Structure Interlacing and Pore Engineering of Zn2GeO4 Nanofibers for

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Structure Interlacing and Pore Engineering of Zn2GeO4 Nanofibers for Achieving High Capacity and Rate Capability as an Anode Material of Lithium Ion Batteries Wei Wang, Jinwen Qin, and Minhua Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10468 • Publication Date (Web): 28 Dec 2015 Downloaded from http://pubs.acs.org on December 29, 2015

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ACS Applied Materials & Interfaces

Structure Interlacing and Pore Engineering of Zn2GeO4 Nanofibers for Achieving High Capacity and Rate Capability as an Anode Material of Lithium Ion Batteries

Wei Wang, Jinwen Qin, Minhua Cao*

[*] Prof. M. H. Cao Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, Department of Chemistry, Beijing Institute of Technology, Beijing 100081, P. R. China. E-mail: [email protected];

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ABSTRACT: An interlaced Zn2GeO4 nanofiber network with continuous and interpenetrated mesoporous structure was prepared using a facile electrospinning method followed by a thermal treatment. The mesoporous structure in Zn2GeO4 nanofibers is directly in-situ constructed by the decomposition of polyvinylpyrolidone (PVP), while the interlaced nanofiber network is achieved by the mutual fusion of the junctions between nanofibers in higher calcination temperatures. When used as an anode material in lithium ion batteries (LIBs), it exhibits superior lithium storage performance in terms of specific capacity, cycling stability and rate capability. The pore engineering and the interlaced network structure are believed to be responsible for the excellent lithium storage performance. The pore structure allows for easy diffusion of electrolyte, shortens the pathway of Li+ transport, and alleviates large volume variation during repeated Li+ extraction/insertion. Moreover, the interlaced network structure can provide continuous electron/ion pathways and effectively accommodate the strain induced by the volume change during the electrochemical reaction, thus maintaining structural stability and mechanical integrity of electrode materials during lithiation/delithiation process. This strategy in current work offers a new perspective in designing high-performance electrodes for LIBs.

KEYWORDS: Zn2GeO4; interlaced; network; mesoporous; nanofiber; lithium-ion batteries

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1. INTRODUCTION Currently, graphite is the most commonly used commercial anode material in lithium-ion batteries (LIBs).1-2 But its low theoretical specific capacity (372 mAh g-1), as well as limited rate capability, remain insufficient to meet the rapidly increasing demand of advanced portable devices.3-6 Therefore, it is indispensable to explore feasible electrode materials with higher energy and power densities for the application of LIBs. Advanced anode materials are being extensively pursued, among which group IV elements, particularly silicon (Si) and germanium (Ge), are most promising candidates due to their high theoretical lithium-storage capacities of 4200 and 1600 mAh g-1, respectively. Although the theoretical capacity of Si is higher than that of Ge, its limited Li+ diffusivity and intrinsic low electronic conductivity (400 and 104 times lower than Ge at room temperature, respectively) result in poor charge transport kinetics, thus leading to rapid capacity fading.7 In view of this fact, Ge and Ge-containing materials have become very competitive candidates. However, Ge is neither abundant nor inexpensive. So developing a stable and high-capacity Ge-based compound based on earth-abundant elements would be highly desirable.8 Zn2GeO4 is a ternary oxide containing much cheaper metal element Zn, and that Ge accounts for only 27 wt%, which could largely reduce the cost of LIBs.8 In addition, the presence of Zn can improve the reactivity towards lithium during charge-discharge process,9,10 and hence enhance lithium storage capacity.11 Moreover, the addition of electrochemically active Zn can prevent the aggregation of materials like Ge into large grains and function as a cushion to buffer volume change and structural stress,12

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thus restricting large capacity fading. Therefore, Zn2GeO4 is a more affordable choice to reduce the cost, improve the capacity, and restrain the degradation during cycling.13-17 As well known, the nanosizing of electrode materials is one of the most effective strategies to achieve high capability and excellent cycling stability because materials with nanometer-scale architectures can not only shorten diffusion distance for Li+ and electron, but also offer large electrode/electrolyte interface for charge-transfer reaction.7 Up to now, some Zn2GeO4 architectures have been developed by facile synthetic strategies.8,13,14 But most of them are one-dimension (1D) nanorods or branched microstructures, probably due to intrinsic hexagonal crystal structure of Zn2GeO4.18,19,20 It has been addressed that 1D structures have some advantages such as large exposed surfaces, improved lithium-ion transport kinetics and excellent electrical connection with current collector.7 Unfortunately, it still remains challenging to prevent the pulverization of the 1D structures during the Li+ insertion-extraction. For electrode materials, porous structure provides additional free volume to alleviate the structural strain during the repeated Li+ insertion-extraction process, which can inhibit the pulverization. Also, the porous structure can largely facilitate the penetration and diffusion of the electrolyte, effectively reducing the Li+ transfer path length and improve their cyclability.21,22 Thus, constructing mesoporous 1D nanostructures can provide large surface area, rapid ion/electron transport, and excellent structure stability, thus resulting in outstanding lithium storage performance. Furthermore, structural stability is another important factor that influences the

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electrochemical performance of electrode materials since repeated insertion/extraction process of Li+ will inevitably induce structural and/or volume changes, thus resulting in capacity fade during cycling.22 As well known, usual fabric has an interlaced structure, which can ensure its higher mechanical strength. This has inspired us that if we could interweave porous 1D Zn2GeO4 nanostructures into an interlaced network structure, high-performance lithium storage of Zn2GeO4 could be expected since this kind of structure can effectively prevent the collapse of the 1D nanostructures during the charge-discharge process, thus providing improved cycling stability.23,24 Herein, we designed and prepared interlaced porous Zn2GeO4 nanofibers by electrospinning technique followed by thermal treatment procedure in air. During the calcination process, Zn-Ge-based nanofiber precursor obtained by the electrospinning technique was decomposed into Zn2GeO4 accompanied with in-situ formation of numerous mesopores. Meanwhile, by finely controlling the heat treatment temperature, the nanofibers were welded with each other to form an interlaced network structure. Benefiting from their unique interlaced porous microstructure, the as-synthesized Zn2GeO4 nanofibers exhibit superior lithium storage performance in terms of specific capacity, cycling stability and rate capability. The charming aspects of our method lie in that it breakthroughs the limitation of the traditional electrospinning technique that only synthesizes 1D nanofibers, and realizes the construction of 3D interconnected nanofiber network that play an important role in the improvement of lithium storage performance of Zn2GeO4. 2. EXPERIMENTAL SECTION

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Synthesis of interlaced porous Zn2GeO4 nanofibers: All chemical reagents were of analytical

grade

and

were

used

as

received.

In

a

typical

procedure,

polyvinylpyrolidone (PVP, MW: 130 000) was dissolved in 20 g dimethylformamide (DMF) to form a 13 wt% transparent solution. Subsequently, Zn(ac)2·2H2O (2 mmol) and germanium tetraethoxide [Ge(OEt)4] (1 mmol) were added to this transparent solution under magnetic stirring to form a homogeneous solution. The resultant solution was loaded into a plastic syringe equipped with a 7-gauge stainless-steel nozzle. A piece of alumina foil as a fiber collector was placed at 15 cm from the tip of the needle. The feeding rate was set at 0.3 mL h-1. A high voltage of 15 kV was exerted to collect nanofibers. After the electrospinning process, the as-electrospun nanofibers were peeled off from the collector and then they were transferred into a furnace for stabilization. The stabilized sample was calcined at 700 °C in air atmosphere for 3 h at a heating rate of 5 °C min-1 to yield final interlaced porous Zn2GeO4 nanofibers. To investigate the influence of calcining temperature, the stabilized sample was also calcined at 500 °C, 600 °C and 800 °C, respectively. For comparison, nonporous Zn2GeO4 nanorods were prepared as well according to reference.24 Briefly, 2 mmol of Zn(ac)2·2H2O and 1 mmol of GeO2 were dispersed in 20 mL of 0.5 M NaOH aqueous solution, and stirred for 30 min. Subsequently, the homogeneous solution was transferred into 50 mL Teflon-lined stainless steel autoclave and maintained at 200 °C for 12 h. The final product was harvested by centrifugation, washed with absolute alcohol, and then dried at 50 °C for 8 h. Structural characterizations: The phase composition of the obtained products were

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characterized on a powder X-ray diffraction (XRD) (Bruker D8 Advance) from 10° to 80° (2θ) with a scanning step of 10° min-1 at a voltage of 40 kV and a current of 40 mA. X-Ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250 spectrometer (PerkinElmer) to characterize surface chemical composition of the products. The morphology of the resulting products was investigated using a field emission scanning electron microscope (FE-SEM, Hitachi, S-4800) and transmission electron microscope (TEM, JEOL 2100F) operating at 200 kV along with energy dispersive spectrometer (EDS). Element mapping images were obtained by using a scanning transmission electron microscope (STEM) (FEI Technai G2 F20). DTG-60AH was used to give the thermo gravimetric and differential scanning calorimetry analysis (TG-DSC). The Brunauer-Emmett-Teller (BET) surface area of as-synthesized samples was measured using a Belsorp-max surface area detecting instrument by N2 physisorption at 77 K. For the testing of the electrode after cycling, the cell was disassembled, and the electrode was taken out. The electrode was washed by dimethyl carbonate (DEC) and then dried at a vacuum condition. Electrochemical measurements: The electrochemical behavior of the obtained products was examined by coin-type cells (2025). For the preparation of the working electrode, the active material, carbonaceous additive (Super P), and polyvinylidene fluoride (PVDF) binder were mixed with a mass ratio of 80:10:10 to make homogeneous slurry by using N-methylpyrrolidone (NMP) as the solvent. The as-resultant slurry was uniformly pasted on a Cu foil and dried at 120 °C for 12 h in vacuum oven. The mass loading of active material was around 0.94-1.05 mg cm-2.

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The cell assembly was performed in an Ar-filled glove box. 1M LiPF6 dissolved in an ethylene carbonate (EC)/dimethyl carbonate (DMC)/ diethyl carbonate (DEC) mixture (1:1:1, in volume) was used as the nonaqueous electrolyte. A Celgard 2400 microporous polypropylene membrane was used as the separator, and Li foil was used as the counter electrode. Galvanostatic cycling experiments of the cells were performed on a LAND CT2001A battery test system in the voltage range of 0.01-3.0 V vs. Li+/Li at room temperature. Cyclic voltammetry tests were conducted on CHI-660D in the potential range of 0.01-3.0 V at a sweep rate of 0.1 mV s-1. Electrochemical impedance spectroscopy (EIS) was also measured on CHI-660D in frequency range 100 kHz to 0.01 Hz. A full Li-ion battery is assembled by coupling the Zn2GeO4 anode with commercially available LiFePO4 cathode. The LiFePO4 electrode film was prepared by pasting a slurry of the active material (80 wt%), Super P (10 wt%) and PVDF (10 wt%) in NMP on aluminum foil, and drying overnight under vacuum at 120 °C. In order to match the cathode/anode capacity, there is a slight excess capacity for the anode compared with the cathode, and the mass ratio of Zn2GeO4 to LiFePO4 was adjusted to 1:4 (3.7-4.2 mg cm-2 for the cathode loading). Prior to full cell assembly, the Zn2GeO4 electrode was pre-lithiated to reduce the initial irreversible capacity. This ex-situ lithiation process was performed by directly contacting the electrode with a lithium metal foil wetted by the electrolyte solution for 30 min.4 The lithium storage properties of the full cells were examined at a 0.1 C rate in voltage windows between 2.0 and 3.9 V with the same experimental system. The full cell capacity was

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calculated according to the weight of the cathode material. 3. RESULT AND DISCUSSION

Figure 1. (a) The schematic illustration of the synthesis procedure for interlaced porous Zn2GeO4 nanofibers. (b) Schematic illustration of the interlaced porous Zn2GeO4 nanofibers with easy diffusion of electrolyte, continuous electron/ion pathways, and facile strain relaxation during Li+ extraction/insertion.

Figure 1a schematically illustrates the preparation process of interlaced porous Zn2GeO4 nanofibers. Briefly, a homogeneous solution was first prepared by dissolving Zn(ac)2·2H2O and Ge(OEt)4 in DMF in the presence of PVP (denoted as Zn-Ge-PVP), which then was electrospun in a closed aqueous system to form Zn-Ge-PVP nanofiber precursor. The precursor was stabilized at 200 °C in air to

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maintain its fiber structure and subsequently calcined at 700 °C in air to obtain the interlaced porous Zn2GeO4 nanofibers. During this thermal treatment process, the decomposition of PVP created a large number of mesopores in the inner of nanofibers, while the junctions between nanofibers also started to fuse with the Zn2GeO4 crystallization, thus resulting in the formation of the interlaced network structure.

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Figure 2. (a) XRD patterns of interlaced porous Zn2GeO4 nanofibers. XPS spectra of interlaced porous Zn2GeO4 nanofibers: (b) survey spectrum, (c) Zn 2p, (d) Ge 3d, and (e) O 1s.

The crystalline structure and chemical composition of as-prepared samples were first studied by powder X-ray diffraction (XRD). As shown in Figure S1a, the Zn-Ge-PVP nanofibers exhibit only one weak and broad diffraction peak at 25° and no other diffraction peaks corresponding to a crystalline phase are detected,

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suggesting that the nanofiber precursor is amorphous.25 However, after thermal treatment of the Zn-Ge-PVP nanofibers precursor at 700 °C in air for 3 h, the amorphous precursor was converted to a crystalline product, as indicated by Figure 2a. All identified diffraction peaks could be assigned to hexagonal Zn2GeO4 (JCPDS card No. 11-0687). To gain further insight into the chemical composition of the calcined sample, X-Ray photoelectron spectroscopy (XPS) measurements were performed. As shown in Figure 2b, the survey XPS spectrum revealed that the as-obtained product was composed of Zn, Ge and O species. Figure 2c shows a high-resolution Zn 2p XPS spectrum, which shows two major peaks with binding energies of 1042.9 (Zn 2p1/2) and 1019.6 eV (Zn 2p3/2), corresponding to the oxidation state of Zn2+.26 The high-resolution Ge 3d XPS spectrum (Figure 2d) displays an obvious peak at 32.5 eV, which can be ascribed to the Ge-O bond.27,28 Finally, the O 1s (Figure 2e) signal shows three types of oxygen with different chemical states, which appear at 530.1 eV (lattice oxygen), 530.9 eV (oxygen of the hydroxide ions) and 532.0 eV (C-O or O-C=O bonds), respectively,29 well consistent with the XRD results. Figure S1b shows field emission scanning electron microscope (FE-SEM) image of Zn-Ge-PVP precursor, which reveals that the sample consists entirely of rather uniform nanofibers. The surface of the nanofibers is extremely smooth and their diameters are in the range of 400-500 nm. More importantly, the nanofibers have a large aspect ratio with lengths ranging up to tens of micrometers. However, after calcining the Zn-Ge-PVP nanofiber precursor in air at 700 °C for 3 h, the resultant Zn2GeO4 exhibits a highly interconnected, flyover-like nanofiber network structure

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(Figure 3a).30,31,32 Besides, it is interesting to note from high-magnification FE-SEM image (Figure 3b) that the nanofibers are porous and composed of inter-linked nanoparticles, and that the diameter of the nanofibers decreases evidently from 400-500 to 150-200 nm compared to that of the nanofiber precursor, which probably results from the thermal decomposition of PVP and the formation of crystalline Zn2GeO4 during the calcination process.33

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Figure 3. (a-e) Typical SEM and TEM images of interlaced porous Zn2GeO4 nanofibers with different magnifications. (f) HR-TEM image and SAED pattern (insert) of interlaced porous Zn2GeO4 nanofibers. (g-k) STEM elemental mapping images and (l) EDS spectrum of interlaced porous Zn2GeO4 nanofibers. (m) Nitrogen adsorption/desorption isotherms of interlaced porous Zn2GeO4 nanofibers and corresponding pore size distribution.

The microstructure of Zn2GeO4 nanofibers was further investigated by transmission electron microscope (TEM) images. Low-magnification TEM (Figure 3c) image

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shows that the nanofibers were interlaced with discernable and uniformly dispersed pores, in well agreement with the observation from previous SEM images. The higher-magnification TEM images (Figure 3d,e) reveal that the pores are in range of 20-100 nm, which will be further proofed by BJH pore-size distribution shown later on. The pore wall consists of Zn2GeO4 nanoparticles with diameters of around 20-50 nm. More importantly, no single nanoparticles can be observed, and these 1D porous nanofibers are closely connected with the adjacent ones to self-assemble into continuous and interpenetrating flyover-like network. This cross-linked structure can effectively inhibit the self-aggregation of nanofibers and prevent mechanical failure upon repeated Li+ intercalation and de-intercalation, thus ensuring durability and good electron/ion contact.31,32,34 The high-resolution TEM (HR-TEM) image (Figure 3f) recorded on the pore wall displays the lattice fringes with a spacing of 0.27 nm, in good agreement with the (410) plane of Zn2GeO4. The corresponding selected area electron diffraction (SAED) pattern (the inset in Figure 3f) indicates the polycrystalline nature of the pore wall. Furthermore, the scanning TEM (STEM) image and the element mapping images combining with the energy dispersive spectrometer (EDS) spectrum of the interlaced porous Zn2GeO4 nanofibers disclose the existence and homogeneous distribution of Zn, Ge and O elements (Figure 3g-l). The porous nature of the interlaced porous Zn2GeO4 nanofibers is further investigated by the nitrogen adsorption/desorption analysis. As shown in Figure 3m, the Zn2GeO4 nanofibers display type IV sorption isotherms, thus indicating this sample exhibits a mesoporous structure. The BET surface area is calculated to be around 18.54 m2 g-1.

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The Barrett-Joyner-Halenda (BJH) pore-size distribution curve (the inset in Figure 3m) indicates that the pore size mainly centers in the range of 20-80 nm, which is well consistent with the observations from above TEM images. We deduce that the decomposition of PVP and the generation of Zn2GeO4 during thermal treatment leads to the formation of the mesopores in the nanofibers.35 Such a highly interconnected, porous interpenetrating network may provide more surface active sites and shorten the transport pathways for both electrons and lithium ions during cycling, thus leading to a high capacity and excellent cycling stability. To highlight the influence of the calcination temperature on the morphology and composition of the final products, temperature-dependent experiments were conducted. The resultant samples with different calcination temperatures of 500 °C, 600 °C and 800 °C (denoted as Zn-Ge-500, Zn-Ge-600 and Zn-Ge-800, respectively) were investigated by XRD and SEM. As shown in Figure 4a,d,g, it is interesting to find that the crystallinity of the samples increases with the increase of the temperature. More specifically, Zn-Ge-500 sample exhibits a typical amorphous structure (Figure 4a). As the calcining temperature was increased to 600 °C, crystalline Zn2GeO4 phase (JCPDS card No. 11-0687) has been formed and at the same time amorphous carbon is also clearly observed (Figure 4d). The carbon contents in Zn-Ge-500 and Zn-Ge-600 were determined to be 19.8 wt% and 15.4 wt% according to thermo gravimetric (TG) analysis (Figure S2), respectively. With further increasing the calcining temperature to 800 °C, the degree of crystallinity of Zn2GeO4 phase is improved significantly and meanwhile the amorphous carbon disappears, which is

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probably due to the complete decomposition of PVP (Figure 4g). Besides, GeO2 phase was also observed in Zn-Ge-800 sample and those diffraction peaks at 20.54°, 25.95°, and 41.8° can be ascribed to (100), (101) and (200) planes of GeO2 (JCPDS card No. 36-1463).

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Figure 4. Representative XRD patterns and FE-SEM images of the products obtained at different calcining temperatures: (a-c) Zn-Ge-500, (d-f) Zn-Ge-600, and (g-i) Zn-Ge-800.

We deduce that the presence of GeO2 is as a result of partial decomposition of Zn2GeO4 at higher temperature, which is consistent well with the results from thermo gravimetric and differential scanning calorimetry (TG-DSC) analysis (Figure S3). The morphology of these samples was further studied by SEM and TEM measurements

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(Figure 4). Obviously, Zn-Ge-500 and Zn-Ge-600 both consist of nonporous, compact nanofibers with fairly uniform diameter (Figure 4b,c,e,f and Figure S4), while Zn-Ge-800 exhibits interlaced nanofiber network structure composing of bigger Zn2GeO4 particles. Moreover, the surface of Zn-Ge-800 nanofibers is rougher and no obvious pores are detected (Figure 4h,i). The porous nature of these samples was further investigated by nitrogen adsorption/desorption analysis (Figure S5). All of these samples give rise to similar adsorption/desorption isotherms, which can be identified as type II according to IUPAC classification, confirming nonporous structure in these products. The formation mechanism of the interlaced porous Zn2GeO4 nanofibers could be explained from following two aspects. The porous structure may result from the diffusion of gases generated by the decomposition of PVP during the calcining process,36 while the interlaced structure is generated by the mutual fusion of

the

junctions between nanofibers in higher calcination temperatures.37 On the one hand, PVP decomposes into CO2 and other gases during the calcination process, and the ballooning of the fiber occurs simultaneously. The formation of the pores is determined mainly by the evaporation rate and the diffusion rate of the gases.38 At lower calcination temperatures, for example, 500 °C or 600 °C, the generation rate of the gases inside the fibers is smaller than the diffusion rate through the surface, and therefore the pressure inside the fibers is less than outside, thus resulting in the formation of solid nanofibers (Figure 4b,c and 4e,f).36,38 With the increase of the calcining temperature (above 700 °C), a large number of gases are rapidly generated

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and the high pressure in the inner of the nanofiber can effectively prompt the gases to diffuse toward the outside, leading to the formation of the porous structure.39,40 On the other hand, during Zn2GeO4 crystallization process (above 700 °C), the joints between adjacent fibers would mutually fuse due to mass transfer, and thus the adjacent fibers will interlace with each other, forming interlaced porous Zn2GeO4 nanofibers (Figure 3). However, for the high calcination temperature of 800 °C, the interlacing of the nanofibers is more obvious, but the porous structure collapses due to the formation of larger Zn2GeO4 particles (Figure 4h,i). Therefore, by the current strategy, the combination of the electrospinning technique with the thermal treatment, we can finely adjust the 1D Zn2GeO4 nanofibers with controllable phases (e.g. amorphous carbon/Zn2GeO4 or crystalline Zn2GeO4) and morphologies (e.g. solid nanofibers, interlaced porous nanofibers and interlaced nanofibers without pores). However, the electrospinning process also suffers from relatively harsh reaction conditions, which include the viscosity of the Zn-Ge-PVP solution and the ambient temperature and humidity. In view of its unique morphology and microstructure, the resultant interlaced porous Zn2GeO4 nanofiber sample was tested as an anode material for LIBs. Figure 5a shows its discharge-charge profiles at a current density of 0.2 A g-1 in a potential window of 0.01-3V (vs. Li+/Li) for 1st, 5th, 10th, 30th and 50th cycles. The initial discharge and charge capacities are 1705.6 and 1405.4 mAh g−1, respectively, giving an initial Coulombic efficiency (CE) of 82.4%. Interestingly, this value is much higher than those of Zn-Ge-500 (49.54%), Zn-Ge-600 (55.53%) and Zn-Ge-800 (66.66%)

17


1.0 0.5

2000

1200

Discharge

Specific capacity (mAh g

(c)

0 0

250

Zn-Ge-800

Zn-Ge-500

Zn-Ge-600

Zn2GeO4 nanorods

1600 0.1A g-1 0.2 0.4 1200

0.8 1.0

800

1.5 2.0

0.2

200

3.0 4.0

60

80

(e)

-1

-1

1.0 A g

-1

2.0 A g

0.4 A g 0.8 A g

-1

900 600 300 0 0

10

20

30

40

Cycle number (n)

Zn-Ge-800

Zn-Ge-600 Zn-Ge-500

60 40 20

Zn2GeO4 nanorods

10

20

30

40

0

50

Cycle number (n)

(d) Zn-Ge-800 Interlaced porous Zn2GeO4 nanofibers

150

Zn-Ge-600

100

Zn-Ge-500 Zn2GeO4 nanorods

0 0

100

Cycle number (n)

1500

-1

40

Interlaced porous Zn2GeO4 nanofibers 80

50

400 20

100

300

-1

)

120

(b)

600

300 600 900 1200 1500 1800 2100

Interlaced porous Zn2GeO4 nanofibers

1200

900

-Z''/ohm

2400

1500

1 st 5 th 10th 30 th 50 th

1.5

1800

Page 18 of 37

Coulombic efficiency (%)

-1

Charge

2.0

0.0 0

Capacity (mAh g )

(a)

2.5

50


+

3.0

-1

Capacity (mAh g )

2100

3.5

0 0

Capacity (mAh g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Voltage (V vs. Li /Li)


120 100

50

100

150

200

250

Z'/ohm

(f)

80

-1

0.4 A g

60

0.8 A g

40

1.0 A g

20

2.0 A g

-1 -1 -1

0 0

10

20

30

40

Cycle number (n)

50

Figure 5. (a) The discharge-charge voltage profiles of interlaced porous Zn2GeO4 nanofiber electrode. (b) The cycling performance of interlaced porous Zn2GeO4 nanofiber electrode as well as Zn-Ge-500, Zn-Ge-600, Zn-Ge-800 and Zn2GeO4 nanorods cycled at 0.2 A g-1. The corresponding CE at 0.2 A g-1 of interlaced porous Zn2GeO4 nanofiber electrode. (c) The rate performance and (d) the Nyquist plots of interlaced porous Zn2GeO4 nanofibers and other control samples. (e) The cycling performance and (f) the corresponding CE of interlaced porous Zn2GeO4 nanofiber electrode at 0.4, 0.8, 1.0 and 2.0 A g-1.

(Figure S6), and is also higher than those of most of previously reported Zn2GeO4 anodes.8,13-15 The possible reason for the significant improvement in initial CE may be that the porous structure of interlaced porous Zn2GeO4 nanofibers is favorable for liquid electrolyte penetration and thus all active Zn2GeO4 nanoparticles could participate in electrochemical reactions to realize a maximum lithium storage (as

18


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shown in Figure 1b). Moreover, the high crystallinity of the as-synthesized Zn2GeO4 also guarantees the fact that the lithium will extract from the host reversibly.13 After the first cycle, the CE rapidly increases to 88.9% and 92.3% in second and third cycles, respectively. From the fifth cycle, the profiles show nearly full reversibility of the capacity, with an average CE of 99.1 % for up to 50 cycles (Figure 5b), indicating rapid stabilization of the solid electrolyte interphase (SEI) layer of interlaced porous Zn2GeO4 nanofiber anode.14 Figure 5b shows the cycling performance of interlaced porous Zn2GeO4 nanofibers, Zn-Ge-500, Zn-Ge-600 and Zn-Ge-800 at a current density of 0.2 A g-1 for 50 cycles. For interlaced porous Zn2GeO4 nanofibers, the capacity loss during the first four cycles can be attributed to an irreversible lithium reaction or the formation of SEI layers, but almost no capacity fading from the fifth cycle was observed. And the capacity retention is 1084.1 mA h g-1 after 50 cycles, which is much higher than those of Zn-Ge-500 (432.9 mAh g-1), Zn-Ge-600 (644.6 mAh g-1) and Zn-Ge-800 (751.3 mAh g-1). Moreover, the rate capability of the interlaced porous Zn2GeO4 nanofibers was also investigated at various current densities (Figure 5c). Good capacity retention was obtained at each current rate ranging from 0.1 to 4.0 A g-1. When the current densities are 0.1, 0.2, 0.4, and 0.8 A g-1, the sample exhibits specific capacities of 1294.2, 1119.2, 1022.4 and 901.6 mAh g-1, respectively. This performance is superior to those of some reported Zn2GeO4 anodes at 0.8 A g-1, for example, 773 mAh g-1 for Zn2GeO4/N-doped graphene composite

13

and 780 mAh g-1 for partially crystalline

Zn2GeO4 nanorod/graphene composite.15 And the capacity comparison of present 19



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Page 20 of 37

work with reported Zn2GeO4-based materials are listed in the Table 1. Even when cycled at high current densities of 2.0 and 4.0 A g-1, the interlaced porous Zn2GeO4 nanofiber electrode could achieve capacities as high as 670 and 509 mAh g-1, respectively. When returned to cycle at 0.2 A g-1, the electrode still maintains the capacity as high as 920 mAh g-1, implying its good reversibility. Meanwhile, the interlaced porous Zn2GeO4 nanofiber electrode shows a significantly enhanced rate performance compared to Zn-Ge-500, Zn-Ge-600 and Zn-Ge-800 at various current densities, as shown in Figure 5c. Table 1 Comparison of the Capacity of present work with reported Zn2GeO4-based materials. Samples

Current density (A g-1)

Cycle number

Capacity (mAh g-1)

Ref.

Amorphous Zn2GeO4 nanoparticles

0.4

200

1090

8

Zn2GeO4/N-doped graphene nanocomposites

0.1

100

1044

13

Sandwiched Zn2GeO4-grapheme oxide nanocomposite

0.2

100

1050

14

Partially crystalline Zn2GeO4 nanorod/graphene composites

0.2

50

768

15

Carbon-coated Zn2GeO4 on Ni foam

0.2

50

933

16

Coaxial Zn2GeO4 @ carbon nanowires

2.0

100

790

17

Zn2GeO4 hollow nanoparticles

0.2

60

1175

18

Zn2GeO4 nanorods

0.2

60

925

18

Mn-doped Zn2GeO4 nanosheets

0.1

100

1301

19

Zn2GeO4/g-C3N4 hybrids

0.2

140

1014

20

Interlaced porous Zn2GeO4 nanofiber

0.2

50

1084

this work

Besides, the interlaced porous Zn2GeO4 nanofibers also exhibit exceptionally improved lithium storage performance compared to pure Zn2GeO4 nanorods (Figure S7), as shown in Figure 5b,c. The excellent rate performance of the interlaced porous 20


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Zn2GeO4 nanofibers should closely associate with their fast kinetics of the redox reactions which involves lithium ion and electron diffusion to/from the electrolyte/particle interface.7,41,42 To confirm this fact, electrochemical impedance spectroscopy (EIS) measurements were conducted on interlaced porous Zn2GeO4 nanofiber electrode after 50 cycles at 0.2 A g-1 (Figure 5d). An equivalent electrical circuit was used to fit Nyquist plots (Figure S8),15 where Re is the electrolyte resistance; R(sf + ct) is the resistance originating from 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; CPE b is the bulk capacitance; Ws is the Warburg resistance; and Ci is the intercalation capacitance. Based on the fitted results, the value of R (sf+ct) for interlaced porous Zn2GeO4 nanofibers was 44.02 Ω, which is far lower than those of Zn-Ge-500 (97.15 Ω), Zn-Ge-600 (66.61Ω), Zn-Ge-800 (56.24 Ω) and pure Zn2GeO4 nanorods (149.93 Ω). Therefore, this result further confirms that the interlaced porous Zn2GeO4 nanofiber electrode possesses a high electrical conductivity, thus resulting in better rate capability and higher reversible capacity compared to other electrodes. Furthermore, the cycling stability of the interlaced porous Zn2GeO4 nanofiber electrode at higher current densities is also investigated as shown in Figure 5e. The recorded discharge capacities were 859.4 mAh g-1 (0.4 A g-1), 769.9 mAh g-1 (0.8 A g-1), 662.5 mAh g-1 (1.0 A g-1), and 502.7 mAh g-1 (2.0 A g-1) after 50 cycles, respectively, all of which are much higher than the theoretical capacity of graphite. For all tested electrodes, even after 100 cycles, relatively higher capacities are still maintained (Figure S9). Although an

21



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Page 22 of 37

irreversibility of capacity was observed in the first charge-discharge cycle, the CE remains over 97 % at each cycle from the fifth cycle to the 50th cycle (Figure 5f), indicating that the interlaced porous Zn2GeO4 nanofibers might be suitable for use at very high power densities. To elucidate the electrochemical reaction mechanism of interlaced porous Zn2GeO4 nanofiber electrode, cyclic voltammetry (CV) curves of the first five cycles in the voltage range of 0.01-3 V were measured (Figure 6a). During the initial cathodic sweep, a sharp reduction peak at around 0.61 V was observed, which can be assigned to the decomposition of Zn2GeO4 as well as irreversible reactions that are related to the formation of SEI films and the decomposition of the electrolyte.8,14 From the second cycle, this peak shifts to 0.86 V and finally fixes at 0.68 V. The irreversible phenomenon in first several cycles may be partially attributed to the multistep reaction of Zn2GeO4. This is also in good agreement with that from the cycling performance profiles (Figure 4b). Another peak that starts at about 0.25 V can be assigned to Li-Zn and Li-Ge alloying reactions.11 In the anodic scan process, two broad oxidation peaks at 0.46 and 1.3 V were observed, which are associated with the delithiation of Li-metal alloys, followed by the re-oxidation of Zn and Ge, respectively, consist with the previous reports.13,15-18 Remarkably, the CV curves do not change significantly after the fourth cycle, revealing a good stability and reversibility for the insertion and exaction of lithium ions. To further investigate the Li storage mechanism of interlaced porous Zn2GeO4 electrode, ex-situ XRD measurements were performed during the first cycle at designated discharge and

22


Page 23 of 37

charge voltages between 0.01 and 3 V (Figure 6b,c). It should be noted that that the very sharp peak at 43.29° in XRD patterns corresponds to Cu foil. Curve a in Figure 6b shows the XRD pattern for the freshly-prepared interlaced porous Zn2GeO4 nanofiber electrode. All of diffraction peaks can be identified as hexagonal Zn2GeO4. After the first discharge to 0.61 V (red curve, curve b in Figure 6b), GeO2 phase was detected, suggesting the partial decomposition of Zn2GeO4.

(b) ∗ Zn GeO 2

Intensity (a.u.)

0.3 0.0 -0.3 -0.6 -0.9

1 st 2 nd 3 rd 4 th 5 th

♠ Li11Ge6 ♠

-1.2 0.0 0.5 1.0 1.5 2.0 2.5+ 3.0 10

Voltage (V vs. Li/Li

(d)

)

(e)

∗

♣

4

♥ Li2CO3 ♦GeO2

♣ Li2ZnGeO4 ♥♠ ♥♥ ♥ ♥ ♥

∗ ∗♦ ∗ ∗ ∗ ∗∗ ♦ ♦∗

20

30

∗

2θ (degree)

40

(c) ∗ Zn GeO

Cu

2

d c b∗ a

Intensity (a.u.)

(a) 0.6 Current (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


♦ GeO2

4

∇ ZnO

Cu

• LiOH·H2O ◊ Li2O

∗ ∗

50 10

∗ ∗♦ ∗ ∗ ∗ ∇&◊ • ∗ ∗♦ ∗• ∗ •∗ ∗ 20

♦

∇

• ∇&◊ • •

30

2θ (degree)

40

g∗ f∗ e 50

(f)

Figure 6 (a) CV curves of interlaced porous Zn2GeO4 nanofiber electrode showing the first five cycles between 0.01 V and 3.0 V at a scan rate of 0.1 mV s-1. (b) XRD patterns of fresh interlaced porous Zn2GeO4 nanofiber electrode (curve a) and discharged to 0.61 V (curve b), to 0.55 V (curve c), and to 0.01 V (curve d) for the 1st cycle. (c) XRD patterns of interlaced porous Zn2GeO4 nanofiber electrode charged to 0.46 V (curve e), to 1.3 V (curve f), and to 3.0 V (curve g) for the 1st cycle. (d) HR-TEM image of interlaced porous Zn2GeO4 nanofibers after 50 charge-discharge cycles at the charged state. (e,f) TEM images of interlaced porous Zn2GeO4 nanofibers after 50 charge-discharge cycles at 0.2 A g-1.

When the electrode was discharged to 0.55 V (green curve, curve c in Figure 6b), the diffraction peaks of Zn2GeO4, GeO2 and Li2ZnGeO4 were found. Further discharging the electrode to 0.01 V (blue curve, curve d in Figure 6b), the diffraction peaks noted above disappeared and Li11Ge6 and Li2CO3 phases emerged. In contrast, when the

23



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Page 24 of 37

interlaced porous Zn2GeO4 nanofiber electrode was charged to 0.46 V, to 1.3 v and to 3.0 V, respectively (curves e, f, and g in Figure 6c), the diffraction peaks belonging to LiOH·H2O and Li2O disappeared gradually, while ZnO, GeO2 and Zn2GeO4 phases appeared again, indicating the re-oxidation of metallic Zn and Ge, which agree well with the relevant reports.15-18 And the disappearance of Li2O in the 1st charge procedure suggested that the decomposition reaction of Li2O was reversible. The HR-TEM image (Figure 6d) of the interlaced porous Zn2GeO4 nanofiber electrode after 50 cycles also confirmed the existence of Zn2GeO4 (d = 0.356 nm), which is consistent with the XRD result. Combined our experimental results with previous works,8,13-20 the possible electrochemical reactions for the interlaced porous Zn2GeO4 nanofiber electrode were proposed as following: (1) (2) (3) (4) (5) (6) Furthermore, to disclose the structure stability of the interlaced porous Zn2GeO4 nanofibers after cycling, TEM analysis was performed on the electrode after 50 cycles in a charged state at 0.2 A g-1 (Figure 6e,f). Clearly, the fiber morphology, the porous structure and the interlaced structure (Figure 6e, green cycles) are well maintained without significant pulverization after 50 charge-discharge cycles. The small lumps on

24


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the nanofibers observed in Figure 6e are super P additive during electrode fabrication.7 These results indicate that the interlaced porous fibers can effectively not only accommodate the large volume variation during repeated Li+ insertion/extraction processes, but also prevent the agglomeration or detachment of the Zn2GeO4 nanoparticles over cycling. On the basis of the above results, the excellent electrochemical performance of the interlaced porous Zn2GeO4 nanofibers may be attributed to their unique microstructures, interlaced network and porous structure, as illustrated in Figure 1b. First, the interlaced nanofiber network, on the one hand, can offer continuous electron transport pathway, and on the other hand, can provide mechanical/structural integrity against large changes in volume and crystal structure over extended cycling, thus resulting in a good reversible capacity (Figure 5b,e). Second, the porous structure not only can act as a reservoir for lithium storage and facilitate the immersion and diffusion of the electrolyte, but also can shorten the pathway with less resistance for both Li+ and electron transport. Meanwhile, the porous structure can offer enough room to reduce the self-aggregation of active materials and accommodate the volume variations during repeated Li+ extraction/insertion.7 Finally, the small-size Zn2GeO4 nanoparticles can reduce the diffusion lengths for electron and ion transport, thus being favorable for increasing the contact area between the electrode and the electrolyte in the cycling process. Therefore, we conclude that the robust structure of the interlaced porous Zn2GeO4 nanofibers is responsible for the excellent electrochemical performance of Zn2GeO4.

25



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Page 26 of 37

To further demonstrate practical application of the as-prepared porous fibers in LIBs, full cells with the interlaced porous Zn2GeO4 nanofiber anode and a commercially available LiFePO4 cathode were fabricated.43,44 In order to approach the cell capacity balance to the 1:1 ratio with a slight excess of anode capacity,4,50 we first obtained the specific capacity ratio of Zn2GeO4 anode to LiFePO4 cathode based on their respective half Li-ion battery tests at the same cycling rate (601.6 mAh g-1 for Zn2GeO4 at 1C, 1C = 1443 mA g-1; 144.7 mAh g-1 for LiFePO4 at 1C, 1C = 170 mA g-1), which was determined to be 4.16:1. According to above principle of capacity balance, the mass ratio of Zn2GeO4 to LiFePO4 thus was determined to be about 1:4 and the loadings of Zn2GeO4 and LiFePO4 in our experiment are 0.94-1.05 and 3.76-4.2 mg cm-2, respectively. Figure 7a shows the full-cell voltage profiles after 1, 5, 10, 20, 30 and 50 cycles with the voltage window of 2.0-3.9 V at 0.1 C with respect to the cathode. The initial charge and discharge capacities were 159.9 and 150.1 mAh g-1, respectively, yielding an initial CE of 93.9% (this initial irreversibility is due to SEI film formation at the LiFePO4 cathode side).4,45 In the subsequent cycles, the high overlapping in the potential trends has been detected, indicating its good operation reversibility. Moreover, it can also be seen that the charge/discharge profiles move to lower voltage plateaus gradually along with galvanostatic cycling. Although the exact reason of such evident drop in the discharge voltage profile currently is not fully understood, it must be related to the rearrangement of the structure and the electrochemical polarization of the interlaced porous Zn2GeO4 electrode during the electrochemical cycling according to the literatures.46-50

26


3.0 2.5 Discharge

2.0 0

30

1 st 5 th 10th 20 th 30 th 50 th

60

90 120 150 180 210 -1 Specific capacity (mAh g )

120

(b)

100

200

80

150 100

60 Charge Discharge 40

50

20

0 0


Charge

3.5

250

-1

(a)

Capacity (mAh g )

4.0

+

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Voltage (V vs. Li /Li)

Page 27 of 37

10

20

30

40

0

50

Cycle number (n)

Figure 7. (a) Voltage profiles of LiFePO4/Zn2GeO4 full cell for 1, 5, 10, 20, 30 and 50 cycles; (b) Corresponding specific capacity versus cycle number of the full cell (left) and CE (right). Cycling rate is 0.1C (1C = 170 mA g-1 vs. LiFePO4).

Figure 7b presents the cycling performance and the CE of the as-developed full cell. It is evident that, the LiFePO4/Zn2GeO4 full cell holds a stable discharge capacity of 128.2 mAh g-1 after 50 cycles, with a good capacity retention of 85.4% (about 0.29% capacity fading per cycle), demonstrating its excellent cycling stability. Moreover, the high CE values beyond the second cycle (>98.4%) also demonstrate the high efficiency of the battery system. From these results, we can see that the LiFePO4/Zn2GeO4 full cell exhibits a better performance in terms of specific capacity, cyclability, and CE, thus confirming the potentiality of the interlaced porous Zn2GeO4 nanofibers as an anode material for LIBs.

4. CONCLUSION In summary, we have demonstrated that the excellent lithium storage of Zn2GeO4 can be achieved by deliberately controlling its microstructures. The constructed interlaced porous Zn2GeO4 nanofibers by using a facile electrospinning method

27



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Page 28 of 37

followed by a thermal treatment exhibit a high reversible capacity, improved rate performance and a good cycling stability even at high rates. The excellent lithium storage performance can be attributed to their pore engineering and interlaced network structure. The novel robust structure and the effective synthesis strategy can be extended to other electrode materials for high-performance energy storage devices. ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author

*Fax: +86-10-68918572. Tel: +86-10-68918468. E-mail: [email protected]

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (21471016 and 21271023) and the 111 Project (B07012). Supporting Information.The XRD pattern, SEM images and TG-DSC data of the

precursor; TEM images of Zn-Ge-600; Nitrogen adsorption-desorption isotherms and the discharge and charge profiles of Zn2GeO4 obtained at 500, 600 and 800 °C; XRD pattern, SEM image, and the discharge and charge profiles of pure Zn2GeO4 nanorods; The cycling performance of interlaced porous Zn2GeO4 nanofiber electrode for 100 cycles; The equivalent electrical circuit of the Zn2GeO4 nanofibers. This information is available free of charge via the Internet at http://pubs.acs.org/ REFERENCES

(1)

Liu, D. H.; Lü, H. Y.; Wu, X. L.; Hou, B. H.; Wan, F.; Bao, S. D.; Yan, Q.;

Xie, H. M.; Wang, R. S., Constructing the Optimal Conductive Network in 28


Page 29 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MnO-based Nanohybrids as High-rate and Long-life Anode Materials for Lithium-ion Batteries. J. Mater. Chem. A 2015, 3, 19738-19746. (2)

Hou, B. H.; Wu, X. L.; Wang, Y. Y.; Lü, H. Y.; Liu, D. H.; Sun, H. Z.; Zhang,

J. P.; Guan, H. Y., Full Protection for Graphene-incorporated Micro/nanocomposites Containing Ultra-small Active Nanoparticles: the Best Li-Storage Properties. Part. Part. Syst. Charact. 2015, 32, 1020-1027. (3)

Zhou, W.; Zheng, J. L.; Yue, Y. H.; Guo, L., Highly Stable RGO-wrapped

Ni3S2 Nanobowls: Structure Fabrication and Superior Long-life Electrochemical Performance in LIBs. Nano Energy 2015, 11, 428-435. (4)

Wang, W.; Sun, Y.; Liu, B.; Wang, S. G.; Cao, M. H., Porous Carbon

Nanofiber Webs Derived from Bacterial Cellulose as an Anode for High Performance Lithium Ion Batteries. Carbon 2015, 91, 56-65. (5)

Li, W. h.; Li, M. S.; Wang, M.; Zeng, L. C.; Yu, Y., Electrospinning with

Partially Carbonization in Air: Highly Porous Carbon Nanofibers Optimized for High-Performance Flexible Lithium-ion Batteries. Nano Energy 2015, 13, 693-701. (6)

Das, S. K.; Patel, M.; Bhattacharyya, A. J., Effect of Nanostructuring and Ex

Situ Amorphous Carbon Coverage on the Lithium Storage and Insertion Kinetics in Anatase Titania. ACS Appl. Mater. Interfaces 2010, 2, 2091-2099. (7)

Wang, W.; Xiao, Y.; Wang, X.; Liu, B.; Cao, M. H., In Situ Encapsulation of

Germanium Clusters in Carbon Nanofibers: High-Performance Anodes for Lithium-ion Batteries. ChemSusChem 2014, 7, 2914-2922.

29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(8)

Page 30 of 37

Yi, R.; Feng, J. K.; Lv, D. P.; Gordin, M. L.; Chen, S. R.; Choi, D.; Wang, D.

H., Amorphous Zn2GeO4 Nanoparticles as Anodes with High Reversible Capacity and Long Cycling Life for Li-ion Batteries. Nano Energy 2013, 2, 498-504. (9)

Qie, L.; Chen, W. M.; Wang, Z. H.; Shao, Q. G.; Li, X.; Yuan, L. X.; Hu, X.

L.; Zhang, W. X.; Huang, Y. H., Nitrogen-doped Porous Carbon Nanofiber Webs as Anodes for Lithium Ion Batteries with a Super High Capacity and Rate Capability. Adv. Mater. 2012, 24, 2047-2050. (10) Bulusheva, L.; Okotrub, A.; Kurenya, A.; Zhang, H. K.; Zhang, H. J.; Chen, X. H.; Song, H. H., Electrochemical Properties of Nitrogen-doped Carbon Nanotube Anode in Li-ion Batteries. Carbon 2011, 49, 4013-4023. (11) Zhang, J.; Hu, Y. S.; Tessonnier, J. P.; Weinberg, G.; Maier, J.; Schlögl, R.; Su, D. S., CNFs@CNTs: Superior Carbon for Electrochemical Energy Storage. Adv. Mater. 2008, 20, 1450-1455. (12) Wang, W.; Xiao, Y.; Zhao, X. Y.; Liu, B.; Cao, M. H., Synthesis of Cd2SnO4-SnO2 Hybrid Micro-cubes with Enhanced Electrochemical Performance for Lithium-ion Batteries. CrystEngComm 2014, 16, 922-929. (13) Zou, F.; Hu, X. L.; Sun, Y. M.; Luo, W.; Xia, F. F.; Qie, L.; Jiang, Y.; Huang, Y. H., Microwave-induced In Situ Synthesis of Zn2GeO4/N-Doped Graphene Nanocomposites and Their Lithium-storage Properties. Chem. Eur. J. 2013, 19, 6027-6033.

30


Page 31 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(14) Zou, F.; Hu, X. L.; Qie, L.; Jiang, Y.; Xiong, X. Q.; Qiao, Y.; Huang, Y. H., Facile Synthesis of Sandwiched Zn2GeO4-graphene Oxide Nanocomposite as A Stable and High-capacity Anode for Lithium-ion Batteries. Nanoscale 2014, 6, 924-930. (15) Wang, R.; Wu, S. P.; Lv, Y. C.; Lin, Z. Q., Partially Crystalline Zn2GeO4 Nanorod/Graphene Composites as Anode Materials for High Performance Lithium Ion Batteries. Langmuir 2014, 30, 8215-8220. (16) Chen, W. M.; Maloney, S.; Wang, W. Y., Carbon-coated Zn2GeO4 on Ni Foam as Lithium Ion Battery Anodes with Exceptional Long Cycling Stability. Electrochim. Acta 2015, 176, 96-102. (17) Chen, W. M.; Lu, L. Y.; Maloney, S.; Yang, Y.; Wang, W. Y., Coaxial Zn2GeO4@carbon Nanowires Directly Grown on Cu Foils as High-performance Anodes for Lithium Ion Batteries. Phys. Chem. Chem. Phys. 2015, 17, 5109-5114. (18) Feng, Y.; Li, X. D.; Shao, Z. P.; Wang, H. T., Morphology-dependent Performance of Zn2GeO4 as a High-Performance Anode Material for Rechargeable Lithium Ion Batteries. J. Mater. Chem. A 2015, 3, 15274-15279. (19) Li, Q.; Miao, X.; Wang, C.; Yin, L., Three-dimensional Mn-doped Zn2GeO4 Nanosheet Array Hierarchical Nanostructures Anchored on Porous Ni Foam as Binder-free

and

Carbon-free

Lithium-ion

Battery

Anodes

with

Enhanced

Electrochemical Performance. J. Mater. Chem. A 2015, 3, 21328-21336. (20) Li, X. D.; Feng, Y.; Li, M. C.; Li, W.; Wei, H.; Song, D. D., Smart Hybrids of Zn2GeO4 Nanoparticles and Ultrathin g-C3N4 Layers: Synergistic Lithium Storage and Excellent Electrochemical Performance. Adv. Funct. Mater. 2015, 25, 6858-6866.

31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 37

(21) Xiao, Y.; Sun, P. P.; Cao, M. H., Core-shell Bimetallic Carbide Nanoparticles Confined in a Three-dimensional N-Doped Carbon Conductive Network for Efficient Lithium Storage. ACS nano 2014, 8, 7846-7857. (22) Pu, X.; Li, L. X.; Song, H. Q.; Du, C. H.; Zhao, Z. F.; Jiang, C. Y.; Cao, G. Z.; Hu, W. G.; Wang, Z. L., A Self-charging Power Unit by Integration of a Textile Triboelectric Nanogenerator and a Flexible Lithium-ion Battery for Wearable Electronics. Adv. Mater. 2015, 27, 2472-2478. (23) Qin, J. W.; Lv, W.; Li, Z. J.; Li, B. H.; Kang, F. Y.; Yang, Q. H., An Interlaced Silver Vanadium Oxide-graphene Hybrid with High Structural Stability for Use in Lithium Ion Batteries. Chem. Commun. 2014, 50, 13447-13450. (24) Liang, J.; Cao, Y. Q.; Lin, H.; Zhang, Z. Z.; Huang, C. C.; Wang, X. X., A Template-Free Solution Route for the Synthesis of Well-Formed One-dimensional Zn2GeO4 Nanocrystals and Its Photocatalytic Behavior. Inorg. Chem. 2013, 52, 6916-6922. (25) Gao, Q.; Zhao, X. Y.; Xiao, Y.; Zhao, D.; Cao, M. H., A Mild Route to Mesoporous Mo2C-C Hybrid Nanospheres for High Performance Lithium-ion Batteries. Nanoscale 2014, 6, 6151-6157. (26) Hou, Y.; Li, X. Y.; Zhao, Q. D.; Quan, X.; Chen, G. H., Electrochemical Method for Synthesis of a ZnFe2O4/TiO2 Composite Nanotube Array Modified Electrode with Enhanced Photoelectrochemical Activity. Adv. Funct. Mater. 2010, 20, 2165-2174.

32


Page 33 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(27) Hanrath, T.; Korgel, B. A., Chemical Surface Passivation of Ge Nanowires. J. Am. Chem. Soc. 2004, 126, 15466-15472. (28) Jin, S. X.; Li, N.; Cui, H.; Wang, C. X., Growth of the Vertically Aligned Graphene@amorphous GeOx Sandwich Nanoflakes and Excellent Li Storage Properties. Nano Energy 2013, 2, 1128-1136. (29) Liu, S.; Tian, J. Q.; Wang, L.; Zhang, Y. W.; Qin, X. Y.; Luo, Y. L.; Asiri, A. M.; Al-Youbi, A. O.; Sun, X. P., Hydrothermal Treatment of Grass: a Low-cost, Green Route to Nitrogen-doped, Carbon-rich, Photoluminescent Polymer Nanodots as an Effective Fluorescent Sensing Platform for Label-free Detection of Cu(II) Ions. Adv. Mater. 2012, 24, 2037-2041. (30) Wen, W.; Wu, J. M.; Cao, M. H., Rapid One-step Synthesis and Electrochemical Performance of NiO/Ni with Tunable Macroporous Architectures. Nano Energy 2013, 2, 1383-1390. (31) Wen, W.; Wu, J. M., Eruption Combustion Synthesis of NiO/Ni Nanocomposites with Enhanced Properties for Dye-absorption and Lithium Storage. ACS Appl. Mater. Interfaces 2011, 3, 4112-4119. (32) Wen, W.; Wu, J. M.; Cao, M. H., NiO/Ni Powders with Effective Architectures as Anode Materials in Li-ion Batteries. J. Mater. Chem. A 2013, 1, 3881-3885. (33) Zhu, C.; Wen, Y.; van Aken, P. A.; Maier, J.; Yu, Y., High Lithium Storage Performance of FeS Nanodots in Porous Graphitic Carbon Nanowires. Adv. Funct. Mater. 2015, 25, 2335-2342.

33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 37

(34) Meng, X. B.; He, K.; Su, D.; Zhang, X. F.; Sun, C. J.; Ren, Y.; Wang, H. H.; Weng, W.; Trahey, L.; Canlas, C. P.; Elam, J. W., Gallium Sulfide-single-walled Carbon Nanotube Composites: High-Performance Anodes for Lithium-Ion Batteries. Adv. Funct. Mater. 2014, 24, 5435-5442. (35) Chen, H. Y.; Di, J. C.; Wang, N.; Dong, H.; Wu, J.; Zhao, Y.; Yu, J. H.; Jiang, L., Fabrication of Hierarchically Porous Inorganic Nanofibers by a General Microemulsion Electrospinning Approach. Small 2011, 7, 1779-1783. (36) Li, G. G.; Hou, Z. Y.; Peng, C.; Wang, W. X.; Cheng, Z. Y.; Li, C. X.; Lian, H. Z.; Lin, J., Electrospinning Derived One-dimensional LaOCl: Ln3+ (Ln= Eu/Sm, Tb, Tm) Nanofibers, Nanotubes and Microbelts with Multicolor-Tunable Emission Properties. Adv. Funct. Mater. 2010, 20, 3446-3456. (37) Raghavan, S. R.; Douglas, J. F., The Conundrum of Gel Formation by Molecular Nanofibers, Wormlike Micelles, and Filamentous Proteins: Gelation without Cross-links? Soft Matter 2012, 8, 8539-8546. (38) Yang, Y.; Wang, H. Y.; Zhou, Q. W.; Kong, M. Q.; Ye, H. T.; Yang, G., Improved Lithium Storage Properties of Electrospun TiO2 with Tunable Morphology: from Porous Anatase to Necklace Rutile. Nanoscale 2013, 5, 10267-10274. (39) Li, L. L.; Peng, S. J.; Cheah, Y. L.; Teh, P. F.; Wang, J.; Wee, G.; Ko, Y.; Wong, C. L.; Srinivasan, M., Electrospun Porous NiCo2O4 Nanotubes as Advanced Electrodes for Electrochemical Capacitors. Chem. Eur. J. 2013, 19, 5892-5898.

34


Page 35 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(40) Hou, H. L.; Wang, L.; Gao, F. M.; Wei, G. D.; Tang, B.; Yang, W. Y.; Wu, T., General Strategy for Fabricating Thoroughly Mesoporous Nanofibers. J. Am. Chem. Soc. 2014, 136, 16716-16719. (41) Xiao, Y.; Zheng, L. R.; Cao, M. H., Hybridization and Pore Engineering for Achieving High-performance Lithium Storage of Carbide as Anode Material. Nano Energy 2015, 12, 152-160. (42) Zhou, W. D.; Xiao, X. C.; Cai, M.; Yang, L., Polydopamine-coated, Nitrogen-doped, Hollow Carbon Sulfur Double-layered Core-shell Structure for Improving Lithium Sulfur Batteries. Nano Lett. 2014, 14, 5250-5256. (43) Kang, S. W.; Xie, H. M.; Zhang, W. M.; Zhang, J. P.; Ma, Z. F.; Wang, R. S.; Wu, X. L., Improve the Overall Performances of Lithium Ion Batteries by a Facile Method of Modifying the Surface of Cu Current Collector with Carbon. Electrochim. Acta 2015, 176, 604-609. (44) Wu, X. L.; Guo, Y. G.; Su, J.; Xiong, J. W.; Zhang, Y. L.; Wan, L. J., Carbon-nanotube-decorated Nano-LiFePO4@C Cathode Material with Superior High-rate and Low-temperature Performances for Lithium-ion Batteries. Adv. Energy Mater. 2013, 3, 1155-1160. (45) Hassoun, J.; Bonaccorso, F.; Agostini, M.; Angelucci, M.; Betti, M. G.; Cingolani, R.; Gemmi, M.; Mariani, C.; Panero, S.; Pellegrini, V., An Advanced Lithium-ion Battery Based on a Graphene Anode and a Lithium Iron Phosphate Cathode. Nano Lett. 2014, 14, 4901-4906. (46) Martha, S. K.; Nanda, J.; Veith, G. M.; Dudney, N. J., Electrochemical and

35



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Rate

Performance

Study

of

High-voltage

Lithium-rich

Page 36 of 37

Composition:

Li1.2Mn0.525Ni0.175Co0.1O2. J. Power Sources 2012, 199, 220-226. (47) Fan, X. Y.; Li, Y.; Wang, J. J.; Gou, L.; Zhao, P.; Li, D. L.; Huang, L.; Sun, S. G., Synthesis and Electrochemical Performance of Porous Li2FeSiO4/C Cathode Material for Long-life Lithium-ion Batteries. J. Alloys Compd. 2010, 493, 77-80. (48) Park,

K.

S.;

Schougaard,

S.

B.;

Goodenough,

J.

B.,

Conducting-polymer/iron-redox-couple Composite Cathodes for Lithium Secondary Batteries. Adv. Mater. 2007, 19, 848-851. (49) Wu, X. L.; Jiang, L. Y.; Cao, F. F.; Guo, Y. G.; Wan, L. J., LiFePO4 Nanoparticles Embedded in a Nanoporous Carbon Matrix: Superior Cathode Material for Electrochemical Energy-storage Devices. Adv. Mater. 2009, 21, 2710-2714. (50) Zhang, M.; Lei, D. N.; Yin, X. M.; Chen, L. B.; Li, Q. H.; Wang, Y. G.; Wang, T. H., Magnetite/graphene Composites: Microwave Irradiation Synthesis and Enhanced Cycling and Rate Performances for Lithium Ion Batteries. J. Mater. Chem. 2010, 20, 5538-5543.

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The table of contents entry

37


Structure Interlacing and Pore Engineering of Zn2GeO4 Nanofibers for

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