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3D graphene networks nanocomposite as lithium ion battery anode with long cycle life and high-rate capability
Journal of Alloys and Compounds 686 (2016) 227e234
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Pomegranate-like Li3VO4/3D graphene networks nanocomposite as lithium ion battery anode with long cycle life and high-rate capability Xin Jin, Bingbing Lei, Jing Wang, Ziliang Chen, Kai Xie, Feilong Wu, Yun Song, Dalin Sun, Fang Fang* Department of Materials Science, Fudan University, 220 Handan Road, Shanghai, 200433, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 April 2016 Received in revised form 30 May 2016 Accepted 4 June 2016 Available online 4 June 2016
Li3VO4 anchored 3D graphene networks (Li3VO4/3DGNs) nanocomposite was constructed via a facile one-pot hydrothermal method. The as-prepared Li3VO4/3DGNs nanocomposite exhibits a unique pomegranate-like structure where nanohole-dotted Li3VO4 nanoparticles resembling pomegranate seeds are firmly anchored and uniformly distributed on 3DGNs. Li3VO4/3DGNs composite presents excellent cycling and rate performance. The initial reversible capacity is 397 mAh g1 at 2 A g1 and remains at 259 mAh g1 even after 2500 cycles. In addition, under current densities of 0.2, 0.4, 0.8, 2, 4, 8, 16 and 32 A g1, the composite delivers large capacities of 471, 404, 359, 318, 290, 247, 191 and 142 mAh g1, respectively. The excellent electrochemical performance is mainly attributed to the enhanced electrical conductivity and structural stability as well as accelerated Liþ diffusion derived from both the 3D continuous conductive channels endowed robust 3DGNs and the nanohole-dotted Li3VO4 nanoparticles. © 2016 Elsevier B.V. All rights reserved.
Keywords: Lithium vanadium oxide 3D graphene Anode Lithium-ion batteries
1. Introduction Lithium-ion batteries (LIBs) are one of the most successful portable energy storage devices in modern society due to their high energy density and environmentally friendly characteristics. However, graphite, the most widely applied commercial anode in LIBs, encounters several demerits such as unsafe Liþ intercalation voltage (~0.2 V), low theoretical capacity (372 mAh g1) and limited rate capability [1e3]. Many efforts have been devoted to finding substitutes for graphite. Among all currently studied anode materials [4e9], anode materials based on intercalation/de-intercalation mechanism have attracted considerable interests for practical application since they can normally conserve structural integrity after repeated Liþ insertion/extraction [10e12]. Li3VO4, a novel anode material based on intercalation/deintercalation mechanism, is currently considered as a promising anode candidate over graphite due to its high Liþ mobility (z104 S m1), safe and energy-efficient Liþ intercalation voltage range of 0.5e1.0 V as well as high theoretical capacity of 394 mAh g1 [13,14]. Nevertheless, the electronic conductivity of Li3VO4 is low, which would lead to a large polarization of the
* Corresponding author. E-mail address: [email protected] (F. Fang). http://dx.doi.org/10.1016/j.jallcom.2016.06.018 0925-8388/© 2016 Elsevier B.V. All rights reserved.
electrode during the charge-discharge process especially under high current density, thus inevitably compromising the high-rate capability and long-term cycling stability of Li3VO4 anode [13]. To overcome this problem, approaches of combining poorly conductive Li3VO4 with conductive carbon matrix have been adopted recently. For example, a Li3VO4/graphite composite synthesized via a quasi sol gel method [15] exhibits improved rate performance. Rate capacities of 364, 278 and 203 mAh g1 are obtained at current densities of 0.5, 1.2 and 2.3 A g1 for Li3VO4/graphite, while those for pure Li3VO4 are only around 140, 100 and 70 mAh g1 at 0.4, 0.8 and 2 A g1 [16]. The enhanced rate performance may be ascribed to the decrease of charge transfer resistance resulted from the good electrical contact between Li3VO4 and graphite. However, at high rates, for example, 12 A g1, the capacity of Li3VO4/graphite fades to below 50 mAh g1. The agglomeration of Li3VO4 particles is believed to be responsible for the capacity deterioration at high rates since the as-formed large aggregates with sizes of more than 5 mm would impede the fast transportation of electrons and Li ions. Hence, inhibiting particle agglomeration to acquire small-sized Li3VO4 particles while keeping the good conductivity is the key for enhancing the high-rate performance of Li3VO4 [13,14]. Compared with graphite or amorphous carbon, reduced graphene oxide nanosheets (rGOs/GNS) are rich in oxygen-containing functional groups, which can serve as preferred anchoring sites for Li3VO4 particles via strong chemical interactions to suppress their
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agglomeration, thus generally obtaining small-sized Li3VO4 particles [17e20]. For example, in the composite of Li3VO4@GNS [21], most of the Li3VO4 particle sizes are reduced to less than 100 nm, but some of them still exceed 500 nm. The reason for the particle aggregation maybe the heavy restacking of GNS, which leads to the loss of many available surface interaction sites between Li3VO4 and GNS [22]. Previous studies [23e25] on graphene have demonstrated that the assembly of GNS into three dimensional graphene networks (3DGNs) is an effective approach to circumvent the restacking of GNS, thus possibly providing more active sites for anchoring Li3VO4 particles and suppressing their agglomeration. Beyond that, the interconnected structure of 3DGNs has two more merits compared with GNS: first, it can provide 3D continuous conductive electron transfer channels inside 3DGNs, thus further facilitating the charge transfer inside the composite [23]. Second, intrinsically robust 3D frameworks of 3DGNs can accommodate lattice strain/stress caused by Liþ insertion/extraction upon continuous cycling [17]. Therefore, it is reasonable to speculate that combining Li3VO4 nanoparticles with 3DGNs is an effective way to further improve the high-rate and long-term cycling performance of Li3VO4. In this work, a unique pomegranate-like Li3VO4/3DGNs nanocomposite was synthesized via a facile one-pot hydrothermal method. As an anode for LIBs, this composite delivers a high rate capacity of 142 mAh g1 at 32 A g1. Meanwhile, at a current density of 2 A g1, a capacity as high as 259 mAh g1 can still be obtained even after 2500 cycles. 2. Experimental 2.1. Mixture preparation Graphene oxide (GO) was prepared from natural graphite (Hengdeli, 20 mm, 99% carbon basis) by a modified Hummers method [26]. Dried GO was then dissolved in deionized water via sonication to prepare 3 mg mL1 GO solution. The synthesis process of Li3VO4/3DGNs is illustrated in Fig. S1 0.468 g NH4VO3 and 1.68 g LiOH were dissolved in 50 mL GO solution using magnetic stirring and ultrasound. The resultant mixture was then transferred to a 100 mL Teflon-lined autoclave and kept at 180 C in a high temperature oven for 11 h. Next, the as-prepared solid product was separated from the autoclave and washed four times (8000 rmp, 5 min each time) in a centrifugal machine (Kaite TGL16M). For the first two times, the product was washed with deionized water (30 mL each time) in a 50 mL centrifugal tube. For the next two times, the product was washed with tertiary butanol (30 mL each time) in a 50 mL centrifugal tube. The product was then freezedried for 24 h. The prepared product is noted as Li3VO4/3DGNs composite and content of 3DGNs in the Li3VO4/3DGNs composite is detected to be 10.1 wt% by thermogravimetric analysis as shown in Fig. S2. For comparison, pure Li3VO4 was prepared via the same hydrothermal method by mixing 0.468 g NH4VO3 and 1.68 g LiOH in 50 mL deionized water instead of GO solution. 3DGNs were also prepared by the same hydrothermal method using 50 mL GO solution without NH4VO3 and LiOH. The optical and SEM images of 3DGNs are shown in Fig. S3. Subsequently, a reference sample named Li3VO4 þ 3DGNs mixture was prepared by simply grinding pure Li3VO4 powder and 3DGNs at a weight ratio of 9:1. 2.2. Material characterization To investigate the phase components and structures, X-ray diffraction (XRD) was carried out on an X-ray diffractometer (D2 PHASER, Bruker AXS). The morphology and microstructure of the samples were investigated by field emission scanning electron
microscopy (FESEM, LEO 1530 Gemini) and transmission electron microscopy (TEM, FEI Tecnai G2 F30 S-Twin). Raman spectra were measured by a LabRAM HR800 spectrograph to characterize the structure of the samples and understand their electron behaviours. 2.3. Electrochemical measurements Electrochemical measurements were performed using Swagelok-type cells. The preparation process of the working electrodes was consisted of three steps. First, active material, Super P and PVDF were mixed applying a weight ratio of 75:20:5 and grounded in a mortar for 0.5 h. Then, a slurry was prepared using Nmethylpyrrolidinone (NMP) and coated onto a Cu foil using the doctor blade technique. Next, the electrode was dried in a vacuum oven at 120 C for 12 h to remove NMP and water. Mass loading of active material on Cu current collector was 0.9e1.2 mg cm2. The Swagelok-type cells were assembled in a glove box filled with argon atmosphere (<1 ppm, H2O and O2) by applying a Whatman GF/D borosilicate glass-fibre sheet as the separator, a lithium pellet as the anode, and 1 M LiPF6 dissolved in a solution of ethylene carbonate/dimethyl carbonate (1:1 in volume) as the electrolyte. Assembled cells were allowed to soak for 12 h before electrochemical tests. Charge/discharge measurements were carried out between 3.0 V and 0.2 V on an automatic battery testing system (CT2001A, LANHE). Electrochemical impedance spectroscopy (EIS) measurements were carried out after 5 cycles in a frequency range of 0.01e100 kHz with AC signal amplitude of 5 mV on a CHI 660e electrochemical workstation. Cyclic voltammetry (CV) measurements were also carried out on the CHI 660e electrochemical workstation at a scan rate of 0.05 mV s1 between 0.2 and 3.0 V. All electrochemical measurements were carried out under room temperature. 3. Results and discussion Fig. 1(a) presents the XRD patterns of the Li3VO4/3DGNs composite and the Li3VO4 þ 3DGNs mixture. Diffraction peaks of the Li3VO4/3DGNs composite and the Li3VO4 þ 3DGNs mixture can be indexed into an orthorhombic Li3VO4 phase (JCPDS No. 38e1247). The typical (002) peak of graphene at 26 is not detected in both samples, which may be eclipsed by the (111) peak of Li3VO4. Raman spectra in Fig. 1(b) confirm the existence of Li3VO4 and graphene in the Li3VO4/3DGNs composite and the Li3VO4 þ 3DGNs mixture. Peaks located between 200 and 500 cm1 and 750950 cm1 correspond to the Raman peaks of Li3VO4 and peaks in ranges of 1250e1470 cm1 and 14901660 cm1 can be attributed to the D band (sp3 hybridization) and G band (sp2 hybridization) of graphitic carbon. From the insert picture in Fig. 1(b), two features can be summarized: i) the peak intensity ratios between peaks of D band and G band (ID/IG) of the Li3VO4/3DGNs composite, the Li3VO4 þ 3DGNs mixture and pure 3DGNs are estimated to be 1.02, 1.05 and 1.15, respectively. The smaller ID/IG ratio indicates an enhanced graphitization degree of 3DGNs, possibly suggesting the improvement of electron transfer properties [27,28]; ii) the peak of the G Raman band of graphene in the Li3VO4/3DGNs composite is observed to be blue-shifted from 1576 cm1 for 3DGNs to 1588 cm1 by 12 cm1, while that for the Li3VO4 þ 3DGNs mixture remains at 1576 cm1. This blueshift of the peak of G Raman band in the Li3VO4/3DGNs composite is attributed to the enhanced charge transfer from Li3VO4 to 3DGNs [29,30], indicating that better electrical contact between Li3VO4 and 3DGNs is achieved via the in situ co-growth of these two components. The morphologies of the Li3VO4/3DGNs composite and the Li3VO4 þ 3DGNs mixture are compared in Fig. 2. From the low magnification SEM image of the Li3VO4/3DGNs composite in
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Fig. 1. (a) XRD patterns of the Li3VO4/3DGNs composite and the Li3VO4 þ 3DGNs mixture; (b) Raman spectra of the Li3VO4/3DGNs composite, the Li3VO4 þ 3DGNs mixture and 3DGNs.
Fig. 2. (a) Low and (b) high magnification FESEM images of the Li3VO4/3DGNs composite; (c) Low and (d) high magnification FESEM images of the Li3VO4 þ 3DGNs mixture.
Fig. 2(a), it can be clearly observed that the whole structure of the Li3VO4/3DGNs composite resembles the interior structure of a pomegranate where pomegranate seeds-like Li3VO4 particles are uniformly anchored on 3DGNs. Magnified SEM image in Fig. 2(b) further confirms this structure. Single Li3VO4 particles with particle
sizes of only 100e200 nm are dispersed uniformly on the voile-like 3DGNs and maintain intimately connected with 3DGNs. For comparison, the surface morphologies of the Li3VO4 þ 3DGNs mixture are also investigated and shown in Fig. 2(c) and (d). Without the in situ growing process, Li3VO4 particles are severely agglomerated to
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form large particles with sizes of up to more than 2 mm, suggesting that the pomegranate-like structure can only be achieved by in situ co-growth and this structure can effectively prohibit the agglomeration of Li3VO4 nanoparticles. Besides, 3DGNs can hardly be observed among Li3VO4 particles, indicating that the ex situ mixing of Li3VO4 and 3DGNs cannot build up an intimate connection between Li3VO4 and 3DGNs, which may hinder fast electron transport between Li3VO4 and 3DGNs. The comparison of these two samples proves that the one-pot hydrothermal co-growth of 3DGNs and Li3VO4 successfully inhibited the agglomeration of Li3VO4 nanoparticles and achieved the uniform distribution of these nanoparticles on 3DGNs with intimate contact. The detailed structure of Li3VO4/3DGNs composite was further characterized by TEM. As can be seen from Fig. 3(a), Li3VO4 particles with a mean size of about 100e200 nm are still anchored on the wrinkled GNS even after a long-time sonication during the preparation of TEM specimens, suggesting the strong interaction between Li3VO4 and 3DGNs. Interestingly, a large number of nanoholes with sizes ranging from several to tens of nanometers are observed to be dotted all over the Li3VO4 particles (marked by white circles in Fig. 3(b)). These nanoholes are resulted from selective water etching during the centrifugal washing processes with deionized water [31]. A similar selective etching phenomenon was also reported by Li et al. in the preparation of hollow structured Li3VO4 [32]. These nanoholes may be beneficial to facilitate electrolyte permeation inside Li3VO4 nanoparticles and shorten the Liþ diffusion distance. HRTEM was applied to study the crystallinity of Li3VO4/3DGNs. As shown in Fig. 3(c), ordered lattice fringes can be clearly observed, with an interplanar crystal spacing of 0.41 nm, corresponding to the (110) plane of Li3VO4. Regular diffraction spots
in SEAD pattern in Fig. 3(d) suggest the well-crystallization of the as-synthesized Li3VO4 in the Li3VO4/3DGNs composite. Fig. 4(a) and (b) show the galvanostatic discharge/charge curves of the Li3VO4/3DGNs composite and the Li3VO4 þ 3DGNs mixture for the first three cycles at 200 mA g1. During the discharge process of the Li3VO4/3DGNs composite (Fig. 4(a)), a wide voltage plateau at approximately 0.8 V and a relatively short plateau at approximately 0.6 V followed by a gentle slope can be clearly observed on the first discharge curve. The discharge curves of the second and third cycles almost overlapped and two plateaus are observed at around 0.9 V and 0.6 V for each curve. During the charge process, the charge curves of all three cycles are quite similar, exhibiting one main charge plateau at around 1.3 V for each curve. During the discharge process for the Li3VO4 þ 3DGNs mixture, as shown in Fig. 4(b), two short plateaus are observed at around 0.8 V and 0.5 V for all three cycles. During the charge process, one main charge plateau at 1.3 V is observed for each charge curve. The peak locations of all main discharge/charge plateaus of the Li3VO4 þ 3DGNs mixture are close to the Li3VO4/3DGNs composite, but the lengths of the plateaus are much shortened. The initial discharge/charge capacities of the Li3VO4/3DGNs composite are 806/519 mAh g1, which are much higher than the 540/293 mAh g1 for the Li3VO4 þ 3DGNs mixture. The resulted initial columbic efficiencies of these two samples are 65% and 54%, respectively. It should be noted that the initial columbic efficiency of Li3VO4/3DGNs is 11% higher than that of Li3VO4 þ 3DGNs. In addition, columbic efficiencies of the second and third cycles for Li3VO4/3DGNs are 93% and 96%, also higher than the 91% and 94% for Li3VO4 þ 3DGNs. The higher columbic efficiencies demonstrate the better reversibility of Li3VO4/3DGNs.
Fig. 3. TEM micrographs of (a) the Li3VO4/3DGNs composite and (b) the enlarged image of the red rectangle marked area; (c) HRTEM image of the Li3VO4/3DGNs composite; (d) SEAD pattern of the Li3VO4/3DGNs composite. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)
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Fig. 4. Galvanostatic discharge and charge profiles of (a) the Li3VO4/3DGNs composite and (b) the Li3VO4 þ 3DGNs mixture; Cyclic voltammetry curves of (c) the Li3VO4/3DGNs composite and (d) the Li3VO4 þ 3DGNs mixture; (e) Cycling performances of the Li3VO4/3DGNs and the Li3VO4 þ 3DGNs composite at 2 A g1, inset is the cycling performance comparison of the Li3VO4/3DGNs composite and the Li3VO4 þ 3DGNs mixture at 2 A g1 for the first 200 cycles.
To further investigate the electrode kinetics of Li3VO4/3DGNs and Li3VO4 þ 3DGNs electrodes, CV is performed and the results are presented in Fig. 4(c) and (d). For Li3VO4/3DGNs electrode, two main reduction peaks are observed at 0.84 V and 0.54 V during the first cathodic scan. The reduction peak at 0.84 V corresponds to the formation of SEI film and the lithiation process of Li3VO4 to form Li3þxVO4 (0 x 2), while the reduction peak at 0.54 V corresponds to the lithiation process of Li3VO4 [33]. In the subsequent two cathodic scans, the two main reduction peaks representing the lithiation process of Li3VO4 change to 0.9 V and 0.56 V, which can be ascribed to the activation of Li3VO4 [34]. During the three anodic scans, one main oxidation peak at around 1.32 V and a small oxidation peak at 1.09 V are observed for each scan, which corresponds to the delithiation process of Li3þxVO4 to Li3VO4. The locations of all main oxidation and reduction peaks consist well with
the voltage plateaus of the discharge/charge curves shown in Fig. 4(a). For Li3VO4 þ 3DGNs electrode, the reduction peak at 0.99 V corresponds to the formation of SEI film and two reduction peaks at 0.79 V and 0.52 V correspond to the lithiation process of Li3VO4 to Li3þxVO4. One main oxidation peak at 1.32 V and a small oxidation peak at 1.11 V represent the delithiation process of Li3þxVO4 to Li3VO4. Peak current densities and integral areas of the CV curves for Li3VO4 þ 3DGNs are much reduced compared to Li3VO4/3DGNs. The higher peak current densities and larger integral areas of Li3VO4/3DGNs demonstrate more reactions with Liþ and better utilization of the active materials. What should be pointed out is that the oxidation peaks of Li3VO4/3DGNs remain steady after the first cycle, while the main oxidation peak at 1.32 V for Li3VO4 þ 3DGNs shows an apparent decrease in the subsequent two cycles. This result suggests the higher capacity retention and
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more stable electrochemical performance of Li3VO4/3DGNs than Li3VO4 þ 3DGNs, which matches well with the discharge/charge profiles. The cycling performances of Li3VO4/3DGNs and Li3VO4 þ 3DGNs are exhibited in Fig. 4(e). Under a high current density of 2 A g1, an initial reversible capacity as high as 397 mAh g1 is achieved for the Li3VO4/3DGNs composite and a capacity of 356 mAh g1 can still be maintained after 200 cycles, while the initial reversible capacity of the Li3VO4 þ 3DGNs mixture is only 144 mAh g1 and quickly fades to 84 mAh g1 after 200 cycles. Noticeably, even after 2500 cycles, the Li3VO4/3DGNs composite can still deliver a capacity of 259 mAh g1. Besides, columbic efficiencies of the Li3VO4/3DGNs composite remain above 99.4% after the first few cycles, suggesting the highly reversible Liþ intercalation/de-intercalation property of the Li3VO4/3DGNs composite. The cycling performance of Li3VO4/ 3DGNs shows superior advantages compared with most of the previous reports (summarized in Table S1), for example, Li3VO41 after 200 cycles, at 200 mA g1), Li3VO4/C d (286 mAh g 1 (205.5 mAh g after 2000 cycles, at 2 A g1), Li3VO4/N-doped graphene (193 mAh g1 after 900 cycles, at 2 A g1), demonstrating the excellent cycling stability of the Li3VO4/3DGNs composite. To understand the reason for this excellent cycling performance, TEM micrographs of a Li3VO4/3DGNs electrode after cycling for 1200 cycles at 2 A g1 are investigated. As can been seen from Fig. 5(a), even after a long-term cycling process plus a long-period sonication process, the Li3VO4 nanoparticles are still anchored on the 3DGNs and the overall structural integrity of single Li3VO4 nanoparticle is well conserved. Moreover, as shown in Fig. 5(b), even the nanoholes (one of the nanoholes is marked by a white circle as a representative) on Li3VO4 nanoparticles are well preserved, suggesting the robust 3D graphene networks may effectively buffer the strain/stress of Li3VO4 nanoparticles occurred during Liþ insertion/ extraction, which accounts for the good cycling stability of Li3VO4/ 3DGNs. Rate performances of the Li3VO4/3DGNs composite and the Li3VO4 þ 3DGNs mixture are compared in Fig. 6(a). At current densities of 0.2, 0.4, 0.8, 2, 4, 8 and 16 A g1, the capacities of the Li3VO4/3DGNs composite can reach up to 471, 404, 359, 318, 290, 247 and 191 mAh g1, respectively. Even at a very high current density of 32 A g1, a capacity as high as 142 mAh g1 can still be retained. When the current density restores to 0.2 A g1, the capacity bounces back to 439 mAh g1, indicating the good rate capability of the Li3VO4/3DGNs composite. For the Li3VO4 þ 3DGNs mixture, capacities of only 29 and 13 mAh g1 are retained at 8 and 16 A g1, while nearly no electrochemical activity is shown at
Fig. 6. (a) Rate performances of the Li3VO4/3DGNs composite and the Li3VO4 þ 3DGNs mixture; (b) Nyquist plots of the Li3VO4/3DGNs composite and the Li3VO4 þ 3DGNs mixture after 5 cycles, inset is the linear fitting line of real parts of the complex impedance versus u1/2; (c) Equivalent circuit.
32 A g1. It should be noted that the high-rate performance of the Li3VO4/3DGNs composite is also superior compared with most of
Fig. 5. TEM micrographs of the Li3VO4/3DGNs electrode after 1200 cycles at 2 A g1.
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the reported excellent Li3VO4 anode materials, for example, the carbon-encapsulated Li3VO4 (106 mAh g1 at 32 A g1) [13], Li3VO4 anchored GNS (133 mAh g1 at 20 A g1) [21] and the carboncoated Li3VO4 (90 mAh g1 at 20 A g1) [34]. EIS measurements are employed to illustrate the electrode kinetics of the Li3VO4/3DGNs composite and the Li3VO4 þ 3DGNs mixture. Each EIS spectrum in Fig. 6(b) exhibits two compressed semicircles from the high to medium frequency range and one line inclined at approximately 45 in the low-frequency range. The compressed semicircle in the high frequency range is relative to the SEI film resistance (RSEI), the diameter of the semicircle in the medium frequency range is related to the charge transfer resistance (Rct) and the 45 inclined line is associated with the Warburg impedance (ZW) [35,36]. After simulating the two compressed semicircles for both samples using the equivalent circuit shown in Fig. 6(c), values of RSEI and Rct after five cycles were calculated to be 36.2 U, 18.9 U for Li3VO4/3DGNs and 60.69 U, 107.6 U for Li3VO4 þ 3DGNs, respectively. Both the RSEI and Rct values of Li3VO4/ 3DGNs are smaller than those of Li3VO4 þ 3DGNs. The smaller RSEI of Li3VO4/3DGNs suggests the pomegranate-like structure derived from the in situ co-growth of 3DGNs and Li3VO4 can possibly protect Li3VO4 from side reactions with the electrolyte, forming a thinner SEI film, while the much lower Rct of the Li3VO4/3DGNs composite indicates a faster electron transportation inside the composite [37]. The lithium diffusion coefficients of the two samples can also be calculated by employing the following formula: D ¼ R2T2/2A2n4F4C2s2
(1)
D is the diffusion coefficient, R is the gas constant, T is the absolute temperature, A is the surface area of the anode, n is the number of electrons transferred in the half-reaction for the redox couple (2), F is the Faraday constant, C is the concentration of Liþ in solid (9.8 103 mol cm3), and s is the Warburg factor, which is related to Zre as shown in the following formula and can be obtained from the slope of the lines in the inset of Fig. 6(b) [38]. Zre ¼ Re þ Rct þ su1/2
(2)
After linear fitting, the lithium diffusion coefficients at 298 K are calculated to be 1.73 1012 cm2 s1 for the Li3VO4/3DGNs composite and 5.88 1013 cm2 s1 for the Li3VO4 þ 3DGNs mixture, respectively. The lithium ion diffusion coefficient of the Li3VO4/ 3DGNs composite is twice that of the Li3VO4 þ 3DGNs mixture, suggesting the pomegranate-like structure is favourable for facilitating Liþ diffusion within the Li3VO4/3DGNs composite. The high-rate capability and good cycling stability of the Li3VO4/ 3DGNs composite can be mainly attributed to the 3D continuous conductive channels endowed robust 3DGNs and the nanoholedotted Li3VO4 nanoparticles as illustrated in Fig. 7. Firstly, Li3VO4 nanoparticles are firmly and uniformly anchored on the highly conductive 3DGNs after the in situ co-growing process, thus intimate electrical contact between Li3VO4 and 3DGNs is built up on a large scale, which effectively improves the electron transportation between the two components. Moreover, the 3D interconnected conductive networks of 3DGNs can provide 3D continuous electron transportation channels, which can effectively accelerate the charge transfer inside 3DGNs [39]. As a result, the charge transfer resistance of the Li3VO4/3DGNs composite is greatly decreased due to the combined effect of above two factors. Secondly, during the hydrothermal process, 3DGNs can act as both physical and chemical spacers to impede the aggregation of Li3VO4 particles, thus providing more accessible interfacial insertion/extraction sites for lithium ions. To be specific, on one hand, the existence of graphene nanosheets can serve as physical barriers to hinder the aggregation
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Fig. 7. Schematic illustration of Liþ and electron transfer behaviour in the Li3VO4/ 3DGNs nanocomposite.
of Li3VO4 particles. On the other hand, oxygen-containing functional groups (hydroxyl group and epoxy group proved by FTIR in Fig. S4) [40e42] on the surface of 3DGNs allow the uniform anchoring of Li3VO4 particles on 3DGNs and immobilize them via strong chemical interactions between the functional groups and Li3VO4 nanoparticles, thus prohibiting the agglomeration of the Li3VO4 nanoparticles [43,44]. Hence, Liþ transport distance is greatly shortened owing to the nanometer-sized Li3VO4 particles. Thirdly, better electrolyte penetration is assured within the composite due to massive nanoholes on Li3VO4 nanoparticles, therefore, the Liþ diffusion distance is further shortened [45]. Fourthly, intrinsically flexible graphene networks can not only protect Li3VO4 from side reactions with the electrolyte, but also act as an elastic buffering cushion to accommodate crystal strain/stress generated upon repetitive Liþ intercalation/de-intercalation, maintaining structural integrity of the nanocomposite and thus ensuring the long-term and high-rate performance.
4. Conclusions A one-pot hydrothermal method was adopted to fabricate the pomegranate-like Li3VO4/3DGNs nanocomposite, in which the nanohole-dotted Li3VO4 nanoparticles with sizes of 100e200 nm are firmly anchored on the 3D conductive graphene networks. This unique structure permits rapid electron and Liþ transportation, leading to superior electrochemical performance. An initial reversible capacity of 397 mAh g1 is achieved and a capacity of 259 mAh g1 can still be retained even after 2500 cycles at 2 A g1. The Li3VO4/3DGNs composite also presents an excellent high-rate performance, delivering a stable capacity of 142 mAh g1 even at a very high current density of 32 A g1. The incorporation of constructing 3D conductive networks and achieving uniform distribution of nanoparticles via a facile one-pot in situ co-growing approach can be applied to a wide range of research fields, such as cathodes design for LIBs, sodium ion batteries, supercapacitors and solar cells.
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Acknowledgement This work was financially supported by the National Natural Science Foundation of China (Nos. 51301002, 51471052, 51571063, U1201241), the Science and Technology Commission of Shanghai Municipality (No. 14JC1490200, 15YF1401300) and the Guangxi Collaborative Innovation Center for Structure and Property of New Energy Materials. We also thank Dr. Liang Chen (Department of chemistry, Fudan University) for his kind assistance with GO preparation. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.06.018. References [1] P. Tartaj, J.M. Amarilla, M.B. V azquez Santo, Aerosol-assisted synthesis of colloidal aggregates with different morphology: toward the electrochemical optimization of Li3VO4 battery anodes using scalable routes, Chem. Mater. 28 (2016) 986e993. [2] S. Ni, J. Zhang, J. Ma, X. Yang, L. Zhang, Superior electrochemical performance of Li3VO4/N-doped C as an anode for Li-ion batteries, J. Mater. Chem. A 3 (2015) 17951e17955. [3] S. Hu, Y. Song, S. Yuan, H. Liu, Q. Xu, Y. Wang, C.X. Wang, Y.Y. Xia, A hierarchical structure of carbon-coated Li3VO4 nanoparticles embedded in expanded graphite for high performance lithium ion battery, J. Power Sources 303 (2016) 333e339. [4] B.D. Polat, O.L. Eryilmaz, O. Keles, SiAg film by magnetron sputtering for high reversible lithium ion storage anodes, J. Alloys Compd. 654 (2016) 363e370. [5] C.Q. Du, J.W. Wu, J. Liu, M. Yang, Q. Xu, Z.Y. Tang, X.H. Zhang, Synthesis of lithium vanadium tetroxide anode material via a fast sol-gel method based on spontaneous chemical reactions, Electrochim. Acta 152 (2015) 473e479. [6] Q. Shi, R. Hu, L. Ouyang, M. Zeng, M. Zhu, High-capacity LiV3O8 thin-film cathode with a mixed amorphous-nanocrystalline microstructure prepared by RF magnetron sputtering, Electrochem. Commun. 11 (2009) 2169e2172. [7] R. Ma, Y. Liu, Y. Yang, K. Pu, M. Gao, H. Pan, LieSiealloy-assisted improvement in the intrinsic cyclability of Mg2Si as an anode material for Li-ion batteries, Acta Mater. 98 (2015) 128e134. [8] C. Liang, M. Gao, H. Pan, Y. Liu, M. Yan, Lithium alloys and metal oxides as high-capacity anode materials for lithium-ion batteries, J. Alloys Compd. 575 (2013) 246e256. [9] M.H. Pyun, Y.J. Park, Graphene/LiMn2O4 nanocomposites for enhanced lithium ion batteries with high rate capability, J. Alloys Compd. 643 (2015) S90eS94. [10] Y.Q. Qiao, X.L. Wang, J.P. Zhou, J. Zhang, C.D. Gu, J.P. Tu, Synthesis and electrochemical performance of rod-like LiV3O8 cathode materials for rechargeable lithium batteries, J. Power Sources 198 (2012) 287e293. [11] S. Huang, Y. Lu, T.Q. Wang, C.D. Gu, X.L. Wang, J.P. Tu, Polyacrylamide-assisted freeze drying synthesis of hierarchical plate-arrayed LiV3O8 for high-rate lithium-ion batteries, J. Power Sources 235 (2013) 256e264. [12] S. Huang, X.L. Wang, Y. Lu, X.M. Jian, X.Y. Zhao, H. Tang, J.B. Cai, C.D. Gu, J.P. Tu, Facile synthesis of cookies-shaped LiV3O8 cathode materials with good cycling performance for lithium-ion batteries, J. Alloys Compd. 584 (2014) 41e46. [13] C. Zhang, H. Song, C. Liu, Y. Liu, C. Zhang, X. Nan, G. Cao, Fast and reversible Li ion insertion in carbon-encapsulated Li3VO4 as anode for lithium-ion battery, Adv. Funct. Mater. 25 (2015) 3497e3504. [14] G. Shao, L. Gan, Y. Ma, H. Li, T. Zhai, Enhancing the performance of Li3VO4 by combining nanotechnology and surface carbon coating for lithium ion batteries, J. Mater. Chem. A 3 (2015) 11253e11260. [15] S. Ni, X. Lv, J. Zhang, J. Ma, X. Yang, L. Zhang, The electrochemical performance of lithium vanadate/natural graphite composite material as anode for lithium ion batteries, Electrochim. Acta 145 (2014) 327e334. [16] D. Zhao, M. Cao, Constructing highly graphitized carbon-wrapped Li3VO4 nanoparticles with hierarchically porous structure as a long life and high capacity anode for lithium-ion batteries, ACS Appl. Mater. Inter. 7 (2015) 25084e25093. [17] S. Ni, J. Zhang, J. Ma, X. Yang, L. Zhang, Li3VO4/N-doped graphene with high capacity and excellent cycle stability as anode for lithium ion batteries, J. Power Sources 296 (2015) 377e382. [18] Y. Shi, J.Z. Wang, S.L. Chou, D. Wexler, H.J. Li, K. Ozawa, H.K. Liu, Y.P. Wu, 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. 13 (2013) 4715e4720.
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Pomegranate-like Li3VO4/3D graphene networks nanocomposite as lithium ion battery anode with long cycle life and high-rate capability Xin Jin, Bingbing Lei, Jing Wang, Ziliang Chen, Kai Xie, Feilong Wu, Yun Song, Dalin Sun, Fang Fang* Department of Materials Science, Fudan University, 220 Handan Road, Shanghai, 200433, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 April 2016 Received in revised form 30 May 2016 Accepted 4 June 2016 Available online 4 June 2016
Li3VO4 anchored 3D graphene networks (Li3VO4/3DGNs) nanocomposite was constructed via a facile one-pot hydrothermal method. The as-prepared Li3VO4/3DGNs nanocomposite exhibits a unique pomegranate-like structure where nanohole-dotted Li3VO4 nanoparticles resembling pomegranate seeds are firmly anchored and uniformly distributed on 3DGNs. Li3VO4/3DGNs composite presents excellent cycling and rate performance. The initial reversible capacity is 397 mAh g1 at 2 A g1 and remains at 259 mAh g1 even after 2500 cycles. In addition, under current densities of 0.2, 0.4, 0.8, 2, 4, 8, 16 and 32 A g1, the composite delivers large capacities of 471, 404, 359, 318, 290, 247, 191 and 142 mAh g1, respectively. The excellent electrochemical performance is mainly attributed to the enhanced electrical conductivity and structural stability as well as accelerated Liþ diffusion derived from both the 3D continuous conductive channels endowed robust 3DGNs and the nanohole-dotted Li3VO4 nanoparticles. © 2016 Elsevier B.V. All rights reserved.
Keywords: Lithium vanadium oxide 3D graphene Anode Lithium-ion batteries
1. Introduction Lithium-ion batteries (LIBs) are one of the most successful portable energy storage devices in modern society due to their high energy density and environmentally friendly characteristics. However, graphite, the most widely applied commercial anode in LIBs, encounters several demerits such as unsafe Liþ intercalation voltage (~0.2 V), low theoretical capacity (372 mAh g1) and limited rate capability [1e3]. Many efforts have been devoted to finding substitutes for graphite. Among all currently studied anode materials [4e9], anode materials based on intercalation/de-intercalation mechanism have attracted considerable interests for practical application since they can normally conserve structural integrity after repeated Liþ insertion/extraction [10e12]. Li3VO4, a novel anode material based on intercalation/deintercalation mechanism, is currently considered as a promising anode candidate over graphite due to its high Liþ mobility (z104 S m1), safe and energy-efficient Liþ intercalation voltage range of 0.5e1.0 V as well as high theoretical capacity of 394 mAh g1 [13,14]. Nevertheless, the electronic conductivity of Li3VO4 is low, which would lead to a large polarization of the
* Corresponding author. E-mail address: [email protected] (F. Fang). http://dx.doi.org/10.1016/j.jallcom.2016.06.018 0925-8388/© 2016 Elsevier B.V. All rights reserved.
electrode during the charge-discharge process especially under high current density, thus inevitably compromising the high-rate capability and long-term cycling stability of Li3VO4 anode [13]. To overcome this problem, approaches of combining poorly conductive Li3VO4 with conductive carbon matrix have been adopted recently. For example, a Li3VO4/graphite composite synthesized via a quasi sol gel method [15] exhibits improved rate performance. Rate capacities of 364, 278 and 203 mAh g1 are obtained at current densities of 0.5, 1.2 and 2.3 A g1 for Li3VO4/graphite, while those for pure Li3VO4 are only around 140, 100 and 70 mAh g1 at 0.4, 0.8 and 2 A g1 [16]. The enhanced rate performance may be ascribed to the decrease of charge transfer resistance resulted from the good electrical contact between Li3VO4 and graphite. However, at high rates, for example, 12 A g1, the capacity of Li3VO4/graphite fades to below 50 mAh g1. The agglomeration of Li3VO4 particles is believed to be responsible for the capacity deterioration at high rates since the as-formed large aggregates with sizes of more than 5 mm would impede the fast transportation of electrons and Li ions. Hence, inhibiting particle agglomeration to acquire small-sized Li3VO4 particles while keeping the good conductivity is the key for enhancing the high-rate performance of Li3VO4 [13,14]. Compared with graphite or amorphous carbon, reduced graphene oxide nanosheets (rGOs/GNS) are rich in oxygen-containing functional groups, which can serve as preferred anchoring sites for Li3VO4 particles via strong chemical interactions to suppress their
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agglomeration, thus generally obtaining small-sized Li3VO4 particles [17e20]. For example, in the composite of Li3VO4@GNS [21], most of the Li3VO4 particle sizes are reduced to less than 100 nm, but some of them still exceed 500 nm. The reason for the particle aggregation maybe the heavy restacking of GNS, which leads to the loss of many available surface interaction sites between Li3VO4 and GNS [22]. Previous studies [23e25] on graphene have demonstrated that the assembly of GNS into three dimensional graphene networks (3DGNs) is an effective approach to circumvent the restacking of GNS, thus possibly providing more active sites for anchoring Li3VO4 particles and suppressing their agglomeration. Beyond that, the interconnected structure of 3DGNs has two more merits compared with GNS: first, it can provide 3D continuous conductive electron transfer channels inside 3DGNs, thus further facilitating the charge transfer inside the composite [23]. Second, intrinsically robust 3D frameworks of 3DGNs can accommodate lattice strain/stress caused by Liþ insertion/extraction upon continuous cycling [17]. Therefore, it is reasonable to speculate that combining Li3VO4 nanoparticles with 3DGNs is an effective way to further improve the high-rate and long-term cycling performance of Li3VO4. In this work, a unique pomegranate-like Li3VO4/3DGNs nanocomposite was synthesized via a facile one-pot hydrothermal method. As an anode for LIBs, this composite delivers a high rate capacity of 142 mAh g1 at 32 A g1. Meanwhile, at a current density of 2 A g1, a capacity as high as 259 mAh g1 can still be obtained even after 2500 cycles. 2. Experimental 2.1. Mixture preparation Graphene oxide (GO) was prepared from natural graphite (Hengdeli, 20 mm, 99% carbon basis) by a modified Hummers method [26]. Dried GO was then dissolved in deionized water via sonication to prepare 3 mg mL1 GO solution. The synthesis process of Li3VO4/3DGNs is illustrated in Fig. S1 0.468 g NH4VO3 and 1.68 g LiOH were dissolved in 50 mL GO solution using magnetic stirring and ultrasound. The resultant mixture was then transferred to a 100 mL Teflon-lined autoclave and kept at 180 C in a high temperature oven for 11 h. Next, the as-prepared solid product was separated from the autoclave and washed four times (8000 rmp, 5 min each time) in a centrifugal machine (Kaite TGL16M). For the first two times, the product was washed with deionized water (30 mL each time) in a 50 mL centrifugal tube. For the next two times, the product was washed with tertiary butanol (30 mL each time) in a 50 mL centrifugal tube. The product was then freezedried for 24 h. The prepared product is noted as Li3VO4/3DGNs composite and content of 3DGNs in the Li3VO4/3DGNs composite is detected to be 10.1 wt% by thermogravimetric analysis as shown in Fig. S2. For comparison, pure Li3VO4 was prepared via the same hydrothermal method by mixing 0.468 g NH4VO3 and 1.68 g LiOH in 50 mL deionized water instead of GO solution. 3DGNs were also prepared by the same hydrothermal method using 50 mL GO solution without NH4VO3 and LiOH. The optical and SEM images of 3DGNs are shown in Fig. S3. Subsequently, a reference sample named Li3VO4 þ 3DGNs mixture was prepared by simply grinding pure Li3VO4 powder and 3DGNs at a weight ratio of 9:1. 2.2. Material characterization To investigate the phase components and structures, X-ray diffraction (XRD) was carried out on an X-ray diffractometer (D2 PHASER, Bruker AXS). The morphology and microstructure of the samples were investigated by field emission scanning electron
microscopy (FESEM, LEO 1530 Gemini) and transmission electron microscopy (TEM, FEI Tecnai G2 F30 S-Twin). Raman spectra were measured by a LabRAM HR800 spectrograph to characterize the structure of the samples and understand their electron behaviours. 2.3. Electrochemical measurements Electrochemical measurements were performed using Swagelok-type cells. The preparation process of the working electrodes was consisted of three steps. First, active material, Super P and PVDF were mixed applying a weight ratio of 75:20:5 and grounded in a mortar for 0.5 h. Then, a slurry was prepared using Nmethylpyrrolidinone (NMP) and coated onto a Cu foil using the doctor blade technique. Next, the electrode was dried in a vacuum oven at 120 C for 12 h to remove NMP and water. Mass loading of active material on Cu current collector was 0.9e1.2 mg cm2. The Swagelok-type cells were assembled in a glove box filled with argon atmosphere (<1 ppm, H2O and O2) by applying a Whatman GF/D borosilicate glass-fibre sheet as the separator, a lithium pellet as the anode, and 1 M LiPF6 dissolved in a solution of ethylene carbonate/dimethyl carbonate (1:1 in volume) as the electrolyte. Assembled cells were allowed to soak for 12 h before electrochemical tests. Charge/discharge measurements were carried out between 3.0 V and 0.2 V on an automatic battery testing system (CT2001A, LANHE). Electrochemical impedance spectroscopy (EIS) measurements were carried out after 5 cycles in a frequency range of 0.01e100 kHz with AC signal amplitude of 5 mV on a CHI 660e electrochemical workstation. Cyclic voltammetry (CV) measurements were also carried out on the CHI 660e electrochemical workstation at a scan rate of 0.05 mV s1 between 0.2 and 3.0 V. All electrochemical measurements were carried out under room temperature. 3. Results and discussion Fig. 1(a) presents the XRD patterns of the Li3VO4/3DGNs composite and the Li3VO4 þ 3DGNs mixture. Diffraction peaks of the Li3VO4/3DGNs composite and the Li3VO4 þ 3DGNs mixture can be indexed into an orthorhombic Li3VO4 phase (JCPDS No. 38e1247). The typical (002) peak of graphene at 26 is not detected in both samples, which may be eclipsed by the (111) peak of Li3VO4. Raman spectra in Fig. 1(b) confirm the existence of Li3VO4 and graphene in the Li3VO4/3DGNs composite and the Li3VO4 þ 3DGNs mixture. Peaks located between 200 and 500 cm1 and 750950 cm1 correspond to the Raman peaks of Li3VO4 and peaks in ranges of 1250e1470 cm1 and 14901660 cm1 can be attributed to the D band (sp3 hybridization) and G band (sp2 hybridization) of graphitic carbon. From the insert picture in Fig. 1(b), two features can be summarized: i) the peak intensity ratios between peaks of D band and G band (ID/IG) of the Li3VO4/3DGNs composite, the Li3VO4 þ 3DGNs mixture and pure 3DGNs are estimated to be 1.02, 1.05 and 1.15, respectively. The smaller ID/IG ratio indicates an enhanced graphitization degree of 3DGNs, possibly suggesting the improvement of electron transfer properties [27,28]; ii) the peak of the G Raman band of graphene in the Li3VO4/3DGNs composite is observed to be blue-shifted from 1576 cm1 for 3DGNs to 1588 cm1 by 12 cm1, while that for the Li3VO4 þ 3DGNs mixture remains at 1576 cm1. This blueshift of the peak of G Raman band in the Li3VO4/3DGNs composite is attributed to the enhanced charge transfer from Li3VO4 to 3DGNs [29,30], indicating that better electrical contact between Li3VO4 and 3DGNs is achieved via the in situ co-growth of these two components. The morphologies of the Li3VO4/3DGNs composite and the Li3VO4 þ 3DGNs mixture are compared in Fig. 2. From the low magnification SEM image of the Li3VO4/3DGNs composite in
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Fig. 1. (a) XRD patterns of the Li3VO4/3DGNs composite and the Li3VO4 þ 3DGNs mixture; (b) Raman spectra of the Li3VO4/3DGNs composite, the Li3VO4 þ 3DGNs mixture and 3DGNs.
Fig. 2. (a) Low and (b) high magnification FESEM images of the Li3VO4/3DGNs composite; (c) Low and (d) high magnification FESEM images of the Li3VO4 þ 3DGNs mixture.
Fig. 2(a), it can be clearly observed that the whole structure of the Li3VO4/3DGNs composite resembles the interior structure of a pomegranate where pomegranate seeds-like Li3VO4 particles are uniformly anchored on 3DGNs. Magnified SEM image in Fig. 2(b) further confirms this structure. Single Li3VO4 particles with particle
sizes of only 100e200 nm are dispersed uniformly on the voile-like 3DGNs and maintain intimately connected with 3DGNs. For comparison, the surface morphologies of the Li3VO4 þ 3DGNs mixture are also investigated and shown in Fig. 2(c) and (d). Without the in situ growing process, Li3VO4 particles are severely agglomerated to
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form large particles with sizes of up to more than 2 mm, suggesting that the pomegranate-like structure can only be achieved by in situ co-growth and this structure can effectively prohibit the agglomeration of Li3VO4 nanoparticles. Besides, 3DGNs can hardly be observed among Li3VO4 particles, indicating that the ex situ mixing of Li3VO4 and 3DGNs cannot build up an intimate connection between Li3VO4 and 3DGNs, which may hinder fast electron transport between Li3VO4 and 3DGNs. The comparison of these two samples proves that the one-pot hydrothermal co-growth of 3DGNs and Li3VO4 successfully inhibited the agglomeration of Li3VO4 nanoparticles and achieved the uniform distribution of these nanoparticles on 3DGNs with intimate contact. The detailed structure of Li3VO4/3DGNs composite was further characterized by TEM. As can be seen from Fig. 3(a), Li3VO4 particles with a mean size of about 100e200 nm are still anchored on the wrinkled GNS even after a long-time sonication during the preparation of TEM specimens, suggesting the strong interaction between Li3VO4 and 3DGNs. Interestingly, a large number of nanoholes with sizes ranging from several to tens of nanometers are observed to be dotted all over the Li3VO4 particles (marked by white circles in Fig. 3(b)). These nanoholes are resulted from selective water etching during the centrifugal washing processes with deionized water [31]. A similar selective etching phenomenon was also reported by Li et al. in the preparation of hollow structured Li3VO4 [32]. These nanoholes may be beneficial to facilitate electrolyte permeation inside Li3VO4 nanoparticles and shorten the Liþ diffusion distance. HRTEM was applied to study the crystallinity of Li3VO4/3DGNs. As shown in Fig. 3(c), ordered lattice fringes can be clearly observed, with an interplanar crystal spacing of 0.41 nm, corresponding to the (110) plane of Li3VO4. Regular diffraction spots
in SEAD pattern in Fig. 3(d) suggest the well-crystallization of the as-synthesized Li3VO4 in the Li3VO4/3DGNs composite. Fig. 4(a) and (b) show the galvanostatic discharge/charge curves of the Li3VO4/3DGNs composite and the Li3VO4 þ 3DGNs mixture for the first three cycles at 200 mA g1. During the discharge process of the Li3VO4/3DGNs composite (Fig. 4(a)), a wide voltage plateau at approximately 0.8 V and a relatively short plateau at approximately 0.6 V followed by a gentle slope can be clearly observed on the first discharge curve. The discharge curves of the second and third cycles almost overlapped and two plateaus are observed at around 0.9 V and 0.6 V for each curve. During the charge process, the charge curves of all three cycles are quite similar, exhibiting one main charge plateau at around 1.3 V for each curve. During the discharge process for the Li3VO4 þ 3DGNs mixture, as shown in Fig. 4(b), two short plateaus are observed at around 0.8 V and 0.5 V for all three cycles. During the charge process, one main charge plateau at 1.3 V is observed for each charge curve. The peak locations of all main discharge/charge plateaus of the Li3VO4 þ 3DGNs mixture are close to the Li3VO4/3DGNs composite, but the lengths of the plateaus are much shortened. The initial discharge/charge capacities of the Li3VO4/3DGNs composite are 806/519 mAh g1, which are much higher than the 540/293 mAh g1 for the Li3VO4 þ 3DGNs mixture. The resulted initial columbic efficiencies of these two samples are 65% and 54%, respectively. It should be noted that the initial columbic efficiency of Li3VO4/3DGNs is 11% higher than that of Li3VO4 þ 3DGNs. In addition, columbic efficiencies of the second and third cycles for Li3VO4/3DGNs are 93% and 96%, also higher than the 91% and 94% for Li3VO4 þ 3DGNs. The higher columbic efficiencies demonstrate the better reversibility of Li3VO4/3DGNs.
Fig. 3. TEM micrographs of (a) the Li3VO4/3DGNs composite and (b) the enlarged image of the red rectangle marked area; (c) HRTEM image of the Li3VO4/3DGNs composite; (d) SEAD pattern of the Li3VO4/3DGNs composite. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)
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Fig. 4. Galvanostatic discharge and charge profiles of (a) the Li3VO4/3DGNs composite and (b) the Li3VO4 þ 3DGNs mixture; Cyclic voltammetry curves of (c) the Li3VO4/3DGNs composite and (d) the Li3VO4 þ 3DGNs mixture; (e) Cycling performances of the Li3VO4/3DGNs and the Li3VO4 þ 3DGNs composite at 2 A g1, inset is the cycling performance comparison of the Li3VO4/3DGNs composite and the Li3VO4 þ 3DGNs mixture at 2 A g1 for the first 200 cycles.
To further investigate the electrode kinetics of Li3VO4/3DGNs and Li3VO4 þ 3DGNs electrodes, CV is performed and the results are presented in Fig. 4(c) and (d). For Li3VO4/3DGNs electrode, two main reduction peaks are observed at 0.84 V and 0.54 V during the first cathodic scan. The reduction peak at 0.84 V corresponds to the formation of SEI film and the lithiation process of Li3VO4 to form Li3þxVO4 (0 x 2), while the reduction peak at 0.54 V corresponds to the lithiation process of Li3VO4 [33]. In the subsequent two cathodic scans, the two main reduction peaks representing the lithiation process of Li3VO4 change to 0.9 V and 0.56 V, which can be ascribed to the activation of Li3VO4 [34]. During the three anodic scans, one main oxidation peak at around 1.32 V and a small oxidation peak at 1.09 V are observed for each scan, which corresponds to the delithiation process of Li3þxVO4 to Li3VO4. The locations of all main oxidation and reduction peaks consist well with
the voltage plateaus of the discharge/charge curves shown in Fig. 4(a). For Li3VO4 þ 3DGNs electrode, the reduction peak at 0.99 V corresponds to the formation of SEI film and two reduction peaks at 0.79 V and 0.52 V correspond to the lithiation process of Li3VO4 to Li3þxVO4. One main oxidation peak at 1.32 V and a small oxidation peak at 1.11 V represent the delithiation process of Li3þxVO4 to Li3VO4. Peak current densities and integral areas of the CV curves for Li3VO4 þ 3DGNs are much reduced compared to Li3VO4/3DGNs. The higher peak current densities and larger integral areas of Li3VO4/3DGNs demonstrate more reactions with Liþ and better utilization of the active materials. What should be pointed out is that the oxidation peaks of Li3VO4/3DGNs remain steady after the first cycle, while the main oxidation peak at 1.32 V for Li3VO4 þ 3DGNs shows an apparent decrease in the subsequent two cycles. This result suggests the higher capacity retention and
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more stable electrochemical performance of Li3VO4/3DGNs than Li3VO4 þ 3DGNs, which matches well with the discharge/charge profiles. The cycling performances of Li3VO4/3DGNs and Li3VO4 þ 3DGNs are exhibited in Fig. 4(e). Under a high current density of 2 A g1, an initial reversible capacity as high as 397 mAh g1 is achieved for the Li3VO4/3DGNs composite and a capacity of 356 mAh g1 can still be maintained after 200 cycles, while the initial reversible capacity of the Li3VO4 þ 3DGNs mixture is only 144 mAh g1 and quickly fades to 84 mAh g1 after 200 cycles. Noticeably, even after 2500 cycles, the Li3VO4/3DGNs composite can still deliver a capacity of 259 mAh g1. Besides, columbic efficiencies of the Li3VO4/3DGNs composite remain above 99.4% after the first few cycles, suggesting the highly reversible Liþ intercalation/de-intercalation property of the Li3VO4/3DGNs composite. The cycling performance of Li3VO4/ 3DGNs shows superior advantages compared with most of the previous reports (summarized in Table S1), for example, Li3VO41 after 200 cycles, at 200 mA g1), Li3VO4/C d (286 mAh g 1 (205.5 mAh g after 2000 cycles, at 2 A g1), Li3VO4/N-doped graphene (193 mAh g1 after 900 cycles, at 2 A g1), demonstrating the excellent cycling stability of the Li3VO4/3DGNs composite. To understand the reason for this excellent cycling performance, TEM micrographs of a Li3VO4/3DGNs electrode after cycling for 1200 cycles at 2 A g1 are investigated. As can been seen from Fig. 5(a), even after a long-term cycling process plus a long-period sonication process, the Li3VO4 nanoparticles are still anchored on the 3DGNs and the overall structural integrity of single Li3VO4 nanoparticle is well conserved. Moreover, as shown in Fig. 5(b), even the nanoholes (one of the nanoholes is marked by a white circle as a representative) on Li3VO4 nanoparticles are well preserved, suggesting the robust 3D graphene networks may effectively buffer the strain/stress of Li3VO4 nanoparticles occurred during Liþ insertion/ extraction, which accounts for the good cycling stability of Li3VO4/ 3DGNs. Rate performances of the Li3VO4/3DGNs composite and the Li3VO4 þ 3DGNs mixture are compared in Fig. 6(a). At current densities of 0.2, 0.4, 0.8, 2, 4, 8 and 16 A g1, the capacities of the Li3VO4/3DGNs composite can reach up to 471, 404, 359, 318, 290, 247 and 191 mAh g1, respectively. Even at a very high current density of 32 A g1, a capacity as high as 142 mAh g1 can still be retained. When the current density restores to 0.2 A g1, the capacity bounces back to 439 mAh g1, indicating the good rate capability of the Li3VO4/3DGNs composite. For the Li3VO4 þ 3DGNs mixture, capacities of only 29 and 13 mAh g1 are retained at 8 and 16 A g1, while nearly no electrochemical activity is shown at
Fig. 6. (a) Rate performances of the Li3VO4/3DGNs composite and the Li3VO4 þ 3DGNs mixture; (b) Nyquist plots of the Li3VO4/3DGNs composite and the Li3VO4 þ 3DGNs mixture after 5 cycles, inset is the linear fitting line of real parts of the complex impedance versus u1/2; (c) Equivalent circuit.
32 A g1. It should be noted that the high-rate performance of the Li3VO4/3DGNs composite is also superior compared with most of
Fig. 5. TEM micrographs of the Li3VO4/3DGNs electrode after 1200 cycles at 2 A g1.
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the reported excellent Li3VO4 anode materials, for example, the carbon-encapsulated Li3VO4 (106 mAh g1 at 32 A g1) [13], Li3VO4 anchored GNS (133 mAh g1 at 20 A g1) [21] and the carboncoated Li3VO4 (90 mAh g1 at 20 A g1) [34]. EIS measurements are employed to illustrate the electrode kinetics of the Li3VO4/3DGNs composite and the Li3VO4 þ 3DGNs mixture. Each EIS spectrum in Fig. 6(b) exhibits two compressed semicircles from the high to medium frequency range and one line inclined at approximately 45 in the low-frequency range. The compressed semicircle in the high frequency range is relative to the SEI film resistance (RSEI), the diameter of the semicircle in the medium frequency range is related to the charge transfer resistance (Rct) and the 45 inclined line is associated with the Warburg impedance (ZW) [35,36]. After simulating the two compressed semicircles for both samples using the equivalent circuit shown in Fig. 6(c), values of RSEI and Rct after five cycles were calculated to be 36.2 U, 18.9 U for Li3VO4/3DGNs and 60.69 U, 107.6 U for Li3VO4 þ 3DGNs, respectively. Both the RSEI and Rct values of Li3VO4/ 3DGNs are smaller than those of Li3VO4 þ 3DGNs. The smaller RSEI of Li3VO4/3DGNs suggests the pomegranate-like structure derived from the in situ co-growth of 3DGNs and Li3VO4 can possibly protect Li3VO4 from side reactions with the electrolyte, forming a thinner SEI film, while the much lower Rct of the Li3VO4/3DGNs composite indicates a faster electron transportation inside the composite [37]. The lithium diffusion coefficients of the two samples can also be calculated by employing the following formula: D ¼ R2T2/2A2n4F4C2s2
(1)
D is the diffusion coefficient, R is the gas constant, T is the absolute temperature, A is the surface area of the anode, n is the number of electrons transferred in the half-reaction for the redox couple (2), F is the Faraday constant, C is the concentration of Liþ in solid (9.8 103 mol cm3), and s is the Warburg factor, which is related to Zre as shown in the following formula and can be obtained from the slope of the lines in the inset of Fig. 6(b) [38]. Zre ¼ Re þ Rct þ su1/2
(2)
After linear fitting, the lithium diffusion coefficients at 298 K are calculated to be 1.73 1012 cm2 s1 for the Li3VO4/3DGNs composite and 5.88 1013 cm2 s1 for the Li3VO4 þ 3DGNs mixture, respectively. The lithium ion diffusion coefficient of the Li3VO4/ 3DGNs composite is twice that of the Li3VO4 þ 3DGNs mixture, suggesting the pomegranate-like structure is favourable for facilitating Liþ diffusion within the Li3VO4/3DGNs composite. The high-rate capability and good cycling stability of the Li3VO4/ 3DGNs composite can be mainly attributed to the 3D continuous conductive channels endowed robust 3DGNs and the nanoholedotted Li3VO4 nanoparticles as illustrated in Fig. 7. Firstly, Li3VO4 nanoparticles are firmly and uniformly anchored on the highly conductive 3DGNs after the in situ co-growing process, thus intimate electrical contact between Li3VO4 and 3DGNs is built up on a large scale, which effectively improves the electron transportation between the two components. Moreover, the 3D interconnected conductive networks of 3DGNs can provide 3D continuous electron transportation channels, which can effectively accelerate the charge transfer inside 3DGNs [39]. As a result, the charge transfer resistance of the Li3VO4/3DGNs composite is greatly decreased due to the combined effect of above two factors. Secondly, during the hydrothermal process, 3DGNs can act as both physical and chemical spacers to impede the aggregation of Li3VO4 particles, thus providing more accessible interfacial insertion/extraction sites for lithium ions. To be specific, on one hand, the existence of graphene nanosheets can serve as physical barriers to hinder the aggregation
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Fig. 7. Schematic illustration of Liþ and electron transfer behaviour in the Li3VO4/ 3DGNs nanocomposite.
of Li3VO4 particles. On the other hand, oxygen-containing functional groups (hydroxyl group and epoxy group proved by FTIR in Fig. S4) [40e42] on the surface of 3DGNs allow the uniform anchoring of Li3VO4 particles on 3DGNs and immobilize them via strong chemical interactions between the functional groups and Li3VO4 nanoparticles, thus prohibiting the agglomeration of the Li3VO4 nanoparticles [43,44]. Hence, Liþ transport distance is greatly shortened owing to the nanometer-sized Li3VO4 particles. Thirdly, better electrolyte penetration is assured within the composite due to massive nanoholes on Li3VO4 nanoparticles, therefore, the Liþ diffusion distance is further shortened [45]. Fourthly, intrinsically flexible graphene networks can not only protect Li3VO4 from side reactions with the electrolyte, but also act as an elastic buffering cushion to accommodate crystal strain/stress generated upon repetitive Liþ intercalation/de-intercalation, maintaining structural integrity of the nanocomposite and thus ensuring the long-term and high-rate performance.
4. Conclusions A one-pot hydrothermal method was adopted to fabricate the pomegranate-like Li3VO4/3DGNs nanocomposite, in which the nanohole-dotted Li3VO4 nanoparticles with sizes of 100e200 nm are firmly anchored on the 3D conductive graphene networks. This unique structure permits rapid electron and Liþ transportation, leading to superior electrochemical performance. An initial reversible capacity of 397 mAh g1 is achieved and a capacity of 259 mAh g1 can still be retained even after 2500 cycles at 2 A g1. The Li3VO4/3DGNs composite also presents an excellent high-rate performance, delivering a stable capacity of 142 mAh g1 even at a very high current density of 32 A g1. The incorporation of constructing 3D conductive networks and achieving uniform distribution of nanoparticles via a facile one-pot in situ co-growing approach can be applied to a wide range of research fields, such as cathodes design for LIBs, sodium ion batteries, supercapacitors and solar cells.
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Acknowledgement This work was financially supported by the National Natural Science Foundation of China (Nos. 51301002, 51471052, 51571063, U1201241), the Science and Technology Commission of Shanghai Municipality (No. 14JC1490200, 15YF1401300) and the Guangxi Collaborative Innovation Center for Structure and Property of New Energy Materials. We also thank Dr. Liang Chen (Department of chemistry, Fudan University) for his kind assistance with GO preparation. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.06.018. References [1] P. Tartaj, J.M. Amarilla, M.B. V azquez Santo, Aerosol-assisted synthesis of colloidal aggregates with different morphology: toward the electrochemical optimization of Li3VO4 battery anodes using scalable routes, Chem. Mater. 28 (2016) 986e993. [2] S. Ni, J. Zhang, J. Ma, X. Yang, L. Zhang, Superior electrochemical performance of Li3VO4/N-doped C as an anode for Li-ion batteries, J. Mater. Chem. A 3 (2015) 17951e17955. [3] S. Hu, Y. Song, S. Yuan, H. Liu, Q. Xu, Y. Wang, C.X. Wang, Y.Y. Xia, A hierarchical structure of carbon-coated Li3VO4 nanoparticles embedded in expanded graphite for high performance lithium ion battery, J. Power Sources 303 (2016) 333e339. [4] B.D. Polat, O.L. Eryilmaz, O. Keles, SiAg film by magnetron sputtering for high reversible lithium ion storage anodes, J. Alloys Compd. 654 (2016) 363e370. [5] C.Q. Du, J.W. Wu, J. Liu, M. Yang, Q. Xu, Z.Y. Tang, X.H. Zhang, Synthesis of lithium vanadium tetroxide anode material via a fast sol-gel method based on spontaneous chemical reactions, Electrochim. Acta 152 (2015) 473e479. [6] Q. Shi, R. Hu, L. Ouyang, M. Zeng, M. Zhu, High-capacity LiV3O8 thin-film cathode with a mixed amorphous-nanocrystalline microstructure prepared by RF magnetron sputtering, Electrochem. Commun. 11 (2009) 2169e2172. [7] R. Ma, Y. Liu, Y. Yang, K. Pu, M. Gao, H. Pan, LieSiealloy-assisted improvement in the intrinsic cyclability of Mg2Si as an anode material for Li-ion batteries, Acta Mater. 98 (2015) 128e134. [8] C. Liang, M. Gao, H. Pan, Y. Liu, M. Yan, Lithium alloys and metal oxides as high-capacity anode materials for lithium-ion batteries, J. Alloys Compd. 575 (2013) 246e256. [9] M.H. Pyun, Y.J. Park, Graphene/LiMn2O4 nanocomposites for enhanced lithium ion batteries with high rate capability, J. Alloys Compd. 643 (2015) S90eS94. [10] Y.Q. Qiao, X.L. Wang, J.P. Zhou, J. Zhang, C.D. Gu, J.P. Tu, Synthesis and electrochemical performance of rod-like LiV3O8 cathode materials for rechargeable lithium batteries, J. Power Sources 198 (2012) 287e293. [11] S. Huang, Y. Lu, T.Q. Wang, C.D. Gu, X.L. Wang, J.P. Tu, Polyacrylamide-assisted freeze drying synthesis of hierarchical plate-arrayed LiV3O8 for high-rate lithium-ion batteries, J. Power Sources 235 (2013) 256e264. [12] S. Huang, X.L. Wang, Y. Lu, X.M. Jian, X.Y. Zhao, H. Tang, J.B. Cai, C.D. Gu, J.P. Tu, Facile synthesis of cookies-shaped LiV3O8 cathode materials with good cycling performance for lithium-ion batteries, J. Alloys Compd. 584 (2014) 41e46. [13] C. Zhang, H. Song, C. Liu, Y. Liu, C. Zhang, X. Nan, G. Cao, Fast and reversible Li ion insertion in carbon-encapsulated Li3VO4 as anode for lithium-ion battery, Adv. Funct. Mater. 25 (2015) 3497e3504. [14] G. Shao, L. Gan, Y. Ma, H. Li, T. Zhai, Enhancing the performance of Li3VO4 by combining nanotechnology and surface carbon coating for lithium ion batteries, J. Mater. Chem. A 3 (2015) 11253e11260. [15] S. Ni, X. Lv, J. Zhang, J. Ma, X. Yang, L. Zhang, The electrochemical performance of lithium vanadate/natural graphite composite material as anode for lithium ion batteries, Electrochim. Acta 145 (2014) 327e334. [16] D. Zhao, M. Cao, Constructing highly graphitized carbon-wrapped Li3VO4 nanoparticles with hierarchically porous structure as a long life and high capacity anode for lithium-ion batteries, ACS Appl. Mater. Inter. 7 (2015) 25084e25093. [17] S. Ni, J. Zhang, J. Ma, X. Yang, L. Zhang, Li3VO4/N-doped graphene with high capacity and excellent cycle stability as anode for lithium ion batteries, J. Power Sources 296 (2015) 377e382. [18] Y. Shi, J.Z. Wang, S.L. Chou, D. Wexler, H.J. Li, K. Ozawa, H.K. Liu, Y.P. Wu, 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. 13 (2013) 4715e4720.
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