Microwave-Assisted Metal-Organic Frameworks Derived Synthesis of Zn2GeO4 Nanowire Bundles for Lithium-Ion Batteries

Germanium-based multi-metallic-oxide materials have advantages of low activation energy, tunable output voltage, and high theoretical capacity. However, they also exhibit unsatisfactory electronic conductivity, sluggish cation kinetics, and severe volume change, resulting in inferior long-cycle stability and rate performance in lithium-ion batteries (LIBs). To solve these problems, we synthesize metal-organic frameworks derived from rice-like Zn2GeO4 nanowire bundles as the anode of LIBs via a microwave-assisted hydrothermal method, minimizing the particle size and enlarging the cation’s transmission channels, as well as, enhancing the electronic conductivity of the materials. The obtained Zn2GeO4 anode exhibits superior electrochemical performance. A high initial charge capacity of 730 mAhg−1 is obtained and maintained at 661 mAhg−1 after 500 cycles at 100 mA g−1 with a small capacity degradation ratio of ~0.02% for each cycle. Moreover, Zn2GeO4 exhibits a good rate performance, delivering a high capacity of 503 mA h g−1 at 5000 mA g−1. The good electrochemical performance of the rice-like Zn2GeO4 electrode can be attributed to its unique wire-bundle structure, the buffering effect of the bimetallic reaction at different potentials, good electrical conductivity, and fast kinetic rate.


Introduction
Rechargeable lithium-ion batteries (LIBs) have been widely used in fields such as electronics, power grids, and electric vehicles, due to their excellent performance. However, the low capacity (~372 mAh g −1 ) of the graphite anode material of LIBs cannot meet the current market demand for high-energy storage [1][2][3][4]. Therefore, there is an urgent need to develop high-energy-density electrode materials to meet the needs of social development [5]. Anode materials such as Si, Ge, Sn, Zn, Ni and their oxides have high theoretical capacities. As a result, they have been intensively studied by many researchers [6][7][8][9][10]. In the above electrode materials, Ge-based electrode materials exhibit a high theoretical capacity of~1600 mAh g −1 due to the reversible reaction of each Ge atom with 4.4 lithium atoms. Moreover, Ge also possesses fascinating features, such as high conductivity and fast Li + diffusivity, making it a promising anode candidate [11,12]. However, the volume of the germanium-based anode changes drastically during the frequent embedding and de-embedding of lithium ions, which seriously affects its structural stability [13]. Decreasing the particle size or dimensions to the nanoscale (such as nanoparticles, nanorods, nanofibers, nanowires, etc. [14,15]) can shorten the lithium ions' diffusion pathway and, to some degree, mitigate physical strains, thereby lowering the capacity fading. Moreover, the reserve of Ge on earth is not abundant, which makes it far from a practical application because of its cost and elemental abundance [16]. The design of bimetal oxides is an excellent option for decreasing material costs and maintaining the low discharging platform. The design of the optimized morphology and other elements such as Zn and O to form Ge-based bimetal oxides have been proven effective for improving the materials' electrochemical performances [17].
Despite great efforts, the practical application of this Zn 2 GeO 4 -based anode is still plagued by its rapid capacity decay and poor rate performance. Combining Zn 2 GeO 4 with carbon materials or other materials such as graphene, carbon nanowires, or carbon nanotubes is an effective way to solve the problem. Zhang et al. successfully synthesized Zn 2 GeO 4 -C-Ag by a simple impregnation/redox method. The electrode exhibited better electrochemical performance, and the specific capacity could be stabilized at 561 mAh g −1 after 50 cycles. In addition, the nanoscale Zn 2 GeO 4 nanorods were prepared through a hydrothermal method by Feng et al. This electrode material exhibits good electrochemical performance with an initial capacity of up to 995 mAh g −1 at 400 mA g −1 [18,19].
Lately, metal-organic framework (MOF) derived materials have gradually developed into promising anode materials for LIBs [20][21][22]. The broad investigation and utilization of MOFs are attributed to the highly-tunable and skeletal structures, and high surface-tovolume ratios, which contribute to the structural stability and electrical conductivity [23][24][25]. Therefore, MOFs with 3D pore architecture are rapidly developed and widely applied in energy storage, gas absorption, electrochemical, etc. [26][27][28]. Inspired by the merits arising from MOFs and Ge-based bimetal compounds, we propose the microwave-assisted method synthesis of Zn 2 GeO 4 anodes for LIBs using MOFs as both structural and compositional precursors. The fabricated Zn 2 GeO 4 could consequently inherit its MOF precursor's morphology and porous structure, which is supposed to facilitate lithium diffusion and improve the structural stability. In addition, the Zn 2 GeO 4 prepared by a microwave hydrothermal method has high phase purity and is suitable for the preparation of small-sized Zn 2 GeO 4 crystals, showing great advantages over the hydrothermal/solvent thermal method. The microwave heating method involves the interaction between the electromagnetic radiation and the molecular dipole moment, and this method allows the temperature of the whole reaction system to be uniform and greatly improves the reaction rate for the preparation of Zn 2 GeO 4 with uniform shapes compared with the conventional hydrothermal/solvent heat method.
In this work, to improve the electron transferability of germanium-based polymetallic oxide materials, accelerate the transport rate of lithium ions in the electrode, and improve the lithium storage performance, we synthesized rice-like Zn 2 GeO 4 nanowire bundles as the anode for LIBs by the microwave-assisted hydrothermal method. The obtained Zn 2 GeO 4 possesses minimized particle sizes, expanded cation transport channels, improved material electronic conductivity, and excellent electrochemical properties. The rice-like Zn 2 GeO 4 -150 nanowire bundles delivered a high reversible capacity of 730 mAh g −1 and maintained it at 661 mAh g −1 after 500 cycles at a current density of 100 mA g −1 . The superior electrochemical performances of Zn 2 GeO 4 -150 nanowire bundles are likely ascribed to the large specific surface area, robust structure, and the buffering effect of bi-metal reactions at different potentials. Since the Zn 2 GeO 4 prepared in this paper has good long-cycle stability and a fast kinetic rate, it is expected to be a lithium-ion battery anode with high-energydensity and long cycle life.

Synthesis of Ge-MOF
Under stirring conditions, 0.2 g of GeO 2 was dispersed in 10 mL of deionized water, and then 7.5 g of DACH was dropped into the GeO 2 suspension. Then, the above solution was transferred to a 50 mL Teflon-lined stainless steel autoclave and heated at 180 • C for 5 days. After cooling to room temperature, the solution was rinsed with acetone and deionized water, and dried under vacuum at 60 • C for 12 h to obtain a white precipitate of Ge-MOF.

Synthesis of Zn 2 GeO 4
An amount of 0.16 g of NaOH was dissolved into 30 mL of deionized water under stirring conditions. Then 0.16 g of ZIF-8 and 0.16 g of Ge-MOF were added into the above solution with stirring for 30 min. Finally, the mixed solution was transferred to a glass reaction vessel and allowed to react for 10 min under continuous magnetic stirring at 150 • C under microwave irradiation. The white precipitate was collected and centrifuged with deionized water several times and dried under a vacuum at 60 • C overnight. Then, the precursor was treated in a tube furnace at 700 • C for 5 h protected by N 2 to form a final product, which was named Zn 2 GeO 4 -150. For comparison, Zn 2 GeO 4 -130 and Zn 2 GeO 4 -180 were also prepared at 130 • C and 180 • C for 10 min under microwave irradiation and treated at 700 • C for 5 h.

Material Characterization
The scanning electron microscope (SEM, JSM-6700F, Tokyo, Japan) with energy dispersive spectroscopy (EDS) was used to characterize the morphology and microstructure of the products with a microscope acceleration voltage of 30 KV. Transmission electron microscopy (TEM, JEOL JEM-200CX, Tokyo, Japan) was used to characterize the internal structure of the products with a microscope acceleration voltage of 200 KV. X-ray diffraction (XRD, Rigaku D/max-2550 V, Cu Ka radiation, Tokyo, Japan) was used to determine the crystals of the product with a test voltage and current of 40 KV/30 MA, a test range of 5-85 • , and a scan speed of 2 • /min. X-ray photoelectron spectroscopy (XPS, PHI-5700, Tokyo, Japan) was used. The equipment of micromeritics instrument Corp, ASAP 2020 was used to measure the Brunauer-Emmett-Teller (BET) surface area of samples at 77 K.

Electrochemical Evaluation
The material in the working electrode consists of the active material, acetylene black, and poly(vinylidene fluoride) (PVDF), with a mass ratio of 8:1:1. The mass load of electrode material on copper foil is maintained at around 2 mg cm −2 , and maintained a thickness of several micrometers. Lithium foil was used as the anode for assembling coin batteries; LiPF 6 is a solvent for using propyl carbonate/ethyl carbonate (PC/EC) as an electrolyte. Constant current charging and discharging are measured at room temperature between 5 mV and 3.0 V (relative to Li + /Li) at different current densities (0.1-5 C) on a LAND-CT 2001, 1 C = 1000 mA g −1 . The voltage ampere (CV) circuit was measured at a scanning rate of 0.1 mV S −1 on a CHI660D. Electrochemical resistance testing (EIS) was performed on CHI660D at a frequency range in 0.01-1 × 10 6 Hz.

Characterization of Anode Materials
The synthesis process of the MOF derived rice-like Zn 2 GeO 4 nanowire bundles is schematically illustrated in Figure 1. XRD was performed on the Zn 2 GeO 4 products, Ge-Nanomaterials 2023, 13, 1432 4 of 13 MOF, and Zn-MOF. The typical XRD pattern of Ge-MOF is shown in Figure 2a, where all peaks are well-matched with the previously reported sample of C 6 H 10 (NH 2 ) 2 Ge 3 O 6 [29,30]. Furthermore, all the prominent peak positions of ZIF-8 are in good agreement with the previous reports [31,32]. Figure 2b shows that the prominent peaks of Zn 2 GeO 4 , including (110), (300), (220), (113), (410), and (223), are in good agreement with the rhombohedral Zn 2 GeO 4 (JCPDS No. 11-0687), indicating a high purity of Zn 2 GeO 4 via the microwaveinduced method [33]. All the Zn 2 GeO 4 -130, -150, and -180 samples exhibited type IV nitrogen adsorption/desorption isotherms with typical H1 hysteresis loops and their specific surface areas were approximately~31,~43 and~26 m 2 g −1 , respectively (Figure 2c,d), and the major pore sizes of all three Zn 2 GeO 4 samples ranged from~3 to 13 nm. The large difference in the specific surface area of Zn 2 GeO 4 is mainly due to the difference in sample morphology and crystallinity caused by the synthesis temperatures of various samples. electrolyte. Constant current charging and discharging are measured at room temperature between 5 mV and 3.0 V (relative to Li + /Li) at different current densities (0.1-5 C) on a LAND-CT 2001, 1 C = 1000 mA g −1 . The voltage ampere (CV) circuit was measured at a scanning rate of 0.1 mV S −1 on a CHI660D. Electrochemical resistance testing (EIS) was performed on CHI660D at a frequency range in 0.01-1 × 10 6 Hz.

Characterization of Anode Materials
The synthesis process of the MOF derived rice-like Zn2GeO4 nanowire bundles is schematically illustrated in Figure 1. XRD was performed on the Zn2GeO4 products, Ge-MOF, and Zn-MOF. The typical XRD pattern of Ge-MOF is shown in Figure 2a, where all peaks are well-matched with the previously reported sample of C6H10(NH2)2Ge3O6 [29,30]. Furthermore, all the prominent peak positions of ZIF-8 are in good agreement with the previous reports [31,32]. Figure 2b shows that the prominent peaks of Zn2GeO4, including (110), (300), (220), (113), (410), and (223), are in good agreement with the rhombohedral Zn2GeO4 (JCPDS No. 11-0687), indicating a high purity of Zn2GeO4 via the microwaveinduced method [33]. All the Zn2GeO4-130, -150, and -180 samples exhibited type IV nitrogen adsorption/desorption isotherms with typical H1 hysteresis loops and their specific surface areas were approximately ~31, ~43 and ~26 m 2 g −1 , respectively (Figure 2c,d), and the major pore sizes of all three Zn2GeO4 samples ranged from ~3 to 13 nm. The large difference in the specific surface area of Zn2GeO4 is mainly due to the difference in sample morphology and crystallinity caused by the synthesis temperatures of various samples.  Because of its highest surface area, the Zn2GeO4-150 sample was chosen for further investigations of the chemical composition and chemical bonding state by XPS analysis (Figure 3a-d). Figure 3a shows the overall XPS spectrum of the Zn2GeO4-150. The highresolution XPS spectra of the binding energies at 1045 and 1023 eV in Figure 3b correspond to the Zn 2p3/2 and Zn 2p1/2 peaks, showing the oxidation state of Zn 2+ [34,35]. Figure 3c,d reveal the binding energies of Ge 3d and O 1s peaks, centered at 32 and 531.2 eV, respec- Because of its highest surface area, the Zn 2 GeO 4 -150 sample was chosen for further investigations of the chemical composition and chemical bonding state by XPS analysis (Figure 3a-d). Figure 3a shows the overall XPS spectrum of the Zn 2 GeO 4 -150. The highresolution XPS spectra of the binding energies at 1045 and 1023 eV in Figure 3b correspond to the Zn 2p 3/2 and Zn 2p 1/2 peaks, showing the oxidation state of Zn 2+ [34,35]. Figure 3c,d reveal the binding energies of Ge 3d and O 1s peaks, centered at 32 and 531.2 eV, respectively, which prove the existence of Ge-O bonding in the form of tetravalent Ge 4+ [36]. In addition, the O 1s split peaks appear at 530.3 eV, 531.0 eV and 532.0 eV, corresponding to lattice oxygen, oxygen of the hydroxide ion and C-O or O-C=O bonds, respectively. All the binding energy results are consistent with the XRD results of Zn 2 GeO 4 [37]. The surface morphology and structure are shown in Figure 4a-h by SEM and TEM images. The Ge-MOF precursor with a plate-like morphology is shown in Figure 4a. A rhombic dodecahedral morphology of the Zn-MOF sample is shown in Figure 4b, with an average size of ~50 nm. As seen in Figure 4c, Zn2GeO4-150 particles are uniformly distributed with a rough surface, uniform size, and a rice grain shape with a diameter of ~90 nm and a length of ~250 nm. A closer observation is presented in Figure 4d-f, rice-like Zn2GeO4 nanowire bundles were accumulated by several nanowire bundles, which were further confirmed by the TEM image (Figure 4g,h). Figures 4g and h presents a mass of small nanowires with an average width of ~20 nm and a length of ~80 nm, mounted on the surface of rice-like Zn2GeO4-150 particles. The rice-like Zn2GeO4 nanowire bundles are accumulated by several nanowire bundles which may mitigate the volume change during cycling and promote a long-cycle stability of the electrode. In addition, the small size of the Zn2GeO4 nanowire bundle particles can shorten the ion and electron transport paths and promote fast reaction kinetics in the electrode. The EDS spectrum shown in Figure 4i demonstrates the presence of Zn, Ge, and O elements. In addition, the C element is also observed, which is derived from the carbon-based adhesive tape substrate, used to support and disperse the SEM samples. The SEM image reveals the structure of Zn2GeO4 under the different microwave irradiation temperatures of 130 and 180 °C in Figure 4j,k. It is confirmed that the single crystallite of Zn2GeO4-130 (Figure 4j) is not yet formed at low temperatures. As the microwave irradiation temperature increases up to 180 °C, the ricelike structure of Zn2GeO4 breaks down and is transformed into a random rectangular structure (Figure 4k). Figure 4l, l-a-l-c shows multiple element mapping images that confirm the uniform distribution of Ge, Zn, and O elements. The surface morphology and structure are shown in Figure 4a-h by SEM and TEM images. The Ge-MOF precursor with a plate-like morphology is shown in Figure 4a. A rhombic dodecahedral morphology of the Zn-MOF sample is shown in Figure 4b, with an average size of~50 nm. As seen in Figure 4c, Zn 2 GeO 4 -150 particles are uniformly distributed with a rough surface, uniform size, and a rice grain shape with a diameter of 90 nm and a length of~250 nm. A closer observation is presented in Figure 4d-f, ricelike Zn 2 GeO 4 nanowire bundles were accumulated by several nanowire bundles, which were further confirmed by the TEM image (Figure 4g,h). Figure 4g,h presents a mass of small nanowires with an average width of~20 nm and a length of~80 nm, mounted on the surface of rice-like Zn 2 GeO 4 -150 particles. The rice-like Zn 2 GeO 4 nanowire bundles are accumulated by several nanowire bundles which may mitigate the volume change during cycling and promote a long-cycle stability of the electrode. In addition, the small size of the Zn 2 GeO 4 nanowire bundle particles can shorten the ion and electron transport paths and promote fast reaction kinetics in the electrode. The EDS spectrum shown in Figure 4i demonstrates the presence of Zn, Ge, and O elements. In addition, the C element is also observed, which is derived from the carbon-based adhesive tape substrate, used to support and disperse the SEM samples. The SEM image reveals the structure of Zn 2 GeO 4 under the different microwave irradiation temperatures of 130 and 180 • C in Figure 4j,k. It is confirmed that the single crystallite of Zn 2 GeO 4 -130 (Figure 4j) is not yet formed at low temperatures. As the microwave irradiation temperature increases up to 180 • C, the rice-like structure of Zn 2 GeO 4 breaks down and is transformed into a random rectangular structure (Figure 4k). Figure 4l, l-a-l-c shows multiple element mapping images that confirm the uniform distribution of Ge, Zn, and O elements.

Electrochemical Performances as Anode Material for LIBs
Rice-like Zn2GeO4-150 and other Zn2GeO4 samples (Zn2GeO4-130 and Zn2GeO4-180) were all investigated as anode materials for LIBs at room temperature. The electrochemical properties are shown in Figure 5a-c. The electrochemical reaction mechanism of

Rice-like Zn 2 GeO 4 -150 and other Zn 2 GeO 4 samples (Zn 2 GeO 4 -130 and Zn 2 GeO 4 -180)
were all investigated as anode materials for LIBs at room temperature. The electrochemical properties are shown in Figure 5a-c. The electrochemical reaction mechanism of Zn 2 GeO 4 -150 was performed by investigating its initial three cyclic voltammograms (CV). During the first cathodic scan, a sharp peak was detected at around 0.6 V, which can be attributed to the formation of a solid electrolyte interface (SEI) film, and the Zn 2 GeO 4 decomposed into Zn, Ge, and Li 2 O. Another peak ranges from 0.3 V to 0.005 V, which corresponds to the formation of Li x Zn and Li y Ge alloys [38][39][40]. The Li 2 O matrix, formed during the discharging process, can be used as a cushioning pad to alleviate the volume changes of Ge and Zn during the Li x Zn and Li y Ge. alloying/de-alloying processes [41,42]. In the first anodic scan, two remarkable broad peaks were observed at~0.45 and 1.2 V, ascribed to the delithiation of Li x Zn and Li y Ge alloys at 0.45 V, and then further partially oxidized to the ZnO and GeO 2 at 1.2 V [43]. For the subsequent anode scan, two major reduction peaks were observed in the range of 0.6 to 1.2 V and 0.005 to 0.6 V. The major 0.75 V peak in the second cycle is shifted to a higher voltage compared to the peak in the first cycle and then returns to a lower voltage in the third cycle. This phenomenon suggests that the subsequent redox reactions occur mainly in the Li 2 O matrix with Zn-ZnO and Ge-GeO 2 , while the reactions between 0.6 and 0 V are due to the alloying of Li with Zn and Ge. In the subsequent cycles, the observed CV peaks almost overlap, indicating the excellent reversible electrochemical behavior of Zn 2 GeO 4 [44]. The initial capacity loss may come from the formation of an SEI layer and the multi-step reactions of Zn 2 GeO 4 [45,46]. Finally, during the discharging and charging processes, all electrochemical reactions can be described by the following Equations (1)-(5) [39][40][41][42][43][44][45][46]: Ge + yLi + + ye − ↔ Li y Ge (0 ≤ y ≤ 4.4) ZnO + 2Li + + 2e − ↔ Zn + Li 2 O (4) The cycle performances of Zn 2 GeO 4 -130, Zn 2 GeO 4 -150, and Zn 2 GeO 4 -180 electrodes are shown in Figure 5c. Among them, Zn 2 GeO 4 -150 exhibited the best electrochemical performance. A high initial reversible capacity of 730 mA h g −1 was obtained at a current density of 100 mA g −1 and remained at 661 mAh g −1 after 500 cycles, showing a highcapacity retention ratio of 90.5% and a low capacity degradation of about 0.02% for each cycle. In the initial cycle, there is an irreversible capacity loss, which is mainly due to the decomposition of the electrolyte and the formation of a solid electrolyte interface (SEI) layer. Moreover, a stable Coulombic efficiency (close to 100%) was obtained after the second cycle and remained stable in subsequent cycles. The excellent cyclic performance of the Zn 2 GeO 4 -150 electrode is probably ascribed to the porous structure, the formation of Li x Zn and Li y Ge alloys with rich redox reactions and favorable buffering effects and enhanced electrical conductivity [47,48]. The two samples of Zn 2 GeO 4 -130 and Zn 2 GeO 4 -180 delivered higher reversible capacities of 700 mAh g −1 and 690 mAh g −1 , respectively, close to the initial capacity of Zn 2 GeO 4 -150. However, capacities of Zn 2 GeO 4 -130 and Zn 2 GeO 4 -180 faded rapidly, with the retained capacities of~410 mA h g −1 and 390 mA h g −1 respectively after 500 cycles, followed by the low-capacity retention ratios.
Ge + yLi + + ye − ↔ LiyGe (0 ≤ y ≤ 4.4) ZnO + 2Li + + 2e − ↔ Zn + Li2O (4) GeO2 + 4Li + + 4e − ↔ Ge + 2Li2O (5) The cycle performances of Zn2GeO4-130, Zn2GeO4-150, and Zn2GeO4-180 electrodes are shown in Figure 5c. Among them, Zn2GeO4-150 exhibited the best electrochemical performance. A high initial reversible capacity of 730 mA h g −1 was obtained at a current density of 100 mA g −1 and remained at 661 mAh g −1 after 500 cycles, showing a high-capacity The rate capabilities of Zn 2 GeO 4 -130, Zn 2 GeO 4 -150, and Zn 2 GeO 4 -180 are shown in Figure 6a. Even though the current densities were changed rapidly, the Zn 2 GeO 4 -150 sample still exhibited the most stable cycling performance and the highest capacity. As shown in Figure 6a, the average specific capacities of the rice-like Zn 2 GeO 4 -150 were 730, 698, 627, 587, 544, and 503 mAh g −1 at step current densities of 0.1 to 5 C. When the current density returned to 0.1 C, the capacity of Zn 2 GeO 4 almost recovered (up to 709 mAh g −1 ), demonstrating the reversible insertion-extraction of Li cations and the stable rice-like structure [49]. In contrast, Zn 2 GeO 4 -130 and Zn 2 GeO 4 -180 showed unfavorable rate capacities with a low and rapid decrease in capacities under the same testing conditions, revealing the significant impacts of synthesis temperatures on the electrochemical performance of samples. The large difference in rate capacity between Zn 2 GeO 4 -130 and Zn 2 GeO 4 -180 is mainly due to the effect of the microwave synthesis temperature on the crystalline structure of the Zn 2 GeO 4 material, which in consequence affects the electrical conductivity, ion diffusion rate, and structural stability of the material.
The obtained Nyquist plots of the AC impedance of the Zn 2 GeO 4 products are shown in Figure 6b. The Nyquist plots of Zn 2 GeO 4 -150 have a similar semi-circle, along with the Zn 2 GeO 4 -130 and Zn 2 GeO 4 -180 samples. However, the charge transfer resistance (R ct ) of Zn 2 GeO 4 -150 is~75 Ω. This is much smaller than Zn 2 GeO 4 -130 (~150 Ω) and Zn 2 GeO 4 -180 (~100 Ω), demonstrating that the rice-like Zn 2 GeO 4 -150 has better electronic conductivity and lower inner electrochemical resistance. In particular, compared to previous Ge-based anodes (Table 1) with all kinds of morphologies, such as nanoflower, nanorod, nanowire, or nanofiber, the rice-like Zn 2 GeO 4 -150 nanowire bundle electrode stands out by its remarkable electrochemical performance, which is ascribed to the robust structure and proper porosity derived from MOF precursors. [49]. In contrast, Zn2GeO4-130 and Zn2GeO4-180 showed unfavorable rate capacities with a low and rapid decrease in capacities under the same testing conditions, revealing the significant impacts of synthesis temperatures on the electrochemical performance of samples. The large difference in rate capacity between Zn2GeO4-130 and Zn2GeO4-180 is mainly due to the effect of the microwave synthesis temperature on the crystalline structure of the Zn2GeO4 material, which in consequence affects the electrical conductivity, ion diffusion rate, and structural stability of the material. The obtained Nyquist plots of the AC impedance of the Zn2GeO4 products are shown in Figure 6b. The Nyquist plots of Zn2GeO4-150 have a similar semi-circle, along with the Zn2GeO4-130 and Zn2GeO4-180 samples. However, the charge transfer resistance (Rct) of Zn2GeO4-150 is ~75 Ω. This is much smaller than Zn2GeO4-130 (~150 Ω) and Zn2GeO4-180 (~100 Ω), demonstrating that the rice-like Zn2GeO4-150 has better electronic conductivity and lower inner electrochemical resistance. In particular, compared to previous Ge-based anodes (Table 1) with all kinds of morphologies, such as nanoflower, nanorod, nanowire, or nanofiber, the rice-like Zn2GeO4-150 nanowire bundle electrode stands out by its  As the excellent cyclic performance and rate performance of Zn 2 GeO 4 -150 draw our attention, the relative lithium kinetics in Zn 2 GeO 4 -150 should also be measured to explain the possible reason. Figure 7a shows the CV of the Zn 2 GeO 4 -150 electrode at different scan rates, from 0.2 to 1.0 mV s −1 . Peak 1 corresponds to the redox reactions occurring mainly in the Li 2 O matrix with Zn-ZnO and Ge-GeO 2 and peak 2 corresponds to the reaction of the Li x Zn and Li y Ge alloys oxidized to the ZnO and GeO 2 at 1.2 V. The lithium storage mechanism could be revealed by the relationship between the measured current (i) and the scan rate (v) from the CV curves, using the equations as follows: i = a × v b , log(i) = b × log(v) + log(a), where (v) represents the scan rate, (i) represents the current; both a and b (ranges from 0.5 to 1) are constants [51]. Figure 7b,c reveal the relationship between log(i) and log(v) of the Zn 2 GeO 4 -150. It showed that the values of b are 0.71 and 0.57 for oxidation along with a reduction state, respectively, suggesting both conversion reaction and capacitive behavior occur during the charge-discharge processes. Furthermore, the capacitive capacity was calculated using the equation: i (v) = k 1 × v + k 2 × v 1/2 , where i (v) represents the measured current at a fixed potential (v), k 1 and k 2 are the adjustable constant parameters, and v represents the scan rate [50]. The capacitive capacity contribution is about 57.9% for the Zn 2 GeO 4 -150 electrode at 0.2 mV s −1 . Particularly, when the value of v was increased up to 1.0 mV s −1 , the capacitive contribution enhanced up to 76.6% (Figure 7d). The capacitive contribution of the Zn 2 GeO 4 -150 electrode increases with the increase in the scan rate, which indicates the large existence of surface absorption mechanism and copious amounts of active spots of the rice-like Zn 2 GeO 4 nanowire bundles.

Conclusions
In summary, this work synthesized the rice-like Zn2GeO4 nanowire bundles using a microwave-induced hydrothermal method with subsequent pyrolysis of the MOF precursor. The Zn2GeO4-150 nanowire bundle electrode delivers a high initial reversible capacity of 730 mAh g −1 at a current density of 100 mA g −1 and retains a high reversible capacity of 661 mA h g −1 after 500 cycles. Moreover, good high-rate capabilities (1685, 698, 627, 577, 544, 503 mAh g −1 at step-up current densities of 0.1 C to 5 C) were also achieved because of the tiny particle size and improved conductivity as well as the robust structural stability of the Zn2GeO4-150 anode. The Zn2GeO4-150 nanowire bundles have also shown superior cycling performance with a high capacity retention ratio of 90.5% (500 cycles), which is attributed to the composition and structure associated merits such as the large specific surface area, robust carbon structure, buffering effect of the bi-metal reactions at different potentials, and excellent electronic conductivity, as well as the fast kinetics.

Conclusions
In summary, this work synthesized the rice-like Zn 2 GeO 4 nanowire bundles using a microwave-induced hydrothermal method with subsequent pyrolysis of the MOF precursor. The Zn 2 GeO 4 -150 nanowire bundle electrode delivers a high initial reversible capacity of 730 mAh g −1 at a current density of 100 mA g −1 and retains a high reversible capacity of 661 mA h g −1 after 500 cycles. Moreover, good high-rate capabilities (1685, 698, 627, 577, 544, 503 mAh g −1 at step-up current densities of 0.1 C to 5 C) were also achieved because of the tiny particle size and improved conductivity as well as the robust structural stability of the Zn 2 GeO 4 -150 anode. The Zn 2 GeO 4 -150 nanowire bundles have also shown superior cycling performance with a high capacity retention ratio of 90.5% (500 cycles), which is attributed to the composition and structure associated merits such as the large specific surface area, robust carbon structure, buffering effect of the bi-metal reactions at different potentials, and excellent electronic conductivity, as well as the fast kinetics.