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Cite this: Chem. Commun., 2014, 50, 13956 Received 24th August 2014, Accepted 11th September 2014
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Coordination complex pyrolyzation for the synthesis of nanostructured GeO2 with high lithium storage properties† Xiaona Li,a Jianwen Liang,a Zhiguo Hou,a Yongchun Zhu,*a Yan Wanga and Yitai Qian*ab
DOI: 10.1039/c4cc06658h www.rsc.org/chemcomm
A new (NH4)3H(Ge7O16)(H2O)2.72 precursor-pyrolyzation approach was designed and developed for the facile synthesis of nanostructured GeO2, avoiding the use of any hazardous or expensive germanium compounds. The products show promising anode application in lithium ion batteries with high capacity and excellent cycling stability.
For the past few years, great attention has been paid to GeO2 nanostructured materials owning to their wide range of applications, such as photosensors,1 optical waveguides,2 connections in optoelectronic communication3 and lithium ion batteries (LIBs).4–6 Meanwhile, considerable advances have been made in the preparation of various GeO2 nanostructures. Typical traditional preparation methods such as electrospinning methods,3,7 thermal vapor deposition methods,8–10 solution-phase approaches,6,11,12 and others,13,14 have been reported in recent years. However, these developed approaches have some restrictions, as they generally need high temperature, complex equipment, or expensive precursors (Ge(OPri)4, Ge(OEt)4, etc.) and hazardous solvents (benzene, tetrahydrofuran, etc.), which limit the use of GeO2 in LIBs and other categories. Therefore, it is urgent to develop a new effective and environmentally friendly strategy for the facile synthesis of GeO2 nanostructures. Electrospinning and thermal vapor deposition methods, using expensive raw precursors such as Ge(OPri)4, or requiring a high temperature range from 700 to 1200 1C, were initially adopted for the synthesis of GeO2, and the products obtained always showed one-dimensional structures. Soon afterwards, the solution-phase approach in aqueous systems was investigated.15
a
Hefei National Laboratory for Physical Science at Micro-scale, Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China. E-mail: [email protected], [email protected] b School of Chemistry and Chemical Engineering, Shandong University, Jinan, Shandong 250100, P. R. China † Electronic supplementary information (ESI) available: Experimental section; auxiliary analysis such as SEM images, TGA curve, XPS and Raman spectra, cycling performance of commercial GeO2. See DOI: 10.1039/c4cc06658h
13956 | Chem. Commun., 2014, 50, 13956--13959
However, the hydrolysis of GeCl4 and other organic germanium compounds proceeds too rapidly to control the final morphology of GeO2 in pure water without any other hydrolytic agents. Recently, the reverse micelle or microemulsion system, which is formed by surfactant solving in a non-polar solvent such as cyclohexane, benzene and octane, has been considered in some studies to prepare GeO2 nanostructures.12,16,17 Using this method, GeO2 hexabranched nanoparticles,17 hollow walnuts,12 and nanocubes,16 were successfully produced. An alternative solution brought forward is to introduce basic amino acid L-lysine or ammonium hydroxide into the aqueous system to slow down the hydrolysis process.11,18 Nevertheless, one challenge that has remained in these two strategies is the use of the expensive and environmentally hazardous germanium compound. Our previous report has shown that a metallic oxide can be obtained from its coordination complex precursor, and the morphology of precursor can be well retained.19 As for Ge(IV), it can also complex with some non-toxic inorganic compound ligands to form coordination complexes. If one of the coordination complexes can be synthesized using a facile, environmentally benign and cost-effective route, nanostructured GeO2 can also be obtained through pyrolyzation of the precursors. As a proof of concept and to address the challenge mentioned above, a new inorganic coordination complex precursor-pyrolyzation approach for the facile synthesis of nanostructured GeO2 was designed (as shown in Fig. 1a). Herein, (NH4)3H(Ge7O16)(H2O)2.72 (the crystal structure is shown in Fig. 1b), which is prepared via the reaction between commercial bulk hexagonal GeO2 (Fig. S1a and b, ESI†) and ammonium hydroxide (28.0–30.0%), is selected as the precursor to obtain GeO2 nanostructures. Neither expensive germanium compounds nor a toxic organic solvent is required. At the same time, this process achieves the synthesis of different kinds of nanostructured GeO2 such as nanoparticles, nanocubes, and nanospindles. For the synthesis of GeO2 nanoparticles (np-GeO2), 0.4 mol of commercial GeO2 and 0.8 mol of tartaric acid were added into 40 mL ammonium hydroxide (the detailed synthesis procedure is shown in the ESI†). Commercial bulk GeO2 was
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Fig. 1 (a) An illustration of the strategy for nanostructured GeO2 through the (NH4)3H(Ge7O16)(H2O)2.72 precursor-pyrolyzation synthetic approach. (b) View of the (NH4)3H(Ge7O16)(H2O)2.72 structure at different orientations.
converted into nanostructured (NH4)3H(Ge7O16)(H2O)2.72 via the following reaction in an autoclave at 150 1C for 6 h:
Fig. 2 (a) SEM image of agglomerated as-prepared reacted products. (b) SEM image, (c) TEM image, (d) HRTEM image, (e) XRD patterns and (f) BJH pores diameter distribution of the np-GeO2 product.
7GeO2 + 4.72NH3H2O = (NH4)3H(Ge7O16)(H2O)2.72 + 1.72NH3 As shown in Fig. 2a, the precursors obtained are mainly dominated by irregular nanoparticles with a size of about 200 nm, thus suggesting that the synthesis process may involve some kind of dissolution of bulk GeO2 in the reaction system. The typical X-ray diffraction (XRD) pattern of the precursors is shown in Fig. S2 (ESI†), and all diffraction peaks can be indexed to the cubic phase of (NH4)3H(Ge7O16)(H2O)2.72 (JCPDS Card No. 83-0957, space group: P4% 3m). Then np-GeO2 were obtained by pyrolysis of (NH4)3H(Ge7O16)(H2O)2.72 at 400 1C for 5 h in air (thermogravimetric analysis curves in Fig. S3, ESI†), and the corresponding XRD pattern presented in Fig. 2e matches well with that of the hexagonal phase structure of a-GeO2 (JCPDS Card No. 85-1515, space group: P3221). In addition, the XPS spectrum and Raman analysis (Fig. S4a and b, ESI†) of the final product further confirm the chemical composition and phase structure of a-GeO2, respectively. As can be seen from Fig. 2b, the resulting GeO2 product has the same irregular particle shape as (NH4)3H(Ge7O16)(H2O)2.72 and has a rough surface morphology. The transmission electron microscopy (TEM) image (Fig. 2c) of the np-GeO2 product clearly reveals the presence of pores in the irregular nanoparticles. High-resolution transmission electron microscopy (HRTEM, Fig. 2d) shows the lattice fringe images of 0.353 nm that can be indexed to the (011) diffraction peaks of hexagonal GeO2, confirming the crystalline nature of np-GeO2. The pore structure of np-GeO2 is characterized using the Brunauer–Emmett–Teller (BET, Fig. S5 (ESI†) for the nitrogen adsorption and desorption isotherms) method, with a specific surface area of 10.7 m2 g 1 and a distributional Barrett–Joyner–Halenda (BJH, Fig. 2f) mesopore size about 6 nm.
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It is noteworthy that the amount of tartaric acid and concentrations of reactants may play key roles in determining the shapes of the precursors, and thus also affect the formation of the final GeO2 in our study. Several experiments were performed regarding the influence of these parameters, and the post-annealing process remained unchanged. When the amount of commercial bulk and tartaric acid were changed to 0.2 and 0.4 mol, respectively, monodisperse GeO2 nanocubes (nc-GeO2) with size less than 100 nm were obtained (Fig. 3a). However, GeO2 nanospindles (ns-GeO2) assembled by much smaller nanoparticles were synthesized when no tartaric acid was added into the reaction system (Fig. 3b, XRD patterns and TEM images shown in the Fig. S6, ESI†). And these GeO2 nanocubes and nanospindles are similar to those prepared in the reverse micelle system12,16,18 or controlled hydrolysis of tetraethoxygermane without a surfactant.11 It is well known that GeO2 is soluble in alkali solution, and in this context, the formation of (NH4)3H(Ge7O16)(H2O)2.72 is in ammonium hydroxide solution. Both the amount of tartaric acid and concentrations of reactants probably
Fig. 3 SEM images of as-prepared (a) GeO2 nanocubes and (b) GeO2 nanospindles.
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influence the coordination between GeO2 and NH4OH by changing the pH and solubility of GeO2 in the reaction system, thus leading to the formation of different morphologies of GeO2. However, the exact mechanism remains unclear and is being investigated in ongoing studies in our laboratory. GeO2 has been considered as one of the emerging anode materials in next generation LIBs due to its high specific capacities, low operating voltage, and high electrical and ionic conductivities.5,6 The theoretical reversible capacity of GeO2 is B1126 mA h g 1, according to the following equations: GeO2 + 4Li+ + 4e - Ge + 2Li2O (irreversible) and Ge + 4.4Li+ + 4.4e 2 Li4.4Ge (reversible). However, it shows a fast capacity fading during cycling due to the large volume change between Ge and Li4.4Ge that causes pulverization and particle cracking.20–23 Thus, research studies have focused on synthesizing GeO2 using different nanostructures to minimize the effects of the stress and pulverization.4,6 Here, the lithium-storage properties of the as-prepared different nanostructured GeO2 anodes were investigated in CR2016 coin cells with lithium foil as a counter electrode, and the properties of commercial bulk GeO2 were also tested for comparison. Fig. 4a and c shows the cycling performance of the np-GeO2 anode at 0.1, 0.5, 1, and 2 C (1 C = 1000 mA h g 1). At a low current density of 0.1 C, in the initial cycle, the np-GeO2 anode delivers a discharge capacity of 2938 mA h g 1, and a charge capacity of 816 mA h g 1, corresponding to a Coulombic efficiency of B28%. The low initial efficiency can be attributed to a larger number of Li+ ions consumed to form Li2O and the formation of a massive solid electrolyte interface (SEI) layer on the surface of the electrodes.5,6,20 Though the low initial
Fig. 4 Electrochemical performances of GeO2 electrodes. (a, c) Cycling stabilities GeO2 anodes at 0.1, 0.5, 1, and 2 C. (b) np-GeO2 electrode cycled at various current densities. (d, e). Derivative galvanostatic dQ/dV profiles at 0.5 C.
13958 | Chem. Commun., 2014, 50, 13956--13959
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Coulombic efficiency may hinder the practical application, it could be tackled by prelithiating the anodes using stabilized lithium metal powder22 or by galvanostatic cycling in lithium half-cells at a low current for several cycles.24 From the second cycle, it can be observed that the reversibility of the electrode is improved significantly, and the Coulombic efficiency stays above 99% from the 5th cycle. The discharge capacity of the np-GeO2 anode can be stabilized at 1340 mA h g 1 at 0.1 C after 50 cycles, and reversible capacities of 1107, 830, 743 mA h g 1 can be achieved after 100 cycles at the rates of 0.5, 1 and 2 C, respectively. Furthermore, the cycling performance of nc-GeO2 and ns-GeO2 were also investigated (Fig. S7, ESI†). Though the capacity of nc-GeO2 is not as high as that of np-GeO2 at the same current density, no obvious capacity fading is observed, whereas the capacity of ns-GeO2 is the lowest among the three samples and fades gradually during cycling. In contrast, commercial bulk GeO2 only shows an initial charge capacity of 864 mA h g 1 with a capacity retention of B5% after 100 cycles (Fig. S8, ESI†). The poor cycling stability of bulk GeO2 may be due to the pulverization of micro-particles induced by unavoidable volume change during cycling, leading to a loss of electrical contact. Moreover, the rate capabilities of the GeO2 anodes were investigated by increasing the current densities from 0.1 C to 5 C. As shown in Fig. 4b, the np-GeO2 anode exhibits considerably good rate capability and still delivers a reversible capacity of 310 mA h g 1 at a high current density of 5 C, while the nc-GeO2 and ns-GeO2 anode show only 165 and 16 mA h g 1, respectively (Fig. S9, ESI†). Remarkably, when the current density is again reduced back to 0.2 C, high stable capacities of 1080 and 720 mA h g 1 can be resumed for np-GeO2 and nc-GeO2 anodes, while low and continuously decreasing capacities are observed for the ns-GeO2 electrode. Based on the lithium-storage properties of the as-prepared nanostructured GeO2 anodes in this context, we considered the question of what caused the variation in the electrochemical properties. The SEM and TEM images of the three samples show that the ns-GeO2 sample is assembled by much smaller nanoparticles than the others, and the particle size of np-GeO2 is the largest. As one of the semiconductor oxide materials, the effect of size on the electrochemical properties of GeO2 should be considered. Commonly, smaller particle sizes induce larger band gap, thus leading to lower electrical conductivities, which is not beneficial for the electrochemical reaction. Fig. S10 (ESI†) exhibits the electrochemical impedance spectroscopy (EIS) images of the as-prepared GeO2 electrodes before cycling. The Nyquist plots for all samples consist of a depressed semicircle in the high-middle-frequency region attributed to the chargetransfer resistance (Rct) and an inclined line in the low-frequency region. As can be seen, the semicircle diameter of np-GeO2 is the smallest, indicating the lowest value of Rct which may improve the process kinetics and achieve higher capacity and better rate capability. Additionally, it is interesting to note that the capacities of all samples increase slowly in the initial several cycles and then remain stable for the next few cycling processes. To reveal the reason for the gradual increase, the mechanism of lithium
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storage in the np-GeO2 anode was investigated by dQ/dV profiles in Fig. 4d and e. At the first discharge curve, the sharp irreversible feature around 0.4 V is related to the formation of the Ge and Li2O matrix due to the reaction between GeO2 and lithium. Then the cathodic current response in the 0.05–0.3 V range is attributed to the Li–Ge alloying process.5,25,26 In the initial charge process, a very broad peak centered at 0.38 V corresponds to the de-alloying of LixGe. The profiles of the 2nd and 3rd cycles are similar to that of the 1st, apart from the disappearance of the cathodic peak around 0.4 V. However, from the 20th cycle, two additional peaks appear during cycling: one is the anodic peak at 1.3 V about the partial re-oxidization of Ge to GeO2, the other is the cathodic peak at 0.6 V, which is attributed to the reduction of the re-oxidized GeO2 during the charge process.25,26 This phenomenon can also be observed in the galvanostatic curves of the np-GeO2 anode shown in Fig. S11 and 12 (ESI†). Based on the reaction between GeO2 anode and lithium, if some of the Ge could be re-oxidized to GeO2 during cycling, the capacity would be higher. From the enlarged derivative galvanostatic dQ/dV profiles in Fig. 4e, the value of dQ/dV at or around 0.6 V and 1.3 V after 20 cycles is much higher than that in the initial several cycles. Apart from that, the capacitive processes of the np-GeO2 anode during cycling might also contribute to the total capacity. This explains why the capacity in the 10th cycle is higher even though there is no obvious appearance of a re-oxidation/reduction peak. In summary, differently nanostructured GeO2 can be synthesized by a new facile coordination complex precursor-pyrolyzation process. The designed synthetic route has been successful in avoiding the use of any toxic or expensive germanium compound. Significantly, the as-prepared np-GeO2 exhibits excellent electrochemical storage properties with high specific reversible capacity of 1340 mA h g 1 up to 50 cycles at a current rate of 0.1 A g 1, good rate capability and stable cycle retention, as a suitable anode candidate material in LIBs. This work was supported by the 973 Project of China (No. 2011CB935901), and the National Natural Science Fund of China (No. 91022033, 21201158).
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Notes and references 1 V. V. Atuchin, T. A. Gavrilova, S. A. Gromilov, V. G. Kostrovsky, L. D. Pokrovsky, I. B. Troitskaia, R. Vemuri, G. Carbajal-Franco and C. Ramana, Cryst. Growth Des., 2009, 9, 1829–1832. ´ndez and J. Piqueras, Nanotechnology, 2007, 18, 155203. 2 P. Hidalgo, B. Me 3 P. Viswanathamurthi, N. Bhattarai, H. Kim, M. Khil, D. Lee and E.-K. Suh, J. Chem. Phys., 2004, 121, 441–445. 4 Y. Son, M. Park, Y. Son, J.-S. Lee, J.-H. Jang, Y. Kim and J. Cho, Nano Lett., 2014, 14, 1005–1010. 5 Y. Chen, C. Yan and O. G. Schmidt, Adv. Energy Mater., 2013, 3, 1269–1274. 6 Y.-M. Lin, K. C. Klavetter, A. Heller and C. B. Mullins, J. Phys. Chem. Lett., 2013, 4, 999–1004. 7 H. Kim, P. Viswanathamurthi, N. Bhattarai and D. Lee, Rev. Adv. Mater. Sci., 2003, 5, 220–223. 8 L. Armelao, F. Heigl, P.-S. G. Kim, R. A. Rosenberg, T. Z. Regier and T.-K. Sham, J. Phys. Chem. C, 2012, 116, 14163–14169. 9 Z. Gu, F. Liu, J. Y. Howe, M. P. Paranthaman and Z. Pan, Cryst. Growth Des., 2008, 9, 35–39. ´ndez and J. Piqueras, Nanotechnology, 2005, 16, 2521. 10 P. Hidalgo, B. Me 11 M. Javadi, Z. Yang and J. G. Veinot, Chem. Commun., 2014, 50, 6101–6104. 12 X. Zou, B. Liu, Q. Li, Z. Li, B. Liu, W. Wu, Q. Zhao, Y. Sui, D. Li and B. Zou, CrystEngComm, 2011, 13, 979–984. 13 Y. Tang, Y. Zhang, N. Wang, I. Bello, C. Lee and S. Lee, Appl. Phys. Lett., 1999, 74, 3824–3826. 14 Z. G. Bai, D. P. Yu, H. Z. Zhang, Y. Ding, Y. P. Wang, X. Z. Gai, Q. L. Hang, G. C. Xiong and S. Q. Feng, Chem. Phys. Lett., 1999, 303, 311–314. 15 M. Adachi, K. Nakagawa, K. Sago, Y. Murata and Y. Nishikawa, Chem. Commun., 2005, 2381–2383. 16 H. Wu, J. Liu, M. Ge, L. Niu, Y. Zeng, Y. Wang, G. Lv, L. Wang, G. Zhang and J. Jiang, Chem. Mater., 2006, 18, 1817–1820. 17 Y.-W. Chiu and M. H. Huang, J. Phys. Chem. C, 2009, 113, 6056–6060. 18 T. M. Davis, M. A. Snyder and M. Tsapatsis, Langmuir, 2007, 23, 12469–12472. 19 X. Li, X. Zhu, Y. Zhu, Z. Yuan, L. Si and Y. Qian, Carbon, 2014, 69, 515–524. 20 W. Wei and L. Guo, Part. Part. Syst. Charact., 2013, 30, 658–661. 21 J. Feng, M. Lai and L. Lu, Electrochim. Acta, 2012, 62, 103–108. 22 X.-L. Wang, W.-Q. Han, H. Chen, J. Bai, T. A. Tyson, X.-Q. Yu, X.-J. Wang and X.-Q. Yang, J. Am. Chem. Soc., 2011, 133, 20692–20695. 23 J. Graetz, C. Ahn, R. Yazami and B. Fultz, J. Electrochem. Soc., 2004, 151, A698–A702. ¨ller and S. Passerini, 24 A. Varzi, D. Bresser, J. von Zamory, F. Mu Adv. Energy Mater., 2014, 4, DOI: 10.1002/aenm.201400054. 25 A. Jahel, A. Darwiche, C. M. Ghimbeu, C. Vix-Guterl and L. Monconduit, J. Power Sources, 2014, 269, 755–759. 26 L. Baggetto and P. H. Notten, J. Electrochem. Soc., 2009, 156, A169–A175.
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Cite this: Chem. Commun., 2014, 50, 13956 Received 24th August 2014, Accepted 11th September 2014
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Coordination complex pyrolyzation for the synthesis of nanostructured GeO2 with high lithium storage properties† Xiaona Li,a Jianwen Liang,a Zhiguo Hou,a Yongchun Zhu,*a Yan Wanga and Yitai Qian*ab
DOI: 10.1039/c4cc06658h www.rsc.org/chemcomm
A new (NH4)3H(Ge7O16)(H2O)2.72 precursor-pyrolyzation approach was designed and developed for the facile synthesis of nanostructured GeO2, avoiding the use of any hazardous or expensive germanium compounds. The products show promising anode application in lithium ion batteries with high capacity and excellent cycling stability.
For the past few years, great attention has been paid to GeO2 nanostructured materials owning to their wide range of applications, such as photosensors,1 optical waveguides,2 connections in optoelectronic communication3 and lithium ion batteries (LIBs).4–6 Meanwhile, considerable advances have been made in the preparation of various GeO2 nanostructures. Typical traditional preparation methods such as electrospinning methods,3,7 thermal vapor deposition methods,8–10 solution-phase approaches,6,11,12 and others,13,14 have been reported in recent years. However, these developed approaches have some restrictions, as they generally need high temperature, complex equipment, or expensive precursors (Ge(OPri)4, Ge(OEt)4, etc.) and hazardous solvents (benzene, tetrahydrofuran, etc.), which limit the use of GeO2 in LIBs and other categories. Therefore, it is urgent to develop a new effective and environmentally friendly strategy for the facile synthesis of GeO2 nanostructures. Electrospinning and thermal vapor deposition methods, using expensive raw precursors such as Ge(OPri)4, or requiring a high temperature range from 700 to 1200 1C, were initially adopted for the synthesis of GeO2, and the products obtained always showed one-dimensional structures. Soon afterwards, the solution-phase approach in aqueous systems was investigated.15
a
Hefei National Laboratory for Physical Science at Micro-scale, Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China. E-mail: [email protected], [email protected] b School of Chemistry and Chemical Engineering, Shandong University, Jinan, Shandong 250100, P. R. China † Electronic supplementary information (ESI) available: Experimental section; auxiliary analysis such as SEM images, TGA curve, XPS and Raman spectra, cycling performance of commercial GeO2. See DOI: 10.1039/c4cc06658h
13956 | Chem. Commun., 2014, 50, 13956--13959
However, the hydrolysis of GeCl4 and other organic germanium compounds proceeds too rapidly to control the final morphology of GeO2 in pure water without any other hydrolytic agents. Recently, the reverse micelle or microemulsion system, which is formed by surfactant solving in a non-polar solvent such as cyclohexane, benzene and octane, has been considered in some studies to prepare GeO2 nanostructures.12,16,17 Using this method, GeO2 hexabranched nanoparticles,17 hollow walnuts,12 and nanocubes,16 were successfully produced. An alternative solution brought forward is to introduce basic amino acid L-lysine or ammonium hydroxide into the aqueous system to slow down the hydrolysis process.11,18 Nevertheless, one challenge that has remained in these two strategies is the use of the expensive and environmentally hazardous germanium compound. Our previous report has shown that a metallic oxide can be obtained from its coordination complex precursor, and the morphology of precursor can be well retained.19 As for Ge(IV), it can also complex with some non-toxic inorganic compound ligands to form coordination complexes. If one of the coordination complexes can be synthesized using a facile, environmentally benign and cost-effective route, nanostructured GeO2 can also be obtained through pyrolyzation of the precursors. As a proof of concept and to address the challenge mentioned above, a new inorganic coordination complex precursor-pyrolyzation approach for the facile synthesis of nanostructured GeO2 was designed (as shown in Fig. 1a). Herein, (NH4)3H(Ge7O16)(H2O)2.72 (the crystal structure is shown in Fig. 1b), which is prepared via the reaction between commercial bulk hexagonal GeO2 (Fig. S1a and b, ESI†) and ammonium hydroxide (28.0–30.0%), is selected as the precursor to obtain GeO2 nanostructures. Neither expensive germanium compounds nor a toxic organic solvent is required. At the same time, this process achieves the synthesis of different kinds of nanostructured GeO2 such as nanoparticles, nanocubes, and nanospindles. For the synthesis of GeO2 nanoparticles (np-GeO2), 0.4 mol of commercial GeO2 and 0.8 mol of tartaric acid were added into 40 mL ammonium hydroxide (the detailed synthesis procedure is shown in the ESI†). Commercial bulk GeO2 was
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Fig. 1 (a) An illustration of the strategy for nanostructured GeO2 through the (NH4)3H(Ge7O16)(H2O)2.72 precursor-pyrolyzation synthetic approach. (b) View of the (NH4)3H(Ge7O16)(H2O)2.72 structure at different orientations.
converted into nanostructured (NH4)3H(Ge7O16)(H2O)2.72 via the following reaction in an autoclave at 150 1C for 6 h:
Fig. 2 (a) SEM image of agglomerated as-prepared reacted products. (b) SEM image, (c) TEM image, (d) HRTEM image, (e) XRD patterns and (f) BJH pores diameter distribution of the np-GeO2 product.
7GeO2 + 4.72NH3H2O = (NH4)3H(Ge7O16)(H2O)2.72 + 1.72NH3 As shown in Fig. 2a, the precursors obtained are mainly dominated by irregular nanoparticles with a size of about 200 nm, thus suggesting that the synthesis process may involve some kind of dissolution of bulk GeO2 in the reaction system. The typical X-ray diffraction (XRD) pattern of the precursors is shown in Fig. S2 (ESI†), and all diffraction peaks can be indexed to the cubic phase of (NH4)3H(Ge7O16)(H2O)2.72 (JCPDS Card No. 83-0957, space group: P4% 3m). Then np-GeO2 were obtained by pyrolysis of (NH4)3H(Ge7O16)(H2O)2.72 at 400 1C for 5 h in air (thermogravimetric analysis curves in Fig. S3, ESI†), and the corresponding XRD pattern presented in Fig. 2e matches well with that of the hexagonal phase structure of a-GeO2 (JCPDS Card No. 85-1515, space group: P3221). In addition, the XPS spectrum and Raman analysis (Fig. S4a and b, ESI†) of the final product further confirm the chemical composition and phase structure of a-GeO2, respectively. As can be seen from Fig. 2b, the resulting GeO2 product has the same irregular particle shape as (NH4)3H(Ge7O16)(H2O)2.72 and has a rough surface morphology. The transmission electron microscopy (TEM) image (Fig. 2c) of the np-GeO2 product clearly reveals the presence of pores in the irregular nanoparticles. High-resolution transmission electron microscopy (HRTEM, Fig. 2d) shows the lattice fringe images of 0.353 nm that can be indexed to the (011) diffraction peaks of hexagonal GeO2, confirming the crystalline nature of np-GeO2. The pore structure of np-GeO2 is characterized using the Brunauer–Emmett–Teller (BET, Fig. S5 (ESI†) for the nitrogen adsorption and desorption isotherms) method, with a specific surface area of 10.7 m2 g 1 and a distributional Barrett–Joyner–Halenda (BJH, Fig. 2f) mesopore size about 6 nm.
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It is noteworthy that the amount of tartaric acid and concentrations of reactants may play key roles in determining the shapes of the precursors, and thus also affect the formation of the final GeO2 in our study. Several experiments were performed regarding the influence of these parameters, and the post-annealing process remained unchanged. When the amount of commercial bulk and tartaric acid were changed to 0.2 and 0.4 mol, respectively, monodisperse GeO2 nanocubes (nc-GeO2) with size less than 100 nm were obtained (Fig. 3a). However, GeO2 nanospindles (ns-GeO2) assembled by much smaller nanoparticles were synthesized when no tartaric acid was added into the reaction system (Fig. 3b, XRD patterns and TEM images shown in the Fig. S6, ESI†). And these GeO2 nanocubes and nanospindles are similar to those prepared in the reverse micelle system12,16,18 or controlled hydrolysis of tetraethoxygermane without a surfactant.11 It is well known that GeO2 is soluble in alkali solution, and in this context, the formation of (NH4)3H(Ge7O16)(H2O)2.72 is in ammonium hydroxide solution. Both the amount of tartaric acid and concentrations of reactants probably
Fig. 3 SEM images of as-prepared (a) GeO2 nanocubes and (b) GeO2 nanospindles.
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influence the coordination between GeO2 and NH4OH by changing the pH and solubility of GeO2 in the reaction system, thus leading to the formation of different morphologies of GeO2. However, the exact mechanism remains unclear and is being investigated in ongoing studies in our laboratory. GeO2 has been considered as one of the emerging anode materials in next generation LIBs due to its high specific capacities, low operating voltage, and high electrical and ionic conductivities.5,6 The theoretical reversible capacity of GeO2 is B1126 mA h g 1, according to the following equations: GeO2 + 4Li+ + 4e - Ge + 2Li2O (irreversible) and Ge + 4.4Li+ + 4.4e 2 Li4.4Ge (reversible). However, it shows a fast capacity fading during cycling due to the large volume change between Ge and Li4.4Ge that causes pulverization and particle cracking.20–23 Thus, research studies have focused on synthesizing GeO2 using different nanostructures to minimize the effects of the stress and pulverization.4,6 Here, the lithium-storage properties of the as-prepared different nanostructured GeO2 anodes were investigated in CR2016 coin cells with lithium foil as a counter electrode, and the properties of commercial bulk GeO2 were also tested for comparison. Fig. 4a and c shows the cycling performance of the np-GeO2 anode at 0.1, 0.5, 1, and 2 C (1 C = 1000 mA h g 1). At a low current density of 0.1 C, in the initial cycle, the np-GeO2 anode delivers a discharge capacity of 2938 mA h g 1, and a charge capacity of 816 mA h g 1, corresponding to a Coulombic efficiency of B28%. The low initial efficiency can be attributed to a larger number of Li+ ions consumed to form Li2O and the formation of a massive solid electrolyte interface (SEI) layer on the surface of the electrodes.5,6,20 Though the low initial
Fig. 4 Electrochemical performances of GeO2 electrodes. (a, c) Cycling stabilities GeO2 anodes at 0.1, 0.5, 1, and 2 C. (b) np-GeO2 electrode cycled at various current densities. (d, e). Derivative galvanostatic dQ/dV profiles at 0.5 C.
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Coulombic efficiency may hinder the practical application, it could be tackled by prelithiating the anodes using stabilized lithium metal powder22 or by galvanostatic cycling in lithium half-cells at a low current for several cycles.24 From the second cycle, it can be observed that the reversibility of the electrode is improved significantly, and the Coulombic efficiency stays above 99% from the 5th cycle. The discharge capacity of the np-GeO2 anode can be stabilized at 1340 mA h g 1 at 0.1 C after 50 cycles, and reversible capacities of 1107, 830, 743 mA h g 1 can be achieved after 100 cycles at the rates of 0.5, 1 and 2 C, respectively. Furthermore, the cycling performance of nc-GeO2 and ns-GeO2 were also investigated (Fig. S7, ESI†). Though the capacity of nc-GeO2 is not as high as that of np-GeO2 at the same current density, no obvious capacity fading is observed, whereas the capacity of ns-GeO2 is the lowest among the three samples and fades gradually during cycling. In contrast, commercial bulk GeO2 only shows an initial charge capacity of 864 mA h g 1 with a capacity retention of B5% after 100 cycles (Fig. S8, ESI†). The poor cycling stability of bulk GeO2 may be due to the pulverization of micro-particles induced by unavoidable volume change during cycling, leading to a loss of electrical contact. Moreover, the rate capabilities of the GeO2 anodes were investigated by increasing the current densities from 0.1 C to 5 C. As shown in Fig. 4b, the np-GeO2 anode exhibits considerably good rate capability and still delivers a reversible capacity of 310 mA h g 1 at a high current density of 5 C, while the nc-GeO2 and ns-GeO2 anode show only 165 and 16 mA h g 1, respectively (Fig. S9, ESI†). Remarkably, when the current density is again reduced back to 0.2 C, high stable capacities of 1080 and 720 mA h g 1 can be resumed for np-GeO2 and nc-GeO2 anodes, while low and continuously decreasing capacities are observed for the ns-GeO2 electrode. Based on the lithium-storage properties of the as-prepared nanostructured GeO2 anodes in this context, we considered the question of what caused the variation in the electrochemical properties. The SEM and TEM images of the three samples show that the ns-GeO2 sample is assembled by much smaller nanoparticles than the others, and the particle size of np-GeO2 is the largest. As one of the semiconductor oxide materials, the effect of size on the electrochemical properties of GeO2 should be considered. Commonly, smaller particle sizes induce larger band gap, thus leading to lower electrical conductivities, which is not beneficial for the electrochemical reaction. Fig. S10 (ESI†) exhibits the electrochemical impedance spectroscopy (EIS) images of the as-prepared GeO2 electrodes before cycling. The Nyquist plots for all samples consist of a depressed semicircle in the high-middle-frequency region attributed to the chargetransfer resistance (Rct) and an inclined line in the low-frequency region. As can be seen, the semicircle diameter of np-GeO2 is the smallest, indicating the lowest value of Rct which may improve the process kinetics and achieve higher capacity and better rate capability. Additionally, it is interesting to note that the capacities of all samples increase slowly in the initial several cycles and then remain stable for the next few cycling processes. To reveal the reason for the gradual increase, the mechanism of lithium
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storage in the np-GeO2 anode was investigated by dQ/dV profiles in Fig. 4d and e. At the first discharge curve, the sharp irreversible feature around 0.4 V is related to the formation of the Ge and Li2O matrix due to the reaction between GeO2 and lithium. Then the cathodic current response in the 0.05–0.3 V range is attributed to the Li–Ge alloying process.5,25,26 In the initial charge process, a very broad peak centered at 0.38 V corresponds to the de-alloying of LixGe. The profiles of the 2nd and 3rd cycles are similar to that of the 1st, apart from the disappearance of the cathodic peak around 0.4 V. However, from the 20th cycle, two additional peaks appear during cycling: one is the anodic peak at 1.3 V about the partial re-oxidization of Ge to GeO2, the other is the cathodic peak at 0.6 V, which is attributed to the reduction of the re-oxidized GeO2 during the charge process.25,26 This phenomenon can also be observed in the galvanostatic curves of the np-GeO2 anode shown in Fig. S11 and 12 (ESI†). Based on the reaction between GeO2 anode and lithium, if some of the Ge could be re-oxidized to GeO2 during cycling, the capacity would be higher. From the enlarged derivative galvanostatic dQ/dV profiles in Fig. 4e, the value of dQ/dV at or around 0.6 V and 1.3 V after 20 cycles is much higher than that in the initial several cycles. Apart from that, the capacitive processes of the np-GeO2 anode during cycling might also contribute to the total capacity. This explains why the capacity in the 10th cycle is higher even though there is no obvious appearance of a re-oxidation/reduction peak. In summary, differently nanostructured GeO2 can be synthesized by a new facile coordination complex precursor-pyrolyzation process. The designed synthetic route has been successful in avoiding the use of any toxic or expensive germanium compound. Significantly, the as-prepared np-GeO2 exhibits excellent electrochemical storage properties with high specific reversible capacity of 1340 mA h g 1 up to 50 cycles at a current rate of 0.1 A g 1, good rate capability and stable cycle retention, as a suitable anode candidate material in LIBs. This work was supported by the 973 Project of China (No. 2011CB935901), and the National Natural Science Fund of China (No. 91022033, 21201158).
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