) and its average pore size is measured to be 18.89 nm (Fig. S6(b)), which further proved the existence of nano-pores. The formation of the 3D hierarchical porous structure can be explained by a process of preferential dissolution of γ phase and subsequent dealloying of β phase. The precursor ingot is composed of primary γ phase and minor β phase, as can be seen in Fig. S7(a). After melt-spinning, the grain size can be diminished to 1-1.5 µm as shown in Fig. S7(b), and the SEAD patterns demonstrate these micro-sized grains are γ phases as shown in Fig. S7(c). These micro-sized γ phase grains would be firstly chemically corroded away in the acid solution with Fe3+ to form micro-pores because of its lower corrosion potential compared with β phase (-0.05 V for β, and -0.15 V for γ in 0.5N Cl- solution at 25℃)27, as can be clearly observed in the area indicated by the white arrow in Fig.S7(b). Secondly, due to the lower corrosion potential of zinc and aluminum elements than copper in acid solutions, zinc and aluminum atoms in the γ and β phases would be selectively corroded off while the left copper atoms would gradually diffuse and assemble together to form a nano-scale pore/ligament

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structure27,28. Moreover, XRD patterns during dealloying processing (as shown in Fig. S4(b) ) further show the variation of β and γ phases in the selected diffraction angle range of 75-88o. The peak intensity ratio of (633) plane in γ phase to (211) plane in β phase decreases with the increasing of dealloying time from Table S1, it further proves that γ phase corrodes faster than β phase. Thus, the 3D hierarchical porous structure can be formed by one step dealloying processing for the Cu-Zn-Al dual-phase precursor ribbons. In order to strengthen the pure copper ligaments, the following heat treatment was employed to form alloy layers with higher hardness. Fig. 3 shows the phase evolution of the sample before and after heat treatment. It can be seen that some diffraction peaks of γ phase fade while the peak intensity of β phase becomes enhanced, as the magnification XRD spectrum of 70-85o clearly shown in Fig. 3(b). It is manifested that a part of Zn and Al atoms from the non-corroded substrate can diffuse into the outer porous Cu layer during heat treatment, which leads to the formation of β phase layer. Moreover, the diffraction peaks of Cu become sharper, which indicates that the copper layer on the surface of the ligaments may be well crystallized after heat-treatment.

Fig. 3 XRD patterns of the sample dealloyed for 90min before and after heat treatment (a) and its magnification view in the selected diffraction angle of 70~85o (b). 12


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To examine the integrity of 3D hierarchical pore structure after heat treatment, Fig. 4(a) and 4(b) show the top view and the sectional view of the as-dealloyed sample for 90 min after

(b)

heat treatment. It can be clearly seen that the 3D micro-nano hierarchical porous structure can be well preserved after heat treatment. The micro-channels still maintain in the scale of 1-2 microns while nano-sized pores still can be uniformly distributed on the ligaments, as shown in Fig. 4(a). The BET results (as shown in Fig. S6(c) and Fig. S6(d)) of the as-dealloyed sample after heat-treatment remain almost unchanged, which can further prove the well-preserved porous structure. Then, the hierarchical porous structure has been further observed by TEM that the micro-channels with 1-2 µm still keep almost unchanged in Fig. 4(c). The magnification of the ligament is given in Fig. 4(d), the nano-pores with 50-80 nm can be clearly observed. Moreover, a layer of particle-like matters appears on the surface of matrix ligament. The layer can be identified to a layer of nanocrystalline copper with the thickness of dozens of nanometers, as the inset HR-TEM image presented in Fig. 4(e), and the grain size is about 5-10 nm. Meanwhile, the SAED pattern of the ligament is given in the Fig. 4(f), polycrystalline rings corresponding to Cu still exist. However, diffraction rings corresponding to the planes of γ and β phases can be clearly observed, indicating that large amount of β and γ phases formed after heat-treatment. Moreover, The formation of β and γ phases can be explained by the diffusion of Zn and Al atoms from the non-corroded substrate to the outer porous pure Cu during heat-treatment, as can be demonstrated by the EDS result in Fig. S4(b). Consequently, 3D hierarchical porous Cu/β/γ composite current collector can be fabricated by one-step dealloying and heat-treatment, as schematically shown in Fig.1. Moreover, because of the existence of non-corroded substrate, the as-prepared 3D hierarchical

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porous Cu/β/γ composite current collector is as flexible as the commercial planar Cu foil, as shown in Fig. S8.

Fig.4 The microstructure of the sample dealloyed for 90min after heat treatment: (a) SEM image from top view; (b) SEM image from sectional view; (c) TEM image in low magnification; (d) TEM image in higher magnification; (e)HR-TEM image; (f) SAED of the selected region D in Fig. (d).

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To verify the enhanced ligament after heat-treatment, a nano-indentation test was conducted to compare the hardness of the 3D hierarchical porous current collector before and after heat-treatment. From Fig.5, the indentation depth of the 3D hierarchical porous Cu/β/γ composite current collector is 1563 nm at a load of 5 mN, while the depth of the 3D hierarchical porous Cu current collector is 1833 nm, which means a higher hardness of the 3D hierarchical porous Cu/β/γ composite current collector than the 3D hierarchical porous Cu current collector. The inset table shows the hardness of different porous current collectors in Fig.5. As can be seen, the hardness of the 3D hierarchical porous Cu/β/γ composite current collector can reach 33.2 Hv, which is nearly twice as high as for 3D hierarchical porous Cu current collector. The improved hardness after heat-treatment is attributed to the formation of β and γ alloy layers, which would act as strain buffer layer to resist plastic deformation during cycling and improve the cycling performance of Sn anodes. Furthermore, the conductivity of the hierarchical porous Cu/β/γ composite current collector was measured to 10400000 S/m, which is much higher than that of the sample before heat treatment (about 3300000 S/m). This is attributed to the well crystallized of Cu. It should be pointed out that although the conductivity of the hierarchical porous Cu/β/γ composite current collector is only 1/5 of common pure Cu foil, it still exhibits 3 orders of magnitudes higher than that of common conductive carbon black or carbon paper current collector31, as shown in Table 1.

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Fig. 5 Nano-indentation results of the 3D hierarchical porous current collector before and after heat-treatment.

Table 1 Electric conductivity of the 3D hierarchical porous Cu-based composite and other reported current collectors Current collector

Electric conductivity （S·m-1）

Copper foil 3D hierarchical porous Cu/β/γ composite after heat treatment 3D hierarchical porous Cu before heat treatment Carbon paper 31 Carbon black 31

59000000 10400000 3300000 4800 1000

In order to access the validity of the hierarchical porous Cu/β/γ composite current collector with enhanced ligaments to further accommodate huge volume expansion of Sn anodes in LIBs, Sn was electroless plated on the sample after heat-treatment in comparison with the sample without heat-treatment and common copper foil. The surface morphologies of three Sn-based anode electrodes are given in Fig. 6(a)-(c). It can be clearly seen that some particle-like materials are uniformly distributed on the surfaces of Cu-based current collectors.

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These particles can be confirmed to be Sn according to the XRD patterns in Fig. S9. However, the Sn particle size on the porous current collector (Fig. 6(a) and 6(b)) is about 100-200nm, obviously smaller than that on the copper foil (Fig. 6(c)). This may attribute to the restriction role of nano-sized pores. Moreover, for both porous current collectors, most of the nano-sized pores are filled with Sn particles, while the micro-channels still preserve with a layer of nano-sized Sn formed on the surface. Additionally, electrochemical impedance spectroscopy (EIS) measurements were conducted to further study the inner resistance of these three different Sn electrodes. The EIS spectra(given in Fig. 6(d)) exhibit a characteristic semicircle in the high and medium frequency ranges and an inclined line in the low-frequency region. The diameter of the semicircle is related to the interfacial charge transfer resistance, and the inclined line is attributed to the Warburg impedance, which is related to the Li+ diffusion kinetics in the electrode32. As can be seen, the diameter of the semicircles for the Sn electrodes with hierarchical porous current collector are obviously smaller than the Sn electrodes with planar Cu foil, indicating the well designed porous structure and good contact between anode materials and current collector could effectively enhance the electrical conductivity .

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Fig. 6 SEM images of the microstructure of different current collectors after electroless plating of Sn at room temperature for 1min, (a) 3D hierarchical porous Cu/β/γ composite ; (b) 3D hierarchical porous Cu; (c) common copper foil, and EIS curves of Sn electrodes with different current collector(d).

Fig.7 (a) show the charge-discharge curves of the 1st, 2nd, and 50th cycle of Sn composite electrode using 3D hierarchical porous Cu/β/γ composite as current collector that tested at a current density of 0.1 mA·cm-2 between 1.5 V and 0.01 V. According to the 1st and 50th discharge curves of Sn composite electrode and their corresponding differential curve displayed in Fig. 7(b), reversible plateau around 0.4 V in the negative current direction corresponds to lithium reacted with Sn to form LixSn phases during discharge, and reproducible plateaus around 0.6 V, 0.7 V and 0.8 V in the positive current direction correspond to the Li+ delithiation from LixSn phases during charge33. However, an irreversible plateau about 1.2 V in the first discharge curve is assigned to the formation of SEI layer due to the decomposition of electrolyte on the Sn surface34. The first discharge capacity of Sn composite electrode using 3D hierarchical porous Cu/β/γ current collector is about 1.45 mAh·cm-2 (the measured mass of Sn on the electrode is about 0.6 mg, the mass specific capacity is estimated to about 942 mAh·g-1) which is 4 times higher than using common

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copper foil as current collector (Fig. 7(d)). After the 1st cycle, the discharge capacity of Sn electrode using hierarchical porous Cu/β/γ as current collector still remains 1.16 mAh·cm-2 (~754 mAh·g-1), which is 80.0% of the initial capacity. However, for Sn electrodes with porous Cu and copper foil as a current collector, their capacity reduced to 77.8% (Fig. 7(c)) and 27.9% (Fig. 7(d)) of their initial capacities, respectively. Fig. 7(e) shows the cycling performance of Sn composite electrodes using three different current collectors, it can be seen that the Sn composite electrode using 3D hierarchical porous Cu/β/γ current collector exhibit a more stable cycling performance compared with the other two composite electrodes. However, the Sn electrode using 3D porous Cu as current collector shows a downward trend of the discharge capacity during cycling. After 100th cycle, the discharge capacity retention of Sn composite electrode using 3D hierarchical porous Cu/β/γ current collector is 0.93 mAh·cm-2 (~604 mAh·g-1), which is 64.1% of the initial capacity. While for the Sn electrodes using 3D porous Cu and common Cu foil as current collectors, their capacity retention sharply decreases to 15.6% and 13.3% of their initial capacity respectively. Moreover, even at a higher current density of 0.5 mA·cm-2, the capacity of Sn electrode using 3D hierarchical porous Cu/β/γ composite current collector still maintains at a capacity of 0.73 mAh·cm-2 after 50 cycles, 51.4% capacity retention of its initial capacity (Fig. 7(f)).

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Fig.7 Charge-discharge curves of Sn electrodes with 3D hierarchical porous Cu/β/γ composite current collector (a) and its differential curves (b); compared with Sn electrode with 3D hierarchical porous Cu current collector (c) and copper foil (d); The cycling performance of Sn electrode with different current collectors (e) and the cycling performance of Sn electrode with 3D hierarchical porous Cu/β/γ composite current collector under different current densities (f).

In order to further investigate the reason for causing sharply reduced capacity of Sn

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electrodes using the 3D porous Cu current collector, SEM images of different electrodes after 50 cycles are given in Fig. 8. The cracks can be distinctly observed on the ligaments of the 3D porous Cu current collector (Fig. 8(a)) while the Sn-coated hierarchical porous Cu/β/γ composite electrode still remains intact (Fig. 8(b)). Moreover, EIS measurements were conducted to further study the impedance variation of the Sn-coated hierarchical porous Cu electrode and the Sn-coated hierarchical porous Cu/β/γ composite electrode after cycling. As shown in Fig.8(c), the Sn coated hierarchical porous Cu electrode exhibits a rapid increase in the impedance after 50th cycles, while the diameter of the semicircle for the Sn-coated hierarchical porous Cu/β/γ composite electrode increases slightly(Fig.8(d)). The notably increased diameter of the semicircle for the Sn-coated hierarchical porous Cu electrode indicates the sharply increased resistance of interfacial charge transfer, which can further demonstrate the destruction of the porous structure in porous Cu current collector after cycling. But because of the enhanced ligaments, resistance of interfacial charge transfer of the Sn-coated hierarchical porous Cu/β/γ composite electrode is only slightly increased, indicating the stability of its porous structure. Thus, the better cycle stability of the Sn-coated 3D hierarchical porous Cu/β/γ composite electrode reveals the synergistic effect of both 3D hierarchical porous structure and composite Cu-based alloys phases in the ligaments. On the one hand, the nano-pores distributed on micro-channel walls can enlarge the specific surface area that can load more active Sn anodes, and the particle size of Sn can be restricted to be nano-sized that can reduce the absolute volume expansion during cycling, as well as ensure good electric contact between Sn and current collector; On the other hand, the micro-channels can facilitate the ion fast diffusion

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and mass transportation, as well as relieve the huge volume expansion of Sn during cycling. It is worth to note that the formation of β and γ composite layer after heat-treatment can accommodate more strain due to the higher strength than pure Cu. In addition, the formation of nanocrystalline pure Cu layer on the surface can improve the conductivity of 3D porous current collector.

Fig. 8 SEM images of the Sn electrodes with 3D hierarchical porous Cu current collector (a) and with Cu/β/γ composite current collector (b) after 50 cycles, and EIS curves showing the impedance variation of the Sn electrodes with 3D hierarchical porous Cu current collector(c) and with Cu/β/γ composite current collector(d) after 50 cycles.

4.Conclusions In summary, a novel 3D hierarchical porous Cu-based composite with a layer of nanocrystalline Cu, β-(CuZn) and γ-(CuZn) phases can be fabricated through one-step 22


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dealloying of Cu-34Zn-6Al melt-spun ribbons in 5 wt.% HCl-FeCl3 solution and subsequent heat treatment. The porous structure of the as-dealloyed samples without heat treatment is 3D, open and bicontinuous with hierarchical ligament-pores structure, which is composed of interconnected micro-scale channels of 0.5-1 µm and nano-pores of 50-80 nm distributed in the ligaments. Through the subsequent heat treatment, the 3D hierarchical porous structure can be well persevered, TEM and EDS investigations confirmed that zinc and aluminum atoms from the non-corroded substrate can diffuse into the porous structure to form a layer of β and γ composite phases, moreover the outer pure Cu layer is well crystallized. The formation of β and γ composite phase can help improve the hardness of the ligaments to about 2 times of pure Cu ligaments and the well crystallized Cu can further improve the conductivity of the 3D porous current collector, which are beneficial for electronic transmission. It is found that using the novel 3D hierarchical porous Cu/β/γ composite as a current collector for binder-free Sn anode in lithium ion batteries shows significantly higher cycle stability than using porous Cu and common Cu foil as current collectors.

Acknowledgments This work was supported by the Foundation for National Natural Science Foundation of China (No. 51571090 and No. 51671088), Foundation for Innovative Research Groups of National Natural Science Foundation of China (No. 51621001), Training Program of Major Basic Research Project of Provincial Natural Science Foundation of Guangdong (2017B030308001), and the Fundamental Research Funds for the Central Universities (2017ZD009).

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Appendix A. Supporting Information Supporting information for publication related to this article is free of charge on the online version. Ternary phase diagram of Cu-Zn-Al alloy; Macro-photograph of Cu-34Zn-6Al (wt.%) melt-spun sample; SEM images showing the microstructure evolution of the precursors during dealloying; XRD patterns of Cu-34Zn-6Al starting ribbon dealloyed in the 5 wt.% HCl-FeCl3 solution at room temperature for 30, 90, and 180 min; The peak intensity of (633) plane for γ phase and (211) plane for β after dealloying for different times; EDS results of the as-dealloyed sample before and after heat-treatment; N2 adsorption/desorption isotherms of the 3D hierarchical porous Cu current collector and the 3D hierarchical porous Cu/β/γ composite current collector after heat-treatment; Metallographic image of the precursor ingots after annealing, TEM bright field image of the non-corroded substrate of the ribbons dealloyed for 90 min and its SAED pattern; Macro-photograph of the 3D hierarchical porous Cu/β/γ composite showing its flexibility in comparison with a planar Cu foil; XRD patterns of the different current collectors after electroless plating of Sn.

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Table and Figure captions Table 1 Electric conductivity of the 3D hierarchical porous Cu-based composite and other reported current collectors Fig. 1 Schematic diagram showing the fabricating processing of 3D hierarchical porous Cu-based alloy composite current collector. Fig. 2 The microstructure of the as-dealloyed samples in 5 wt.% HCl-FeCl3 solution at RT for 90min: (a)cross-sectional view of SEM image; (b)top view of SEM image; (c) cross-sectional FIB SEM image showing the 3D porous layer; (d) TEM bright field image; (e) its SAED pattern of region C in Fig. (d); and (f) HR-TEM image corresponding to the selected region D in Fig. (d). Fig.3 XRD patterns of the sample dealloyed for 90min before and after heat treatment (a) and its magnification view in the selected diffraction angle of 70~85o (b). Fig. 4 The microstructure of the sample dealloyed for 90min after heat treatment: (a) SEM image from top view; (b) SEM image from sectional view; (c) TEM image in low magnification; (d) TEM image in higher magnification; (e)HR-TEM image; (f) SAED of the selected region D in Fig. (d). Fig. 5 Nano-indentation results of the 3D hierarchical porous current collector before and after heat-treatment. Fig. 6 SEM images of the microstructure of different current collectors after electroless plating of Sn at room temperature for 1min, (a) 3D hierarchical porous Cu/β/γ composite ; (b) 3D hierarchical porous Cu; (c) common copper foil, and and EIS curves of Sn electrodes with different current collector(d).

Fig. 7 Charge-discharge curves of Sn electrodes with 3D hierarchical porous Cu/β/γ composite current collector (a) and its differential curves (b); compared with Sn electrode with 3D hierarchical porous Cu current collector (c) and copper foil (d); The cycling performance of Sn electrode with different current collectors (e) and the cycling performance of Sn electrode with 3D hierarchical porous Cu/β/γ composite current collector under different current densities (f). Fig. 8 SEM images of the Sn electrodes with 3D hierarchical porous Cu current collector (a) and with Cu/β/γ composite current collector (b) after 50 cycles, and EIS curves showing the impedance variation of the Sn electrodes with 3D hierarchical porous Cu current collector(c) and with Cu/β/γ composite current collector(d) after 50 cycles.

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Graphical abstract

81x110mm (120 x 120 DPI)

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3D hierarchical porous Cu-based composite ... - ACS Publications

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