Mesoporous Hollow Ge Microspheres Prepared via Molten-Salt

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Mesoporous Hollow Ge Microspheres Prepared via Molten-Salt Metallothermic Reaction for High-Performance Li-Storage Anode Ning Lin,* Tieqiang Li, Ying Han, Qianliang Zhang, Tianjun Xu, and Yitai Qian* Department of Chemistry, University of Science and Technology of China, Hefei, Anhui Province 230026, P. R. China S Supporting Information *

ABSTRACT: Generally, Ge-based anodes are prepared by metallothermic reduction of GeO2 with Mg at 650 °C. Herein, a molten-salt system is developed a low-temperature metallothermic reduction of GeO2 to prepare nanostructured Ge based anode materials. Typically, mesoporous hollow Ge microspheres are prepared by reduction of GeO2 with metallic Mg in molten ZnCl2 (mp 292) at 350 °C. Monodispersed Ge particles are synthesized through reduction of GeO2 with Mg in molten AlCl3 (mp 192 °C) at 250 °C. The meso-porous Ge anode delivers the reversible capacity of 1291 mA h g−1 at 0.2 C after 150 cycles with a retention of 97.3%, 1217 mA h g−1 at 0.8 C after 400 cycles with a retention of 91.9%, and superior rate capability with a capacity of 673 mA h g−1 even at 10 C. Then, the reaction mechanism and full-cell performance of as-prepared Ge anodes are studied systemically. KEYWORDS: molten salt, metallothermic reduction, mesoporous structure, Ge anode, hollow microspheres Zn at 450 °C exhibits a reversible capacity of ∼1400 mA h g−1 after 300 cycles at a rate of 0.5 C.12 Despite great developments made in the field of fabricating methodology, developing a more facile and simpler synthetic strategy is still significant. Over the past decades, the molten salt system has aroused intense interest as reaction media for inorganic synthetic chemistry. Generally, the inorganic salts could act as catalyst, as simple solvent, or as active reagent to participate in the chemical reaction.13,14 Liu carried out the magnesiothermic reduction of bulk GeO2 in molten KCl/LiCl (mp 353 °C) to prepare Ge crystals above 450 °C.15 The molten chlorides served as a solvent to generate solvated electrons to promote this reduction. Very recently, Qian reported the reduction of SiCl4 or SiO2 into Si nanocrystals even at 200 °C with Mg (or Al) in molten AlCl3 (mp 190 °C).16,17 The molten AlCl3 could effectively promote the reaction to progress at low temperature. Therefore, it is possible to further lower the reaction temperature for reducing GeO2 assisted by molten salt. In this work, we introduce a series of low-temperature molten salt system for metallothermal (Mg or Zn) reduction of GeO2 into nanostructured Ge, as shown in Scheme 1. After optimizing the reaction conditions, mesoporous hollow Ge microspheres composed of fine nanoparticles were prepared by reduction of bulk GeO2 (Figure S1) with metallic Mg in molten ZnCl2 at 350 °C, denoted as Z-Ge. The BET surface area is 110 m2 g−1, with an average pore size of 5.7 nm. Monodispersed Ge particles were obtained through reduction of bulk GeO2 with

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ecently, nanostructured germanium (Ge)-based materials have been studied extensively as anodes for rechargeable Li-ion batteries (LIBs), because Ge anode exhibits a theoretical capacity of 1600 mA h g−1, high lithium ion diffusivity (400 times higher than Si), and high electronic conductivity (104 times higher than Si).1−4 Accordingly, various synthetic strategies were developed for preparing nanostructured Ge materials. Previously, Ge nanoparticles were synthesized via reduction of GeCl4 using strongly reducing reagents including lithium (or sodium)-napthalide, and Zintl salts NaGe (or KGe, Mg2Ge) in organic solvents such as glyme, dimethyl ether, dimethoxyethane, or diglyme at room temperature.5−8 For instance, 3D porous Ge prepared through reduction of GeCl4 with sodium-napthalide and followed by annealing with SiO2 templates delivered a reversible capacity of 1415 mA h g−1 at 1 C after 100 cycles.9 However, the GeCl4 is highly sensitive to moisture and very expensive, which makes these methods above hard to carry out. Therefore, reports on the Ge anode material for LIBs is seldom on the basis of the reduction of GeCl4. On the other hand, metallothermic reduction of air-stable GeO2 with Zn or Mg was one type of widely used strategy to produce nanostructured Ge. For example, mesoporous Ge anode was prepared by zincothermic reduction of GeO2 at 450 °C, exhibiting a specific capacity about 1200 mA h g−1 at current density of 0.5 C after 100 cycles, and an initial Coulombic efficiency over 80%.10 Three-dimensional macroporous Ge prepared by magnesiothermic reduction of GeO2 at 650 °C delivered a reversible capacity of 1131 mAh g−1 at a rate of 1.0 C after 200 cycles, associated with an ICE of 80.3%.11 Mesoporous Ge particles prepared via reduction of GeO2 with © XXXX American Chemical Society

Received: January 11, 2018 Accepted: February 26, 2018 Published: February 26, 2018 A

DOI: 10.1021/acsami.8b00567 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces Scheme 1. Illustration of Synthesis of Ge Particles and Hollow Ge Microspheres in Different Molten Salts

particles diameter about 15 μm. Noted, it can be seen from the broken spheres that these microparticles have hollow framework with submicrosized shell (Figure 2C). The enlarged SEM (Figure 2d) and the TEM images (Figure 2e) reveal that these microspheres are constructed by interconnected nanoparticles. Some grain boundary and defects can be observed in the HRTEM image (Figure 2f), further indicating the interconnected structure of fine primary particles. Figure 2g, h shows the SEM and TEM pictures of the A-Ge sample. Clearly, the A-Ge is consisted of monodispersed particles which are dense and smooth. The particle size is generally below 500 nm, which is bigger than that of the primary particles of the Z-Ge sample. The high-resolution TEM images further indicate that the obtained Z-Ge particles are well-crystallized, as shown in Figure 2i. A layer of amorphous oxides was formed on the particle surface of both samples because of the direct exposure to air atmosphere. The Brunauer−Emmett−Teller (BET) surface area of the synthesized Z-Ge and A-Ge are 110 and 9.8 m2 g−1, respectively (Figure S3a, b). The pore size distribution of Z-Ge microparticles is ranging from 3 to 60 with an average pore size of 5.7 nm (Figure S3c). Generally, the pores were formed from the left void space by leaching byproduct of MgO. As we can see, these results agree well with the SEM images. Obviously, different morphology and dimension of Ge were obtained in molten ZnCl2 and AlCl3, respectively. This phenomenon may be caused by the unique physical properties of different salts. The ZnCl2 melt has a high boiling temperature (732 °C) and high viscosity. Therefore, the molten ZnCl2 could well-support the hierarchical microstructure. On the other hand, the AlCl3 has a boiling temperature as low as 181 °C. At the reaction temperature of 250 °C, the AlCl3 would vapor as gas phase dimer in the autoclave, which cannot maintain the framework. The self-generated vapor pressure would promote the Ge crystals to grow into bigger particles, thus destroying the initial structure with low BET surface. It is therefore speculated that molten ZnCl2 could not only promote the reduction to occur at low temperature just as molten AlCl3, but also can maintain the spherical microstructures. Then, the effects of the temperature on the reaction and structure were studied systematically. It is demonstrated that reduction of GeO2 with metallic Mg can be initiated even at 300 and 200 °C in molten ZnCl2 and AlCl3, respectively, as shown in Figure S4. However, direct reaction between GeO2 and metallic Mg is unable to be ignited below 350 °C. Rationally, the molten salt plays a key role in promoting the reaction. On one hand, the molten salt provides a liquid

metallic Mg in molten AlCl3 at 250 °C, denoted as A-Ge. It is noted first that the melting/boiling point of ZnCl2 and AlCl3 are 292/732 and 192/181 °C, respectively, which are important for the reduction temperature and the morphology evolution. When the as-prepared Ge anodes were used for LIBs, the A-Ge and Z-Ge electrodes deliver the reversible capacities of 1329 and 1291 mA h g−1 at 0.2 C after 150 cycles, 1124 and 1217 mA h g−1 at 0.8 C after 400 cycles, respectively. The relatively better electrochemical performance of the porous hollow Ge microspheres may be attributed its unique structure and smaller primary particle size. Figure 1a shows the X-ray diffraction (XRD) patterns of the as-prepared Z-Ge and A-Ge samples. All of the diffraction peaks

Figure 1. (a) XRD patterns and (b) Raman spectrum of the asprepared Z-Ge and A-Ge samples.

can be well-defined and designated as the 111, 220, 311, 400, 331, and 422 lattice planes of cubic phase Ge crystalline (JSPDS PDF card, No. 04−0545). As one can see, the peak width at half height of the Z-Ge is wider than that of the A-Ge. Accordingly, it is reasonable to speculate that the primary particle size of the Z-Ge is smaller than that of the A-Ge on the basis of the Sherrery equation. Figure 1b shows the Raman spectrum of the obtained Z-Ge and A-Ge samples. The strong peak at around 295 cm−1 agrees well with the typical Raman E2 vibrational mode of Ge.18 The XPS 3D spectrum shows that the obtained Z-Ge and A-Ge are oxidized slightly on the surface under the air atmosphere (Figure S2). Therefore, the molten salt system is able to lower the reaction temperature of metallothermal reduction of GeO2. The morphology of the as-prepared products was observed by electronic microscopy images, as shown in Figure 2. Figure 2a−d shows the SEM images of the as-prepared Z-Ge at different magnifications, which displays uniform microspheres. As we can see, the Z-Ge has spherical microstructure with a B


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ACS Applied Materials & Interfaces

Figure 2. SEM pictures of (a−d) the Z-Ge with different magnification and (g) the A-Ge. The TEM images of (e) Z-Ge and (h) A-Ge samples. And, HR-TEM images of (f) Z-Ge and (i) A-Ge particles.

GeOx, which may consume a lot Li+.19 The corresponding initial Coulombic efficiencies of A-Ge is 79.7% that is higher than that of Z-Ge (65.5%). On one hand, the A-Ge material have lower contact area with electrolyte compared to Z-Ge, which can prevent side reactions occurred from a decomposition of electrolytes at a low potential. On the other hand, the more residual GeOx surface and inner defects of the Z-Ge particle would lead to high irreversible capacity, as demonstrated by CV curves (Figure S7). The cycling stability at low current density was measured, as shown in Figure 3b. The Z-Ge electrode delivers a reversible capacity of 1291 mA h g−1 at 0.2 C (1 C = 1600 mA g−1) after 150 cycles, which retains 97.3%. The A-Ge anode delivers a reversible capacity of 1329 mA h g−1 at a rate of 0.2 C (1 C = 1600 mA g−1) after 150 cycles, associated with a capacity retention of 86.3%. The capacity fading of A-Ge electrode is mainly occurred during initial 30 cycles, which may be caused by the gradual pulverization of these big Ge particles. The Coulombic efficiencies of both samples are all maintained at above 98.5% after initial three cycles. Next, the rate capability of these Ge electrodes was investigated with increasing current density from 0.5 to 10.0 C, and then back to 0.5 C, as presented in Figure 3c, d. As one can see, the Z-Ge electrode displays better capacity retention at high current density. The specific capacities of Z-Ge based electrode are maintained at 1315, 1252, 1213, 1181, 1137, 1024, 872, and 673 mA h g−1 at current densities of 0.5, 1.0, 1.5,

reaction media, facilitating the close contact of these reagents. On the other hand, an ionization process of metal reductant would produce ions and solvated electrons which would attack and break the Ge−O bond.15 The Ge sample obtained by reducing GeO2 in ZnCl2 at 300 °C is irregular nanoparticles that is different from the Z-Ge, as shown in Figure S5. It is speculated that the reduction reaction at 300 °C is uncompleted and low-yield. After removal of the residual GeO2, the framework of the product would collapse. In addition, it is also demonstrated that the low-temperature molten salts system was suitable for metallic reductant Zn, as demonstrated in Figure S6. As important Li-storage anode material, the Z-Ge and A-Ge samples were taken as examples to evaluate the electrochemical Li-storage performance. Figure 3a shows the voltage-capacity plots of the Z-Ge and A-Ge based electrodes. Both of them have similar potential profiles. The discharge plateau in the first cycle is ranging from 0.6 to 0.01 V that are assigned to the lithiation of Ge to form LixGe alloys. In the charge procedure, the delithiation plateau is located at about 0.5−0.6 V, which is corresponding to dealloying of LixGe. The voltage-capacity plots agree well with the CV curves (Figure S7). In the first cycle, the charge/discharge capacity of Z-Ge and A-Ge were measured to be 1340/2047 and 1508/1893 mA h g−1, respectively. The irreversible capacity is attributable to the formation of the SEI layer, trapping Li+ in the defects of active materials and the side reaction between the Li+ and residual C


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Figure 3. (a) Initial galvanostatic discharge/charge profiles of A-Ge and Z-Ge based electrodes at 0.2 C. (b) Charge capacity and Coulombic efficiency of the of A-Ge- and Z-Ge-based electrodes at 0.2 C. (c) Capacity potential profiles at different rate of Z-Ge-based electrode. (d) Rate capability of A-Ge and Z-Ge based electrodes a from 0.5 to 10 C. (e) Cycling behavior of the A-Ge and Z-Ge based electrodes at 0.8 C.

2.0, 3.0, 5.0, 7.0, and 10.0 C, reespectively. When the current density is returned back to 0.5 C, an average capacity as high as 1290 mA h g−1 is recovered. Besides, it should be mentioned that big potential polarization occurred at high current density, as shown in Figure 3c. The better rate performance of the Z-Ge electrode is attributed to the high-specific surface area and porous structure which would provide more active reaction sites and fast diffusion path during high-rate cycling.20,21 The long-term cycling stability was evaluated at high current density of 0.8 C, as exhibited in Figure 3e. It should be noted that the electrode was first activated at low current density of 0.2 C for two cycles. The Z-Ge anode delivers a specific capacity of 1217 mA h g−1 after 400 cycles, with a capacity retention of 91.9% at 0.8 C. The A-Ge anode delivers a reversible of 1124 mA h g−1, and a capacity retention of 76.7% at 0.8 C. As one can see, both A-Ge and Z-Ge based electrodes show fine cycling stability. EIS measurements were conducted to further disclose the electrochemical performance. Figure 4 shows the experimental and fitted Nyquist plots and the corresponding equivalent circuit of fresh A-Ge and Z-Ge based electrodes. These Nyquist plots consist of a semicircle in the high-frequency region, and a straight line in the low-frequency region, which are corresponding to the charge transfer resistance on the interface of the electrolyte and active material (Rct) and the lithium ion diffusion in the solid electrodes, respectively. As we can see, the Rct of Z-Ge electrode is lower than that of the A-Ge electrode, because the Z-Ge has higher surface area and active reaction

Figure 4. Experimental and fitted Nyquist plots of fresh A-Ge and ZGe based electrode, associated with the equivalent circuit for the EIS.

sites, facilitating the contact of electrolyte and active materials and electrochemical Li alloy/dealloy reactions. After different cycles, the Z-Ge electrode exhibits relatively lower resistance in the SEI film and interfacial charge transfer than that of the AGe electrode, indicating better preservation of the electrode integrity (Figure S8). Above all, it is rationally concluded that the enhanced electrochemical Li-storage performance in terms of cycling stability and rate capability of Z-Ge electrode may be attributed to following aspects. First, the primary particles size of Z-Ge is smaller than that of the A-Ge. Therefore, the strain stress caused by volume change can be well released during repeated lithiation/delithiation process, improving the cycling stability. Second, the porous structure with high specific surface area and active sites provides transfer channel for fast ion conduction, enhancing the rate capability. On the other hand, the Z-Ge D



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crystal has more defects than that of the well-crystallized A-Ge particles, which would trap Li+ and lower the Coulombic efficiency. Therefore, both crystal structure and micro/ nanostructure are important in improving the electrochemical performance of electrode materials. For practical application, these aspects should be well-balanced and controlled. Finally, The Z-Ge anode was taken as example to couple with LiCoO2 cathode for evaluating the full-cell performance. The commercial LiCoO2 shows a working potential plateau at around 4.0 V and a reversible capacity about 160 mA h g−1, as exhibited in Figure S9. The electrochemical reaction for the full cell could be illustrated as LiCoO2+Ge = Li1−xCoO2 + LixGe. Noticeably, before assembling the full cell, the Ge anode electrode was prelithiated in half cell to eliminate the effect of lithium loss in the first cycles.22 The capacity of full-cell is limited by anode part. Figure 5a shows the discharge/charge

Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b00567. XPS, N2 adsorption−desorption isotherm curves, XRD patterns, SEM images, and discharge/charge plots (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ning Lin: 0000-0002-8029-5595 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Postdoctoral Program for Innovative Talents (BX201600140), the China Postdoctoral Science Foundation (2016M600484), the Fundamental Research Funds for the Central Universities (WK2060190078), and the National Natural Science Foundation of China (21701163, 21671181). Dr. N. Lin would like to express heartfelt gratitude to Miss Y. R. Zhao for her constant encouragement.

Figure 5. (a) The voltage−capacity profiles and (b) cycling properties at 0.2 C of the full cell.

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REFERENCES

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capacity-voltage plots of commercial LiCoO2, Z-Ge and the corresponding full cell, the working potential of the full cell is above 3.2 V that agrees well with the different value of the LiCoO2 cathode and the Z-Ge anode. Figure 5b exhibits the cycling properties and the Coulombic efficiency at 0.2C. The initial reversible charge capacity and Coulombic efficiency are 1223 mA h g−1 and about 90.8%. After 50 cycles, the Z-Ge anode delivers a capacity of 1150 mA h g−1, associated with a capacity retention of 94%. To further improve the full-cell performance, the assembly technique needs to be optimized carefully. In summary, a low-temperature molten-salt metallothermic reduction method is developed for reduction of GeO2 to Ge. Porous hollow Ge microspheres was prepared through reduction of GeO2 in molten ZnCl2 with metallic Mg at 350 °C. Monodispersed Ge particles were synthesized through reduction with Mg in molten AlCl3 at 250 °C. It is demonstrated that the reaction temperature and the physical properties of the molten salts play decisive role in determining the morphology and structure of the products. When the assynthesized Ge anode materials were used for Li ion batteries, both of them exhibit good electrochemical Li-storage performance. It is noteworthy that the Z-Ge product with high surface area and smaller primary particle size shows better cycling stability and rate capability. As a contrast, the well-crystallized A-Ge has higher initial Coulombic efficiency due to its wellcrystallized structure. This work develops a new molten-salt methodology to produce micro/nanostructured Ge anode materials, which may be also applicable for preparing other functional materials. E


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Mesoporous Hollow Ge Microspheres Prepared via Molten-Salt

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