Constructing Highly Graphitized Carbon-Wrapped Li3VO4

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Constructing highly graphitized carbon-wrapped Li3VO4 nanoparticles with hierarchically porous structure as a long life and high capacity anode for lithium-ion batteries Di Zhao, and Minhua Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 26 Oct 2015 Downloaded from http://pubs.acs.org on October 27, 2015

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ACS Applied Materials & Interfaces

Constructing highly graphitized carbon-wrapped Li3VO4 nanoparticles with hierarchically porous structure as a long life and high capacity anode for lithium-ion batteries Di Zhao and Minhua 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. ABSTRACT: Li3VO4 nanoparticles (NPs) embedded in a continuous, highly graphitized carbon network with an interconnected hierarchically porous structure (HP-Li3VO4/C) were prepared using a facile, green freeze drying method followed by in situ carbonizing. Because of its unique microstructure, the resultant HP-Li3VO4/C exhibits excellent lithium storage performance in terms of specific capacity, cycling stability, and rate capability when used as an anode material in lithium ion batteries (LIBs). Specifically, it delivers an extremely high capacity of 381 mAh g-1 for up to 300 cycles at 0.2 A g−1, and even at a rate as high as 4 A g−1, a high reversible capacity of 275 mAh g-1 can be retained after tested for 500 cycles. This excellent electrochemical performance can be attributed to Li3VO4 NPs wrapped with highly graphitized carbon conductive framework and hierarchically porous structure. This work may offer a new methodology for the preparation of other electrode materials for LIBs. KEYWORDS: Li3VO4; graphitized carbon; hierarchical pore; anode materials; lithium ion batteries

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1. INTRODUCTION The increasing demand for long-term and high-performance rechargeable lithium-ion batteries (LIBs) for a wide range of applications ranging from consumer devices, hybrid electric vehicles, and large-scale grid energy storage are urgent.1 As the key component of LIBs, the electrode materials mainly dominate the electrochemical properties of LIBs. The anode materials mainly have three categories according to their reaction mechanisms: typical graphite, Ti-based oxides, and layered V-based or Mo-based oxides with the intercalation/de-intercalation mechanism; Siand Sn-based alloys based on the alloying-dealloying reaction; most transition metal oxides associated with the conversion (redox) reaction.2,3 Unfortunately, among them, the anodes based on alloying-dealloying reaction are capable to host 4.4 mol Li per Si or Sn accompanied by enormous volume changes (>300%),4 while for those anodes based on conversion (redox) reaction, the M-O bonds will rupture in the first discharge process, which causes a large electrode polarization, thus leading to poor energy efficiency.5 On the contrary, the anodes based on the Li insertion/de-insertion reaction refer to the reversible insertion of Li+ into the interconnected framework of empty lattice sites during the Li insertion/de-insertion processes, and the structural integrity of the host lattice generally is conserved. This intercalation mechanism has been proved to be the most attractive and successful mechanism in the history of LIBs due to its high reversibility and high energy efficiency. However, the intercalation-type graphite, which is the current commercial anode material, remains insufficient for nextgeneration LIBs due to its low theoretical capacity (372 mAh g-1). Especially, it suffers from severe safety risk of dendritic lithium growth and short circuits due to its low potential of lithium ion insertion (only about 0.2 V vs. Li+/Li) and polarization at high rates.2,6 Therefore, much


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effort has been devoted to developing electrochemically active materials with high capacity, long cycling stability, and high rate capability to replace the commercial graphite.7-10 The spinel Li4Ti5O12 is another well recognized intercalation-type anode material, which is known for good reversibility and no volume change during the intercalation process. However, its limited capacity (~150 mAh g-1 ) along with a high operation voltage (about 1.6 V vs. Li+/Li) halve the overall cell voltage and cell energy seriously.6,11 Therefore, developing hosts with a lower intercalation/de-intercalation voltage of Li ion is crucial for future applications in electronics and electric vehicles.12 Just recently (2013), Li et al. for the first time reported that Li ions can be intercalated into layered Li3VO4 in the voltage range of 0.5∼1.0 V vs. Li/Li+ and that its high density is comparable to that of graphite, thus resulting in its high theoretical volumetric energy density, which is safe and important for future applications in portable electronics and electric vehicles.2 From then on, Li3VO4 has received great interest due to its outstanding features as anode materials. For instance, Kim et al. successfully synthesized irregular Li3VO4 particles by a two-step method, and when used as an anode in LIBs, 190 mAh g-1 of charge capacity was obtained after 100 cycles at 1.0 C.11 Kim et al. prepared spherical sub-micron sized Li3VO4 particles with Li/V atom ratios in the range of 3 to 3.08 by ultrasonic spray pyrolysis. Compared to their previously reported results, enhanced capacities and rate capabilities were achieved with the specific capacity of 250 mAh g−1 after 100 cycles at a rate of 0.5 C.13 Recently, Ni et al. reported a facile hydrothermal method followed by annealing treatment for synthesizing Li3VO4 NPs with a small amount of orthorhombic V4O9, which showed excellent electrochemical properties.14 Although great progress has been made on Li3VO4 as an anode material, there are still some barriers that limit its practical applications. For example, Li3VO4 possesses rather low electronic conductivity, which may cause large resistance polarization and


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poor rate capability.6,15 As is well known, the electronic conductivity of materials can be effectively improved by doping or carbon coating. Just recently, carbon-based materials have been used to enhance the electronic conductivity of Li3VO4, such as carbon nanotube/Li3VO4,16 graphene/Li3VO46,17,18

Li3VO4/N-doped

graphene19

and

carbon

encapsulated

Li3VO4

composites20,21 and significantly improved lithium storage performance has been achieved. From aforementioned reports on Li3VO4, it can be seen that several approaches have been developed for synthesizing different Li3VO4 nanostructures, mainly including ultrasonic nebulization, solidstate reaction and solution-based precipitation, and hydrothermal method (with high-temperature or vacuum pressure) followed by high-temperature annealing treatment.6 However, the lack of efficient approach for synthesizing the Li3VO4 hybrids with homogeneous carbon, as well as efficient control over the morphology, is a serious problem for further design and optimization its practical application as anode materials for LIBs. Inspired by the above studies, we for the first time propose a facile and waste-free strategy to fabricate bare Li3VO4 NPs embedded homogeneously in hierarchically porous carbon conductive network (defined as HP-Li3VO4/C) via a facile freeze drying followed by in situ carbonizing the precursor. In contrast with previous methods, for instance solid state reaction, that needs to macroscopically mix vanadium and carbon sources, the freeze drying-assisted method in our experiment can ensure homogeneously mixing of the reactive species at the atomic and molecular level, which will have advantages of good stoichiometric control, low synthesis temperature, short-heating time, good crystallinity, and small particle size, even down to nanometer level.22 As a result, vanadium and lithium are homogenously distributed in the organic framework at molecular dimension. During the pyrolysis process, the organic functional groups are carbonized into a porous carbon network, while the metal precursors are in situ transformed


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into uniform Li3VO4 NPs with an average diameter of ∼30 nm, generating the final HPLi3VO4/C hybrid. Furthermore, unlike nano-sized particle materials, porous carbon materials with a micrometer size can not only provide porous transport channels for the rapid access of lithium ions, but also offer high packing densities.9,23-25 When evaluated as an anode for LIBs, the HP-Li3VO4/C hybrid exhibits higher specific capacity and significantly enhanced cycling performance compared to bare Li3VO4. 2. EXPERIMENTAL SECTION 2.1. Synthesis of HP-Li3VO4/C hybrid: In a typical synthesis for HP-Li3VO4/C hybrid, vanadium(IV)oxy acetylacetonate ([VO(acac)2], 0.265 g, 1 mmol; Aldrich) and lithium hydroxide (LiOH, 0.126 g, 3 mmol; Aldrich) were added to distilled water (15 mL; Beijing Chemical Reagent Ltd.) to give a palm red transparent solution with the hand shake. Then, the mixture was freeze-dried to form a light brown fluffy floc. As a result, the homogeneous distribution of the constituent ions favors easy crystallization during heat-treatment. Finally, the floc was heated at 550 oC for 2 h under a N2 atmosphere at a heating rate of 3 oC min-1. To obtain bare Li3VO4, the floc was treated at 550 oC under air atmosphere for 2 h at the same heating rate of 3 oC min-1. 2.2. Sample Characterizations: The obtained products were characterized on powder X-ray diffraction (XRD) (Bruker D8 X-ray power diffractometer) operated at 40 kV voltage and 50 mA current. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet 170SXFTIR spectrometer using pressed KBr pellets to test the chemical bonding of the samples from 500 to 3900 cm-1. The general size and morphology of the products were characterized by a fieldemission scanning electron microscope (FESEM, Hitachi S-4800). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were


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carried on a H-8100 transmission electron microscope (TEM) operating at a 200 kV accelerating voltage. The energy dispersive spectroscopy (EDS) element mapping images were taken on Hitachi S-4800 SEM unit. Raman spectra were recorded on an Invia Raman spectrometer, with an excitation laser wavelength of 514.5 nm. Thermogravimetric and differential scanning calorimetry analysis (TG/DSC) was carried out with a DTG-60AH instrument. X-ray photoelectron spectra (XPS) were recorded on an ESCALAB 250 spectrometer (Perkin Elmer) to characterize the chemical composition of the samples. Raman spectra were recorded on an Invia Raman spectrometer, with an excitation laser wavelength of 514.5 nm. The Brunauer-EmmettTeller (BET) surface area of as-synthesized samples was measured using a Belsorp-max surface area detecting instrument by N2 physisorption at 77 K. The pore size distribution was calculated via a non-local density functional theory (NLDFT) method using nitrogen adsorption data with a slit pore model. 2.3. Electrochemical Measurements: The electrochemical behavior of the obtained products was examined by coin-type cells (2025) assembled in an argon filled glovebox. The working electrode was obtained by mixture of active material, carbon black, and carboxymethylcellulose sodium (CMC) binder with a weight ratio of 80:10:10 onto a copper foil current collector. The typical electrode was dried at 120 oC for 24 h under vacuum before being assembled into a coin cell in an argon-filled glovebox. The mass loading of the active material was about 0.65 mg cm-2. A Celgard 2400 microporous polypropylene membrane was used as the separator, and Li foil was used as the counter electrode. The nonaqueous electrolyte used was 1 M LiPF6 dissolved in an ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl carbonate (DEC) mixture (1:1:1, in vol. %). 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 and 0.2-3.0 vs. Li+/Li at room temperature.


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Cyclic voltammetry (CV) curves were recorded on a CHI-760E workstation at a scanning rate of 0.5 mV s-1. The impedance spectra of the cell were measured by applying a sine wave with amplitude of 5 mV over the frequency range from 100 kHz to 0.01 Hz. The specific capacity for HP-Li3VO4/C hybrid is calculated based on the total weight including Li3VO4, amorphous carbon, and graphitized carbon. 3. RESULT AND DISCUSSION

LiOH

Freeze drying

N2

Li insertion /extraction

Annealing VO(acac)2

Li3VO4

Air

Hierarchically porous carbon network

Scheme 1. Schematic illustration of the fabrication of HP-Li3VO4/C hybrid and its application in LIBs.

Scheme 1 shows a schematic diagram of the formation process for HP-Li3VO4/C hybrid. Vanadium (IV) oxy acetylacetonate ([VO(acac)2]) was used as vanadium and carbon sources, and lithium hydroxide (LiOH) as lithium source. VO(acac)2-LiOH aqueous solution was first freezing-dried to form a precursor (Figure S1a), which then was thermally treated at different atmospheres (N2 and air) at 550 oC to form HP-Li3VO4/C hybrid and bare Li3VO4, respectively. This method needs not post-washing process and therefore no solid wastes were formed. Detailed preparation process was described in the experimental section. Fourier transform infrared spectroscopy (FT-IR) clearly shows the structure change from the VO(acac)2 to the precursor and then to the HP-Li3VO4/C hybrid (Figure S1b). It can be seen that the organic functional groups and water gradually disappear accompanied by the formation of inorganic HPLi3VO4/C hybrid. At the same time a large number of pores are also formed due to shrinkage of


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texture on calcining, which can be clearly observed by field-emission scanning electron microscope (FE-SEM) images (Figure 1a,b). The enlarged FE-SEM images (① and ② marked in Figure 1a,b) show obvious nanometer-sized transport pores, which can ensure easy accessibility of functional species.23 However, the control sample, which was obtained by thermally treating the precursor in air at 550 oC, is composed of irregular particles (Figure 1c). The nitrogen adsorption-desorption isotherms in Figure 1d further confirm the hierarchically

a)

c)

b)

①

②

①

② e) f)

-1

110 101

d)

131 230

200

Li3VO4 221 212

50 100 150 Pore size (nm)

020 211002 121 112

0

10

011 200

20

010

3

Intensity (a.u.)

-1

30

dV/dD (cm g nm )

Quantity absorbed (cm3 g-1)

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

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HP-Li3VO4/C

Adsorption

JCPDS No. 39-0378

Desorption

0 0.0

0.3 0.6 0.9 Relative pressure (p/p0)

20

30 40 2 θ (degree)

50

60

Figure 1. a,b) FE-SEM images of HP-Li3VO4/C hybrid and ① and ② are enlarged FE-SEM images; c) FESEM image of bare Li3VO4 particles. d) N2 adsorption-desorption isotherms of HP-Li3VO4/C hybrid and pore size distribution pattern calculated from the desorption branch according to NLDFT model; e) XRD patterns of HP-Li3VO4/C hybrid and bare Li3VO4; f) Crystal structure of Li3VO4.

porous structure of HP-Li3VO4/C hybrid. The uptake at very low P/Po (< 0.1) is derived from the existence of micropores, while the hysteresis loop and the rapid rise of adsorption curve in the higher P/Po indicate the presence of mesopores and macropores in the resultant materials.1,26 The inset in Figure 1d displays pore size distribution curve obtained by applying nonlocal density functional theory (NLDFT) model. Evidently, the curve exhibits a series of peaks centered at ∼1.7, 3, 7, 14, 19, 49, and 97 nm, respectively, indicating that the HP-Li3VO4/C hybrid prepared


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by the current method possesses a hierarchically microporous/mesoporous/macroporous structure. Moreover, the specific surface area of the HP-Li3VO4/C hybrid is 25.4 m2 g-1. Compared to pure micropore, which would lead to an enhanced irreversible Li storage and thus results in a poor cycling performance, the hierarchical pores existing in HP-Li3VO4/C hybrid are beneficial for electrolyte accessibility, rapid lithium ion diffusion and volume buffering.23,26-31 Figure 1e shows XRD patterns of samples obtained at different atmospheres (N2 and air) at 550 oC. For both cases, strong diffraction peaks with high intensity and resolution indicate a high degree of crystallization of the obtained materials. All the diffraction peaks can be well indexed to an orthorhombic Li3VO4 phase (JCPDS No. 39-0378) with the lattice parameters of a = 6.319 Å, b = 5.448 Å, c = 4.940 Å, and α = β = γ = 90°, consistent to a β polymorph with space group Pmn21. Furthermore, as shown in Figure 1f, the crystal structure of Li3VO4 is composed of corner-shared VO4 and LiO4 tetrahedrons. In spite of this fact, it still has empty cationic sites for further insertion of lithium ions. From above XRD patterns, it can also be seen that no V4O9

a)

100

Li3VO4

D 760

800

840

G

HP-Li3VO4/C

Weight loss (%)

HP-Li3VO4/C

Intensity (a.u.)

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


b)

1.2%

Li3VO4

14.5%

HP-Li3VO4/C

80

60

57%

Precursor

Li3VO4

40 500

1000 1500 Wavenumber (cm-1)

2000

100 200 300 400 500 600 700 Temperature (°C)

Figure 2. a) Raman spectra of HP-Li3VO4/C hybrid and bare Li3VO4 and the insets are corresponding enlarged spectra; b) TGA curves for the precursor, HP-Li3VO4/C hybrid and bare Li3VO4.

phase or other vanadium oxides were found,14 suggesting that pure Li3VO4 phase can be obtained at 550 oC for 2 h, which can greatly reduce production costs as compared to the


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conventional solid-state approach or solution-based method which generally require higher temperatures (about 1000 oC) for about 5 h or more.2,11,13 Raman spectroscopy was employed to further confirm the crystalline phase of Li3VO4 and the existence of carbon in the hybrid obtained in N2 atmosphere. As shown in Figure 2a, a clear difference can be observed between these two samples. Compared with that of bare Li3VO4, the Raman spectroscopy of HP-Li3VO4/C hybrid has an evident blue shift at about 817 cm-1 (the vibration of VO4 tetrahedron in Li3VO4), which is probably due to a phonon confinement effect and/or a masking effect. This fact implies that Li3VO4 NPs are covered with a carbon layer,32-34 which is further confirmed by its narrow Raman spectrum in the range of 1000-2000 cm-1. Two obvious peaks at 1323 and 1587 cm-1 can be clearly observed, which can be assigned to characteristic D-band (defects and disordered carbon) and G-band (graphitic carbon).35 It is worth noting that the intensity ratio (R = ID/IG) of the D-band to the G-band is ∼1.02, showing the existence of partially graphitized carbon.1,36,37 To some extent, it reveals that the Li3VO4 NPs were embedded into the hierarchically porous conductive network with both amorphous carbon and graphitic carbon. This fact has also been proved by X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) measurements, which will be described below. Thermogravimetric analysis (TGA) curves, carried out in air at a heating rate of 10 °C min-1, were used to determine the carbon content in as-synthesized samples (Figure 2b). The results showed that the HP-Li3VO4/C hybrid contains ∼14.5 wt.% carbon, while the reference sample (bare Li3VO4) almost does not involve carbon (just 1.2 wt.%). The TGA curve of the precursor, which was performed in N2 at a heating rate of 10 °C min-1, indicates that after calcining, it can still maintain 43% weight, indicating its high yield and the possibility of practical application in LIBs.


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To further determine the chemical composition of HP-Li3VO4/C hybrid, XPS analyses were conducted. As clearly disclosed by the survey XPS spectrum (Figure S2), four elements including V, Li, O, and C are detected. Figure 3a shows the high-resolution V 2p XPS spectrum, in which the binding energies at 517.2 and 524.5 eV agree well with those of V5+ in Li3VO4,

517.2 eV

-1

V5+ 2P1/2 524.5 eV

515 520 525 Binding energy (eV)

531.8 eV OH

50

c)

-1

530.0 eV O22

528

533.3 eV H2O

531 534 Binding energy (eV)

537

540

b)

54.9 eV

52

54 56 58 Binding energy (eV)

C 1s 284.4 eV

-1

O 1s

530

Intensity (Counts s )

510

Li 1s


-1


V 2p

Li+ 1 s

a)

V5+ 2P3/2


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


284.9 eV C-C

60

d)

Graphitic carbon

280

285.8 eV C-OH 289.2 eV C(O)-O

282

284 286 288 290 Binding energy (eV)

292

Figure 3. XPS spectra of HP-Li3VO4/C hybrid: high-resolution spectra for a) V 2p, b) Li 1s, c) O 1s and d) C 1s.

respectively,38 while the peak at 54.9 eV (Figure 3b) corresponds to the Li 1s characteristic peak of Li3VO4.39 The corresponding O 1s XPS spectrum is shown in Figure 3c, which could be deconvoluted into three peaks, indicating the existence of three different oxygen species. The peak with the binding energy of 530.0 eV is attributed to O 1s in metal oxides, while the other two peaks at 531.8 and 533.3 eV can be assigned to OH and H2O molecules, respectively.38 The high-resolution C 1s spectrum is shown in Figure 3d. The strong C 1s peak at 284.4 eV corresponds to graphitic carbon, and the other three peaks arise from the oxygenated carbon atoms (C-C at 284.8 eV, C-OH at 285.8 eV, and C(O)-O at 289.2 eV).38,40,41


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The unique structure and morphology of the HP-Li3VO4/C hybrid were further investigated using TEM and energy-dispersive spectroscopy (EDS) element mapping, as shown in Figure 4. The low-magnification TEM images in Figure 4a,b show that the HP-Li3VO4/C hybrid exhibits a porous structure. Some irregular macropores with diameters of about 100-150 nm (the yellow circles) can be clearly seen. Moreover, smaller pores can also be observed (the inset in Figure 4a, red circle) From high-magnification TEM images (Figure 4c-e), it can be clearly seen that the small NPs with diameters of 25-35 nm embed in the continuous wormhole-like hierarchically

a)

c)

b)

c

d)

e)

f)

f

Figure 4. a,b) TEM images of HP-Li3VO4/C hybrid at different magnifications; the inset in a) is an amplifying image; c-f) representative HRTEM images and the insets in f) are SEM image and the corresponding element mapping images of C and V elements; the blue circles represent amorphous carbon.

porous carbon, which can provide a large volume for electrolyte storage and ensure Li+ diffusion in channels across the HP-Li3VO4/C hybrid anode. This will endow remarkable rate capability and cycling performance.42 In addition, from the TEM and high-resolution TEM (HRTEM) images (Figure 4b,c), it can be observed that a thin graphitized carbon layer (marked as g-C) was formed on the surface of Li3VO4 NPs with a thickness of ~ 3 nm. Additionally, HRTEM images


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(the insets in Figure 4b,c) show that the g-C layer consists of many defects between discontinuous graphene sheets. Moreover, the amorphous carbon (marked as a-C) can be observed in the inner of the hybrid and around the Li3VO4 NPs, which may be due to that the organic groups on the surface can be easily converted from amorphous carbon into graphitized carbon under metal catalysis in thermal decomposition process.37 The characteristic lattice fringes corresponding to 0.55 and 0.41nm in the HRTEM images (Figure 4c-f) should be attributed to the (010) and (110) planes of Li3VO4, respectively. Furthermore, the element mapping images obtained by EDS attached with an electron micro-scope confirm the presence of V, C, and O elements, and that these elements were uniformly distributed in the hybrid (Figure 4f and Figure S3). In a word, above experimental results clearly demonstrate that Li3VO4 NPs are closely and uniformly embedded in the a-C, and meanwhile the surface of the hybrid is wrapped by g-C. The HP-Li3VO4/C hybrid with a unique Li3VO4@a-C@g-C structure may function well as an anode due to its following several aspects: (i) the carbon conductive network specially with the surface wall of g-C favors electron conduction and buffers the stress produced from the intercalation-deintercalation process; (ii) the a-C layer around the Li3VO4 NPs has been well demonstrated to possess more Li storage sites than graphitic carbon;43 (iii) the presence of the carbonaceous materials between the metal oxide NPs prevents their aggregation into bulk materials, which will ensure excellent cycle performance; (IV) the hierarchically porous structure is beneficial for electrolyte accessibility, rapid lithium ion diffusion and volume buffering for the Li3VO4; (V) the high yield and low mass density of the carbon lower the total weight of the electrode, thus providing a higher mass ratio of Li3VO4 in the electrode and a high practical specific capacity.


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The electrochemical performance of HP-Li3VO4/C hybrid was tested as an anode for LIBs. Figure 5a,b show the cyclic voltammogram (CV) curves of HP-Li3VO4/C hybrid and bare Li3VO4 at a scan rate of 0.5 mV s−1 in the potential window of 3.0 V to 0.01 V. By careful observation, it can be seen that both the samples exhibit the same anodic and cathode peaks, implying their identical electrochemical reaction mechanism. However, it is worthy of noticing that, for the HP-Li3VO4/C hybrid, the CV profiles remain steady after the first cycle, suggesting 0.4

b)

0.2

0.0 1st 2nd

-0.4

-0.4

3rd 4th

-0.6

Current (mA)

Current (mA)

a)

0.0 1st 2nd 3rd 4th 5th 6th

5th

0.0 Voltage (V)

c)

2

0.5

0.5 1.0 1.5 2.0 + Voltage (V vs. Li/Li )

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7 th

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200

7 th

0.0 3.0

HP-Li3VO4/C

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1 0 30

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-0.8 0.0

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Li3VO4

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-0.8

Voltage (V)

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2.0 1.5

0.2-3.0 V 25 mA g-1 0.01-3.0 V 200 mA g-1 0.2-3.0 V 100 mA g-1

1.0 0.5 0.0

0 0

200 400 Specific capacity(mAh g-1)

600

0

100 200 300 400 500 Specific capacity (mAh g-1)

600

Figure 5. a,b) The first six consecutive CVs of HP-Li3VO4/C and bare Li3VO4 electrodes at a scan rate of 0.5 mV s-1 in the voltage range of 0.01-3.0 V vs. Li+/Li. c) Galvanostatic discharge-charge profiles of HPLi3VO4/C and Li3VO4 electrodes for the first seven cycles at a current rate of 0.1 A g-1 in the voltage range of 0.2-3.0 V vs. Li+/Li. d) The comparison of typical discharge-charge profiles HP-Li3VO4/C electrode at current densities of 25 and 100 mA g-1 in the voltage range of 0.2-3.0 V vs. Li+/Li, and at a current density of 200 mA g-1 in the voltage range of 0.01-3.00 V vs. Li+/Li.

highly reversible electrochemical reactions. On the contrary, bare Li3VO4 still remains changeable till the 6th cycle. As clearly shown in Figure 5a, in the 1st cathodic scan, one reduction peak at around 0.28 V is observed, which is attributed to the lithiation process and the formation of solid electrolyte interphase (SEI) film.14 Then, from the 2nd cycle, the reduction


14

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peak shift to 0.5 and 0.92 V, indicating a multi-step lithium ion insertion process in the active materials. And this process corresponds to the phase changes from Li3VO4 to Li3+xVO4.44 The observed anodic peaks at the potentials of 0.86 and 1.3 V correspond to the delithiation process that involves the reversible formation of Li3VO4 and the release of lithium ions.14 Figure 5c display the galvanostatic discharge-charge profiles of the electrodes for the first seven cycles at a current rate of 0.1 A g-1 in the voltage range of 0.2-3.0 V vs. Li+/Li. Obviously, the initial discharge capacity (540 mAh g-1) of the HP-Li3VO4/C electrode is far higher than that (330 mAh g-1) of bare Li3VO4 electrode. The intercalation of Li+ occurs at a voltage range mainly between 0.5∼1.0 V vs. Li/Li+, lower than that of Li4Ti5O12 (about 1.5 V) and higher than that of graphite, which intercalates Li at a low potential close to that of the Li-plating (< 0.2 V). Meanwhile, consistent with the above CV results, HP-Li3VO4/C electrode shows high stable discharge capacity of 420 mAh g-1 and charge capacity of 390 mAh g-1 after the 1st cycle. However, the discharge and charge capacities of bare Li3VO4 electrode decrease continuously, revealing its poor cycle stability. As we know, the energy density of a battery is the product of its specific capacity and output voltage during discharge-charge process. For comparison, the galvanostatic discharge-charge profiles obtained at different current densities and different cut-off voltage windows were investigated. As shown in Figure 5d, the discharge capacity at a current density of 200 mA g-1 in the voltage range of 0.01-3.0 V vs. Li/Li+ reached approximately 520 mAh g-1, which is higher than those of previous reports.11,15 In general, for the same sample, the wider cutoff voltage window gives the higher capacity at the same current density. Impressively, when cycled at a rate of 25 mA g-1 and 100 mA g-1 in the voltage range of 0.2-3.0 V, the HP-Li3VO4/C anode still retained its capacities as high as 530 and 450 mAh g-1, respectively. These values are also much higher than those of the reported Li3VO4, whose initial discharge capacities are about


15


425, 323, and 420 mAh g-1 at a rate of 20 mA g-1.2,6,15 Considering that the anode material intercalates Li at a low potential close to that of the Li plating, which easily results in Li dendrites and short circuits2, the cut-off potential of Li3VO4 was set as 0.2 V. Figure 6 shows the cycling performance of HP-Li3VO4/C hybrid and bare Li3VO4 at a current density of 200 mA g-1. The initial discharge and charge specific capacities of HP-Li3VO4/C electrode are 450 and 390 mAh g-1, respectively. Although a large irreversible capacity loss is observed in the first cycle, which can be partly attributed to the formation of SEI

-1

HP-Li3VO4/C 200 mAh g

300

80

3 1st 10th 25th 50th 75th 100th 150th 200th 250th 300th

Voltage (V)

-1

100

450

2

150

-1

Li3VO4 200 mAh g

1

0

0 0

50

100 150 Cycle number

200 400 Specific capacity(mAh g-1)

200

250

60 40 20 0 300

Coulombic efficiency (%)

films on the surface of the electrode, the reversible capacity is still as high as 405 mAh g-1 in the

Specific 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

Page 16 of 29

Figure 6. Cycling performance of HP-Li3VO4/C and bare Li3VO4 electrodes at a current density of 200 mA g-1 and the corresponding coulombic efficiency of HP-Li3VO4/C electrode for 300 cycles. The inset is the corresponding charge and discharge profiles of HP-Li3VO4/C electrode at different cycles with a current density of 200 mA g-1.

second cycle and could be maintained at 381 mAh g-1 in the 300th cycle, indicating the excellent reversibility of this material. While for bare Li3VO4, it delivered only 180 mAh g-1 after 150 cycles. Moreover, for HP-Li3VO4/C hybrid, during the tested 300 cycles, the initial coulombic efficiency is as high as 86.7% for the first cycle, and then it increases sharply to the 96.3% during the second cycle and reaches over 99.8% after the 300th cycle. This result is indicative of the good stability of the SEI films in the subsequent cycles, which is closely related to the


16

Page 17 of 29

lithiation-induced capacity degradation and reactivation, indicating excellent cycling stability of this material.45 Meanwhile, the discharge-charge curves from the first to the 300th cycles almost coincide with each other (the inset in Figure 6), confirming the good cycling stability as well.46 Therefore, the HP-Li3VO4/C hybrid features a safer voltage, higher capacity and long-life cycle stability. Furthermore the electrochemical performance of HP-Li3VO4/C and bare Li3VO4 electrodes are further investigated. For HP-Li3VO4/C electrode, at the current densities of 200 and 1000 mA g1

, 408 and 380 mAh g-1 of the discharge capacities can be maintained after 100 cycles,

respectively, both much higher than 250 mAh g-1 of bare Li3VO4 electrode at a lower current density of 0.1 A g-1 (Figure 7a). Moreover, the rate capability is also evaluated with current

a)

450 300 150

600 0.01-3 V 200 mA g-1

0.01-3 V 1000 mA g-1

HP-Li3VO4/C

0.01-3 V 100 mA g-1

Li3VO4

Specific capacity (mAh g-1)

-1

Specific capacity (mAh g )

densities ranging

0 0

400

20

40 60 Cycle number

80

b)

HP-Li3VO4/C

450 0.025 A g-1 Li3VO4 0.05 0.1 0.2 0.4 0.8 1 2 300

0.025 4

150 0

100

c)

Specific capacity ( m Ah g -1 )

0

20

80

100

d)

800

300

40 60 Cycle number

600 HP-Li3VO4/C

HP-Li3VO4/C

200

100

-Z'' (ohm)

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


-1

4Ag

Li3VO4

400 200

Li3VO4

0

0 0

100


400

500

0

200

400 600 Z' (ohm)

800

Figure 7. Electrochemical properties of the as-prepared electrodes for Li storage: a) cycling performance of HP-Li3VO4/C and Li3VO4 electrodes at a current density of 0.2 A g-1 and 1 A g-1 in the voltage range 0.01-3.00 V vs. Li+/Li; b) rate performance at varied current densities from 0.025 to 4 A g-1 in the voltage range of 0.203.00 V vs. Li+/Li; c) long-life cycling performance of HP-Li3VO4/C electrode at 4 A g-1 for 500 cycles; d) electrochemical impedance spectroscopies of HP-Li3VO4/C and bare Li3VO4 electrodes.


17


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Page 18 of 29

from 25 to 4000 mA g-1 in the voltage range of 0.2-3.0 V vs. Li+/Li, the HP-Li3VO4/C electrode delivers stable capacity at each rate, varying from 403 to 250 mAh g−1. When the current density finally returns to 25 mA g−1, the charge capacity can recover to 398 mAh g−1, indicating the excellent capability of the HP-Li3VO4/C hybrid (Figure 7b). Even at a higher current density of 4000 mA g-1, corresponding to a time of 11 min to finish a discharge/charge process, the HPLi3VO4/C electrode still delivers a capacity as high as 383 mAh g-1 (Figure 7c). From the 50th cycle onwards, HP-Li3VO4/C electrode exhibits stable capacity retention of nearly 280 mAh g-1. Encouragingly, its cycling life can be extended to as long as 500 cycles with a high reversible capacity of 275 mAh g-1. For comparison, we also tested bare Li3VO4 electrode and as shown in Figure7b,c, it exhibits much poorer lithium storage performance. This result supports the fact that the Li3VO4 NPs embedded in hierarchically porous carbon conductive networks could successfully enhance electronic/ionic transport within the anode, thus resulting in improved electrochemical kinetics, which is further proved by electrochemical impedance spectroscopy (EIS). As shown in Figure 7d, the compressed semicircle of HP-Li3VO4/C hybrid in the high frequency range, which represents the charge transfer resistance (Rct) of the electrode, is found to be much smaller than that of bare Li3VO4 electrode. The electrochemical performance of our HP-Li3VO4/C hybrid is compared with those of other Li3VO4-based materials reported previously (Table 1) and these materials include various Li3VO4 nanostructures, Li3VO4/C composites, and Li3VO4/graphene hybrids. Clearly, our HP-Li3VO4/C shows the best lithium storage performance when the capacity, the cycle life, and the applied current density are comprehensively considered. To further explore the Li ion insertion mechanism, an examination for XRD patterns was performed to characterize phase changes of HP-Li3VO4/C hybrid electrode during the cycling


18

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process. Figure 8a and Figure S4 show XRD patterns of HP-Li3VO4/C electrode after the 1st cycle and the 6th cycle at different cut-off voltages, respectively. Obviously, for each case, no Table 1. Comparison of the cycling performance of HP-Li3VO4/C in this work with reported Li3VO4-based materials as anodes for LIBs. Capacity [mAh g-1]

Current density [A g-1]

Cycle number

Voltage window (V)

381

0.2

300

0.2-3

275

4

500

0.2-3

Li3VO4 with carbon coating

245

0.02

50

0.05-3

2015

20

Li3VO4@anchored nanosheets

163

2

5000

0.2-3

2015

17

Li3VO4/N-doped graphene

193

2

900

0.02-3

2015

19

Li3VO4-δ

286

200

200

0.2-3

2015

47

247

500

400

0.2-3

Hollow Li3VO4/rGO spheres

257.9

4

1500

0.2-3

2015

48

Li3VO4 nanoribbon/graphene

440

0.08

200

0.2-3

2015

18

Carbon-encapsulated Li 3VO4 particles

401

0.04

50

0.2-3

2015

21

Hollow Li3VO4/carbon nanotube composite

250

2

2000

0.2-3

2014

16

Li3VO4 irregular agglomerates

190

0.08

100

0.01-2

2013

11

Li3VO4 particles

398

0.1

100

0.02-3

2014

14

Li3VO4 spherical particles

250

0.08

100

0.1-2

2014

13

Hollow-structured Li3VO4

110

4

1000

0.2-3

2014

15

Li3VO4 microboxes wrapped with graphene nanosheets

350

0.02

50

0.2-3

2013

6

210

4

500

0.2-3

Li3VO4 particles

283

0.02

25

0.2-3

2013

2

Materials HP-Li3VO4/C

graphene

Year

Ref. This work

extra peaks appear besides those for Li3VO4, indicating an intercalation-type lithium storage mechanism of Li3VO4. Figure 8b shows O 1s XPS spectra. It can be clearly seen that when the HP-Li3VO4/C electrode was discharged to 0.01 V, the binding energy of O 1s is slightly lower than that emitted from the electrode before discharging (OCV). However, when the electrode


19


was charged to 3 V, the binding energy of O 1s comes back (the black dotted line), which is similar to OCV, indicating the reversible intercalation-type process. Moreover, the O 1s at the state charged to 3 V can be fitted with three peaks at 530, 531.7, and 532.67 eV, respectively, corresponding to oxygen in metal oxides, OH- and H2O molecules.38 Based on this result, it can be seen that there is no Li2O formation (528.8 eV) during Li insertion/de-insertion process,49 further confirming that HP-Li3VO4/C electrode is a promising insertion anode material.

b) -1


a) OCV Intensity (a.u.)

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 20 of 29

D to 0.01 V C to 1.5 V C to 3 V

JCPDS No. 39-0378: Li3VO4 20

40 60 2 θ (degree)

80

O 1s

528

530.0 eV

OCV C to 3 V

531.7 eV 532.67 eV

D to 0.01 V

531 534 Binding energy (eV)

Figure 8. (a) XRD patterns of HP-Li3VO4/C electrode after the 1st cycle at different cut-off voltages; OCV represents the electrode before discharging; D and C represent discharged and charged states, respectively; (b) comparison of O 1s XPS spectra of the as-prepared HP-Li3VO4/C electrode at OCV, discharged to 0.01 V and charged to 3 V at the 6th cycle at 1 A g-1, respectively.

The high specific capacity, superior cyclability, and good rate capability of the resultant HPLi3VO4/C hybrid are unprecedented among all reported Li3VO4 materials. We deduce that the excellent electrochemical performance may be attributed to the synergy of two characteristics of HP-Li3VO4/C: continuous conductive framework and hierarchically porous structure. For these two aspects, continuous conductive framework might play a crucial role due to following reasons: 1) the continuous a-C network wrapped with highly conductive g-C not only could provide a continuous pathway for electron transport and Li+ diffusion but also could effectively stabilize the as-formed SEI films;50,51 2) the highly conductive carbon network possesses high mechanical stability and could maintain the integrity of the electrode during cycling, thus ensuring the excellent stability performance;23,30,52 3) all Li3VO4 NPs are uniformly confined in


20

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the carbon matrix and thus the particle aggregation can be effectively avoided, ensuring a long cycle stability.10 Furthermore, the hierarchically mesoporous-macroporous structure is beneficial for electrolyte accessibility, fast electron/ion transport and volume buffering for Li3VO4, leading to a remarkably improved electrochemical performance.23,26,30,31 In short, the aforementioned experimental results strongly suggest the structural robustness of the HP-Li3VO4/C hybrid, endowing it high reversible capacity and stable cycle performance. 4. CONCLUSION We have proposed a facile and green strategy for fabricating Li3VO4 NPs embedded homogeneously in hierarchically porous carbon conductive network via a facile freeze drying followed by in situ carburization. The resultant HP-Li3VO4/C hybrid exhibits significantly improved lithium storage performance in terms of specific capacity, cycle stability and rate capability in comparison with bare Li3VO4 NPs in this work and other reported Li3VO4 materials. The continuous conductive network with interconnected abundant hierarchically porous can offer fast electron/ion transport and release the stress of volume expansion, leading to such an excellent electrochemical performance. Moreover, the current fabrication method is very simple and highly scalable, which can be extended to prepare other electrode materials. ASSOCIATED CONTENT Supporting Information. The XRD pattern of the precursor. FT-IR spectra of VO(acac)2, the precursor and HP-Li3VO4/C hybrid. The survey XPS spectrum of HP-Li3VO4/C hybrid. The corresponding element mapping image of O for HP-Li3VO4/C hybrid. The XRD patterns of the HP-Li3VO4/C electrode after the 6th cycle at different cut-off voltages. These material are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION


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Corresponding Author *[email protected] ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 21471016 and 21271023) and the 111 Project (B07012). REFERENCES (1) Chen, Y.; Li, X.; Park, K.; Song, J.; Hong, J.; Zhou, L.; Mai, Y. W.; Huang, H.; Goodenough, J. B. Hollow Carbon-Nanotube/Carbon-Nanofiber Hybrid Anodes for Li-Ion Batteries J. Am. Chem. Soc. 2013, 135, 16280-162833. (2) Li, H.; Liu, X.; Zhai, T.; Li, D.; Zhou, H. Li3VO4: A Promising Insertion Anode Material for LithiumIon Batteries Adv. Energy Mater. 2013, 3, 428-432. (3) Reddy, M. V.; Subba Rao, G. V.; Chowdari, B. V. Metal Oxides and Oxysalts as Anode Materials for Li Ion Batteries Chem. Rev. 2013, 113, 5364-5457. (4) Manthiram, A. Materials Challenges and Opportunities of Lithium Ion Batteries J. Phys. Chem. Lett. 2010, 2, 176-184. (5) Bruce, P. G.; Scrosati, B.; Tarascon, J. M. Nanomaterials for Rechargeable Lithium Batteries Angew. Chem. Int. Ed. 2008, 47, 2930-2946. (6) Shi, Y.; Wang, J. Z.; Chou, S. L.; Wexler, D.; Li, H. J.; Ozawa, K.; Liu, H. K.; Wu, Y. P. Hollow structured Li3VO4 Wrapped with Graphene Nanosheets in Situ Prepared by a One-Pot Template-Free Method as an Anode for Lithium-Ion Batteries Nano Lett. 2013, 13, 4715-4720. (7) Yang, G. Z.; Cui, H.; Yang, G. W.; Wang, C. X. Self-Assembly of Co3V2O8 Multilayered Nanosheets: Controllable Synthesis, Excellent Li-Storage Properties, and Investigation of Electrochemical Mechanism ACS Nano 2014, 8, 4474-4487. (8) Zhou, F.; Xin, S.; Liang, H. W.; Song, L. T.; Yu, S. H. Carbon Nanofibers Decorated with


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Table of Contents Graphic (TOC)

Constructing

highly

graphitized

carbon-wrapped

Li3VO4

nanoparticles with hierarchically porous structure as a long life and high capacity anode for lithium-ion batteries Di Zhao and Minhua Cao*

Li insertion /extraction

100

450

80

600

HP-Li3VO4/C

300

200 mAh g-1 150

Li3VO4

Specific capacity (mAh g-1)

-1

Hierarchically porous carbon network

HP-Li3VO4/C Li3VO4

450 0.025 A g-1 0.05 0.1 0.2 0.4 0.8 1 300

50

0.025

60

4

40

150

20 0

0

0 0

2


20

40 60 Cycle number

200

80

250

100

0 300

Coulombic efficiency (%)

Li3VO4 Specific 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

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Highly graphitized carbon-wrapped Li3VO4 nanoparticles with hierarchically porous structure were prepared for the first time using a facile, green method. The as-synthesized hybrid showed


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an excellent Li-storage performance, which can be attributed to the combined effect of the Li3VO4 NPs wrapped with highly graphitized carbon conductive framework and the hierarchically porous structure.


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Constructing Highly Graphitized Carbon-Wrapped Li3VO4

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