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Bottom-up synthesis of mesoporous germanium as anodes for lithium-ion batteries
Journal of Colloid and Interface Science xxx (xxxx) xxx
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Bottom-up synthesis of mesoporous germanium as anodes for lithium-ion batteries Duihai Tang a,⇑, Huan Yu a, Jiawei Zhao b, Wentao Liu b, Wenting Zhang a, Sijia Miao a, Zhen-An Qiao c, Jiangxuan Song b,*, Zhen Zhao a,⇑ a b c
Institute of Catalysis for Energy and Environment, College of Chemistry and Chemical Engineering, Shenyang Normal University, Shenyang 110034, China State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Mesoporous germanium materials
Mesoporous germanium materials were synthesized via the self-templating method. When used as the anode for lithium ion batteries, the mesoporous germanium exhibits excellent cycling stability with a high reversible specific capacity (803 mA h g 1) within 100 cycles at 0.5 C rate, in addition to improved rate performance (655 mA h g 1 at 1 C rate).
were synthesized via self-templating mothed. The in-situ formed salts was used as templates, which can totally be removed. The calcination temperatures could determine the porosities of the final products. When used as anode for LIBs, mGe500 showed excellent performance.
a r t i c l e
i n f o
Article history: Received 27 August 2019 Revised 5 November 2019 Accepted 6 November 2019 Available online xxxx Keywords: Mesoporous germanium Sodium potassium alloy
a b s t r a c t A series of mesoporous germanium materials were synthesized via the self-templating method. Germanium tetrachloride and sodium potassium alloy were utilized as germanium precursor and reducing agent, respectively. The by-products, NaCl and KCl, could be considered as the in-situ templates. The characterization results showed that the mesopores could be obtained, when the salts were removed by water washing. Moreover, the crystalline germanium could also be achieved, when the calcination temperature is as high as 500 °C. However, when the calcination temperatures are 300 °C, the as-received mesoporous germanium materials are amorphous. When evaluated as anode for lithium-ion batteries (LIBs), the obtained mesoporous germanium exhibits outstanding cycling stability, showing a high
⇑ Corresponding authors. E-mail addresses: [email protected] (D. Tang), [email protected] (J. Song), [email protected], [email protected] (Z. Zhao). https://doi.org/10.1016/j.jcis.2019.11.024 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
Please cite this article as: D. Tang, H. Yu, J. Zhao et al., Bottom-up synthesis of mesoporous germanium as anodes for lithium-ion batteries, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.024
2 Self-templating method Bottom-up synthesis Lithium ion batteries
D. Tang et al. / Journal of Colloid and Interface Science xxx (xxxx) xxx
reversible specific capacity of 803 mA h g (655 mA h g 1 at 1 C rate).
1
after 100 cycles, as well as enhanced rate performance Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction Lithium ion batteries (LIBs) are considered as one of the most effective energy storage devices for electric vehicles due to their high energy density, which have been developed for decades [1–5]. The developments of novel electrode materials have always been the focus of research in lithium-ion batteries [6–9]. The group IV elements can be utilized as the anode for LIBs [10–12]. Silicon possesses the high theoretical capacity of 4200 mA h g 1, which is much higher than that of graphite (372 mA h g 1) [12,13]. However, large-scale application of silicon-based materials is hindered by the huge volume expansion during charging and discharging process, leading to fast capacity fading [14,15]. Moreover, as a typical semiconductor, silicon possesses poor electrical conductivity, which also causes bad rate performance [16,17]. As a contrast, Ge has a lower capacity of 1600 mA h g 1, which is lower than that of Si. However, Ge has better electrical conductivity than Si, leading to better cell performances [18,19]. Moreover, compared with other anode materials, Ge exhibits higher capacity [20–22]. Mesoporous materials have been paid more and more attentions in many fields, because of the high specific surface area, pore volume and pore size [23–25]. Moreover, the synthesis of mesoporous crystalline materials is still a challenge in the material chemistry [26]. Furthermore, the fabrication of mesoporous materials should be facile. The surface area, pore size, and pore volume should also be feasible to tune, by varying the synthetic conditions, such as reaction time, reaction temperature, and calcination temperature [27–30]. Traditional synthesis methods of mesoporous materials are hard template and soft template, which are both time-consuming and cause environmental pollution [31–33]. The template-free methods to synthesize mesoporous germanium via the bottom-up process are desirable [34–36].
Herein, a novel strategy was developed to synthesize mesoporous germanium materials via the bottom-up self-templating mothed. Germanium tetrachloride was used as the germanium precursor, and NaK alloy was utilized as the reducing agent. The synthetic process consisted of stirring at room temperature and calcination at high temperature. The nitrogen adsorptiondesorption isotherms and the XRD patterns showed that the asreceived materials were mesoporous, which were comprised of Ge nanoparticles. The calcination temperature could both affect the porosities and the crystalline structures of the as-prepared materials. The higher calcination temperature could lead to lower surface area. When utilized as anode for LIBs, the optional mesoporous germanium material showed high specified capacity and outstanding Coulombic efficiency within 100 cycles at 0.5 C rate, as well as improved rate performance.
2. Experimental section 2.1. Synthesis of mesoporous germanium All the procedures besides the washing and annealing processes were performed in the Ar-filled glove box. The sodium, potassium, toluene, and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. Germanium tetrachloride (GeCl4) was purchased from Aldrich. As shown in Scheme 1, sodium potassium alloy (NaK) was synthesized by blending 0.4 g (0.017 mol) of sodium and 1.6 g (0.041 mol) of potassium. The as-prepared NaK alloy was added to 50 ml of toluene solution of GeCl4 (2.79 g, 0.013 mol). This mixed solution was stirred at room temperature for 4 h, by the end of which time black particles formed. The stirring was then stopped and 20 ml of ethanol was added slowly.
Scheme 1. Synthetic route of the mesoporous germanium.
Please cite this article as: D. Tang, H. Yu, J. Zhao et al., Bottom-up synthesis of mesoporous germanium as anodes for lithium-ion batteries, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.024
D. Tang et al. / Journal of Colloid and Interface Science xxx (xxxx) xxx
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The black powders were collected by filtering, and the intermediates were obtained via annealing at the desired temperature with a heating rate of 10 °C min 1 for 40 min under Ar. After addition of 200 ml of water, the intermediates were placed in a sonic bath for 10 mins to dissolve the salts. The products were then filtrated, and then washed with deionized water twice, and dried at 40 °C under vacuum for 12 h. The final products of mesoporous germanium materials are designated as mGe-x, where x means the calcination temperature. 2.2. Characterization The crystalline structures of the intermediates and the final products were tested via X-ray diffraction (XRD) patterns (Rigaku Ultima IV using Cu Ka radiation (k = 1.5418 Å). The degrees of graphitization were investigated by Raman spectra (HORIBA LabRAM HR Evolution Raman spectrometer with 532 nm laser). The surface area, pore volume, and pore size were achieved via the N2 adsorption and desorption isotherm measurements at 77 K (Micromeritics Tristar II 3020 physisorption analyzer). The surface compositions and chemical valence states were investigated via XPS spectra (Thermo Scientific ESCALAB 250Xi). The morphology and microstructure were measured by scanning electron microscope (SEM, HITACHI SU8020) and transmission electron microscope (TEM, JEOL-1200), respectively. 2.3. Electrochemical measurements For half-cell tests, CR2016 coin-type cells were employed to evaluate the electrochemical measurements, which were assembled in an Ar-filled glove box. In details, the as-prepared sample, Li foil, and Celgard 2400 membranes were used as the working electrode, the counter electrode, and the separator, respectively. Slurries were prepared by mixing the mesoporous Ge, Super-P carbon, and sodium-carboxymethyl cellulose (Na-CMC) in a mass ratio of 8:1:1, which were stirred for 6 h. The as-formed slurries were casted on copper foil, and then dried in vacuum at 100 °C for 12 h. 1 M LiPF6 in a mixture of ethylene carbonate, diethyl carbonate, and dimethyl carbonate (1: 1: 1 v/v) was used as the electrolyte. Galvanostatic charge-discharge tests were performed via a battery tester (BTS-5V1mA, Neware) at room temperature. specific capacity and charge/discharge rate were calculated based on the mass of the mesoporous Ge. 3. Results and discussion The crystalline structures of the intermediates and the asprepared materials are revealed in Figs. S1 and 1a, respectively. In details, as exhibited in Fig. S1, the characteristic peaks of NaCl and KCl could both be observed clearly. The diffraction peaks at 28.4°, 40.5°, 50.2°, 58.7°, 66.4°, and 73.8° are ascribed to diffraction peaks of KCl (2 0 0), (2 2 0), (2 2 2), (4 0 0), (4 2 0), and (4 2 2) (JCPDS Card No. 41-1476), respectively [37]. Moreover, the diffraction peaks at 31.6° and 45.4° are ascribed to the (2 0 0) and (2 2 0) lattice planes of NaCl (JCPDS Card No. 05-0628), respectively [38]. Besides the diffraction peaks of NaCl and KCl, the (1 1 1) diffraction peak of crystalline germanium can be clearly observed in the mGe500 intermediate, which is absent in both mGe-RT and mGe-300. This result indicates that the high temperature calcination can lead to the formation of the crystalline germanium. The NaK alloy can be used as the reducing agent. The resultant salts (NaCl and KCl) could act as template, which can be removed by water washing. As revealed in Fig. 1a, no diffraction peaks of NaCl and KCl could be detected after dissolving the in-situ formed salts, which indicates that the water washing process can completely remove the
Fig. 1. (a) XRD patterns and (b) Raman spectra of mGe-RT, mGe-300, and mGe-500.
reduction generated salts. The characteristic peaks of crystalline germanium can only be observed in the XRD pattern of mGe500. All the diffraction peaks could be indexed to diffraction peaks of crystalline germanium of (1 1 1), (2 2 0), (3 3 1), (4 0 0), and (3 3 1) (JCPDS Card No. 04-0545) [39]. However, there are only two board peaks at 27.3° and 50.2° in the XRD patterns of mGeRT and mGe-300, indicating that the mesoporous germanium calcined at low temperature possesses amorphous structures. As shown in Fig. 1b, Raman spectra are shown to further confirm the amorphous structures of mGe-RT and mGe-300. The board peak at 270 cm 1 can be shown in both mGe-RT and mGe-300, reflecting the formation of the amorphous germanium. Moreover, when the calcination temperature is 500 °C, the Raman spectrum of mGe-500 exhibits a peak at 298 cm 1, which corresponds to the crystalline germanium [40]. According to the XRD patterns and the Raman spectra, when the calcination temperature is low, amorphous structure can be obtained. However, when the calcination temperature is as high as 500 °C, the crystalline germanium can be obtained. The porosities of the mesoporous geranium materials were determined by nitrogen adsorption-desorption isotherms. As exhibited in Fig. 2a, the as-prepared materials show type IV isotherms, indicating the mesopores in the structures. Moreover, the narrow H1 hysteresis loops indicate that mGe-RT, mGe-300, and mGe-500 have good pore connectivity [35]. As exhibited in Table 1, the BET surface area increases, with the decrease of the annealing temperature. In details, mGe-RT has the highest BET surface area of 155.2 m2 g 1. However, mGe-500 possesses the lowest BET surface
Please cite this article as: D. Tang, H. Yu, J. Zhao et al., Bottom-up synthesis of mesoporous germanium as anodes for lithium-ion batteries, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.024
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Fig. 3. High-resolution XPS spectra of Ge 3d for mGe-RT, mGe-300, and mGe-500.
Table 2 Deconvolution of XPS spectra of Ge 3d for mGe-RT, mGe-300, and mGe-500.
Fig. 2. (a) Nitrogen adsorption-desorption isotherms and (b) BJH pore-size distributions of mGe-RT, mGe-300, and mGe-500.
Table 1 The textural properties of the mesoporous germanium materials.
a b
Sample
SBETa (m2 g
mGe-RT mGe-300 mGe-500
155.2 140.3 85.8
1
)
Vtotalb (cm3 g
1
)
0.38 0.36 0.27
Da (nm) 9.2 10.0 12.7
SBET and average pore size (D) are calculated by BET equation. Vtotal (pore volume) is calculated at P/P0 = 0.950–0.995.
area of 85.8 m2 g 1. Furthermore, the pore size increases with the increase of the calcination temperature (Fig. 2b). mGe-500 has the largest pore size of 12.7 nm, while mGe-RT possesses the smallest pore size of 9.2 nm. According to the above results, it is concluded that the calcination temperature can affect the porosities of these materials. The XPS spectra are shown to confirm the composition on the as-synthesized mesoporous germanium materials. As revealed in Fig. 3, germanium and oxygen can both be detected on the surfaces of the final samples. In details, the Ge 3d peak could be divided into two peaks located at 29.7 and 33.1 eV. The peak located at 29.7 eV can be clearly observed, corresponding to the bonding of Ge-Ge. Moreover, the peak at 33.1 eV can be assigned to GeOx, indicating that the as-obtained samples are oxidized on the surfaces under air [41]. The surface oxides are commonly observed in nanoparticles
Samples
Ge-O (%)
Ge-Ge (%)
Ge-RT Ge-300 Ge-500
50.5 45.5 31.7
49.5 54.5 68.3
[35]. Furthermore, the materials show different ratios of GeOx/Ge (Table 2), which may be due to the different calcination temperatures and surface areas. The higher surface area leads to higher ratio of GeOx/Ge. mGe-RT shows the highest ratio of GeOx/Ge, while mGe-500 shows the lowest ratio of GeOx/Ge. The BET surface areas can determine the ratios of GeOx/Ge on the surface of the materials, and then affect the electrochemical performance for LIBs. The morphologies of mGe-RT, mGe-300, and mGe-500 were determined by SEM. As revealed in Fig. 4a, when the sample was prepared at room temperature, mGe-RT is composed of the germanium nanoparticles. When the sample is calcined at 300 °C, the SEM image reveals that the sample of mGe-300 also consists of the nanoparticles, which is larger than that of mGe-RT (Fig. 4b). It is noted that when the sample is annealed at 500 °C, the particle size of mGe-500 is much larger than those of mGe-RT and mGe300 (Fig. 4c). It could be concluded that the sizes of the nanoparticles increase, with the increase of the annealing temperature. The structures and the porosities of the mesoporous germanium materials were investigated by the TEM and HRTEM images. mGeRT was studied first. The TEM image in Fig. 5a clearly exhibits that mGe-RT possesses a disordered mesoporous structure. Moreover, the Ge framework of mGe-RT consist of interconnected particles, whose sizes are between 20 and 50 nm (Fig. S3a). As shown in Fig. 5b, the HRTEM image of mGe-RT reveals that no lattice fringe can be observed, indicating that mGe-RT has amorphous structure. When the calcination temperature is 300 °C, the as-made sample of mGe-300 also has a disordered mesoporous structure, which consists of amorphous Ge nanoparticle (Fig. 4c). Moreover, the pore size of mGe-300 is a little lager than that of mGe-RT, which is consistent with the N2 adsorption/desorption isotherms (Fig. S3b). As exhibited in Fig. 4e, when the calcination temperature is 500 °C, the sample of mGe-500 is composed of interconnected Ge nanoparticles, with the particle size of 20–50 nm (Fig. S2c). This is consistent with crystallite size of 37 nm, which is calculated from the XRD pattern. Furthermore, the HRTEM image of mGe-500 reveals the lattice fringe of 0.33 nm, which can be indexed to the [1 1 1] plane of crystalline germanium (Fig. 4f) [42].
Please cite this article as: D. Tang, H. Yu, J. Zhao et al., Bottom-up synthesis of mesoporous germanium as anodes for lithium-ion batteries, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.024
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Fig. 4. SEM images of (a) mGe-RT, (b) mGe-300, and (c) mGe-500.
Fig. 5. (a) TEM and (b) HRTEM images of mGe-RT. (c) TEM and (d) HRTEM images of mGe-300. (e) TEM and (f) HRTEM images of mGe-500. The insets of (b, d, and f) show FFT images.
The electrochemical performance of mGe-RT, mGe-300, and mGe-500 was tested. As revealed in Fig. 5a, the discharge and charge capacities of mGe-RT are 1664 and 1161 mA h g 1, respectively, corresponding to an initial Coulombic efficiency of 70%. Moreover, the mGe-300 electrode shows initial discharge and charge capacities of 1380 and 980 mA h g 1, respectively, indicating an initial coulombic efficiency of 71%. Furthermore, the mGe-500 electrode shows initial discharge and charge capacities
of 1337 and 800 mA h g 1, respectively, showing an initial coulombic efficiency of 60%. All of these mesoporous materials show low efficiencies likely due to the high surface areas and the formation of the GeOx impurity on the surface of the materials. As shown in Fig. 6b, during the subsequent cycles, charge-discharge efficiencies of over 99% were obtained for both mGe-300 and mGe-500. However, mGe-RT could only show a Coulombic efficiency of 97% during the subsequent cycles.
Please cite this article as: D. Tang, H. Yu, J. Zhao et al., Bottom-up synthesis of mesoporous germanium as anodes for lithium-ion batteries, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.024
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Fig. 6. (a) cycling performance of mGe-RT, mGe-300, and mGe-500 at 0.5 C. (b) Coulombic efficiency of mGe-RT, mGe-300, and mGe-500. (c) Rate capability of mGe-300 and mGe-500.
The cycling performances of mGe-RT, mGe-300, and mGe-500 were investigated at 0.5 C rate (Fig. 5a). In details, the mGe-RT anode shows fast capacity loss, which reveals a discharge capacity retention of only 36.4% after 100 cycles. This can be due to the formation of SEI layer, leading to the decreasing electrical conductivity. Similarly, the mGe-300 anode exhibits fast capacity fading, with a discharge capacity retention of 68.7% after 100 cycles. However, the mGe-500 anode reveals much better cyclability, showing 83.0% capacity retention (785 mA h g 1) after 100 cycles. Moreover, the rate capability of mGe-500 was investigated at rates between 0.1 C and 1 C (Fig. 6c). The mGe-300 anode has a capacity of 500 mA h g 1 at 1 C rate. However, the mGe-500 electrode achieves a capacity of 655 mA h g 1 under the same condition.
According to the above results, mGe-500 shows better electrochemical Li-storage performance than mGe-RT and mGe-300, which could be due to two reasons. On one hand, mGe-500 possesses the lowest BET surface areas among the as-prepared samples, which lead to the lowest content of GeOx. Moreover, GeOx on the surface of the electrode material can consume Li+, leading to lower Coulombic efficiencies. On the other hand, the excellent rate performance of mGe-500 anode can be attributed to the lower BET surface area and the crystalline structure, which could improve the conductivity of the electrode. Cyclic voltammetry and charge/discharge curves are revealed in Fig. 7 to investigate the conversion reaction of mGe-500 in the first three cycles. A shown in Fig. 7a, the cathodic curve of the initial cycle is unlike to those of the following two cycles, which could be due to the formation of SEI layer [43]. Subsequently, the cathodic peaks at 0.08 and 0.50 eV can be attributed to the formation of LixGe [43]. Furthermore, the anodic curves of the initial three cycles all show two main anodic peaks located at 0.46 and 1.13 V, which can be due to the removal of Li from LixGe. In addition, there are no obvious changes in these redox peaks, leading to outstanding cycling stability. Fig. 7b shows charge/discharge curves of mGe-500 during the first cycle. For the initial cycle, a plateau ranges from 0.5 to 0.1 V can be obviously observed, which can be assigned to the lithiation of crystalline Ge to produce LixGe. In the subsequent processes, the charge curves exhibit characteristics of amorphous Ge [44]. 4. Conclusion A series of mesoporous germanium materials were synthesized via the self-templating method and utilized as the anode for LIBs. GeCl4 was utilized as the germanium precursor. NaK alloy was used as the reducing agent. Moreover, the in-situ formed salts, NaCl and KCl, could be considered as the templates, which can totally be dissolved in water. As compared to the soft template and the hard template methods, this water washing process is much easier to remove the templates. The as-prepared samples show mesoporous structures. The crystalline and porous structures could be controlled by varying the calcination temperature. When the calcination temperature was as high as 500 °C, the sample of mGe-500 showed high specific capacity and excellent rate capability, compared with those of mGe-RT and mGe-300. Acknowledgements
Fig. 7. (a) Cyclic voltammetry curves of three charge/discharge cycles of mGe-500 electrode. (b) Charge/discharge curves of mGe-500 during three charge/discharge cycles.
This research was supported by the National Nature Science Foundation of China (Nos 91845201 and 21601128), Natural Science Foundation of Liaoning Province of China (Materials Joint Foundation, No. 20180510031), Liaoning Provincial Instrument and Equipment Sharing Service Platform Building Project, Support
Please cite this article as: D. Tang, H. Yu, J. Zhao et al., Bottom-up synthesis of mesoporous germanium as anodes for lithium-ion batteries, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.024
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Bottom-up synthesis of mesoporous germanium as anodes for lithium-ion batteries Duihai Tang a,⇑, Huan Yu a, Jiawei Zhao b, Wentao Liu b, Wenting Zhang a, Sijia Miao a, Zhen-An Qiao c, Jiangxuan Song b,*, Zhen Zhao a,⇑ a b c
Institute of Catalysis for Energy and Environment, College of Chemistry and Chemical Engineering, Shenyang Normal University, Shenyang 110034, China State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Mesoporous germanium materials
Mesoporous germanium materials were synthesized via the self-templating method. When used as the anode for lithium ion batteries, the mesoporous germanium exhibits excellent cycling stability with a high reversible specific capacity (803 mA h g 1) within 100 cycles at 0.5 C rate, in addition to improved rate performance (655 mA h g 1 at 1 C rate).
were synthesized via self-templating mothed. The in-situ formed salts was used as templates, which can totally be removed. The calcination temperatures could determine the porosities of the final products. When used as anode for LIBs, mGe500 showed excellent performance.
a r t i c l e
i n f o
Article history: Received 27 August 2019 Revised 5 November 2019 Accepted 6 November 2019 Available online xxxx Keywords: Mesoporous germanium Sodium potassium alloy
a b s t r a c t A series of mesoporous germanium materials were synthesized via the self-templating method. Germanium tetrachloride and sodium potassium alloy were utilized as germanium precursor and reducing agent, respectively. The by-products, NaCl and KCl, could be considered as the in-situ templates. The characterization results showed that the mesopores could be obtained, when the salts were removed by water washing. Moreover, the crystalline germanium could also be achieved, when the calcination temperature is as high as 500 °C. However, when the calcination temperatures are 300 °C, the as-received mesoporous germanium materials are amorphous. When evaluated as anode for lithium-ion batteries (LIBs), the obtained mesoporous germanium exhibits outstanding cycling stability, showing a high
⇑ Corresponding authors. E-mail addresses: [email protected] (D. Tang), [email protected] (J. Song), [email protected], [email protected] (Z. Zhao). https://doi.org/10.1016/j.jcis.2019.11.024 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
Please cite this article as: D. Tang, H. Yu, J. Zhao et al., Bottom-up synthesis of mesoporous germanium as anodes for lithium-ion batteries, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.024
2 Self-templating method Bottom-up synthesis Lithium ion batteries
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reversible specific capacity of 803 mA h g (655 mA h g 1 at 1 C rate).
1
after 100 cycles, as well as enhanced rate performance Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction Lithium ion batteries (LIBs) are considered as one of the most effective energy storage devices for electric vehicles due to their high energy density, which have been developed for decades [1–5]. The developments of novel electrode materials have always been the focus of research in lithium-ion batteries [6–9]. The group IV elements can be utilized as the anode for LIBs [10–12]. Silicon possesses the high theoretical capacity of 4200 mA h g 1, which is much higher than that of graphite (372 mA h g 1) [12,13]. However, large-scale application of silicon-based materials is hindered by the huge volume expansion during charging and discharging process, leading to fast capacity fading [14,15]. Moreover, as a typical semiconductor, silicon possesses poor electrical conductivity, which also causes bad rate performance [16,17]. As a contrast, Ge has a lower capacity of 1600 mA h g 1, which is lower than that of Si. However, Ge has better electrical conductivity than Si, leading to better cell performances [18,19]. Moreover, compared with other anode materials, Ge exhibits higher capacity [20–22]. Mesoporous materials have been paid more and more attentions in many fields, because of the high specific surface area, pore volume and pore size [23–25]. Moreover, the synthesis of mesoporous crystalline materials is still a challenge in the material chemistry [26]. Furthermore, the fabrication of mesoporous materials should be facile. The surface area, pore size, and pore volume should also be feasible to tune, by varying the synthetic conditions, such as reaction time, reaction temperature, and calcination temperature [27–30]. Traditional synthesis methods of mesoporous materials are hard template and soft template, which are both time-consuming and cause environmental pollution [31–33]. The template-free methods to synthesize mesoporous germanium via the bottom-up process are desirable [34–36].
Herein, a novel strategy was developed to synthesize mesoporous germanium materials via the bottom-up self-templating mothed. Germanium tetrachloride was used as the germanium precursor, and NaK alloy was utilized as the reducing agent. The synthetic process consisted of stirring at room temperature and calcination at high temperature. The nitrogen adsorptiondesorption isotherms and the XRD patterns showed that the asreceived materials were mesoporous, which were comprised of Ge nanoparticles. The calcination temperature could both affect the porosities and the crystalline structures of the as-prepared materials. The higher calcination temperature could lead to lower surface area. When utilized as anode for LIBs, the optional mesoporous germanium material showed high specified capacity and outstanding Coulombic efficiency within 100 cycles at 0.5 C rate, as well as improved rate performance.
2. Experimental section 2.1. Synthesis of mesoporous germanium All the procedures besides the washing and annealing processes were performed in the Ar-filled glove box. The sodium, potassium, toluene, and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. Germanium tetrachloride (GeCl4) was purchased from Aldrich. As shown in Scheme 1, sodium potassium alloy (NaK) was synthesized by blending 0.4 g (0.017 mol) of sodium and 1.6 g (0.041 mol) of potassium. The as-prepared NaK alloy was added to 50 ml of toluene solution of GeCl4 (2.79 g, 0.013 mol). This mixed solution was stirred at room temperature for 4 h, by the end of which time black particles formed. The stirring was then stopped and 20 ml of ethanol was added slowly.
Scheme 1. Synthetic route of the mesoporous germanium.
Please cite this article as: D. Tang, H. Yu, J. Zhao et al., Bottom-up synthesis of mesoporous germanium as anodes for lithium-ion batteries, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.024
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The black powders were collected by filtering, and the intermediates were obtained via annealing at the desired temperature with a heating rate of 10 °C min 1 for 40 min under Ar. After addition of 200 ml of water, the intermediates were placed in a sonic bath for 10 mins to dissolve the salts. The products were then filtrated, and then washed with deionized water twice, and dried at 40 °C under vacuum for 12 h. The final products of mesoporous germanium materials are designated as mGe-x, where x means the calcination temperature. 2.2. Characterization The crystalline structures of the intermediates and the final products were tested via X-ray diffraction (XRD) patterns (Rigaku Ultima IV using Cu Ka radiation (k = 1.5418 Å). The degrees of graphitization were investigated by Raman spectra (HORIBA LabRAM HR Evolution Raman spectrometer with 532 nm laser). The surface area, pore volume, and pore size were achieved via the N2 adsorption and desorption isotherm measurements at 77 K (Micromeritics Tristar II 3020 physisorption analyzer). The surface compositions and chemical valence states were investigated via XPS spectra (Thermo Scientific ESCALAB 250Xi). The morphology and microstructure were measured by scanning electron microscope (SEM, HITACHI SU8020) and transmission electron microscope (TEM, JEOL-1200), respectively. 2.3. Electrochemical measurements For half-cell tests, CR2016 coin-type cells were employed to evaluate the electrochemical measurements, which were assembled in an Ar-filled glove box. In details, the as-prepared sample, Li foil, and Celgard 2400 membranes were used as the working electrode, the counter electrode, and the separator, respectively. Slurries were prepared by mixing the mesoporous Ge, Super-P carbon, and sodium-carboxymethyl cellulose (Na-CMC) in a mass ratio of 8:1:1, which were stirred for 6 h. The as-formed slurries were casted on copper foil, and then dried in vacuum at 100 °C for 12 h. 1 M LiPF6 in a mixture of ethylene carbonate, diethyl carbonate, and dimethyl carbonate (1: 1: 1 v/v) was used as the electrolyte. Galvanostatic charge-discharge tests were performed via a battery tester (BTS-5V1mA, Neware) at room temperature. specific capacity and charge/discharge rate were calculated based on the mass of the mesoporous Ge. 3. Results and discussion The crystalline structures of the intermediates and the asprepared materials are revealed in Figs. S1 and 1a, respectively. In details, as exhibited in Fig. S1, the characteristic peaks of NaCl and KCl could both be observed clearly. The diffraction peaks at 28.4°, 40.5°, 50.2°, 58.7°, 66.4°, and 73.8° are ascribed to diffraction peaks of KCl (2 0 0), (2 2 0), (2 2 2), (4 0 0), (4 2 0), and (4 2 2) (JCPDS Card No. 41-1476), respectively [37]. Moreover, the diffraction peaks at 31.6° and 45.4° are ascribed to the (2 0 0) and (2 2 0) lattice planes of NaCl (JCPDS Card No. 05-0628), respectively [38]. Besides the diffraction peaks of NaCl and KCl, the (1 1 1) diffraction peak of crystalline germanium can be clearly observed in the mGe500 intermediate, which is absent in both mGe-RT and mGe-300. This result indicates that the high temperature calcination can lead to the formation of the crystalline germanium. The NaK alloy can be used as the reducing agent. The resultant salts (NaCl and KCl) could act as template, which can be removed by water washing. As revealed in Fig. 1a, no diffraction peaks of NaCl and KCl could be detected after dissolving the in-situ formed salts, which indicates that the water washing process can completely remove the
Fig. 1. (a) XRD patterns and (b) Raman spectra of mGe-RT, mGe-300, and mGe-500.
reduction generated salts. The characteristic peaks of crystalline germanium can only be observed in the XRD pattern of mGe500. All the diffraction peaks could be indexed to diffraction peaks of crystalline germanium of (1 1 1), (2 2 0), (3 3 1), (4 0 0), and (3 3 1) (JCPDS Card No. 04-0545) [39]. However, there are only two board peaks at 27.3° and 50.2° in the XRD patterns of mGeRT and mGe-300, indicating that the mesoporous germanium calcined at low temperature possesses amorphous structures. As shown in Fig. 1b, Raman spectra are shown to further confirm the amorphous structures of mGe-RT and mGe-300. The board peak at 270 cm 1 can be shown in both mGe-RT and mGe-300, reflecting the formation of the amorphous germanium. Moreover, when the calcination temperature is 500 °C, the Raman spectrum of mGe-500 exhibits a peak at 298 cm 1, which corresponds to the crystalline germanium [40]. According to the XRD patterns and the Raman spectra, when the calcination temperature is low, amorphous structure can be obtained. However, when the calcination temperature is as high as 500 °C, the crystalline germanium can be obtained. The porosities of the mesoporous geranium materials were determined by nitrogen adsorption-desorption isotherms. As exhibited in Fig. 2a, the as-prepared materials show type IV isotherms, indicating the mesopores in the structures. Moreover, the narrow H1 hysteresis loops indicate that mGe-RT, mGe-300, and mGe-500 have good pore connectivity [35]. As exhibited in Table 1, the BET surface area increases, with the decrease of the annealing temperature. In details, mGe-RT has the highest BET surface area of 155.2 m2 g 1. However, mGe-500 possesses the lowest BET surface
Please cite this article as: D. Tang, H. Yu, J. Zhao et al., Bottom-up synthesis of mesoporous germanium as anodes for lithium-ion batteries, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.024
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Fig. 3. High-resolution XPS spectra of Ge 3d for mGe-RT, mGe-300, and mGe-500.
Table 2 Deconvolution of XPS spectra of Ge 3d for mGe-RT, mGe-300, and mGe-500.
Fig. 2. (a) Nitrogen adsorption-desorption isotherms and (b) BJH pore-size distributions of mGe-RT, mGe-300, and mGe-500.
Table 1 The textural properties of the mesoporous germanium materials.
a b
Sample
SBETa (m2 g
mGe-RT mGe-300 mGe-500
155.2 140.3 85.8
1
)
Vtotalb (cm3 g
1
)
0.38 0.36 0.27
Da (nm) 9.2 10.0 12.7
SBET and average pore size (D) are calculated by BET equation. Vtotal (pore volume) is calculated at P/P0 = 0.950–0.995.
area of 85.8 m2 g 1. Furthermore, the pore size increases with the increase of the calcination temperature (Fig. 2b). mGe-500 has the largest pore size of 12.7 nm, while mGe-RT possesses the smallest pore size of 9.2 nm. According to the above results, it is concluded that the calcination temperature can affect the porosities of these materials. The XPS spectra are shown to confirm the composition on the as-synthesized mesoporous germanium materials. As revealed in Fig. 3, germanium and oxygen can both be detected on the surfaces of the final samples. In details, the Ge 3d peak could be divided into two peaks located at 29.7 and 33.1 eV. The peak located at 29.7 eV can be clearly observed, corresponding to the bonding of Ge-Ge. Moreover, the peak at 33.1 eV can be assigned to GeOx, indicating that the as-obtained samples are oxidized on the surfaces under air [41]. The surface oxides are commonly observed in nanoparticles
Samples
Ge-O (%)
Ge-Ge (%)
Ge-RT Ge-300 Ge-500
50.5 45.5 31.7
49.5 54.5 68.3
[35]. Furthermore, the materials show different ratios of GeOx/Ge (Table 2), which may be due to the different calcination temperatures and surface areas. The higher surface area leads to higher ratio of GeOx/Ge. mGe-RT shows the highest ratio of GeOx/Ge, while mGe-500 shows the lowest ratio of GeOx/Ge. The BET surface areas can determine the ratios of GeOx/Ge on the surface of the materials, and then affect the electrochemical performance for LIBs. The morphologies of mGe-RT, mGe-300, and mGe-500 were determined by SEM. As revealed in Fig. 4a, when the sample was prepared at room temperature, mGe-RT is composed of the germanium nanoparticles. When the sample is calcined at 300 °C, the SEM image reveals that the sample of mGe-300 also consists of the nanoparticles, which is larger than that of mGe-RT (Fig. 4b). It is noted that when the sample is annealed at 500 °C, the particle size of mGe-500 is much larger than those of mGe-RT and mGe300 (Fig. 4c). It could be concluded that the sizes of the nanoparticles increase, with the increase of the annealing temperature. The structures and the porosities of the mesoporous germanium materials were investigated by the TEM and HRTEM images. mGeRT was studied first. The TEM image in Fig. 5a clearly exhibits that mGe-RT possesses a disordered mesoporous structure. Moreover, the Ge framework of mGe-RT consist of interconnected particles, whose sizes are between 20 and 50 nm (Fig. S3a). As shown in Fig. 5b, the HRTEM image of mGe-RT reveals that no lattice fringe can be observed, indicating that mGe-RT has amorphous structure. When the calcination temperature is 300 °C, the as-made sample of mGe-300 also has a disordered mesoporous structure, which consists of amorphous Ge nanoparticle (Fig. 4c). Moreover, the pore size of mGe-300 is a little lager than that of mGe-RT, which is consistent with the N2 adsorption/desorption isotherms (Fig. S3b). As exhibited in Fig. 4e, when the calcination temperature is 500 °C, the sample of mGe-500 is composed of interconnected Ge nanoparticles, with the particle size of 20–50 nm (Fig. S2c). This is consistent with crystallite size of 37 nm, which is calculated from the XRD pattern. Furthermore, the HRTEM image of mGe-500 reveals the lattice fringe of 0.33 nm, which can be indexed to the [1 1 1] plane of crystalline germanium (Fig. 4f) [42].
Please cite this article as: D. Tang, H. Yu, J. Zhao et al., Bottom-up synthesis of mesoporous germanium as anodes for lithium-ion batteries, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.024
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Fig. 4. SEM images of (a) mGe-RT, (b) mGe-300, and (c) mGe-500.
Fig. 5. (a) TEM and (b) HRTEM images of mGe-RT. (c) TEM and (d) HRTEM images of mGe-300. (e) TEM and (f) HRTEM images of mGe-500. The insets of (b, d, and f) show FFT images.
The electrochemical performance of mGe-RT, mGe-300, and mGe-500 was tested. As revealed in Fig. 5a, the discharge and charge capacities of mGe-RT are 1664 and 1161 mA h g 1, respectively, corresponding to an initial Coulombic efficiency of 70%. Moreover, the mGe-300 electrode shows initial discharge and charge capacities of 1380 and 980 mA h g 1, respectively, indicating an initial coulombic efficiency of 71%. Furthermore, the mGe-500 electrode shows initial discharge and charge capacities
of 1337 and 800 mA h g 1, respectively, showing an initial coulombic efficiency of 60%. All of these mesoporous materials show low efficiencies likely due to the high surface areas and the formation of the GeOx impurity on the surface of the materials. As shown in Fig. 6b, during the subsequent cycles, charge-discharge efficiencies of over 99% were obtained for both mGe-300 and mGe-500. However, mGe-RT could only show a Coulombic efficiency of 97% during the subsequent cycles.
Please cite this article as: D. Tang, H. Yu, J. Zhao et al., Bottom-up synthesis of mesoporous germanium as anodes for lithium-ion batteries, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.024
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Fig. 6. (a) cycling performance of mGe-RT, mGe-300, and mGe-500 at 0.5 C. (b) Coulombic efficiency of mGe-RT, mGe-300, and mGe-500. (c) Rate capability of mGe-300 and mGe-500.
The cycling performances of mGe-RT, mGe-300, and mGe-500 were investigated at 0.5 C rate (Fig. 5a). In details, the mGe-RT anode shows fast capacity loss, which reveals a discharge capacity retention of only 36.4% after 100 cycles. This can be due to the formation of SEI layer, leading to the decreasing electrical conductivity. Similarly, the mGe-300 anode exhibits fast capacity fading, with a discharge capacity retention of 68.7% after 100 cycles. However, the mGe-500 anode reveals much better cyclability, showing 83.0% capacity retention (785 mA h g 1) after 100 cycles. Moreover, the rate capability of mGe-500 was investigated at rates between 0.1 C and 1 C (Fig. 6c). The mGe-300 anode has a capacity of 500 mA h g 1 at 1 C rate. However, the mGe-500 electrode achieves a capacity of 655 mA h g 1 under the same condition.
According to the above results, mGe-500 shows better electrochemical Li-storage performance than mGe-RT and mGe-300, which could be due to two reasons. On one hand, mGe-500 possesses the lowest BET surface areas among the as-prepared samples, which lead to the lowest content of GeOx. Moreover, GeOx on the surface of the electrode material can consume Li+, leading to lower Coulombic efficiencies. On the other hand, the excellent rate performance of mGe-500 anode can be attributed to the lower BET surface area and the crystalline structure, which could improve the conductivity of the electrode. Cyclic voltammetry and charge/discharge curves are revealed in Fig. 7 to investigate the conversion reaction of mGe-500 in the first three cycles. A shown in Fig. 7a, the cathodic curve of the initial cycle is unlike to those of the following two cycles, which could be due to the formation of SEI layer [43]. Subsequently, the cathodic peaks at 0.08 and 0.50 eV can be attributed to the formation of LixGe [43]. Furthermore, the anodic curves of the initial three cycles all show two main anodic peaks located at 0.46 and 1.13 V, which can be due to the removal of Li from LixGe. In addition, there are no obvious changes in these redox peaks, leading to outstanding cycling stability. Fig. 7b shows charge/discharge curves of mGe-500 during the first cycle. For the initial cycle, a plateau ranges from 0.5 to 0.1 V can be obviously observed, which can be assigned to the lithiation of crystalline Ge to produce LixGe. In the subsequent processes, the charge curves exhibit characteristics of amorphous Ge [44]. 4. Conclusion A series of mesoporous germanium materials were synthesized via the self-templating method and utilized as the anode for LIBs. GeCl4 was utilized as the germanium precursor. NaK alloy was used as the reducing agent. Moreover, the in-situ formed salts, NaCl and KCl, could be considered as the templates, which can totally be dissolved in water. As compared to the soft template and the hard template methods, this water washing process is much easier to remove the templates. The as-prepared samples show mesoporous structures. The crystalline and porous structures could be controlled by varying the calcination temperature. When the calcination temperature was as high as 500 °C, the sample of mGe-500 showed high specific capacity and excellent rate capability, compared with those of mGe-RT and mGe-300. Acknowledgements
Fig. 7. (a) Cyclic voltammetry curves of three charge/discharge cycles of mGe-500 electrode. (b) Charge/discharge curves of mGe-500 during three charge/discharge cycles.
This research was supported by the National Nature Science Foundation of China (Nos 91845201 and 21601128), Natural Science Foundation of Liaoning Province of China (Materials Joint Foundation, No. 20180510031), Liaoning Provincial Instrument and Equipment Sharing Service Platform Building Project, Support
Please cite this article as: D. Tang, H. Yu, J. Zhao et al., Bottom-up synthesis of mesoporous germanium as anodes for lithium-ion batteries, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.024
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Plan for Innovative Talents in Colleges and Universities of Liaoning Province (No. 22), The Open Project of State Key Laboratory of Supramolecular Structure and Materials (sklssm2019024), Highlevel Innovative Talents Project of Shenyang City (No. RC180171), Program for Excellent Talents in Shenyang Normal University (No. 054-51600210, BS201621), Engineering Technology Research Center of Catalysis for Energy and Environment, Major Platform for Science and Technology of the Universities in Liaoning Province, Liaoning Province Key Laboratory for Highly Efficient Conversion and Clean Utilization of Oil and Gas Resources, and the Engineering Research Center for Highly Efficient Conversion and Clean Use of Oil and Gas Resources of Liaoning Province. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.11.024. References [1] Y. Wang, G. Cao, Developments in nanostructured cathode materials for highperformance lithium-ion batteries, Adv. Mater. 20 (2008) 2251–2269. [2] H. Li, Z. Wang, L. Chen, X. Huang, Research on advanced materials for Li-ion batteries, Adv. Mater. 21 (2009) 4593–4607. [3] V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Challenges in the development of advanced Li-ion batteries: a review, Energy Environ. Sci. 4 (2011) 3243–3262. [4] X. Jiao, Y. Liu, T. Li, C. Zhang, X. Xu, O.O. Kapitanova, C. He, B. Li, S. Xiong, J. Song, Crumpled nitrogen-doped graphene-wrapped phosphorus composite as a promising anode for lithium-ion batteries, ACS Appl. Mater. Interfaces 11 (2019) 30858–30864. [5] Y. Liu, S. Xiong, J. Wang, X. Jiao, S. Li, C. Zhang, Z. Song, J. Song, Dendrite-free lithium metal anode enabled by separator engineering via uniform loading of lithiophilic nucleation sites, Energy Storage Mater. 19 (2019) 24–30. [6] Y. Hu, B. Li, X. Jiao, C. Zhang, X. Dai, J. Song, Stable cycling of phosphorus anode for sodium-ion batteries through chemical bonding with sulfurized polyacrylonitrile, Adv. Funct. Mater. 28 (2018) 1801010–1801017. [7] B.L. Ellis, K.T. Lee, L.F. Nazar, Positive electrode materials for Li-ion and Libatteries, Chem. Mater. 22 (2010) 691–714. [8] M.V. Reddy, G.V. Subba Rao, B.V.R. Chowdari, Metal oxides and oxysalts as anode materials for Li ion batteries, Chem. Rev. 113 (2013) 5364–5457. [9] Z. Song, H. Zhou, Towards sustainable and versatile energy storage devices: an overview of organic electrode materials, Energy Environ. Sci. 6 (2013) 2280– 2301. [10] X. Su, Q. Wu, J. Li, X. Xiao, A. Lott, W. Lu, B.W. Sheldon, J. Wu, Silicon-based nanomaterials for lithium-ion Batteries: a review, Adv. Energy Mater. 4 (2014) 1300882–1300904. [11] S. Gupta, R. Chen, Y.-C. Huang, Y. Kim, E. Sanchez, J.S. Harris, K.C. Saraswat, Highly selective dry etching of germanium over germanium-tin (Ge1-xSnx): A novel route for Ge1-xSnx nanostructure fabrication, Nano Lett. 13 (2013) 3783– 3790. [12] R. Yi, M.L. Gordin, D.H. Wang, Integrating Si nanoscale building blocks into micro-sized materials to enable practical application in lithium-ion batteries, Nanoscale 8 (2016) 1834–1848. [13] M.L. Terranova, S. Orlanducci, E. Tamburri, V. Guglielmotti, M. Rossi, Si/C hybrid nanostructures for Li-ion anodes: an overview, J. Power Sources 246 (2014) 167–177. [14] J.R. Szczech, S. Jin, Nanostructured silicon for high capacity lithium battery anodes, Energy Environ. Sci. 4 (2011) 56–72. [15] J. Song, S. Chen, M. Zhou, T. Xu, D. Lv, M.L. Gordin, T. Long, M. Melnyk, D. Wang, Micro-sized silicon-carbon composites composed of carbon-coated sub-10 nm Si primary particles as high-performance anode materials for lithium-ion batteries, J. Mater. Chem. A 2 (2014) 1257–1262. [16] R. Yi, F. Dai, M.L. Gordin, S. Chen, D. Wang, Micro-sized Si-C composite with interconnected nanoscale building blocks as high-performance anodes for practical application in lithium-ion batteries, Adv. Energy Mater. 3 (2013) 295–300. [17] R. Yi, J. Zai, F. Dai, M.L. Gordin, D. Wang, Improved rate capability of Si-C composite anodes by boron doping for lithium-ion batteries, Electrochem. Commun. 36 (2013) 29–32. [18] H. Lee, M.G. Kim, C.H. Choi, Y.-K. Sun, C.S. Yoon, J. Cho, Surface-stabilized amorphous germanium nanoparticles for lithium-storage material, J. Phys. Chem. B 109 (2005) 20719–20723.
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Please cite this article as: D. Tang, H. Yu, J. Zhao et al., Bottom-up synthesis of mesoporous germanium as anodes for lithium-ion batteries, Journal of Colloid and Interface Science, https://doi.org/10.1016/j.jcis.2019.11.024