Cu3Ge composite and its high performances towards lithium storage

Journal of Colloid and Interface Science 539 (2019) 665–671

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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis

Regular Article

Easy preparation of nanoporous Ge/Cu3Ge composite and its high performances towards lithium storage Qin Hao a, Qiang Liu a, Yuanyuan Zhang a, Caixia Xu a,⇑, Jiagang Hou b,⇑ a b

Institute for Advanced Interdisciplinary Research, School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, Shandong Province, China

g r a p h i c a l a b s t r a c t Nanoporous Ge/Cu3Ge composite is easily prepared through etching GeCuAl alloy in NaOH solution, which exhibits satisfactory electrochemical performance as an advanced anode material for Li-ion batteries in terms of high specific capacity, excellent rate capability, as well as outstanding cycling stability compared with pure porous Ge.

a r t i c l e

i n f o

Article history: Received 13 October 2018 Revised 28 December 2018 Accepted 29 December 2018 Available online 31 December 2018 Keywords: Germanium Nanoporous Dealloying Anode Lithium ion batteries

a b s t r a c t Nanoporous Ge/Cu3Ge composite is fabricated simply through selective dealloying of GeCuAl precursor alloy in dilute alkaline solution. The as-made Ge/Cu3Ge is characterized by three dimensional (3D) bicontinuous network nanostructure which comprises of substantial nanoscale pore voids and ligaments. Owing to the 3D porous architecture and the introduction of well-conductive Cu3Ge, the lithium storage performances of Ge are dramatically enhanced in terms of higher cycling stability and superior rate performance. Nanoporous Ge/Cu3Ge anode delivers steady capacities above 1000 mA h g 1 upon cycling for 70 loops at 400 mA g 1. In particular, after 300 cycles at the high rate of 3200 mA g 1 the capacity retention for Ge/Cu3Ge is able to reach a maximum of 99.3%. On the contrary, the pure nanoporous Ge encounters severe capacity decay. In view of the outstanding energy storage performances and easy preparation, nanoporous Ge/Cu3Ge exhibits great application potential as an advanced anode in lithium storage related technologies. Ó 2018 Elsevier Inc. All rights reserved.

1. Introduction

⇑ Corresponding authors. E-mail addresses: [email protected] (C. Xu), [email protected] (J. Hou). https://doi.org/10.1016/j.jcis.2018.12.104 0021-9797/Ó 2018 Elsevier Inc. All rights reserved.

Lithium-ion batteries (LIBs) have been widely employed as portable electronic devices and even the vehicle power supply due to their rapid charge and discharge abilities, long cycle life,

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and environmental friendliness. However, with the increasing demands for much higher power and energy output, considerable efforts have been devoted to searching for alternatives to replace low-capacity graphite anodes with those of higher capacity and cycling stability [1]. In recent years, group IVA elements have drawn more and more attention as the alternative anode candidates due to their high theoretical capacities, especially Si with the theoretical capacity as high as 4200 mA h g 1 [2]. Ge, in comparison, possesses a relatively low theoretical capacity of 1600 mA h g 1 [3]. However, the lithium ion diffusivity in Ge is 400 times faster and the electron conductivity is 104 times higher than Si [4,5]. Unfortunately, Ge also suffers from a drastic volume change during the Li-alloying process similar to Si, usually resulting in fast capacity decay. Engineering the nanostructure for the Ge anode materials offers abundant opportunities to address the challenges as mentioned above in terms of shortened Li+ diffusion distance, enhanced electroactivity, and relieved volume destroys [6]. Up to now, numerous nanostructured Ge materials have been explored to improve the lithium storage performances of Ge, inclusive of nanoparticles, nanowires, nanotubes, and porous structures, etc [7–11]. Hereinto, porous nanostructure represents one of the most preferable nanoscale materials. It provides fluent transport paths for both Li ions and electrons along the integrated network skeleton, as well as sufficient buffer space to mitigate the volume expansion [12,13]. For example, Liu et al. discovered that the macroporous Ge possessed much improved cycling stability and rate performances as compared with the Ge nanoparticle film [14]. So far, there are some methods which have been used to prepare porous Ge materials, but usually involve complicated procedures or/and high temperature operation, such as metallothermic reduction above 650 °C under a protective atmosphere [15], and the removal of SiO2 template in toxic HF solution [16,17]. Consequently, it is essential to develop a convenient, controllable, and environmentally benign approach to fabricate the porous Ge anodes. At present, the dealloying method has been proven versatile and effective in the preparation of a variety of porous nanostructured materials [18– 20]. The dealloyed products always present an interesting bulky three dimensional (3D) nanoporous architecture which consists of numerous interlinked nanoscale pore channels and ligaments as building blocks. More importantly, the dealloying process has the advantages of convenient and safe operation, absolute yielding, and controllable components for mass preparation. In addition, combining Ge nanomaterials with a wellconductive substance to form hybrid composites is another efficient approach to further boost the lithium storage performances of Ge on account of both alleviation of the volume change over cycling and the modified electrical conductivity. For instance, Ge nanowires coated with few layers of graphene exhibit much better rate capability than pure Ge nanowires at the current density of 20 C [21]. Ge/Cu3Ge-based anode materials have been attracting growing research interests considering that the introduction of Cu3Ge can effectively improve the electrical conductivity of Ge electrode as well as cushion the volume variation in order to obtain the substantial performance enhancements as compared with pure Ge [22–25]. Inspired by the aforementioned statement and pioneering researches, our focus is on the fabrication of nanoporous Ge/Cu3Ge composite through facile dealloying of the well-designed GeCuAl precursor alloy. Selectively removing the more reactive Al atoms from GeCuAl alloy will generate nanoporous (NP) Ge/Cu3Ge composite, which is made up of 3D interconnected nanoscale skeleton and rich open channels. Compared with the pure NP Ge anode, the NP Ge/Cu3Ge exhibits much higher reversible capacities, enhanced cycling stability, and superior rate capability for lithium storage. In the light of the advantages of simple and scalable fabrication as

well as the outstanding energy storage performances, the asmade Ge/Cu3Ge composite manifests promising application potential as an alternative anode candidate for LIBs. 2. Experimental section 2.1. Sample preparation All reagents (purchased from Shanghai Sinopharm Chemical Reagent Ltd. Co of China) with analytical grade were used without

Fig. 1. XRD patterns of the Ge8.5Cu1.5Al90 ternary alloy and the dealloying product obtained by immersing GeCuAl alloy in 0.5 mol L 1 NaOH solution for 24 h. The standard patterns of pure Ge, Al, and Cu3Ge are attached for comparison.

Fig. 2. XPS data of (a) Ge 3d and (b) Cu 2p for the Ge/Cu3Ge composite.

Q. Hao et al. / Journal of Colloid and Interface Science 539 (2019) 665–671

further purification. Certain quantities of Ge, Cu, and Al were weighed in accordance to the atomic ratio of 8.5:1.5:90. Thereafter, these metals were refined in a medium frequency induction furnace and further underwent a melt-spinning process at the speed of 1600 r/s by a single roller melt-spinning instrument under Ar atmosphere. Subsequently, the Ge8.5Cu1.5Al90 alloy foils with the thickness of 50 lm were immersed in 0.5 mol L 1 NaOH solution for 24 h at room temperature. Ge10Zn90 alloy foils were also prepared through the same procedures, and further etched in 2 mol L 1 aqueous solution of ammonia for 30 h to obtain the pure porous Ge material. Finally, the dealloyed samples were washed for several times with ultra-pure water and dried at room temperature in the air. The reproducibility of the dealloying process was confirmed by repeated experiments.

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D8 advanced X-ray diffractometer using Cu Ka radiation at 0.04° s 1. The morphology and composition were characterized by field emission scanning electron microscopy (SEM, JEOL JSM7600F) with an energy-dispersive X-ray spectrometer. Elemental mapping analysis was operated by a FEI QUANTA FEG250 scanning electron microscope equipped with an INCA Energy X-MAX-50 Xray spectroscopy analyzer. Transmission electron microscopy (TEM) detection was conducted in a JEM-2100 high resolution transmission electron microscope (200 kV). X-ray photoelectron spectrometer was operated on a Thermo Scientific ESCALAB 250 X-ray photo electronic spectrometer, using a monochromatized Mg Ka X-ray as the excitation source and C1s (284.60 eV) as the reference line. 2.3. Electrochemical tests

2.2. Characterization The structures of the source alloy and the dealloyed product were analyzed by X-ray diffraction (XRD) technique in a Bruker

The as-made Ge/Cu3Ge and Ge samples were used as the active materials, and further mixed with sodium alginate and super p in a weight ratio of 6:2:2 in ultrapure water. Subsequently, the

Fig. 3. (a–c) SEM images, (d & e) element mapping, and (f) TEM image of the Ge/Cu3Ge composite.

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obtained slurry was smeared onto Cu foil and dried under vacuum at 80 °C for 8 h to prepare the working electrode. For assembling the CR 2032 coin-type cells, Cellgard 2300 was used as the separator with lithium metal foil as the reference electrode, and 1 M LiPF6 mixed solution as the electrolyte (in ethylene carbonate, dimethyl carbonate, and ethylene methyl carbonate with a volumetric ratio of 1:1:1 and 3 wt% vinylene carbonate). The cells were galvanostatically cycled between 0.01 and 1.5 V (vs. Li/Li+) at various densities using a NEWARE BTS 5 V-5 mA computer-controlled galvanostat. All the electrochemical parallel tests were carried out using at least five cells in order to ensure the reliability of the corresponding performance. Electrochemical impedance spectroscopy (EIS) was tested using a Princeton Applied Research spectrometer (0.01–100 kHz). The cyclic voltammetry (CV) tests were operated on CHI 760D electrochemical workstation (Shanghai CH Instruments Co., China) with a scan rate of 0.1 mV s 1 in the range of 0.01–1.5 V.

3. Results and discussions Dealloying method has been proven highly efficient and reproducible in creating functional porous nanomaterials [18–20]. Taking into account the low cost and easy reaction of Al with alkaline solution, Ge8.5Cu1.5Al90 ternary alloy was designed as the precursor. XRD technique was first utilized to examine the structures of the GeCuAl alloy and the resulting dealloyed product. As shown in Fig. 1, in doing aprecise comparison, the three sets of diffraction peaks for the GeCuAl alloy can be indexed to the phase structure of Cu3Ge, pure Ge, and Al, respectively. After dealloying in 0.5 mol L 1 NaOH solution for 24 h, the diffraction peaks of Al disappeared, implying that Al atoms have been almost completely removed while the Ge and Cu3Ge phase still remained.

X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical state of Ge and Cu elements in the dealloyed product. The core level region for the Ge 3d in Fig. 2a are characterized by two peaks locating at 29.9 eV and 32.5 eV, which can be ascribed to metallic and oxidized Ge, respectively, where the oxidation state of Ge may be produced due to the slight oxidation of surface Ge atoms during the dealloying and drying process [26]. As presented in Fig. 2b, the Cu 2p core level region reveals the strong Cu 2p peaks around 933.3 and 953.2 eV, which can be assigned to the metallic Cu. There is a possibility that few CuI coexisted with metallic Cu because of the reactive property of Cu with oxidative objects, but this cannot be absolutely prevented due to their strong binding and a slight difference by only 0.1 eV [27]. According to the experimental observations above [22,25], the Ge/Cu3Ge composite has been successfully generated by dealloying the GeCuAl alloy in NaOH solution. SEM was first employed to provide the morphology information of the Ge/Cu3Ge composite. As displayed in Fig. 3a, upon etching Al from GeCuAl source alloy, a bulky porous structure generated with 3D interconnected network skeleton and rich porosity penetrated through the whole sample. Another high magnification SEM image in Fig. 3b illustrates that the resulting porous structure has the typical pore size distribution at 200 nm in diameter. Energy dispersive X-ray spectroscopy (EDS) was used to analyze the elemental composition of the Ge/Cu3Ge composite. As indicated in Fig. S1, a majority of Al atoms have dissolved, meanwhile the atomic ratio of Ge to Cu keeps at 85:15, which is well consistent with its predetermined ratio in the Ge8.5Cu1.5Al90 precursor alloy. Accordingly, the mole ratio of Ge to Cu3Ge is about 94:6. The results above suggest the highly controllable feature for the eventual component by this simple dealloying method. The location and identity of Cu and Ge elements in the Ge/Cu3Ge product were detected by element mapping. As shown in Fig. 3c–e, the elements of Ge and Cu dis-

Fig. 4. (a) Cycling performances, (b) CEs, (c) CV curves, and (d) Nyquist profiles of Ge/Cu3Ge and Ge electrodes.

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cycles. As a contrast, the NP Ge/Cu3Ge anode delivers a lower initial capacity of 1762.2 mA h g 1, however, it maintains a steady capacity of about 1000 mA h g 1 after the second cycle, demonstrating a capacity retentions of 57.1% from the initial capacity and 96.5% relative to that in the 3nd cycle. Fig. 4b further provides the corresponding coulombic efficiencies (CEs) of both electrodes. It is clearly stated that the pure Ge shows a low initial CE of 42.5%, while the Ge/Cu3Ge composite performs at a much higher value of 63.5%, suggesting the dramatically enhanced reversibility at the first cycle upon the introduction of Cu3Ge phase. In order to further understand its electrochemical behaviors for lithium storage, Fig. 4c shows the first three CV curves of the Ge/ Cu3Ge electrode. Among the three cathodic curves, there is an evident difference in the potential region below 0.25 V, which stems from the irreversible formation of the SEI film and amorphous LixGe species induced by the lithiation process [28–30]. Additionally, these three repeatable peaks, which are located at 0.49, 0.36, and 0.15 V respectively, suggest a multi-step alloying process of Ge with lithium to form LixGe alloys [29,30]. During the anodic scan, the delithiation peaks at about 0.4 and 0.6 V can be assigned to the gradual extraction of lithium from the Li-Ge alloys [15,31,32]. Most importantly, the CV curves show the highly overlapped feature after the first cycle, which illustrates the good reversibility of the lithiation-delithiation reaction in NP Ge/Cu3Ge anode. To verify the effect of Cu3Ge on the conductivity of the Ge electrode, EIS measurements for both electrodes were conducted at an open circuit voltage state. As shown in Fig. 4d, the Nyquist curves for NP Ge/Cu3Ge and Ge anodes are much different, where the depressed semicircles in the high-frequency region corresponds to the charge transfer impedance in the electrode/electrolyte interface [33]. Evidently, the diameter of the semicircle for Ge/Cu3Ge anode is much smaller than that for Ge, which indicates the more effective electric connectivity due to the introduction of Cu3Ge, thus resulting in the better lithium storage performances. The galvanostatic cycling test was further carried out at 3200 mA g 1 to evaluate the influence of Cu3Ge at high current density for lithium storage. For the purpose of activating the Ge and Ge/Cu3Ge electrodes, they were charged and discharged at 160 mA g 1 for the first 10 cycles. As seen in Fig. 5a, the pure Ge anode delivers a high initial capacity above 2200 mA h g 1 but eventually undergoes a severe capacity decline, maintaining only 126.4 mA h g 1 at 300th cycle. In contrast, the Ge/Cu3Ge shows dramatically enhanced cycling reversibility, holding a capacity retention up to 99.3% against that at 11th cycle at 3200 mA g 1. In addition, the corresponding CEs of both anodes in Fig. 5b also suggest that the initial CE of Ge electrode is greatly enhanced due to the Cu3Ge incorporation. More importantly, the Ge/Cu3Ge anode still shows a high CE of 92.1% when the rate is increased tremendously to 3200 mA g 1 at 11th cycle, far beyond the value of 30.2% for the pure Ge. More detailed lithium storage performances are summarized in Table 1. Rate capability is also an important parameter for the practical application of any anode material. Therefore, the rate performances of both NP Ge/Cu3Ge and Ge electrodes were tested in discrete steps from 100 to 2000 mA g 1, which is illustrated in Fig. 6.

tribute uniformly in the whole NP structure, indicating the homogeneous intergrowth of Ge and Cu3Ge during the self-assembling process of dealloying. TEM image provides further details for this structure. As shown in Fig. 3f, the dark skeletons provide verification in the formation of 3D interconnected network structure at nanoscale, while the inner bright regions prove the formation of hollow pore channels embedded in the ligament network. The 3D bicontinuous nanoporous structure and the introduction of the well-conductive Cu3Ge are favorable for the achievement of high lithium storage performances of Ge. Consequently, it is interesting to explore the lithium storage behaviors of the Ge/Cu3Ge nanocomposite. For the clear insights on the effects of Cu3Ge addition, the pure porous Ge was also prepared by the dealloying method as shown in Fig. S2. The cycling stabilities of both electrodes were first evaluated at 400 mA g 1 for 70 cycles. As depicted in Fig. 4a, the initial capacity of NP Ge is 1901.5 mA h g 1, which is higher than the theoretical capacity of Ge anode [3]. It should be attributed to the capacity contribution from the formation of solid electrolyte interphase (SEI) film. Subsequently, the capacity experiences a rapid fading with only 25.7% being preserved after 70

Fig. 5. (a) Cycling performances and (b) CEs of the Ge/Cu3Ge and Ge electrodes at 3200 mA g 1 (the current density during the first 10 cycles is 160 mA g 1).

Table 1 The detailed performances of the NP Ge/Cu3Ge and Ge anodes tested at 3200 mA g conducted based on the electrochemical parallel tests using five cells.). Sample

NP Ge/Cu3Ge NP Ge

160 mA g

1

3200 mA g

1

(The current density during the first 10 cycles is 160 mA g

1

1

. And the error analysis is

Capacity retention

1st capacity (mA h g 1)

1st CE (%)

11th capacity (mA h g 1)

11th CE (%)

300th capacity (mA h g 1)

Against 1st cycle (%)

Against 11th cycle (%)

1845.4 ± 60.2 2228.2 ± 81.3

67.3 ± 0.3 43.6 ± 0.2

859.6 ± 38.7 418.3 ± 37.6

92.1 ± 0.1 57.4 ± 0.1

853.6 ± 15.8 126.4 ± 5.3

46.3 ± 0.2 5.67 ± 0.1

99.3 ± 0.1 30.2 ± 0.1

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operation realizes the simultaneous construction of rich porosity and introduction of Cu3Ge without the usage of extra toxic agents and complicated procedures. Benifiting from the 3D interconnected network skeleton, rich pore voids, as well as the incorporation of well-conductive Cu3Ge, the lithium storage performances of Ge were dramatically improved in terms of unique cycling stability and exceptional rate capability. In particular, the as-made Ge/Cu3Ge composite shows the superior lithium storage performances compared with some other porous Ge and Ge/Cu3Ge-based anodes previously reported. Our work presents the attractive application prospect of nanoporous Ge/Cu3Ge composite as the advanced anode material for LIBs, and provides the expanded opportunities for the design and achievement of high-performance energy storage materials through the dealloying strategy.

Acknowledgments Fig. 6. Rate performances of the Ge/Cu3Ge and Ge electrodes.

When the rates are increased gradually from 100 to 300, 500, 1000, and 2000 mA g 1 in various stages, the Ge/Cu3Ge anode exhibits the stable capacities at about 1200, 1100, 1070, 1050, and 900 mA h g 1, respectively. Once the current density is switched back to 100 mA g 1, the Ge/Cu3Ge maintains a capacity of 1175.7 mA h g 1, which recovers approximately 99.7% of the capacity in relative to that at 10th cycle under the initial 100 mA g 1. By comparison, the pure Ge performs much poorer rate performances. Note that its capacities are only about 340 and 230 mA h g 1 at 1000 and 2000 mA g 1 respectively. Especially, the pure Ge holds a low capacity of 495.3 mA h g 1 when the rate is set back to 100 mA g 1, regaining only 58.8% against the capacity at 10th cycle. What’s more, the as-made NP Ge/Cu3Ge composite presents superior lithium storage performances as compared with many reported porous Ge and other Ge/Cu3Ge-based materials. For example, the Ge/Cu3Ge nanoparticles only possessed the capacity of 608 mA h g 1 after being cycled at 138.4 mA g 1 for 10 cycles, 276.8 mA g 1 for 10 cycles, and 1384 mA g 1 for 80 cycles in sequence [22]. The Ge/Cu3Ge/C powder delivered an initial capacity of 1150 mA h g 1 at a low current density of 100 mA g 1, and remained only 500 mA h g 1 at 50th cycle [23]. The macroporous Ge exhibited a large capacity loss from 1748 to 844 mA h g 1 at 320 mA g 1 after 300 cycles [14]. In contrast, our Ge/Cu3Ge material can still maintain a highly reversible capacity of about 860 mA h g 1 after 300 cycles at a high rate up to 3200 mA g 1. It has been verified by an in situ dilatometry experiment which concludes that Cu3Ge incorporation can effectively alleviate the volume destroy of Ge/C electrode [22]. Therefore, the wellconductive Cu3Ge in the as-made Ge/Cu3Ge composite not only directly modifies the electrical conductivity but also acts as a buffer to alleviate the absolute volume change of Ge during long term charging/discharging cycling. In addition, the 3D bulky porous structure can provide the fluent transfer channels for the delivery of both Li ions and electrons. Meanwhile, the open voids and rich porosity enable the comprehensive contact of electrolytes and ions with the electroactive sites [17,34–36]. It is conclusive that the as-made Ge/Cu3Ge composite possesses superior electrochemical performances towards lithium storage as compared with the NP Ge and many reported similar materials. 4. Conclusions In summary, nanoporous Ge/Cu3Ge composite was fabricated successfully by a simple and green dealloying approach with the only use of common alkaline solution. The one-step dealloying

This work was supported by National Natural Science Foundation of China (51772133) and Shandong Province (ZR2017JL022), China postdoctoral science foundation (2017M622117), and the program for Taishan Scholar of Shandong province (ts201712048).

Appendix A. Supplementary material Supplementary data related to this article (EDS result of the Ge/ Cu3Ge composite and SEM image of the pure Ge). Supplementary data to this article can be found online at https://doi.org/10. 1016/j.jcis.2018.12.104.

References [1] D.X. Cao, Y.Z. Dai, S.M. Xie, H.K. Wang, C.M. Niu, Pyrolytic synthesis of MoO3 nanoplates within foam-like carbon nanoflakes for enhanced lithium ion storage, J. Colloid Interface Sci. 514 (2018) 686–693. [2] S. Prakash, C.F. Zhang, J.D. Park, F. Razmjooei, J.S. Yu, Silicon core-mesoporous shell carbon spheres as high stability lithium ion battery anode, J. Colloid Interface Sci. 534 (2019) 47–54. [3] Z.L. Hu, S. Zhang, C.J. Zhang, G.L. Cui, High performance germanium-based anode materials, Coordin. Chem. Rev. 326 (2016) 34–85. [4] J. Graetz, C.C. Ahn, R. Yazami, B. Fultz, Nanocrystalline and thin film germanium electrodes with high lithium capacity and high rate capabilities, J. Electrochem. Soc. 151 (2004) A698–A702. [5] D. Wang, Y.L. Chang, Q. Wang, J. Cao, D.B. Farmer, R.G. Gordon, H.J. Dai, Surface chemistry and electrical properties of germanium nanowires, J. Am. Chem. Soc. 126 (2004) 11602–11611. [6] J.W. Qin, M.H. Cao, Multidimensional germanium-based materials as anodes for lithium-ion batteries, Chem. Asian J. 11 (2016) 1169–1182. [7] S.W. Kim, D.T. Ngo, J. Heo, C.N. Park, C.J. Park, Electrodeposited germanium/carbon composite as an anode material for lithium ion batteries, Electrochim. Acta 238 (2017) 319–329. [8] M.H. Park, Y.H. Cho, K. Kim, J. Kim, M. Liu, J. Cho, Germanium nanotubes prepared by using the kirkendall effect as anodes for high-rate lithium batteries, Angew. Chem. Int. Ed. 50 (2011) 9647–9650. [9] S. Choi, Y.G. Cho, J. Kim, N.S. Choi, H.K. Song, G.X. Wang, S. Park, Mesoporous germanium anode materials for lithium-ion battery with exceptional cycling stability in wide temperature range, Small 13 (2017) 1603045. [10] S. Yoon, S.H. Jung, K.N. Jung, S.Gi. Woo, W. Cho, Y.N. Jo, K.Y. Cho, Preparation of nanostructured Ge/GeO2 composite in carbon matrix as an anode material for lithium-ion batteries, Electrochim. Acta 188 (2016) 120–125. [11] A. González-Macías, F. Salazar, A. Miranda, A. Trejo-Baños, L.A. Pérez, E. Carvajal, M. Cruz-Irisson, Lithium effects on the mechanical and electronic properties of germanium nanowires, Nanotechnology 29 (2018). [12] A. Vu, Y.Q. Qian, A. Stein, Porous electrode materials for lithium-ion batterieshow to prepare them and what makes them special, Adv. Energy Mater. 2 (2012) 1056–1085. [13] Y.H. Yang, S. Liu, X.F. Bian, J.K. Feng, Y.L. An, C. Yuan, Morphology- and porosity-tunable synthesis of 3D nanoporous SiGe alloy as a high-performance lithium-ion battery anode, ACS Nano 12 (2018) 2900–2908. [14] X. Liu, J.P. Zhao, J. Hao, B.L. Su, Y. Li, 3D ordered macroporous germanium fabricated by electrodeposition from an ionic liquid and its lithium storage properties, J. Mater. Chem. A 1 (2013) 15076–15081. [15] H.P. Jia, R. Kloepsch, X. He, J.P. Badillo, P.F. Gao, O. Fromm, T. Placke, M. Winter, Reversible storage of lithium in three-dimensional macroporous germanium, Chem. Mater. 26 (2014) 5683–5688.

Q. Hao et al. / Journal of Colloid and Interface Science 539 (2019) 665–671 [16] M.H. Park, K. Kim, J. Kim, J. Cho, Flexible dimensional control of high-capacity Li-ion-battery anodes: from 0D hollow to 3D porous germanium nanoparticle assemblies, Adv. Mater. 22 (2010) 415–418. [17] D. Kwon, J. Ryu, M. Shin, G. Song, D. Hong, K.S. Kim, S. Park, Synthesis of dual porous structured germanium anodes with exceptional lithium-ion storage performance, J. Power Sources 374 (2018) 217–224. [18] C.X. Xu, J.X. Su, X.H. Xu, P.P. Liu, H.J. Zhao, F. Tian, Y. Ding, Low temperature CO oxidation over unsupported nanoporous gold, J. Am. Chem. Soc. 129 (2007) 42–43. [19] H.M. Duan, C.X. Xu, Nanoporous PdZr surface alloy as highly active noneplatinum electrocatalyst toward oxygen reduction reaction with unique structure stability and methanol tolerance, J. Power Sources 316 (2016) 106– 113. [20] S. Liu, J.K. Feng, X.F. Bian, Y.T. Qian, J. Liu, H. Xu, Nanoporous germanium as high-capacity lithium-ion battery anode, Nano Energy 13 (2015) 651–657. [21] H. Kim, Y. Son, C. Park, J. Cho, H.C. Choi, Catalyst-free direct growth of a single to a few layers of graphene on a germanium nanowire for the anode material of a lithium battery, Angew. Chem. 125 (2013) 6113–6117. [22] C.J. Zhang, F.L. Chai, L. Fu, P. Hu, S.P. Pang, G.L. Cui, Lithium storage in a highly conductive Cu3Ge boosted Ge/graphene aerogel, J. Mater. Chem. A 3 (2015) 22552–22556. [23] O.B. Chae, S. Park, J.H. Ku, J.H. Ryu, S.M. Oh, Nano-scale uniform distribution of Ge/Cu3Ge phase and its electrochemical performance for lithium-ion batteries, Electrochim. Acta 55 (2010) 2894–2900. [24] J.W. Liang, X.N. Li, Z.G. Hou, J. Jiang, L. Hu, W.Q. Zhang, Y.C. Zhu, Y.T. Qian, A composite structure of Cu3Ge/Ge/C anode promise better rate property for lithium battery, Small 12 (2016) 6024–6032. [25] X.Y. Liu, N. Lin, K.L. Xu, Y. Han, Y. Lu, Y.Y. Zhao, J.B. Zhou, Z. Yi, C.H. Cao, Y.T. Qian, Cu3Ge/Ge@C nanocomposites crosslinked by the in situ formed carbon nanotubes for high-rate lithium storage, Chem. Eng. J. 352 (2018) 206–213. [26] N. Lin, T. Li, Y. Han, Q. Zhang, T. Xu, Y. Qian, Mesoporous hollow Ge microspheres prepared via molten-salt metallothermic reaction for highperformance Li-storage anode, ACS Appl. Mater. Interfaces 10 (2018) 8399– 8404.

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[27] T. Ghodselahi, M.A. Vesaghi, A. Shafiekhani, A. Baghizadeh, M. Lameii, XPS study of the Cu@Cu2O core-shell nanoparticles, Appl. Surf. Sci. 255 (2008) 2730–2734. [28] C. Zhang, Z. Lin, Z. Yang, D. Xiao, P. Hu, H. Xu, Y. Duan, S. Pang, L. Gu, G. Cui, Hierarchically designed germanium microcubes with high initial coulombic efficiency towards highly reversible lithium storage, Chem. Mater. 27 (2015) 2189–2194. [29] X.N. Li, J.W. Liang, Z.G. Hou, Y.C. Zhu, Y. Wang, Y.T. Qian, A synchronous approach for facile production of Ge-carbon hybrid nanoparticles for highperformance lithium batteries, Chem. Commun. 51 (2015) 3882–3885. [30] D.T. Ngo, H.T. Le, C. Kim, J.Y. Lee, J.G. Fisher, I.D. Kim, C.J. Park, Mass-scalable synthesis of 3D porous germanium-carbon composite particles as an ultrahigh rate anode for lithium ion batteries, Energy Environ. Sci. 8 (2015) 3577– 3588. [31] X.L. Sun, W.P. Si, L.X. Xi, B. Liu, X.J. Liu, C.L. Yan, O.G. Schmidt, In situ-formed, amorphous, oxygen-enabled germanium anode with robust cycle life for reversible lithium storage, ChemElectroChem 2 (2015) 737–742. [32] F.W. Yuan, H.Y. Tuan, Scalable solution-grown high-germanium-nanoparticleloading graphene nanocomposites as high-performance lithium-ion battery electrodes: an example of a graphene-based platform towards practical fullcell applications, Chem. Mater. 26 (2014) 2172–2179. [33] J. Liang, X. Li, Z. Hou, T. Zhang, Y. Zhu, X. Yan, Y. Qian, Honeycomb-like macrogermanium as high-capacity anodes for lithium-ion batteries with good cycling and rate performance, Chem. Mater. 27 (2015) 4156–4164. [34] S. Prakash, C.F. Zhang, J.D. Park, F. Razmjooei, J.S. Yu, Silicon core-mesoporous shell carbon spheres as high stability lithium-ion battery anode, J. Colloid Interface Sci. 534 (2019) 47–54. [35] K.W. Tseng, S.B. Huang, W.C. Chang, H.Y. Tuan, Synthesis of mesoporous germanium phosphide microspheres for high-performance lithium-ion and sodium-ion battery anodes, Chem. Mater. 30 (2018) 4440–4447. [36] J. Lee, Y. Wu, Z.B. Peng, Hetero-nanostructured materials for high-power lithium ion batteries, J. Colloid Interface Sci. 529 (2018) 505–519.