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Hierarchical Structural Evolution of Zn2GeO4 in Binary Solvent and its Effect on Li-ion Storage Performance Wei Liu, Tengfei Zhou, Yang Zheng, Jianwen Liu, Chuanqi Feng, Yue Shen, Yunhui Huang, and Zaiping Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00582 • Publication Date (Web): 01 Mar 2017 Downloaded from http://pubs.acs.org on March 7, 2017

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ACS Applied Materials & Interfaces

Hierarchical Structural Evolution of Zn2GeO4 in Binary Solvent and its Effect on Li-ion Storage Performance Wei Liu,† Tengfei Zhou,‡ Yang Zheng,‡ Jianwen Liu,† Chuanqi Feng,† Yue Shen,§ Yunhui Huang,§ and Zaiping Guo,*,†,‡

†

Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry-of-Education

Key Laboratory for Synthesis and Applications of Organic Functional Molecules, Hubei University, Wuhan 430062, China. ‡

Institute for Superconducting and Electronic Materials, School of Mechanical, Materials, and Mechatronics

Engineering, University of Wollongong, North Wollongong, NSW 2500, Australia. §

State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and

Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China.

ACS Paragon Plus Environment

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ABSTRACT:

Zinc germinate (Zn2GeO4) with hierarchical structure was successfully synthesized in a binary ethylenediamine/water (En/H2O) solvent system by wet chemistry methods. The morphological evolution process of the Zn2GeO4 was investigated in detail by tuning the ratio of En to H2O in different solvent systems, and a series of compounds with awl-shaped, fascicular, and cross-linked hierarchical structures were obtained and employed as anode materials in lithium ion batteries. The materials with fascicular structure exhibited excellent electrochemical performance, and a specific reversible capacity of 1034 mA h g-1 was retained at a current density of 0.5 A g-1 after 160 cycles. In addition, the as-prepared nanostructured electrode also delivered impressive rate capability of 315 mA h g-1 at the current density of 10 A g-1. The remarkable electrochemical performances could be ascribed to the following aspects. First, each unit in the threedimensional fascicular structure can effectively buffer the volume expansions during the Li+ extraction/insertion process, accommodate the strain induced by the volume variation, and stabilize its whole configuration. Meanwhile, the small fascicular units can enlarge the electrode/electrolyte contact area and form an integrated interlaced conductive network, which provides continuous electron/ion pathways.

KEYWORDS: Zn2GeO4, hierarchical, fascicular, network, lithium-ion batteries


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INTRODUCTION Due to the scarcity of fossil fuels and global warming problems, rechargeable lithium-ion batteries (LIBs) are becoming the most widely used energy sources in electronic devices. The rapidly growing demand for electric vehicles, portable electronic devices, and other energy storage devices have triggered the current urgent pursuit of advanced batteries with high energy density, high power density, long and stable cycle life, and fast chargedischarge.1–5 In the past decade, a plenty of anode materials have been studied for high capacity LIBs.6–10 But recently, group IV elements, such as silicon (Si) and germanium (Ge) materials have received much more attention as promising alternative active materials to the conventional carbonaceous anodes (372 mA h g-1), owing to their high theoretical specific capacities (4200 mA h g-1 and 1600 mA h g-1 for Si and Ge, respectively) and excellent rate capabilities.11,12 Compared with Si, Ge has some characteristic advantages, including the faster lithium diffusivity in Ge (∼400 times greater than in Si at room temperature), and higher electrical conductivity (∼100 times higher than in Si).13 Nevertheless, to develop superior Ge anode materials with high reversible capacity and long cycle life, two main problems need to be overcome: (1) it suffers from serious capacity fading even a few cycles due to cracking of the active materials, which is caused by the vast volume expansion (∼400% for Si and ~300% for Ge) during insertion and extraction process, (2) the price of Ge anode material is too high for the commercialization.13 Various methods have been reported to enhance the cycling stability of Ge in previous studies, including better stabilized structures, such as nanoparticles,14 nanowires,15 nanotubes,16 and porous17,18 structures. The most feasible materials, however, are germanium oxides,19 germanium-based composites,20–22 and germinates,23–28 which not only can greatly improve the volume expansion, but also efficiently lower the cost via reducing the use of germanium, making these materials attractive for further research. As a very important ternary oxide material, zinc germinate (Zn2GeO4, ZGO) has been accepted as a promising candidate for LIBs due to its high theoretical capacity and excellent rate capability, although the pristine crystalline ZGO shows poor behavior in terms of its cycle life and coulombic efficiency.29–32 Vast numbers of researchers have attempted to design ideal structures of active material and essentially improve its drawbacks33-35. For instance, urchin-like Ca2Ge7O16 hierarchical hollow microspheres were proposed to shorten lithium-ion diffusion pathways and facilitate the penetration of electrolyte,23 Na2Ge4O9 nanoparticles encapsulated in three-dimensional (3D) carbon


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networks can effectively buffer volume change and prevent the aggregation of active materials,24 and layered crystalline CuGeO3 nanowires/graphene prevented the structures from detaching and aggregating during the charge and discharge processes.26 So, we propose that the electrochemical performance and lithium storage capacity of ZGO materials can be further optimized by tuning their morphology and structure. Herein, forest-like hierarchical structures, with the secondary structure based on awl-shaped, fascicular, and cross-linked ZGO, and ZGO nanorods, have been successfully prepared in an ethylenediamine/water (En/H2O) binary solvent system. En was demonstrated to have a great influence on the morphology and self-assembled secondary structure of ZGO. The fascicular ZGO exhibited excellent electro-chemical performance. Discharge capacity of 1034 mA h g-1 was retained at a current density of 0.5 A g-1, even after 160 cycles, and capacity of 1336 and 315 mA h g-1 was recorded at a current density of 0.1 and 10 A g-1, respectively, in the rate capability test. The superior electrochemical properties of fascicular ZGO are likely to be due to its stabilized special secondary structure, integrated interlaced conductive network, large electrolyte contact area, and continuous lithium-ion and electron pathways. This strategy can effectively enhance both the cycling stability and the rate performance of fascicular ZGO anode materials for lithium-ion batteries.

EXPERIMENTAL SECTION Synthesis of Zn2GeO4. In a typical synthesis, 1.10 g of Zn(CH3COO)2·2H2O (5 mmol) was dissolved in a mixed solution of En/H2O with different ratios, and then, 0.26 g of GeO2 (2.5 mmol) was added into that solution. The mixture was sealed, stirred for 60 min, and finally transferred into a stainless steel Teflon-lined autoclave. The reaction was performed under auto-generated pressure at 180 °C for 24 h in an electric oven, followed by natural cooling to room temperature. The product was collected by centrifugation, washed with deionized water and alcohol several times, and dried at 80 °C for 24 hours. Finally, a white powder was obtained. Other ZGO samples were prepared under the same conditions, except for different ratios of En/H2O. The ratios of H2O and En were set to 1:0, 1:1, 1:2, and 1:3 and correspond to the samples denoted as ZGO-0, ZGO-1, ZGO-2 and ZGO-3. The detial informations about Materials Characterization and Electrochemical Measurements could be found in Supporting Informations.

RESULTS AND DISCUSSION


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In the binary En/H2O solvent system, the divalent metal ion Zn2+ has a strong coordination capability towards the ethylenediamine molecules, so a complex cation of [Zn(en)3]2+ is formed with three ethylenediamine molecules chelated to the metal center Zn2+ to yield an octahedral geometry, which plays a role as a controlled release formulation to regulate sequential reactions in the solvent system. In addition, the different morphologies and structures of the resultant materials indicated that the small organic ethylenediamine component acts as a template guiding the growth direction and self-assembly of the ZGO nanorods36. Therefore, the ethylenediamine molecule is believed to have a vital role in the assembly of different structures. The formation processes in En/H2O solvent can be described by the following steps (R[1a]–R[3]), and the preparation of ZGO is schematically illustrated in Scheme 1：

Zn(CH3COO)2 + 3en → [Zn(en)3]2+ + 2CH3COO− NH2CH2CH2NH2 + H2O ↔ [NH3CH2CH2NH2]+ + OH− GeO2 + OH− ↔ HGeO3− 2[Zn(en)3]2+ + HGeO3− + OH− ↔ Zn2GeO4 + 6en+ H2O

R[1a] R[1b] R[2] R[3]

Scheme 1. Preparation process for ZGO in En/H2O solution solvent system.


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Figure 1. (a) X-ray diffraction patterns of the as-prepared ZGO-0, ZGO-1, ZGO-2, and ZGO-3; (b) Raman spectra of ZGO-0 and ZGO-2; (c) XPS survey spectrum of ZGO-2.

The crystalline structures and chemical compositions of the as-prepared samples were examined by powder X-ray diffraction (XRD), as shown in Figure 1a. All the diffraction peaks of ZGO-0, ZGO-1, ZGO-2, and ZGO-3 were perfectly indexed to the rhombohedral phase of Zn2GeO4 (JCPDS, #11-0687, a = b = 1.423 nm, c = 0.953 nm, α = 90°, β = 90°, γ =120°), and there were no other detectable impurity diffraction peaks such as ZGO–ethylenediamine inorganic–organic hybrid, which has been reported in previous studies.37,38 Compared with the ZGO-1, the relative intensity of (113) and (410) peaks in ZGO-2 and ZGO-3 are different,


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it means the nanorod in ZGO hierarchical structure shows different exposed faces and the preferred growth orientations (a mainly dominant growth direction along (113) crystal face).39 The ZGO-0 and ZGO-2 samples were chosen for characterization via Raman spectroscopy (Figure 1b), and there were no detectable Raman peaks of carbon in the ZGO-2 sample from 1200 - 1600 cm-1. The detailed Raman peaks of ZGO-0 and ZGO-2 from 700 to 840 cm-1 are shown in the inset to Figure 1b. The peaks at 748 cm-1, 754 cm-1, and 778 cm-1 are assigned to Ge-O-Zn symmetric stretching, a defect oxygen mode, and Ge-O-Zn asymmetric stretching.39 The three samples (ZGO-1, ZGO-2 and ZGO-3) were prepared in En/H2O solvent, and the chemical states of the elements in ZGO-2 were chosen for further investigation by X-ray photoelectron spectroscopy (XPS). The whole spectrum (Figure 1c) of the ZGO-2 sample revealed that the as-obtained materials were composed of Zn, Ge, and O species. Figure S1a in the Supporting Information contains high-resolution XPS spectra of the major peaks with binding energies of 1044.9 (Zn 2p1/2) and 1021.8 eV (Zn 2p3/2), which correspond to the oxidation state of Zn2+, the high-resolution Ge 3d XPS spectrum shows an obvious peak at 32.1 eV, which can be ascribed to the Ge-O bond (Figure S1b), and the O 1s signal (Figure S1c) appears at 531.1 eV.32,40 All of the results are consistent with general Zn2GeO4. The microstructures of the as-prepared ZGO samples were examined via field-emission scanning electron microscopy (FE-SEM). A panoramic view of uniform ZGO-0 nanorods with a length of about 150 to 200 nm and a diameter of about 50 to 80 nm is shown in Figure 2a, and there no other morphologies were obtained in the hydrothermal reaction (En: H2O = 1:0). Awl-shaped ZGO-1 (Figure 2b) was obtained with a broken and rough surface in the mixed solution (En/H2O) with a ratio of En: H2O = 1:1, but when the ratio was increased to 1:2 and 1:3, the smooth and uniform nanorods self-assemble to fascicular ZGO-2 (Figure 2c) and cross-linked ZGO-3 (Figure 2d). Comparing the four samples, there are clearly huge differences in their morphology and secondary structures, which demonstrates that the concentration of ethylenediamine has a great effect on the insitu growth of ZGO in the binary En/H2O solvent system. The organic ethylenediamine component acts as a source of organic templates for directing the growth direction and self-assembly of the ZGO nanorods.36 In particular, when the ratio increased to 1:2, the small units of the 3D fascicular structure would grow more complete than in the 1:1 and 1:3 samples. Predictably, this type of structure can maintain a structural stability, adequate electrode/electrolyte contact, and an interlaced network structure in the electrode. In addition, the energy dispersive spectroscopy (EDS) spectra of the four samples revealed the presence of Zn, Ge, and O elements, as shown in Figure S2a–d.


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Figure 2e and f show typical transmission electron microscope (TEM) images of ZGO-0 and ZGO-2. A single nanorod of ZGO-0 can be seen in Figure 2e, which possesses a smooth surface, a semi-circular closed tip, and a diameter of approximately 70 nm. A high-resolution TEM (HRTEM) image and the corresponding fast Fourier transform-electron diffraction (FFT-ED) pattern of ZGO-0 are shown in Figure S3a and b. The corresponding inverse FFT (IFFT)-ED pattern (Figure 2f) generated from Figure S3a shows uniform lattice fringes with a spacing of 0.205 nm, corresponding to the exposed (600) crystal facets of rhombic hexahedral Zn2GeO4. The nanorods of fascicular ZGO-2 shown in Figure 2g, and the corresponding inverse FFT (IFFT)-ED pattern (Figure 2f) generated from Figure S3c display a spacing of 0.290 nm, in good agreement with exposed (113) crystal facets of rhombic hexahedral Zn2GeO4. Most importantly, the (113) crystal facet has a higher surface energy (1.56 Jm-2) compared with the other crystal facets, and which is more convenient and accessible for electronic transmission and intercalation of Li+ ions into this active phase.41

Figure 2. Detailed morphology and structural characterization of samples. SEM images of ZGO-0 (a), ZGO-1 (b), and ZGO-2 (c), and ZGO-3 (d) in the En/H2O solvent system. TEM images of ZGO-0 (e), and ZGO-2 (g), with the corresponding inverse FFT (IFFT)-ED patterns (f, h), respectively.

In view of their unique microstructures, all of the resultant ZGO samples were assembled into Li half-cells to investigate their electrochemical performance. Figure 3a shows discharge/charge profiles of ZGO-2 at a current


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density of 0.5 A g−1 in the potential window of 0.01−2.80 V (vs Li+/Li) for the 1st, 2nd, 50th and 150th cycles. The initial discharge and charge capacities are 1143 and 1455 mA h g−1, respectively, giving an initial coulombic efficiency (CE) of 65%, which can be assigned to the reduction process to metallic Ge and Zn, as well as the formation of a solid electrolyte interphase (SEI) layer. Interestingly, all of the four samples show a similar initial CE, although they possess disparate morphologies and microstructures. The discharge and charge profiles for the other three samples are shown in Figure S4.

Figure 3. (a) Galvanostatic charge-discharge profiles of ZGO-2 for selected cycles at a current density of 0.5 A g-1 with the potential window from 0.01 V to 2.80 V; (b) Rate performance of ZGO-0, ZGO-1, ZGO-2, and ZGO-3 at the discharge/charge current densities of 0.1, 0.5, 1, 3, 5, and 10 A g-1; (c) Cycling performances of ZGO-0, ZGO-1, ZGO-2, and ZGO-3 at the charge/discharge current density of 0.5 A g-1; (d) CV spectra of ZGO-2 electrode with the potential window from 0.01 V to 2.80 V; (e) EIS spectra of ZGO-0 and ZGO-2, with the inset showing the equivalent circuit.


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Figure 3c shows the cycling performances of ZGO-0, ZGO-1, ZGO-2, and ZGO-3 at a current density of 0.5 A g−1 for 160 cycles. During the charge/discharge cycles, the specific capacity of ZGO-2 remained stable above 1000 mA h g−1 at the current density of 0.5 A g−1, and the CE was stabilized at∼99% after the third cycle. The specific capacity shows a little fading after 10 cycles, however, and then the capacity keeps increasing gradually from the 60th cycle, indicating a long-standing activation process in the ZGO material. In contrast, the specific capacity of ZGO-0 decreases quickly from 1143 to 494 mA h g−1 at 0.5 A g−1 after 160 cycles, and the CE is around 51% after second cycle. Apparently, ZGO-0 suffers from severe capacity degradation and pulverization during charge/discharge cycling, which is related to the large volume changes in ZGO materials, causing by cracking in the active materials during lithium ion insertion and extraction. The specific capacities of ZGO-1 and ZGO-3 were also recorded: they showed better cycling stability than ZGO-0, but lower capacities than ZGO-2. The different electrochemical performance results should be ascribed to the different designs of their structures.

The rate capabilities were further investigated at various current densities (Figure 3b). The charge/discharge current densities were varied from 0.1 to 0.5, 1, 3, 5, and 10 A g-1, and the average discharge capacities of ZGO-2 were 1336, 940, 753, 627, 503, and 315 mA h g-1. When the current density was returned to 0.1 A g−1, the reversible capacity of ZGO-2 returned to 1325mA h g−1 immediately, but ZGO-0 could only deliver the capacity of 967 mA h g−1 under the same conditions, revealing the good capacity recovery and stable structure of ZGO-2. In summary, according to this cycling behavior, fascicular ZGO-2 possesses a large specific surface area, ideal configuration, and stabilized interlaced structure, which guarantee higher lithium storage capacity and better cycling stability. Cyclic voltammetry (CV) of ZGO-0 and ZGO-2 was also performed for comparison, and the results were shown in Figure S5a and b at the slow scan rate of 0.10 mV/s. The third cycle curves were selected for study and are shown in Figure 3d. During the third cathodic process (discharge process), a sharp reduction peak at around 0.70 V (φ1) in both curves was observed, which can be assigned to the decomposition of ZGO, while in the anodic scan process, two broad oxidation peaks at 0.46 and 1.30 V (φ2) were observed, which are associated with the delithiation of Limetal alloys, followed by the re-oxidation of Zn and Ge, respectively. The better Li+ storage performance of ZGO-2 in comparison with ZGO-0 can be observed from the cyclic voltammograms (CVs). Remarkably, the oxidation reduction potential (∆φ = φ2-φ1) between ZGO-0 and ZGO-2 suggeststhe degree of polarization of the electrode, and we determined that the values of ∆φZGO-0 and ∆φZGO-2 were 0.73 and 0.57 V. ZGO-2 has a much lower voltage


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difference value than ZGO-0. Therefore, ZGO-2 was less polarized and had faster electrochemical reaction kinetics. This result could be closely associated with the fast kinetics of the redox reactions, which involve lithium ion and electron diffusion to/from the electrolyte/solid particle interface. Therefore, electrochemical impedance spectroscopy (EIS) measurements of ZGO-0 and ZGO-2 anodes were carried out. An equivalent electrical circuit was used to fit the Nyquist plots (Figure 3e), which were each composed of one semicircle in the high and medium frequency region and a sloping line in the low-frequency region. In the equivalent circuit, Re is the electrolyte resistance; R(sf+ct) is the resistance corresponding to the surface film and charge transfer; CPE (sf+dl) is the constant phase element resulting from the surface film and double layer capacitance; Rb is the bulk resistance; CPEb is the bulk capacitance, and Wo is the Warburg resistance. On the basis of the fitted results, the value of R(sf+ct) for fascicular ZGO-2 was 104.2 Ω, which is far lower than that for ZGO-0 (131.0 Ω).40,42 Therefore, this result further confirms that the fascicular ZGO-2 possesses high electrical conductivity, thus resulting in better rate capability and higher reversible capacity compared to other samples, which is consistent with its CV performance. To better understand the capacity fading of ZGO anode materials in different samples, we proposed an assumption of pulverization, agglomeration, and then loss of electrical contact in the ZGO electrodes, as presented in the schematic illustration in Figure 4a. Before the discharge/charge process, the nanorods of every small structural unit are well dispersed, with smooth surfaces and uniform size, while dozens or hundreds of cycles later, the nanorods become shaggy and swollen due to the insertion and extraction of Li+; with further cycling, the anode material suffers from pulverization and agglomeration, and even loss of electrical contact between the active material and the Cu collector, leading to serious capacity fading and short battery lifetime. Post-cycling SEM pictures have been studied and proved this assumption. Many large cracks clearly appear on the ZGO-0 electrode, and the nanorods of ZGO-0 suffer from serious pulverization after 160 cycles, with few nanorods keeping their original structure in Figure 4b. Some small cracks still can be observed after 160 cycles in ZGO-2, as shown in Figure 4c, but the fascicular ZGO-2 could maintain its basic structure, and every small fascicular unit still exists. So, compared with the other samples, we can attribute the excellent long-term cycling performance of the ZGO-2 sample to its firm secondary structure. It’s not only exhibited excellent performance in terms of capacity, but also a greatly improved response to the volume change and pulverization in ZGO.


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Figure 4. Schematic illustration of morphological changes during electrochemical cycling (a), post-cycling SEM images of ZGO-0 (b) and ZGO-2 (c) electrodes after 160 cycles, with the insets showing higher magnification.

CONCLUSION In summary, a series of compounds with awl-shaped, fascicular, and cross-linked ZGO and ZGO nanorods have been successfully synthesized in an ethylenediamine/water binary solvent system via a simple solvothermal route. The morphological evolution process of the ZGO was investigated in detail by tuning the different ratios of En and H2O in the solvent system. The fascicular ZGO exhibited excellent electrochemical performance. The discharge capacity was 1034 mA h g-1 with a current density of 0.5 A g-1 even after 160 cycles, and the capacity of 1336 and 315 mA h g-1 was recorded at a current density of 0.1 and 10 A g-1, respectively, in the rate capability test. We attribute the greatly enhanced electrochemical properties of fascicular ZGO to its stabilized special secondary structure, integrated interlaced conductive


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network, large electrolyte contact area, and continuous lithium-ion and electron pathways. Undoubtedly, our mixed solvent thermal system could be applied for preparing other advanced functional materials.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.xxxxxxx. Figures of XPS spectra, EDS spectrum, HRTEM and corresponding FFT pattern, the charge-discharge behaviour and cyclic voltammograms of prepared materials.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS Support from the National Natural Science Foundation of China (NSFC, 21476063) is gratefully acknowledged. Financial support provided by the Australian Research Council (ARC) (DP170102406 and FT150100109) are gratefully acknowledged. The authors would also like to thank the Analytical and Testing Center of Huazhong University of Science and Technology (HUST) for equipment access, and Dr. Tania Silver and Dr. Ruoying Zheng for critical reading and valuable remarks on the manuscript.

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Hierarchical Structural Evolution of Zn2GeO4 in ... - ACS Publications

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