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Electrospun Lotus Root-like CoMoO4@Graphene Nanofibers as High-Performance Anode for Lithium Ion Batteries
Accepted Manuscript Title: Electrospun Lotus Root-like CoMoO4 @Graphene Nanofibers as High-Performance Anode for Lithium Ion Batteries Author: Jing Xu Shaozhen Gu Ling Fan Patrick Xu Bingan Lu PII: DOI: Reference:
S0013-4686(16)30251-1 http://dx.doi.org/doi:10.1016/j.electacta.2016.01.228 EA 26697
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
11-12-2015 19-1-2016 30-1-2016
Please cite this article as: Jing Xu, Shaozhen Gu, Ling Fan, Patrick Xu, Bingan Lu, Electrospun Lotus Root-like CoMoO4@Graphene Nanofibers as High-Performance Anode for Lithium Ion Batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.01.228 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrospun Lotus Root-like CoMoO4@Graphene High-Performance Anode for Lithium Ion Batteries
Nanofibers
as
Jing Xu1,2, Shaozhen Gu2, Ling Fan2, Patrick Xu3, Bingan Lu2*
1
School of Materials Science and Engineering, Hunan University, Changsha 410082,
P. R. China 2
School of Physics and Electronics, Hunan University, Changsha 410082, P. R.
China. 3
Number 4 High School, Beijing 100088, P. R. China.
Corresponding Author *E-mail: [email protected]
ABSTRACT Lotus root-like CoMoO4@graphene nanofibers (CoMoO4@G NFs) was prepared by electrospinning along with post heat treatment, and used as high-performance anode for lithium ion batteries (LIBs). Compared with the pure CoMoO4 NFs, the prepared lotus root-like CoMoO4@G NFs electrode displays higher reversible capacity of 735 mA h g-1 at 100 mA g-1, better rate capability and cycling stability (capacity retains 80% based on the second cycle even after 200 cycles,). These results highlight the importance of combination of conductive graphene nanosheets with multi-level porous CoMoO4 NFs for high-performance LIBs.
■ INTRODUCTION Lithium ion batteries (LIBs) are considered as the most vital candidates for portable electronic devices and hybrid electric vehicles because of their high energy density and high electro-motive force.[1-5] Transition-metal oxides have been widely investigated as promising anode materials for LIBs due to their high theoretical capacity (600-1000 mA h g-1).[2] However, their large volume expansion/contraction and severe particle aggregation during the Li+ intercalation/deintercalation processes
would result in a large irreversible capacity loss and poor cycling stability.[2, 6-8] In order to improve the cycling stability of anode materials, many researchers have focused on nanostructured transition-metal oxide/graphene composites.[9-16] Graphene, a two-dimensional (2D) carbon atom monolayer, has been extensively used as anode material because of its large specific surface area (2630 m2 g−1), superior electrical conductivity, excellent chemical stability and structural flexibility.[17-19] Therefore, it is reasonable to believe nanostructured transition-metal oxide/graphene can efficiently deliver superior electrochemical performance for LIBs.[10,
15, 20-22]
Among various
transition-metal oxides, metal molybdates (MMoO4, M=Co, Ni, Mn, Zn) are promising candidates for LIBs applications due to their multiple oxidation states and reversible small ion storage.[23-26] Particularly, CoMoO4 has received much interest as supercapacitor or anode for LIBs.[24,
26-28]
It is reported that CoMoO4/graphene
nanocomposite has demonstrated enhanced electrochemical performance.[27,
29, 30]
However, these preparation methods are involved with complex processes and the cycling performance still need to be improved. One-dimensional (1D) nanostructured materials have advantages of excellent cycling performance due to its efficient transport pathways for electrons and ions.[31-33] Electrospinning is a simple and effective technique to fabricate 1D nanofibers (NFs) with controllable diameter,[31, 34, 35]
and using this method along with certain post-treatments could fabricate many
interesting multilevel 1D structures (branched nanowires and necklacelike nanowires).[36-39] So it is desirable to use electrospinning for the preparation of novel 1D CoMoO4/graphene nanostructure as high-performance anode for LIBs.
Herein,
we
report
a
facile
strategy
to
synthesize
lotus
root-like
CoMoO4@graphene nanofibers (CoMoO4@G NFs) as advanced anode materials for high performance LIBs. And compared with the previous reports[23, 26-27, 30], this lotus root-like CoMoO4@G electrode has shown better performances. The CoMoO4@G NFs is fabricated by a classical electrospinning along with post heat treatment. The embedded graphene in the CoMoO4 NFs is the key to improving its electrochemical performance. The multi-level porous nanostructure and high surface area of the lotus root-like CoMoO4@G NFs could also improve stability and shorten lithium path length during the Li+ intercalation/deintercalation processes, leading to high performance. The lotus root-like CoMoO4@G NFs displays superior LIBs performance with high reversible capacity (735 mAh g-1 at 100 mA g-1), excellent Coulombic efficiency (＞99%), impressive cycling performance (capacity retains 80% based on the second cycle even after 200 cycles, ) and rate capability. The superior electrochemical performance of the CoMoO4@G NFs is attributed to its multi-level 1D nanostructure and the synergetic chemical coupling effects between the conductive graphene and the high capacity of CoMoO4.
■ EXPERIMENTAL SECTION Materials preparation. All reagents were received commercial and used without further purification. The graphene was prepared by micromechanical cleavage and then intense ultrasound to form graphene nanosheets (Figure S1). CoMoO4 and CoMoO4@G NFs were prepared by electrospinning. Typically, the spinneret had an inner diameter of 0.6 mm, grounding steel strip were used as the collectors with a
distance of 15 cm and a direct current voltage of 20 kV. After electrospinning, the fibers were heated from room temperature to 450 °C at a heating rate of 2.5 °C min-1, and then held at 450 °C for 2 h in air.
Synthesis of CoMoO4 NFs and CoMoO4@G NFs. Electrospinning solution was prepared as follows: Co(NO3)2·6H2O and H3Mo12O40P·xH2O were added to ethanol and N,N-dimethyl formamide (DMF) solution (1:1, w/w) and stirred for 1 h at room temperature. Graphene was then added into the resulting solution and under intense ultrasound for 20 min. Then 10 wt% polyvinyl pyrrolidone (PVP, Sigma Aldrich) was added to the above solution and stirred vigorously for another 3 h. The as-prepared solution was loaded into a syringe with a stainless needle. The as-spun PVP/Co2+, Mo6+/graphene NFs were collected and dried in vacuum for 12 h. Followed by heat treatment, the CoMoO4@G NFs nanocomposite was obtained. For comparation, CoMoO4 NFs was prepared in similar way without the adding graphene step. Material Characterizations. The morphology and microstructure were characterized by scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEOL 2010). The crystal structure of the materials was determined by X-Ray diffraction (XRD, RIGAKUD/Max-2550 with Cu Kα radiation). The specific surface area and pore diameter were confirmed by Belsorp-Mini II analyser (Japan). Raman spectra were acquired with a Labram-010 Raman spectrometer under an excitation of the 633 nm laser. Electrochemical Measurements. The CoMoO4 and CoMoO4@G NFs electrodes
were prepared by using a mixture of the active material (CoMoO4@G NFs or CoMoO4), carbon black, and binder (CMC) in the mass ratio 80:10:10. The slurry was then uniformly cast on Cu-foil, and the as-prepared electrodes were dried overnight at 60 °C under vacuum. The mass of active material was about 0.8 mg cm-2. Half cells were assembled in an Ar-filled glove box using Li foil as the counter electrode, celgard 2300 membrane as the separator, and 1 M LiPF6 in ethylene carbonate and diethyl carbonate (1:1, v/v) as the electrolyte. The cells were charged and discharged at various current densities between 0.01 and 3 V (vs. Li/Li+) on a computer controlled battery tester (ArbinBT-2000). Cyclic voltammograms (CVs) were recorded from 3.0 to 0.01V (vs. Li/Li+) at a scan rate of 0.5 mV s-1 by using a CHI660C electrochemistry work-station. Electrochemical impedance spectroscopy (EIS) measurements were performed by applying an alternating voltage of 5 mV amplitude in the frequency range from 0.01Hz–100 kHz at de-lithiation states. ■ RESULTS AND DISCUSSION The method for preparing the lotus root-like structure of CoMoO4@G NFs and pure CoMoO4 NFs as anode materials for LIBs is illustrated in Figure 1. The precursor solution was firstly electrospun to form nanofibers, dried in vacuum oven and then followed by the annealing process. Finally, CoMoO4 or CoMoO4@G NFs with different nanostructures was obtained, respectively. X-ray diffraction (XRD) was used to investigate the phase structure of the as-prepared nanocomposites. Figure 2a shows the XRD patterns of the lotus root-like CoMoO4@G NFs and pure CoMoO4 NFs. All the diffraction peaks can be indexed to
monoclinic CoMoO4 (JCPDS: 21-0868) for pure CoMoO4. For CoMoO4@G NFs, besides the typical CoMoO4 peaks, an additional small and broad peak appears at 2θ of 24.5-27.5, which can be attributed to the disorderedly stacked graphene sheets.[13, 40, 41]
To demonstrate the existence of graphene nanosheets, Raman measurement was
carried out (as shown in Figure 2b). As reported, the fundamental Raman scattering peaks for CoMoO4 are observed at 366, 815, 872 and 930 cm-1. These main bands of CoMoO4 were presented in the Raman spectra of CoMoO4 and CoMoO4@G NFs indicating the successful synthesis of CoMoO4.[24, 26] The Raman spectrum of the CoMoO4@G NFs shows typical peaks of magnetite and characteristic peaks of the D and G bands from graphene nanosheets at around 1350 and 1580 cm-1.[29, 30] The morphology and detailed structural information of CoMoO4 and CoMoO4@G NFs were determined by SEM and TEM (Figure 3a-d). The as-spun CoMoO4 and CoMoO4@G NFs precursor fibers were continuous nanofibers with smooth surface and uniform diameter of about 400 nm (Figure S2). Figure 3a and 3b show the SEM of CoMoO4 and CoMoO4@G NFs after annealing, respectively. It is clearly seen that the 1D continuous nanostructure can be maintained perfectly with reduced diameters of around 100 nm and rough surface. High magnification SEM (the inset of Figure 3b) demonstrates that the CoMoO4@G NFs display a lotus root-like nanostructure and there are many pores on the surface. For both nanocomposites, the apparent reduction in diameter is attributed to the removal of PVP and the decomposition of metal precursors.[34, 42, 43] For CoMoO4@G NFs, lotus root-like nanostructure is formed due to the movement of graphene nanosheets during the annealing process.
To gain further insight into the morphology and microstructure of the porous CoMoO4@G NFs, TEM analysis was performed (Figure 3c). From the relative light contrast, it can be clearly observed the existence of lotus root-like nanostructure and numerous pores with diameter of around 8 nm, in good agreement with the BET analysis (Figure S3). In addition, the nitrogen adsorption and desorption isotherms measurement showed that the specific surface area of CoMoO4 NFs and lotus root-like CoMoO4@G NFs was 15.4, 32.2 m2 g-1, respectively. A representative high-resolution TEM (HRTEM) image is shown in Figure 3d, the lattice fringe with an interplanar distance of around 0.279 nm, corresponding to the spacing of the (-131) plane of CoMoO4. Figure 4a displays the CV curves of lotus root-like CoMoO4@G NFs of the 1st, 2nd, 5th, 10th cycles. It is obvious from the CV curves that there are substantial differences between the first and subsequent cycles mainly due to the formation of solid electrolyte interface (SEI). The first cathodic sweep shows peaks at 1.24, 0.36, and 0.028 V, the 1.24 and 0.36 V related to the crystal structure destruction accompanied by the complete reduction to Co0 and Mo0 metal nanoparticles, respectively, as described in Eq. (1).[12, 44, 45] The peak at 0.028 V can be ascribed to the insertion of lithium into graphene nanosheets (Eq. (2)). The first anodic sweep shows peaks at 0.22, 1.4 and 1.8 V, corresponding to lithium extraction from graphene nanosheets (Eq. (2)) and oxidation of Mo and Co to form MoO3 and CoO, respectively. The peak at 1.4 V can be attributed to the oxidation of Mo0 to Mo4+, but the peak at 1.8 V can be account for the oxidation of Co to Co2+ and Mo4+ to Mo6+ (eq.
(3) and (4)).[26, 27] The second and following cathodic scans of CoMoO4@G NFs show peaks at 1.55, 0.64 and 0.014 V, which represent the reduction of Mo6+ to Mo4+, complete reduction to Co0 and Mo0 metal and insertion of lithium in G nanosheets, respectively, while the anodic scans also show peaks at 0.2, 1.4 and 1.83 V. CoMoO4 8Li Co Mo 4Li2O C (graphene) xLi xe LixC
(2)
Co Li2O CoO 2Li
(3)
Mo 3Li2O MoO3 6Li
(4)
(1)
The CV curves of the lotus root-like CoMoO4@G NFs are mostly overlapped from the second cycle onward, indicating good reversibility of the electrochemical reactions. While both the peaks and integral area for CoMoO4 obviously decreases during the subsequent cycles (as shown in Figure S4), indicating that there has severe capacity fading. These results indicate that the electrochemical reversibility of CoMoO4@G NFs is gradually built after the initial cycle and much better than that of CoMoO4. Figure 4b presents the charge/discharge profiles of the as-prepared lotus root-like CoMoO4@G NFs in the 1st, 2nd, 100th, and 200th cycles in the voltage window 0.01−3.0 V (vs. Li/Li+) at a current density of 100 mA g−1. In the first discharge process, the discharge curve starts from the open circuit voltage (Voc≈ 2.0 V) and shows a sharp decrease to 1.4 V where a sloping voltage plateau sets in, corresponding to the first cathodic peak in the CV sweep. The plateau is followed by a sloping region up to the 0.4 V. A small voltage plateau can be seen at 0.4 V followed by continuous decrease in voltage up to the cut-off voltage, 0.01 V. Upon charging to 3V, a smooth voltage profile is observed till 1.0 V which is followed by upward
sloping voltage plateau up to 2.0 V and a gradual rise to 3.0 V. Galvanostatic charge-discharge testing was also carried out at a current density of 100 mA g-1 with the voltage window ranging from 0.01 to 3 V. Figure 4c displays the comparative cycling performance of as-prepared CoMoO4 NFs and lotus root-like CoMoO4@G NFs electrodes. The initial discharge and charge capacities were 1113 and 909 mA h g-1 for CoMoO4@G NFs, 1199 and 910mAh g-1 for CoMoO4, respectively. Compared to the theoretical capacity of CoMoO4 (980 mA h g-1) and graphite (372 mA h g-1), the extra discharge capacity of the lotus root-like CoMoO4@G NFs may be attributed to the larger electrochemical active surface area of CoMoO4@G NFs and/or grain boundary area of the nanosized CoMoO4 NFs. The Coulombic efficiency of CoMoO4@G NFs and CoMoO4 NFs in the first cycle is 81.65 % and 75.88 %, respectively, and both are nearly 100 % during the subsequent cycles. The initial capacity loss may result from the incomplete conversion reaction and irreversible lithium loss due to the formation SEI layer and an organic polymeric layer on the metal nanoparticles during first discharge.[2, 3, 44] Obviously, the lotus root-like CoMoO4@G NFs possesses superior cycling performance compared with pure CoMoO4 NFs. It is clearly that pure CoMoO4 shows a sharp decrease from the initial cycle. After 85 discharge-charge cycles, the pure CoMoO4 NFs can only deliver a capacity of 321 mA h g-1, while a reversible capacity of 735 mA h g-1 of CoMoO4@G NFs still can be retained even after 200 cycles. Therefore, it can be concluded that the unique structure of CoMoO4@G NFs and the presence of graphene nanosheets are significant to improve the capacity and cycling performance for LIBs.
The rate capability of the two as-prepared electrodes was measured at different current densities (100, 200, 400, 600, 800 and 1000 mA g-1), as shown in Figure 4d. The lotus root-like CoMoO4@G NFs exhibits excellent rate capability. With the increase current density ranging from 100, 200, 400, 600, 800 to 1000 mA g-1, the capacity of the lotus root-like CoMoO4@G NFs decreases from 740, 580, 485, 430, 390 to 360 mA h g-1, while the capacity of CoMoO4 NFs decreases from 750, 405, 280, 225, 200 to 160 mA h g-1. Moreover, even after the current density returned to 100 mA g-1 the third time, the lotus root-like CoMoO4@G NFs can recover the stable cycling performance, about 700 mA h g-1. The improvement in electrochemical performance of CoMoO4@G NFs can be attributed to the following reasons: (1) the multi-level porous structure of lotus root-like CoMoO4@G NFs can provide a large number of active sites, large electrode/electrolyte contact area and short path length for Li+; (2) the graphene sheets embedded in the CoMoO4 NFs not only can improve the electronic and ionic conductivity of the nanocomposite, but also provide a mechanical buffer space to accommodate the volume change of the nanofibers during charge and discharge, which can efficiently prevent the aggregation of CoMoO4 NFs upon continuous cycling. Electrochemical impedance spectroscopy (EIS) was further carried out to understand the reasons for enhanced electrochemical performance of the lotus root-like CoMoO4@G NFs. As shown in Figure 5a, every plot is composed of a semicircle in the high-frequency region and a straight sloping line in the
low-frequency region. In general, the high-frequency semicircle corresponds to the SEI layer resistance and the inclined line at the low-frequency accounts for Warburg characteristic behavior related to the mass transfer resistance of Li+ within the electrode material.[27] The semicircle diameter lotus root-like of CoMoO4@G NFs was much smaller than that of CoMoO4 NFs, and the charge transfer resistance (Rct) for lotus root-like CoMoO4@G NFs is also smaller than that of pure CoMoO4 NFs. So it can be concluded that CoMoO4@G NFs possesses much higher electrical conductivity and faster charge-transfer reaction for Li+ insertion and extraction compared with CoMoO4 NFs. Moreover, as shown in Figure 5b, the lotus root-like CoMoO4@G NFs still retains well defined nanofiber architecture after ten cycles at the current density of 100 mA g-1, except its slightly volume increases. While the nanofiber structure of CoMoO4 NFs (Figure 5c) breaks down. ■ CONCLUSIONS In summary, a novel lotus root-like structure of CoMoO4@G NFs was successfully prepared by a facile method, electrospinning along with post annealing. It has been demonstrated that the combination of CoMoO4 with graphene nanosheets exhibits enhanced electrochemical performance for LIBs: high reversible capacity of 735 mA h g-1 at 100 mA g-1, excellent Coulombic efficiency, impressive cycling stability (almost no capacity loss after the 50th cycle) and rate capability. Such attractive capacitive behaviors are attributed to the unique lotus root-like structure and the addition of graphene nanosheets. The excellent electrochemical performance and facile preparation of the novel lotus root-like CoMoO4@G NFs make it possible to be
a promising anode material for high-performance LIBs applications.
■ ACKNOWLEDGEMENTS B. Lu thanks to financially supported by National Natural Science Foundation of China (No. 21303046，21473052), the Natural Science Foundation of Hunan Province (No. 201324), the Research Fund for the Doctoral Program of Higher Education (No. 20130161120014) and Hunan University Young Scientists fund.
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Figure 1. Schematic illustration of the fabrication processes of CoMoO4 NFs and lotus root-like CoMoO4@G NFs.
Figure 2. a) XRD patterns and b) Raman spectra of the pure CoMoO4 NFs and the lotus root-like CoMoO4@G NFs, respectively.
Figure 3. a) SEM image of pure CoMoO4 NFs and the inset is enlarged image of CoMoO4 NFs. b) SEM image of the lotus root-like CoMoO4@G NFs and the inset is enlarged image of a CoMoO4@G NFs. c) TEM image and the enlarged image of the lotus root-like CoMoO4@G NFs. d) High-magnification TEM image of CoMoO4@G NFs.
Figure 4. a) CVs of the lotus root-like CoMoO4@G NFs at a scanning rate of 0.5 mV s-1. b) Galvanostatic charge-discharge curves of lotus root-like CoMoO4@G NFs cycled at the 1st, 2nd, 100th, and 200th between 3 and 0.01 V (vs. Li/Li+) at a current density of 100 mA g-1. c) Comparison of the cycling performance of lotus root-like CoMoO4@G NFs and pure CoMoO4 NFs. d) Rate capability of the lotus root-like CoMoO4@G NFs and pure CoMoO4 NFs electrodes at various current densities between 100 and 1000 mA g-1.
Figure 5. a) Electrochemical impedance spectroscopy of the lotus root-like CoMoO4@G NFs and pure CoMoO4 NFs after cycling. b) SEM of the lotus root-like CoMoO4@G NFs and c) pure CoMoO4 NFs after ten charge and discharge processes.
S0013-4686(16)30251-1 http://dx.doi.org/doi:10.1016/j.electacta.2016.01.228 EA 26697
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
11-12-2015 19-1-2016 30-1-2016
Please cite this article as: Jing Xu, Shaozhen Gu, Ling Fan, Patrick Xu, Bingan Lu, Electrospun Lotus Root-like CoMoO4@Graphene Nanofibers as High-Performance Anode for Lithium Ion Batteries, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.01.228 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrospun Lotus Root-like CoMoO4@Graphene High-Performance Anode for Lithium Ion Batteries
Nanofibers
as
Jing Xu1,2, Shaozhen Gu2, Ling Fan2, Patrick Xu3, Bingan Lu2*
1
School of Materials Science and Engineering, Hunan University, Changsha 410082,
P. R. China 2
School of Physics and Electronics, Hunan University, Changsha 410082, P. R.
China. 3
Number 4 High School, Beijing 100088, P. R. China.
Corresponding Author *E-mail: [email protected]
ABSTRACT Lotus root-like CoMoO4@graphene nanofibers (CoMoO4@G NFs) was prepared by electrospinning along with post heat treatment, and used as high-performance anode for lithium ion batteries (LIBs). Compared with the pure CoMoO4 NFs, the prepared lotus root-like CoMoO4@G NFs electrode displays higher reversible capacity of 735 mA h g-1 at 100 mA g-1, better rate capability and cycling stability (capacity retains 80% based on the second cycle even after 200 cycles,). These results highlight the importance of combination of conductive graphene nanosheets with multi-level porous CoMoO4 NFs for high-performance LIBs.
■ INTRODUCTION Lithium ion batteries (LIBs) are considered as the most vital candidates for portable electronic devices and hybrid electric vehicles because of their high energy density and high electro-motive force.[1-5] Transition-metal oxides have been widely investigated as promising anode materials for LIBs due to their high theoretical capacity (600-1000 mA h g-1).[2] However, their large volume expansion/contraction and severe particle aggregation during the Li+ intercalation/deintercalation processes
would result in a large irreversible capacity loss and poor cycling stability.[2, 6-8] In order to improve the cycling stability of anode materials, many researchers have focused on nanostructured transition-metal oxide/graphene composites.[9-16] Graphene, a two-dimensional (2D) carbon atom monolayer, has been extensively used as anode material because of its large specific surface area (2630 m2 g−1), superior electrical conductivity, excellent chemical stability and structural flexibility.[17-19] Therefore, it is reasonable to believe nanostructured transition-metal oxide/graphene can efficiently deliver superior electrochemical performance for LIBs.[10,
15, 20-22]
Among various
transition-metal oxides, metal molybdates (MMoO4, M=Co, Ni, Mn, Zn) are promising candidates for LIBs applications due to their multiple oxidation states and reversible small ion storage.[23-26] Particularly, CoMoO4 has received much interest as supercapacitor or anode for LIBs.[24,
26-28]
It is reported that CoMoO4/graphene
nanocomposite has demonstrated enhanced electrochemical performance.[27,
29, 30]
However, these preparation methods are involved with complex processes and the cycling performance still need to be improved. One-dimensional (1D) nanostructured materials have advantages of excellent cycling performance due to its efficient transport pathways for electrons and ions.[31-33] Electrospinning is a simple and effective technique to fabricate 1D nanofibers (NFs) with controllable diameter,[31, 34, 35]
and using this method along with certain post-treatments could fabricate many
interesting multilevel 1D structures (branched nanowires and necklacelike nanowires).[36-39] So it is desirable to use electrospinning for the preparation of novel 1D CoMoO4/graphene nanostructure as high-performance anode for LIBs.
Herein,
we
report
a
facile
strategy
to
synthesize
lotus
root-like
CoMoO4@graphene nanofibers (CoMoO4@G NFs) as advanced anode materials for high performance LIBs. And compared with the previous reports[23, 26-27, 30], this lotus root-like CoMoO4@G electrode has shown better performances. The CoMoO4@G NFs is fabricated by a classical electrospinning along with post heat treatment. The embedded graphene in the CoMoO4 NFs is the key to improving its electrochemical performance. The multi-level porous nanostructure and high surface area of the lotus root-like CoMoO4@G NFs could also improve stability and shorten lithium path length during the Li+ intercalation/deintercalation processes, leading to high performance. The lotus root-like CoMoO4@G NFs displays superior LIBs performance with high reversible capacity (735 mAh g-1 at 100 mA g-1), excellent Coulombic efficiency (＞99%), impressive cycling performance (capacity retains 80% based on the second cycle even after 200 cycles, ) and rate capability. The superior electrochemical performance of the CoMoO4@G NFs is attributed to its multi-level 1D nanostructure and the synergetic chemical coupling effects between the conductive graphene and the high capacity of CoMoO4.
■ EXPERIMENTAL SECTION Materials preparation. All reagents were received commercial and used without further purification. The graphene was prepared by micromechanical cleavage and then intense ultrasound to form graphene nanosheets (Figure S1). CoMoO4 and CoMoO4@G NFs were prepared by electrospinning. Typically, the spinneret had an inner diameter of 0.6 mm, grounding steel strip were used as the collectors with a
distance of 15 cm and a direct current voltage of 20 kV. After electrospinning, the fibers were heated from room temperature to 450 °C at a heating rate of 2.5 °C min-1, and then held at 450 °C for 2 h in air.
Synthesis of CoMoO4 NFs and CoMoO4@G NFs. Electrospinning solution was prepared as follows: Co(NO3)2·6H2O and H3Mo12O40P·xH2O were added to ethanol and N,N-dimethyl formamide (DMF) solution (1:1, w/w) and stirred for 1 h at room temperature. Graphene was then added into the resulting solution and under intense ultrasound for 20 min. Then 10 wt% polyvinyl pyrrolidone (PVP, Sigma Aldrich) was added to the above solution and stirred vigorously for another 3 h. The as-prepared solution was loaded into a syringe with a stainless needle. The as-spun PVP/Co2+, Mo6+/graphene NFs were collected and dried in vacuum for 12 h. Followed by heat treatment, the CoMoO4@G NFs nanocomposite was obtained. For comparation, CoMoO4 NFs was prepared in similar way without the adding graphene step. Material Characterizations. The morphology and microstructure were characterized by scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEOL 2010). The crystal structure of the materials was determined by X-Ray diffraction (XRD, RIGAKUD/Max-2550 with Cu Kα radiation). The specific surface area and pore diameter were confirmed by Belsorp-Mini II analyser (Japan). Raman spectra were acquired with a Labram-010 Raman spectrometer under an excitation of the 633 nm laser. Electrochemical Measurements. The CoMoO4 and CoMoO4@G NFs electrodes
were prepared by using a mixture of the active material (CoMoO4@G NFs or CoMoO4), carbon black, and binder (CMC) in the mass ratio 80:10:10. The slurry was then uniformly cast on Cu-foil, and the as-prepared electrodes were dried overnight at 60 °C under vacuum. The mass of active material was about 0.8 mg cm-2. Half cells were assembled in an Ar-filled glove box using Li foil as the counter electrode, celgard 2300 membrane as the separator, and 1 M LiPF6 in ethylene carbonate and diethyl carbonate (1:1, v/v) as the electrolyte. The cells were charged and discharged at various current densities between 0.01 and 3 V (vs. Li/Li+) on a computer controlled battery tester (ArbinBT-2000). Cyclic voltammograms (CVs) were recorded from 3.0 to 0.01V (vs. Li/Li+) at a scan rate of 0.5 mV s-1 by using a CHI660C electrochemistry work-station. Electrochemical impedance spectroscopy (EIS) measurements were performed by applying an alternating voltage of 5 mV amplitude in the frequency range from 0.01Hz–100 kHz at de-lithiation states. ■ RESULTS AND DISCUSSION The method for preparing the lotus root-like structure of CoMoO4@G NFs and pure CoMoO4 NFs as anode materials for LIBs is illustrated in Figure 1. The precursor solution was firstly electrospun to form nanofibers, dried in vacuum oven and then followed by the annealing process. Finally, CoMoO4 or CoMoO4@G NFs with different nanostructures was obtained, respectively. X-ray diffraction (XRD) was used to investigate the phase structure of the as-prepared nanocomposites. Figure 2a shows the XRD patterns of the lotus root-like CoMoO4@G NFs and pure CoMoO4 NFs. All the diffraction peaks can be indexed to
monoclinic CoMoO4 (JCPDS: 21-0868) for pure CoMoO4. For CoMoO4@G NFs, besides the typical CoMoO4 peaks, an additional small and broad peak appears at 2θ of 24.5-27.5, which can be attributed to the disorderedly stacked graphene sheets.[13, 40, 41]
To demonstrate the existence of graphene nanosheets, Raman measurement was
carried out (as shown in Figure 2b). As reported, the fundamental Raman scattering peaks for CoMoO4 are observed at 366, 815, 872 and 930 cm-1. These main bands of CoMoO4 were presented in the Raman spectra of CoMoO4 and CoMoO4@G NFs indicating the successful synthesis of CoMoO4.[24, 26] The Raman spectrum of the CoMoO4@G NFs shows typical peaks of magnetite and characteristic peaks of the D and G bands from graphene nanosheets at around 1350 and 1580 cm-1.[29, 30] The morphology and detailed structural information of CoMoO4 and CoMoO4@G NFs were determined by SEM and TEM (Figure 3a-d). The as-spun CoMoO4 and CoMoO4@G NFs precursor fibers were continuous nanofibers with smooth surface and uniform diameter of about 400 nm (Figure S2). Figure 3a and 3b show the SEM of CoMoO4 and CoMoO4@G NFs after annealing, respectively. It is clearly seen that the 1D continuous nanostructure can be maintained perfectly with reduced diameters of around 100 nm and rough surface. High magnification SEM (the inset of Figure 3b) demonstrates that the CoMoO4@G NFs display a lotus root-like nanostructure and there are many pores on the surface. For both nanocomposites, the apparent reduction in diameter is attributed to the removal of PVP and the decomposition of metal precursors.[34, 42, 43] For CoMoO4@G NFs, lotus root-like nanostructure is formed due to the movement of graphene nanosheets during the annealing process.
To gain further insight into the morphology and microstructure of the porous CoMoO4@G NFs, TEM analysis was performed (Figure 3c). From the relative light contrast, it can be clearly observed the existence of lotus root-like nanostructure and numerous pores with diameter of around 8 nm, in good agreement with the BET analysis (Figure S3). In addition, the nitrogen adsorption and desorption isotherms measurement showed that the specific surface area of CoMoO4 NFs and lotus root-like CoMoO4@G NFs was 15.4, 32.2 m2 g-1, respectively. A representative high-resolution TEM (HRTEM) image is shown in Figure 3d, the lattice fringe with an interplanar distance of around 0.279 nm, corresponding to the spacing of the (-131) plane of CoMoO4. Figure 4a displays the CV curves of lotus root-like CoMoO4@G NFs of the 1st, 2nd, 5th, 10th cycles. It is obvious from the CV curves that there are substantial differences between the first and subsequent cycles mainly due to the formation of solid electrolyte interface (SEI). The first cathodic sweep shows peaks at 1.24, 0.36, and 0.028 V, the 1.24 and 0.36 V related to the crystal structure destruction accompanied by the complete reduction to Co0 and Mo0 metal nanoparticles, respectively, as described in Eq. (1).[12, 44, 45] The peak at 0.028 V can be ascribed to the insertion of lithium into graphene nanosheets (Eq. (2)). The first anodic sweep shows peaks at 0.22, 1.4 and 1.8 V, corresponding to lithium extraction from graphene nanosheets (Eq. (2)) and oxidation of Mo and Co to form MoO3 and CoO, respectively. The peak at 1.4 V can be attributed to the oxidation of Mo0 to Mo4+, but the peak at 1.8 V can be account for the oxidation of Co to Co2+ and Mo4+ to Mo6+ (eq.
(3) and (4)).[26, 27] The second and following cathodic scans of CoMoO4@G NFs show peaks at 1.55, 0.64 and 0.014 V, which represent the reduction of Mo6+ to Mo4+, complete reduction to Co0 and Mo0 metal and insertion of lithium in G nanosheets, respectively, while the anodic scans also show peaks at 0.2, 1.4 and 1.83 V. CoMoO4 8Li Co Mo 4Li2O C (graphene) xLi xe LixC
(2)
Co Li2O CoO 2Li
(3)
Mo 3Li2O MoO3 6Li
(4)
(1)
The CV curves of the lotus root-like CoMoO4@G NFs are mostly overlapped from the second cycle onward, indicating good reversibility of the electrochemical reactions. While both the peaks and integral area for CoMoO4 obviously decreases during the subsequent cycles (as shown in Figure S4), indicating that there has severe capacity fading. These results indicate that the electrochemical reversibility of CoMoO4@G NFs is gradually built after the initial cycle and much better than that of CoMoO4. Figure 4b presents the charge/discharge profiles of the as-prepared lotus root-like CoMoO4@G NFs in the 1st, 2nd, 100th, and 200th cycles in the voltage window 0.01−3.0 V (vs. Li/Li+) at a current density of 100 mA g−1. In the first discharge process, the discharge curve starts from the open circuit voltage (Voc≈ 2.0 V) and shows a sharp decrease to 1.4 V where a sloping voltage plateau sets in, corresponding to the first cathodic peak in the CV sweep. The plateau is followed by a sloping region up to the 0.4 V. A small voltage plateau can be seen at 0.4 V followed by continuous decrease in voltage up to the cut-off voltage, 0.01 V. Upon charging to 3V, a smooth voltage profile is observed till 1.0 V which is followed by upward
sloping voltage plateau up to 2.0 V and a gradual rise to 3.0 V. Galvanostatic charge-discharge testing was also carried out at a current density of 100 mA g-1 with the voltage window ranging from 0.01 to 3 V. Figure 4c displays the comparative cycling performance of as-prepared CoMoO4 NFs and lotus root-like CoMoO4@G NFs electrodes. The initial discharge and charge capacities were 1113 and 909 mA h g-1 for CoMoO4@G NFs, 1199 and 910mAh g-1 for CoMoO4, respectively. Compared to the theoretical capacity of CoMoO4 (980 mA h g-1) and graphite (372 mA h g-1), the extra discharge capacity of the lotus root-like CoMoO4@G NFs may be attributed to the larger electrochemical active surface area of CoMoO4@G NFs and/or grain boundary area of the nanosized CoMoO4 NFs. The Coulombic efficiency of CoMoO4@G NFs and CoMoO4 NFs in the first cycle is 81.65 % and 75.88 %, respectively, and both are nearly 100 % during the subsequent cycles. The initial capacity loss may result from the incomplete conversion reaction and irreversible lithium loss due to the formation SEI layer and an organic polymeric layer on the metal nanoparticles during first discharge.[2, 3, 44] Obviously, the lotus root-like CoMoO4@G NFs possesses superior cycling performance compared with pure CoMoO4 NFs. It is clearly that pure CoMoO4 shows a sharp decrease from the initial cycle. After 85 discharge-charge cycles, the pure CoMoO4 NFs can only deliver a capacity of 321 mA h g-1, while a reversible capacity of 735 mA h g-1 of CoMoO4@G NFs still can be retained even after 200 cycles. Therefore, it can be concluded that the unique structure of CoMoO4@G NFs and the presence of graphene nanosheets are significant to improve the capacity and cycling performance for LIBs.
The rate capability of the two as-prepared electrodes was measured at different current densities (100, 200, 400, 600, 800 and 1000 mA g-1), as shown in Figure 4d. The lotus root-like CoMoO4@G NFs exhibits excellent rate capability. With the increase current density ranging from 100, 200, 400, 600, 800 to 1000 mA g-1, the capacity of the lotus root-like CoMoO4@G NFs decreases from 740, 580, 485, 430, 390 to 360 mA h g-1, while the capacity of CoMoO4 NFs decreases from 750, 405, 280, 225, 200 to 160 mA h g-1. Moreover, even after the current density returned to 100 mA g-1 the third time, the lotus root-like CoMoO4@G NFs can recover the stable cycling performance, about 700 mA h g-1. The improvement in electrochemical performance of CoMoO4@G NFs can be attributed to the following reasons: (1) the multi-level porous structure of lotus root-like CoMoO4@G NFs can provide a large number of active sites, large electrode/electrolyte contact area and short path length for Li+; (2) the graphene sheets embedded in the CoMoO4 NFs not only can improve the electronic and ionic conductivity of the nanocomposite, but also provide a mechanical buffer space to accommodate the volume change of the nanofibers during charge and discharge, which can efficiently prevent the aggregation of CoMoO4 NFs upon continuous cycling. Electrochemical impedance spectroscopy (EIS) was further carried out to understand the reasons for enhanced electrochemical performance of the lotus root-like CoMoO4@G NFs. As shown in Figure 5a, every plot is composed of a semicircle in the high-frequency region and a straight sloping line in the
low-frequency region. In general, the high-frequency semicircle corresponds to the SEI layer resistance and the inclined line at the low-frequency accounts for Warburg characteristic behavior related to the mass transfer resistance of Li+ within the electrode material.[27] The semicircle diameter lotus root-like of CoMoO4@G NFs was much smaller than that of CoMoO4 NFs, and the charge transfer resistance (Rct) for lotus root-like CoMoO4@G NFs is also smaller than that of pure CoMoO4 NFs. So it can be concluded that CoMoO4@G NFs possesses much higher electrical conductivity and faster charge-transfer reaction for Li+ insertion and extraction compared with CoMoO4 NFs. Moreover, as shown in Figure 5b, the lotus root-like CoMoO4@G NFs still retains well defined nanofiber architecture after ten cycles at the current density of 100 mA g-1, except its slightly volume increases. While the nanofiber structure of CoMoO4 NFs (Figure 5c) breaks down. ■ CONCLUSIONS In summary, a novel lotus root-like structure of CoMoO4@G NFs was successfully prepared by a facile method, electrospinning along with post annealing. It has been demonstrated that the combination of CoMoO4 with graphene nanosheets exhibits enhanced electrochemical performance for LIBs: high reversible capacity of 735 mA h g-1 at 100 mA g-1, excellent Coulombic efficiency, impressive cycling stability (almost no capacity loss after the 50th cycle) and rate capability. Such attractive capacitive behaviors are attributed to the unique lotus root-like structure and the addition of graphene nanosheets. The excellent electrochemical performance and facile preparation of the novel lotus root-like CoMoO4@G NFs make it possible to be
a promising anode material for high-performance LIBs applications.
■ ACKNOWLEDGEMENTS B. Lu thanks to financially supported by National Natural Science Foundation of China (No. 21303046，21473052), the Natural Science Foundation of Hunan Province (No. 201324), the Research Fund for the Doctoral Program of Higher Education (No. 20130161120014) and Hunan University Young Scientists fund.
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Figure 1. Schematic illustration of the fabrication processes of CoMoO4 NFs and lotus root-like CoMoO4@G NFs.
Figure 2. a) XRD patterns and b) Raman spectra of the pure CoMoO4 NFs and the lotus root-like CoMoO4@G NFs, respectively.
Figure 3. a) SEM image of pure CoMoO4 NFs and the inset is enlarged image of CoMoO4 NFs. b) SEM image of the lotus root-like CoMoO4@G NFs and the inset is enlarged image of a CoMoO4@G NFs. c) TEM image and the enlarged image of the lotus root-like CoMoO4@G NFs. d) High-magnification TEM image of CoMoO4@G NFs.
Figure 4. a) CVs of the lotus root-like CoMoO4@G NFs at a scanning rate of 0.5 mV s-1. b) Galvanostatic charge-discharge curves of lotus root-like CoMoO4@G NFs cycled at the 1st, 2nd, 100th, and 200th between 3 and 0.01 V (vs. Li/Li+) at a current density of 100 mA g-1. c) Comparison of the cycling performance of lotus root-like CoMoO4@G NFs and pure CoMoO4 NFs. d) Rate capability of the lotus root-like CoMoO4@G NFs and pure CoMoO4 NFs electrodes at various current densities between 100 and 1000 mA g-1.
Figure 5. a) Electrochemical impedance spectroscopy of the lotus root-like CoMoO4@G NFs and pure CoMoO4 NFs after cycling. b) SEM of the lotus root-like CoMoO4@G NFs and c) pure CoMoO4 NFs after ten charge and discharge processes.