Carbon-Free Porous Zn2GeO4 Nanofibers as Advanced Anode

Nov 2, 2016 - ... National & Local United Engineering Laboratory for Power Batteries, ... *E-mail: [email protected]., *E-mail: [email protected]...
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Carbon-Free Porous Zn2GeO4 Nanofibers as Advanced Anode Materials for High-Performance Lithium Ion Batteries Huan-Huan Li,†,§ Xing-Long Wu,†,‡,§ Lin-Lin Zhang,† Chao-Ying Fan,† Hai-Feng Wang,† Xiao-Ying Li,† Hai-Zhu Sun,*,† Jing-Ping Zhang,*,† and Qingyu Yan*,‡ †

Faculty of Chemistry, National & Local United Engineering Laboratory for Power Batteries, Northeast Normal University, Changchun, Jilin 130024, China ‡ School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore S Supporting Information *

ABSTRACT: In this work, carbon-free, porous, and micro/nanostructural Zn2GeO4 nanofibers (p-ZGONFs) have been prepared via a dissolution-recrystallization-assisted electrospinning technology. The successful electrospinning to fabricate the uniform p-ZGONFs mainly benefits from the preparation of completely dissolved solution, which avoids the sedimentation of common Ge-containing solid-state precursors. Electrochemical tests demonstrate that the as-prepared pZGONFs exhibit superior Li-storage properties in terms of high initial reversible capacity of 1075.6 mA h g−1, outstanding cycling stability (no capacity decay after 130 cycles at 0.2 A g−1), and excellent high-rate capabilities (e.g., still delivering a capacity of 384.7 mA h g−1 at a very high current density of 10 A g−1) when used as anode materials for lithium ion batteries (LIBs). All these Li-storage properties are much better than those of Zn2GeO4 nanorods prepared by a hydrothermal process. The much enhanced Li-storage properties should be attributed to its distinctive structural characteristics including the carbon-free composition, plentiful pores, and macro/nanostructures. Carbon-free composition promises its high theoretical Listorage capacity, and plentiful pores cannot only accommodate the volumetric variations during the successive lithiation/ delithiation but can also serve as the electrolyte reservoirs to facilitate Li interaction with electrode materials. KEYWORDS: lithium ion batteries, anode materials, Zn2GeO4, electrospinning, carbon-free

1. INTRODUCTION

in the severe loss of Li-storage capacity and hence poor cycle life, restricting their application in practical LIBs.25−27 To improve the cycling performance of TMOs, two strategies have been commonly used in recent years. The first one is to prepare diverse nanoscale structures, such as nanoparticles,28 nanorods,29 nanofibers,30,31 nanosheets,27,32 and so on. In comparison to bulk, nanostructural materials can provide much shorter distance/time of Li diffusion and higher surface-tovolume ratio, which are beneficial to the rate-performance enhancement as well as to the increase of actual capacity. Regarding the promising Zn2GeO4 anode, almost all of the previous studies mainly focus on the preparation of nanorods via the hydrothermal method to improve its Li-storage properties.33−35 For example, Zn2GeO4 nanorods (20−30 nm in diameter and 100−150 nm in length), prepared hydrothermally, deliver a high initial capacity of 995 mA h g−1, but the specific capacities decrease rapidly to about 616 mA h g−1 after 100 cycles.29 The capacity decay implies the unstable

With today’s modern technology, lithium ion batteries (LIBs) have been widely used as a main energy storage system to power portable electronics and electric vehicles.1−7 To meet the requirements of the rapidly developed applications, further improvements in terms of higher power and energy densities as well as longer cycling stability compared to present LIBs are pursued.6,8−11 In this context, it is critical to explore advanced electrode materials to improve the overall performance of LIBs. Among various electrode materials, Zn2GeO4 is proposed as a promising anode materials for LIBs because of its high theoretical capacity.12,13 This high value originates from the dual Li-alloying reactions of Zn and Ge atoms in addition to the conversion reaction14−21 in comparison to other Ge-containing metal oxides, such as Ca 2 Ge 7 O 16 , 22 Co 2 GeO 4 , 23 and BaGe4O9,24 in which only Ge atoms can alloy with Li. Unfortunately, similar to other transition-metal oxides (TMOs) used as anode materials for LIBs, Zn2GeO4 anode will also encounter large volumetric variations and sluggish kinetics of ionic transports and conversion reactions during the successive Li-uptake/release processes. These drawbacks will finally result © XXXX American Chemical Society

Received: September 10, 2016 Accepted: November 2, 2016 Published: November 2, 2016 A

DOI: 10.1021/acsami.6b11503 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces nature of nanomaterials in electrochemical processes.36 For nanoscale structures used as the anode in LIBs, there are several severe issues:36,37 (1) hard to handle for electrode preparation during scalable processes, (2) easy coalescence during the longterm successive lithiation/delithiation, (3) inability to stack densely in the electrode, making the energy density of the whole electrode relatively low, and (4) the complex, expensive, and low-yielding preparation procedures of nanostructures, limiting their practical applications. Hence, engineering micro/ nanostructures, in which these nanostructures are the nanoscale building blocks, is thought to be an ideal strategy to further improve the electrochemical properties of ordinary nanostructures.32,37 The second strategy is to construct conductive carbonaceous networks to form the TMO-containing composites.38−40 Although such networks optimize effectively the e− transfer pathway and thereby enhance the high-rate and cycling properties, the introduction of carbonaceous materials sometimes decreases the overall Li-storage capacity because of the much lower theoretical capacity (372 mA h g−1) of the carbon anode compared to that of the TMOs. For example, the pristine Zn2GeO4 delivered much higher specific capacity than the Zn2GeO4/g-C3N4 composite in the initial 40 cycles as previously reported.19 In such a situation, it is necessary to optimize the component ratio of carbon in the composite. Therefore, it is proposed that a micro/nanostructure with the absence of carbon and the presence of proper pores should be an ideal architecture to avoid the earlier mentioned issues. From previous reports, electrospinning technology has been demonstrated as an effective approach to fabricate micro/ nanostructures with uniform one-dimensional (1D) morphology.41,42 It also has ease of scalable production because of the macroscopic and simple handling processes. However, although few Zn2GeO4-based materials prepared by electrospinning technology have been reported,20 a long continuous nanofiber structure with evenly distributed porosity for Zn2GeO4 is still absent up to now. This may be mainly because almost all of the Ge-containing precursors (such as metallic Ge, GeO2, and organic germanium salts) are insoluble or can hydrolyze easily to become solid state in the precursor solution, which makes it very difficult to electrospin uniform 1D nanofibers (illustrated in Figure S1 of the Supporting Information). In this work, we, for the first time, prepare the uniform 1D porous Zn2GeO4 nanofibers (abbreviated as p-ZGONFs) with the characteristics of carbon-free, porous, and micro/nanostructures via the dissolution-recrystallization-assisted electrospinning technology. Electrochemical tests demonstrate that the as-prepared p-ZGONFs exhibit superior Li-storage properties in terms of the high reversible capacity and excellent highrate capabilities. More significantly, all these Li-storage properties are much better than those of Zn2GeO4 nanorods (ZGONRs) prepared by a commonly employed hydrothermal process. This demonstrates the advantages of as-prepared pZGONFs as anode materials for LIBs.

water with a mole ratio of 1:2 to form a suspension under ultrasonication. Subsequently, the obtained suspension was transferred into a Teflon-lined stainless steel autoclave (15 mL in volume) and was treated at 185 °C for 20 h. The resulting ZGONRs products were purified by centrifugation with distilled water and then were dried at 80 °C overnight. 2.3. Preparation of p-ZGONFs. Three hundred milligrams of the as-prepared ZGONRs was added into the 7.5 wt % aqueous PAA solution and then was stirred for 1 h at room temperature. After that, citric acid (CA, about 0.99 g) was added slowly until the mixture became clear and transparent (the dissolution process). The resulting viscous solution was loaded into a plastic syringe equipped with a flat stainless steel needle of 0.9 mm in diameter and was electrospun on the electrospinning devices. Finally, the obtained ZGONRs/PAACA fibers were annealed at 700 °C for 4 h under air atmosphere to gain the p-ZGONFs production (the recrystallization process). 2.4. Material Characterization. The structures of the asprepared ZGONRs and p-ZGONFs were characterized by powder X-ray diffraction (PXRD, Rigaku P/max 2200VPC) using Cu Kα radiation. X-ray photoelectron spectra (XPS) were performed with Al Kα radiation and energy step size of 0.1 eV. Field emission scanning electron microscope (FESEM, JEOL JSM-6700F field emission) and transmission electron microscope (TEM, JEM-2010F, 200 kV) were used to study the morphology of the products. To confirm the porous structure of p-ZGONFs, nitrogen (N2) adsorption isotherm was adopted at −196 °C. 2.5. Electrochemical Measurement. The working electrodes were prepared by mixing the active materials, acetylene black, and polyvinyldifluoride (PVDF, mass ratio 70:15:15) in N-methylpyrrolidone to form a slurry. The loading mass of the active material was about 1.2 mg cm−2, which is a common value for evaluating the electrochemical properties of an electrode material. Pure lithium foil was used as the counter electrode, and a 1.0 mol L−1 LiPF6 in ethylene carbonate (EC)/ dimethyl carbonate (DMC) was used as the electrolyte. The 2032 coin cells were assembled in an Ar-filled glovebox (SUniversal 2440/750, from MIKROUNA) with the concentrations of moisture and oxygen below 0.1 ppm. The specific capacity and current density were calculated on the basis of the mass of the active materials. Galvanostatic cycling measurements were made using a Land battery test system (LAND CT2001A) from 0.005 to 2.9 V at room temperature. Cyclic voltammetry (CV) curves were taken using a VersaSTAT 3 (Princeton Applied Research) over the potential range of 0.005−2.9 V at a scan rate of 0.1 mV s−1. Electrochemical impedance spectroscopy (EIS) measurements were carried out in half cells on P4000 Potentiostats Electrochemistry Workstation with a ±5 mV ac signal amplitude and a frequency that ranged from 10 kHz to 0.01 Hz.

3. RESULTS AND DISCUSSION Scheme 1 illustrates the preparation processes of p-ZGONFs. As shown, the precursor of Zn2GeO4 nanorods (ZGONRs) was first synthesized by a common hydrothermal method and then was dispersed uniformly into the poly(acrylic acid) (PAA) aqueous solution. Subsequently, CA was added slowly into the obtained dispersion until all ZGONRs were dissolved completely. The resulting viscous solution can be easily electrospun into well-defined nanofibers because of the absence of any solid-state precipitates in the precursor solution. Finally,

2. EXPERIMENTAL SECTION 2.1. Materials. All raw materials, including germanium dioxide (GeO 2 , 99.99%), zinc acetate dihydrate (Zn(CH3COOH)2·2H2O, 99.99%), citric acid monohydrate (AR grade), and poly(acrylic acid) (PAA, molecular weight is ∼450 000), were purchased from Beijing Chemical Reagent Factory without further purification. 2.2. Preparation of ZGONRs. First, GeO2 and Zn(CH3COOH)2·2H2O were dispersed/dissolved in the distilled B

DOI: 10.1021/acsami.6b11503 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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which NPs can provide short Li diffusion paths when used as anode materials for LIBs. Moreover, TEM and high-resolution TEM (HRTEM) were implemented to reveal the nanostructures. As the representative TEM and HRTEM images show in Figure 2a and b, the

Scheme 1. Schematic Illustration for the Preparation Processes of p-ZGONFs

p-ZGONFs were obtained after annealing the as-obtained fiber precursors in air at 700 °C for 4 h. Such a preparation strategy is simple and easy to scale up. The PXRD were performed to verify the phase and purity of the as-prepared samples. As shown in Figure S2, all diffraction peaks in the PXRD pattern can be well indexed to the orthorhombic Zn2GeO4 phase (JCPDS Card No. 011-0687, the standard diffraction pattern for the orthorhombic Zn2GeO4 material) with an R3 space group without any detectable impurities. The XPS tests were also carried out to investigate the final products. The XPS spectrum (Figure S3) confirms that the sample is mainly composed of Zn, Ge, and O elements without the presence of C. The size and morphology were further checked using FESEM. As demonstrated in Figure 1a, the as-prepared samples are uniform nanofibers with a high aspect ratio (the average diameter is about 200 nm, and the length can be up to the range of several centimeters). In addition, the corresponding images of elemental mappings (Figure 1c) show that all Zn, Ge, and O elements are homogeneously distributed in the whole nanofiber. The C element is also absent in the results of mapping tests, which agree well with the XPS results. Both demonstrate the carbonfree nature of the nanofibers. More interestingly, the magnified FESEM image (Figure 1b) discloses that the 1D fibers consist of much smaller primary Zn2GeO4 nanoparticles (NPs). This suggests that such nanofibers are micro/nanostructures in

Figure 2. (a, b) Low- and high-magnification TEM images of pZGONFs. (c) Nitrogen adsorption−desorption isotherms and (d) corresponding pore size distribution curves for p-ZGONFs.

primary Zn2GeO4 NPs (about 15 nm) connect with each other. The clear fringes with an interplanar spacing of about 0.39 nm in the HRTEM image (Figure 2b) correspond to the (113) lattice planes of Zn2GeO4 material.43 The pores in the asprepared nanofibers are visible, and they originated from the interspaces between the connecting Zn2GeO4 NPs, which is illustrated by the dashed line in Figure 2b. Thus, with the already mentioned compositional and morphological investigations, the product is denoted as porous Zn2GeO4 nanofibers with the abbreviation of p-ZGONFs. Furthermore, the pores were further investigated by nitrogen adsorption−desorption isotherm (Figure 2c) and by the corresponding pore size distribution of Barrett−Joyner−Halenda (BJH, Figure 2d) curves. The hysteresis of the desorption curve in comparison to the adsorption one discloses the presence of plentiful mesopores, which results in a specific surface area of about 112.5 m2 g−1 (Table S1). The BJH fitting suggests that the

Figure 1. (a, b) SEM images of p-ZGONFs under different magnifications. (c) SEM image of p-ZGONFs and corresponding energy-dispersive system (EDS) mapping for Zn, Ge, and O elements. C

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Figure 3. (a) CV curves of the initial five cycles and (b) galvanostatic profiles for the p-ZGONFs. (c) The comparison of cycling performances between p-ZGONFs and ZGONRs at 200 mA g−1. (d) The difference of specific capacity between p-ZGONFs and ZGONRs cycled at 200 mA g−1. (e, f) High-rate capabilities of p-ZGONFs at various current densities from 0.2 A g−1 to 10 A g−1.

p-ZGONFs, which promotes the occurrence of the conversion reaction. In the following CV cycles, the cathodic peak at 0.72 V shifts to about 0.9 V because of the changes of the microstructures and the composition of the electrodes.12 All CV curves overlap well, demonstrating the outstanding reversibility of redox reactions of conversion and alloying/ dealloying processes. The Li-storage performances of p-ZGONFs were further investigated by the galvanostatic tests in the potential window of 0.005−2.9 V versus Li+/Li at a current density of 200 mA g−1. Figure 3b presents the galvanostatic discharge−charge (GDC) profiles of the initial three cycles. In the first discharge process, there are two apparent plateaus at about 0.8 V and below 0.25 V, which is consistent with the results of CV tests. Moreover, the specific capacities delivered by p-ZGONFs are up to 1695.8 and 1075.6 mA h g−1 in the first discharge and charge processes, respectively. The corresponding initial Coulombic efficiency (CE) is about 63.4%. The 36.6% irreversible capacity should be mainly associated with the irreversible formation of SEI during the first discharge process.44 In comparison, the ZGONR control can only deliver a reversible Li-storage capacity of about 428.9 mA h g−1 with the CE of 30.8% during the first cycle (Figure S6), both of which are much lower than those of p-ZGONFs. This should be caused by its poorer conductivity, making its electrochemical

mesopores in the p-ZGONFs are mainly centered at 3−20 nm (Figure 2d). In addition to p-ZGONFs, the product obtained from the hydrothermal process as described in section 2.2 is used as a control for the comparison of Li-storage properties. As shown in Figure S4, the product is Zn2GeO4 phase with the nanorod shape, which is named as ZGONRs. The length is about 3 μm, and the diameter is about 200 nm. The p-ZGONFs as anode material for LIBs and their Listorage processes were first studied by the CV curves. Figure 3a shows the initial five CV curves of p-ZGONFs at a scanning rate of 0.2 mV s−1 with the voltage range of 0.005−2.9 V versus Li+/Li. During the first cathodic scan that starts from the opencircuit voltage, one obvious peak is observed at around 0.72 V, which corresponds to the conversion lithiation of Zn2GeO4 to Zn, Ge, and Li2O accompanied by the formation of solid electrolyte interface films (SEI).13 The peak below 0.25 V indicates the alloying reactions between Li ions and the formed Ge and Zn.33,43 In the following first anodic scan, two obvious peaks are observed at 0.3 and 1.35 V, which can be assigned to the delithiation processes of Li−Ge (Li−Zn) alloys and reoxidation to metal oxides including GeO2 and ZnO,15 respectively. In comparison, for the ZGONRs control, the anodic peak at 1.35 V, presenting the reoxidation of Zn and Ge to the corresponding oxides, does not appear in its CV pattern (Figure S5). This implies the better Li+/e− transport kinetics of D

DOI: 10.1021/acsami.6b11503 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. (a) Schematic illustration of the faster Li ion transportation of p-ZGONFs compared to ZGONRs. EIS spectra of (b) p-ZGONF and (c) ZGONR electrode materials in half cells where lithium metal was used as counter electrodes under different conditions. (d) The relationship between real resistance (Z′) and frequencies of the fresh cells, in which the slope of the linear fitting line can be adopted to evaluate the apparent Li diffusion coefficient.

A g−1 (Figure 3f). To the best of our knowledge, such superior rate performance is among the best results for the Zn2GeO4based anode materials reported previously (disclosed in the comparison data in Figure S8).13,15,17,19,33,43,45,46 For the as-prepared p-ZGONFs, the high Li-storage capacity, the excellent cycling stability, and the superior rate performance can be attributed to the carbon-free and hierarchically porous 1D structure with nanoscale primary Zn2GeO4 NPs as the building blocks. First, as discussed in the Introduction, the carbon-free design will ensure high specific capacity. Second, the porous structure enables the accommodation of the volume changes during the successive Li-uptake/release processes of Zn2GeO4 nanofibers, which allows the integral electrode structures to achieve excellent cycling stability. From the SEM images (Figure S9) of the p-ZGONF electrode after 100 cycles, it can be clearly observed that the p-ZGONFs still remain as one-dimensional structures, further suggesting their structural stability during cycling. Moreover, hierarchical pores in the p-ZGONFs also serve as the electrolyte reservoirs, which can effectively provide sufficient Li ions the electrochemical processes (Figure 4a), especially the ones under high current densities. Third, the nanoscale Zn2GeO4 primary particles significantly shorten the e−/Li+-transfer distance and time. To examine the apparent Li diffusion kinetics of the pZGONF anode and the ZGONR control, electrochemical impedance spectroscopy (EIS) was further carried out on the half cells. Figure 4b and c is the Nyquist plots of EIS of pZGONF and ZGONR electrodes, respectively. As disclosed, for the p-ZGONFs electrodes, the Nyquist profiles during the 5th and the 10th cycles overlap each other, both of which are without obvious increase of diameters of semicircles in comparison to the profiles during the first cycle. This suggests that the p-ZGONF electrodes can rapidly get a stable electrochemical interphase with almost invariable charge transfer resistances. However, for the ZGONR control, the semicircles in the Nyquist profiles increase gradually from 95 Ω during the 1st cycle to 120 Ω during the 5th cycle and then to 150 Ω during the 10th cycle. Moreover, all Nyquist profiles of p-ZGONFs before and after cycling show the smaller diameters of semicircles (e.g., 80 Ω for p-ZGONFs vs 150 Ω for ZGONRs during the 10th cycle), suggesting their smaller charge-transfer resistance in the lithiation/delithiation processes. For the ionic diffusion in the whole electrodes, it is known that the diffusion time (t) is determined by the diffusion length (L) and the apparent Li diffusion coefficient (Dapparent)47 according to the following formula:

reduction processes for the formation of alloyed LixGe and LixZn much slower for the Zn2GeO4 nanorods. More importantly, as disclosed in Figure 3c, the as-prepared pZGONFs exhibit much better cycling stability compared to the ZGONR control. For example, p-ZGONFs deliver a specific capacity of about 1242 mA h g−1 after 130 cycles without any capacity decay at 200 mA g−1. In the cycling processes, the CE remains above 99% from the third cycle onward. In contrast, the specific capacities delivered by ZGONRs decrease rapidly, and the ZGONR electrodes become inactive for Li-storage within 50 cycles. Figure 3d shows the capacity differences between p-ZGONFs and ZGONRs during cycling at a current density of 200 mA g−1, further demonstrating the superior cycling stability of p-ZGONFs in comparison to ZGONRs. In addition, the p-ZGONF electrodes also exhibit outstanding cycling stability when cycled at higher current densities. Taking the current density of 2 A g−1 as an example (Figure S7), a high specific capacity of about 679.6 mA h g−1 can be delivered after 100 cycles, and the average CE is close to 100% except for the initial three cycles. To clarify the effect of carbon, the carboncontaining ZGONF (ZGONFs/C) composites, which were derived from similar preparation procedures with p-ZGONFs just with the different annealing atmosphere of an inert N2, were also studied. As shown in Figure S7, the 1st and 100th reversible capacities of p-ZGONFs and ZGONFs/C are 766.7 (741.2) and 679.6 (643.6) mA h g−1, with capacity retentions of 88.64% and 86.83%, respectively. There is an interesting phenomenon that although the capacity versus cycle number curve of ZGONFs/C seems smoother than that of p-ZGONFs, it displays a little lower capacity retention than that of pZGONFs. This might be caused by the irreversible extraction of lithium ions deposited on the amorphous carbon in the ZGONFs/C composite, especially for the initial few cycles. Therefore, it can be concluded that the carbon-free electrode materials with a tailored structure can demonstrate a better cycle stability in comparison to the materials with carbon incorporation. In addition, the rate performance of p-ZGONFs was further studied under various current densities from 0.2 A g−1 to 10 A g−1. As displayed in Figure 3e and f, the p-ZGONF electrodes can deliver the specific capacities of 1128, 1087.1, 976.9, 796.1, 649.1, and 450.3 mA h g−1 at current densities of 0.2, 0.4, 0.8, 1.6, 3.2, and 6.4 A g−1, respectively, with high CEs of above 99%. Moreover, even at an ultrahigh current density of 10 A g−1, it still exhibits a specific capacity of 384.7 mA h g−1. Most significantly, the specific capacity can recover to about 1082.5 mA h g−1 when the current density changes back to 0.2 E

DOI: 10.1021/acsami.6b11503 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

ACS Applied Materials & Interfaces t = L2 /Dapparent

(1)

Dapparent = (R2T 2)/(2A2 n 4F 4C 2σ 2)

(2)

Z′ ∝ σω−1/2

(3)

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (21573036, 21574018, and 51602048), the Science Technology Program of Jilin Province (20140101087JC) and Singapore MOE AcRF Tier 1 grants RG2/13, RG113/15, and Singapore A*STAR Pharos program SERC 1527200022 are gratefully acknowledged.

where ω, A, n, C, F, R, and T stand for the angular frequencies, electrode area, electrons number, the molar concentration of Li ions, Faraday constant, gas constant, and the absolute temperature, respectively. From the linear Z′-ω1/2 fitting results as shown in Figure 4d, the σ value for p-ZGONF electrode is calculated to be 61.71 Ω rad1/2 s1/2, which is smaller than that (158.25 Ω rad1/2 s1/2) of ZGONRs. According to eq 2, Dapparent is proportional to 1/σ2 because other parameters are almost the same for the two cell systems. Hence, the Dapparent value of pZGONF electrode is about 6.6 times larger than that of ZGONR electrode. The much enhanced apparent diffusion processes in the electrode of p-ZGONFs should benefit from the shorter Li-diffusion distance in the Zn2GeO4 crystal lattice because of the much smaller primary particle size in p-ZGONFs than that in ZGONRs.

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CONCLUSIONS In summary, we have successfully developed a dissolutionrecrystallization-assisted electrospinning technology to prepare the carbon-free, porous, and micro/nanostructural 1D Zn2GeO4 fibers. When used as anode materials for LIBs, the as-prepared p-ZGONFs exhibit much improved Li-storage properties in terms of higher specific capacity, outstanding cycling stability, and high-rate performance compared to the common ZGONRs. These superior electrochemical properties should be attributed to their unique carbon-free porous micro/ nanostructures composed of smaller Zn2GeO4 NPs. In view of the feasibility and scalable production of the preparation procedures, the present work may pave a new way for the largescale preparation of other carbon-free TMO nanofibers with porous structures composed of primary nanoparticles, which can be widely used in battery materials as well as in catalytic, optical, and electronic fields. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b11503. XPS spectrum of the p-ZGONFs, voltage profiles of the initial few cycles, electrochemical performance for ZGONRs, and the table of comparison of electrochemical performance of Zn2GeO4-based anode materials (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions §

These authors contributed equally.

Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acsami.6b11503 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.6b11503 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX