Article pubs.acs.org/IC
Self-Assembled Sandwich-like Vanadium Oxide/Graphene Mesoporous Composite as High-Capacity Anode Material for Lithium Ion Batteries Xingchao Wang,† Yudai Huang,*,† Dianzeng Jia,*,† Wei Kong Pang,‡,§ Zaiping Guo,‡ Yaping Du,∥ Xincun Tang,†,⊥ and Yali Cao† †
Key Laboratory of Energy Materials Chemistry, Ministry of Education, Key Laboratory of Advanced Functional Materials, Autonomous Region, Institute of Applied Chemistry, Xinjiang University, Urumqi, Xinjiang 830046, China ‡ Institute for Superconducting & Electronic Materials, University of Wollongong, Wollongong, New South Wales 2522, Australia § Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, New South Wales 2232, Australia ∥ Frontier Institute of Science and Technology jointly with College of Science, Xi’an Jiaotong University, 99 Yanxiang Road, Yanta District, Xi’an, Shaanxi Province 710054, China ⊥ School of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, China S Supporting Information *
ABSTRACT: Sandwich-like V2O5/graphene mesoporous composite has been synthesized by a facile solvothermal approach. The crystalline structure, morphology, and electrochemical performance of the as-prepared materials have been investigated in detail. The results demonstrate that the 30−50 nm V2O5 particles are homogeneously anchored on conducting graphene sheets, which allow the V2O5 nanoparticles to be wired up to a current collector through the underlying conducting graphene layers. As an anode material for lithium ion batteries, the composite exhibits a high reversible capacity of 1006 mAh g−1 at a current density of 0.5 A g−1 after 300 cycles. It also exhibits excellent rate performance with a discharge capacity of 500 mAh g−1 at the current density of 3.0 A g−1, which is superior to the performance of the vanadium-based materials reported previously. The electrochemical properties demonstrate that the sandwich-like V2O5/graphene mesoporous composite could be a promising candidate material for high-capacity anode in lithium ion batteries. ions in its layered structure.8,9 More interestingly, V2O5 can be studied as a potential anode or cathode material for LIBs.10−15 As a cathode material for LIBs, V2O5 can accommodate 2 Li+ per mole of V2O5, offering a high theoretical capacity of ∼296 mAh g−1.16,17 In the voltage window of 0−3.0 V (vs Li/Li+), V2O5 may participate in a conversion reaction. In principle, large capacity in TMOs can be delivered by utilizing all possible oxidation states of a compound via the “conversion reaction” or “displacement redox reaction”, where the TMOs can reversibly react with Li+ to form the corresponding metallic nanoparticles (NPs).18 Therefore, V2O5 may be reduced to metallic V within the potential window of 0−3.0 V when used as an anode material for LIBs, which can yield a theoretical capacity of 1471 mAh g−1, higher than that of other TMOs, such as SnO2, Fe2O3, Co3O4, etc.10,19−21 Unfortunately, the intrinsically low diffusion coefficient of Li+ (D ≈ 10−12 cm2 s−1), poor structural
1. INTRODUCTION Lithium ion batteries (LIBs) are undoubtedly the most promising energy conversion devices and have conquered the market in the field of advanced portable electronic devices such as mobile phones, digital cameras, laptops, etc. The performance of LIBs, however, cannot meet the requirements of plug-in hybrid electric vehicles (PHEVs) or all-electric vehicles (EVs) with respect to high power density, high rate performance, and safety concerns.1−4 Commercial graphite anode with low theoretical capacity (372 mAh g−1) and poor rate capability is one of the main reasons why the LIBs cannot completely meet the above demands.5 Hence, developing new electrode materials with larger reversible capacity and higher energy density is vitally important.6 Electrochemically active transition metal oxides (TMOs) with their large theoretical capacity are considered as promising anode materials for next-generation LIBs.7 Among them, V2O5 is a well-known TMO and has been extensively explored in various fields, due to the ease of accommodating molecules or © XXXX American Chemical Society
Received: August 27, 2015
A
DOI: 10.1021/acs.inorgchem.5b01914 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry stability, and low electronic conductivity (10−2 to 10−3 S cm−1) of V2O5 limit its rate capability as well as its long-term cycling stability.22,23 Extensive research has been conducted to improve the electrochemical kinetics. One widely practiced strategy to effectively mitigate the above problems is to synthesize nanosized V2O5 with various morphologies which endow it with shorter Li+ diffusion pathways and easier Li+/electron transport, leading to marked improvement in electrochemical performance.24−26 Therefore, substantial efforts are being made to fabricate nanoscale materials with better cyclability and superior rate performance.27,28 Nevertheless, the nanostructured materials feature low packing density, resulting in low volumetric density and the formation of insulating layers, as well as unwanted poisonous side reactions.29 In order to mitigate these issues associated with nanomaterials, confining the nanomaterials in a conductive and porous robust framework is a favorable choice. Namely, the mesostructured materials, containing both nanoparticles and porous structure, combining the merits and mitigating the shortcomings of both the components, are ideal candidates. The uniform nanostructure provides a large contact area and short diffusion paths for both ions and electrons, leading to a better rate capability.30 In addition, the porous structure can accommodate the volume changes during Li+ insertion and extraction, contributing to good cyclability.31 Carbonaceous materials, with good electrical conductivity and excellent mechanical/electrochemical stability, are commonly introduced as matrices to buffer the volume changes, and enhance the structural stability, as well as conductivity of electrodes containing TMOs.32−34 Compared to the commonly used graphite, carbon black, and carbon nanotube, graphene exhibits the advantages of maintaining higher electrode conductivity of the overall electrode due to its higher surface area, excellent electronic properties, and good thermal and mechanical stability, which render it a good candidate for hybridizing with metallic or TMO NPs to facilitate the overall conductivity of the anode, prevent agglomeration of particles, and buffer the associated volume changes during cycling.35−38 Co 3 O 4 -graphene, Fe 3 O 4 -graphene, SnO 2 -graphene, and Mn3O4-graphene composites were reported as anode materials for LIBs recently, with better performances compared to those of previously reported Co3O4-, Fe3O4-, SnO2-, and Mn3O4based anodes.39−43 Lee et al.44 synthesized V2O5-nanowiregraphene, Rui et al.45 reported reduced-graphene-oxide supporting porous V2O5 material, and Chen et al.46 synthesized a composite of graphene and V2O5 NPs, which featured better performances compared to bare V2O5. Recently, Xie et al.47 synthesized amorphous V2O5-graphene composite via atomic layer deposition, and the composite displayed capacity of 900 mAh g−1 at 200 mA g−1 when used as an anode material for LIBs. To the best of our knowledge, however, there are no reports on a sandwich-like V2O5/graphene mesoporous composite, especially in terms of systematically exploring the potential of the composite as anode for LIBs. Furthermore, it is still a challenge to develop simple techniques to prepare V2O5/ graphene mesoporous composite for high performance LIBs.48 Herein, a facile solvothermal approach is introduced for growing V2O5 NPs on graphene to form a novel sandwich-like V2O5/graphene mesoporous composite. The composite exhibits many structural advantages. Specifically, the graphene sheets not only act as a highly conductive skeleton to sustain the sandwich structure but also immobilize the V2O5 NPs and suppress particle aggregation, as well as absorb the large volume
changes of V2O5 NPs during cycling. In addition, the wrinkled graphene nanosheets cross-link and form a mesoporous structure, allowing good penetration of the electrolyte into the active materials. On the other hand, the V2O5 NPs are confined to the regions between the graphene layers, which restricts growth during the synthesis process, leading to easily available, well-distributed uniform V2O5 NPs, which promotes good Li+/electron transport. As a result, when evaluated as an anode material for LIBs, the composite demonstrates significantly superior electrochemical performance compared to pristine V2O5 and to the materials in previous reports in terms of specific capacity, cyclability, and rate performance.15,49,50
2. EXPERIMENTAL SECTION 2.1. Material Synthesis. 2.1.1. Synthesis of Vanadium Precursor. 4.0 g of commercial V2O5 was dispersed in 40 mL of deionized water with stirring at 0−5 °C in an ice bath, and then 12 mL of 30% H2O2 was added dropwise to form a reddish brown solution. The solution was adjusted to pH 7.0−8.0 with 1 M NaOH aqueous solution, and 15 mL of acetylacetone was (within 20 min) slowly injected into the above solution. During the whole procedure, the solution was magnetically stirred at 800 rpm. Finally, the resultant blue precipitates were centrifuged, washed several times with ethanol, and dried under vacuum at 80 °C for 24 h, yielding the vanadyl precursor. 2.1.2. Synthesis of V2O5/Graphene Mesoporous Composite. First, 20 mg of graphene (Shenzhen Nano Port Co. Ltd.) was suspended in 30 mL of N,N-dimethylformamide (DMF) solution with dispersion by ultrasound in an ultrasonic cleaner (KQ-100, frequency 40 kHz, output power 100 W) at room temperature for 2 h to form a stable graphene nanosheet dispersion. 0.346 g of vanadyl precursor was diluted in another 30 mL of DMF solution via constant stirring for 1 h, after which the solution was added into the above graphene nanosheet dispersion and ultrasonicated for 2 h. The mixture was transferred into a 100 mL Teflon-lined autoclave and maintained at 200 °C for 20 h. After cooling down naturally, the resulting precipitate was centrifuged, washed several times with ethanol solution, and then dried under vacuum at 80 °C overnight to yield the V2O5/graphene composite precursor. The sandwich-like V2O5/graphene mesoporous composite was obtained after thermal pyrolysis of the as-prepared precursor at 350 °C for 0.5 h, with a heating rate of 1 °C min−1 in air. For comparison, pristine V2O5 was obtained by a similar process, but in the absence of graphene. 2.2. Materials Characterization. The crystal structures of the samples were investigated via a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation. The collected X-ray diffraction (XRD) patterns were analyzed by the Rietveld method using Rietica ver. 1.7.7. In the refinement, the optimized parameters included the background coefficients, zero-shift, peak shape parameters, phase lattice, positional parameters, and isotropic atomic displacement parameters. The figures of merit for the refinement include the Bragg statistical reliability factor (RB) and the weighted profile factor (Rwp). The morphology of the sample was analyzed by field emission scanning electron microscopy (FE-SEM, S-4800, Hitachi, Japan), transmission electron microscopy (TEM, H-600, Hitachi, Japan), and high resolution transmission electron microscopy (HRTEM, 2100F, JEOL, Japan). X-ray photoelectron spectroscopy (XPS) measurements were carried out on a PHI-5300 ESCA1610 SAM instrument equipped with a Mg Kα X-ray excitation source (1253.6 eV) operating at 10 kV and 10 mA. Raman spectroscopy was conducted on a Bruker Senterra R200-L spectrometer (532 nm). The weight percent of graphene in the composite was investigated by a thermogravimetry−differential scanning calorimetry (TG−DSC) instrument (NETZSCH STA 449 F3) in air at a heating rate of 10 °C min−1 from 25 to 800 °C. Brunauer−Emmett−Teller (BET) absorption and the pore size distribution were measured on an Xtended Pressure Sorption Analyzer (ASAP 2020, Micromeritics, USA) at 77 K. The specific surface area B
DOI: 10.1021/acs.inorgchem.5b01914 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry and pore diameter distribution of the as-obtained material were analyzed by BET and Barrett−Joyner−Halenda (BJH) methods. 2.3. Electrochemical Measurements. The electrochemical properties of the as-prepared materials were characterized using 2032 type coin cells, wherein Celgard 2300 was used as separator and lithium foil as the counter/reference electrode. The working electrodes were prepared by mixing the active material, acetylene black, and polyvinylidene fluoride (PVDF) in the weight ratio of 70:20:10 in Nmethyl-2-pyrrolidinone (NMP). The mixture was then pasted onto Cu foil for use as anode in LIBs. The cast electrode was dried at 110 °C in vacuum for 10 h and then cut into individual electrodes ∼1.13 cm2 in area by a punching machine. A 1 M solution of LiPF6 in ethylene carbonate:diethyl carbonate (EC:DEC) in the volume ratio of 1:1 was used as the standard organic electrolyte. Laboratory-built twoelectrode coin-type cells were assembled in an argon-filled glovebox (Mbraun, Germany), in which the moisture and oxygen concentrations were kept below 1 ppm. Charge/discharge tests were conducted on a multichannel battery test system (Land 2100, China) at different current densities. Cyclic voltammogram (CHI660D, Chenhua, China) experiments were performed in the voltage range of 0 to 3.0 V at 0.1 mV s−1. Electrochemical impedance spectroscopy (EIS) was carried out by applying an ac voltage of 5 mV on a Zahner Elektrik electrochemical workstation in the frequency range from 0.01 to 100 kHz. The specific capacities are reported based on the total mass of active materials, including V2O5 and graphene.
Specifically, the sandwich-like V2O5/graphene mesoporous composite was obtained through a one-pot reaction to form the V2O5/graphene precursor, followed by thermal annealing in an air atmosphere. Since graphene sheets have extremely large surface areas and are easy to disperse in solution, a small amount is sufficient to anchor the V2O5 NPs. Furthermore, the V2O5 NPs are well dispersed between/on the graphene sheets, preventing stacking of the graphene sheets and forming a novel sandwich-like structure. The phase purity and crystal structure of the as-synthesized pristine V2O5 and sandwich-like V2O5/graphene mesoporous composite were examined by XRD. With the aid of Rietveld refinement, the crystallographic details of the samples were extracted. It was found that both samples are single-phase and crystalline in the Pmmn space group. The lattice parameters of the orthorhombic structure that were determined are shown in Figure 1. It was found that the V2O5 NPs anchored on the graphene layers exhibit larger lattice parameters (a, b, and c) than those of the pristine V2O5. Scherrer’s equation was also used to determine the crystallite size of the samples, and it was found that the pristine V2O5 has a crystallite size more than double that of the V2O5/graphene. The smaller size of V2O5 in the composite provides a larger surface-to-volume ratio. It is also notable that a higher background is observed in the composite, and no characteristic peak of graphene at 22−28° is observed, testifying that the graphene nanosheets in the composite are in a homogeneous dispersion and successfully covered with V2O5, which is similar to what has been previously reported.41,51,52 In terms of the crystallographic details (i.e., positional parameters, isotropic atomic displacement parameters, and occupancy), the two samples are nearly identical (Table S1 and Figure S1). In other words, there is no significant difference between the structure of the composite and that of the pristine V2O5, except for the lattice parameters and the crystallite sizes, which could mainly be responsible for the different electrochemical performances of the samples. Raman spectroscopy was conducted to characterize the ordered/disordered structure of graphene (Figure S2). Both the G (∼1591.3 cm−1, E2g phonons of C sp2 atoms) and the D (∼1365.6 cm−1, κ-point phonons of A1g symmetry) bands are respectively detected in the Raman spectrum for the sandwichlike V2O5/graphene mesoporous composite, which further demonstrates the presence of graphene in the as-prepared material. The graphene content in the composite was about
3. RESULTS AND DISCUSSION The V2O5/graphene mesoporous composite was prepared by the facile solvothermal approach illustrated in Scheme 1. Scheme 1. Schematic Illustration of the Synthesis of the Sandwich-like V2O5/Graphene Mesoporous Composite
Figure 1. Rietveld-refined fits using XRD data of (a) pristine V2O5 and (b) V2O5/graphene mesoporous composite. The crosses (+) represent the experimental data, the red solid line is the calculated pattern, the green line is the difference between the experimental data and the calculated pattern, and the blue bars at the bottom show the Bragg positions of the orthorhombic V2O5. C
DOI: 10.1021/acs.inorgchem.5b01914 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 2. XPS spectra of the sandwich-like V2O5/graphene mesoporous composite: (a) survey spectrum, (b) V 2p, (c) O 1s, and (d) C 1s.
average diameter of about 30−50 nm are well-dispersed on the surfaces of the graphene sheets and the interfaces of the graphene, which self-assemble into a sandwich structure. Such a sandwich nanostructure gives rise to a high specific surface area. Meanwhile, the intimate contact of the V2O5 NPs with the conductive graphene framework is able to afford rapid electron transfer to the active material during repeated cycling. TEM images (Figure 3b,c) and HRTEM mapping images (Figure S4) further confirm that the V2O5 NPs are homogeneously anchored on the thin graphene nanosheets almost without any aggregation. Collection of HRTEM images was carried out to determine the lattice spacing (Figure 3d). The obvious lattice fringes with spacing of 0.437 nm match very well with the d-spacing of the (001) planes of V2O5. The well-dispersed V2O5 NPs benefit from the unstacked graphene nanosheets, which not only are favorable for preventing the V2O5 NPs from aggregating but also act as a framework to support the sandwich structure. Meanwhile, the V2O5 NPs anchored on the surfaces of the graphene sheets can act as spacers to efficiently prevent the graphene sheets from restacking, achieving a high active surface area. Nitrogen isothermal adsorption−desorption measurements were carried out to determine the BET surface area and the porosity of the materials. As shown in Figure 4, the isotherm curve of the as-synthesized composite is a typical type IV isotherm, indicating a mesoporous structure with a sharp capillary condensation at relative pressure P/P0 of ∼0.85, which proves the presence of large and uniform mesopores in the composite. The formation of the porous structure is probably attributable to the crystalline shrinkage and disconnection in the precursor, as well as graphene decomposition, during the annealing process. The interconnected mesopores will provide rapid transport channels for solvated ions to the nanopores throughout the electrode, which is essential to achieve high rate performance. The average pore size is ∼20.4 nm, calculated
4.1% by thermogravimetric (TG) analysis (Figure S3). To determine the chemical composition of the composite, XPS measurements were conducted (Figure 2). Figure 2b displays the XPS spectra of 2p3/2 at 516.4 and 517.8 eV together with 2p1/2 at 525.0 eV of V 2p, corresponding to the V4+ and V5+ states, respectively. The mixed valence (V4+/5+) is crucial for the higher electric conductivity.26,53 The O 1s excitation spectrum of vanadyl oxygen is characterized by a large peak at 530.6 eV, corresponding to V−O(2)−V bridging oxygen (Figure 2c). The high-resolution C 1s spectrum of the composite presented in Figure 2d can be deconvolved into two peaks, corresponding to two oxygen-containing functional groups: (a) carbon in C− C at 284.5 eV and (b) carbon in C−O at 286.3 eV. The general morphology of the sandwich-like V2O5/ graphene mesoporous composite was characterized by FESEM. As shown in Figure 3a, uniform V2O5 NPs with an
Figure 3. (a) SEM, (b, c) low-magnification TEM, and (d) HRTEM images of the sandwich-like V2O5/graphene mesoporous composite. D
DOI: 10.1021/acs.inorgchem.5b01914 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
thereby forming LixV2O5 phases. Meanwhile, three oxidization peaks centered at about 2.0, 2.4, and 2.6 V can be associated with the Li+ deintercalation from the LixV2O5 phases. Notably, the CV curves almost overlap after the first cycle, suggesting that the composite possesses highly reversible phase transition processes. Figure 5b displays the galvanostatic discharge/charge curves for the composite at a constant current rate of 0.2 A g−1, with a cutoff voltage of 0−3.0 V. For the first discharge curve, a discernible plateau centered at 2.6 V represents the Li+ insertion process, while the other sloping plateau between 0.7 and 0.8 V is related to the formation of the solid electrolyte interphase (SEI) film. It is noteworthy that the first discharge/ charge cycle can deliver capacities of 1903 and 871 mAh g−1, respectively. The relatively low initial Coulombic efficiency may arise from the incomplete conversion reaction and decomposition of the electrolyte, as well as the formation of the SEI and Li2O, which is common to most anode materials.57,58 After the first cycle, however, the second and the third discharge profiles overlap, indicating the highly reversible performance of the V2O5/graphene mesoporous composite. Li+ insertion/ extraction into/from V2O5 may be described by eqs 1 and 2. It is assumed that V2O5 could be reduced to metallic V based on a conversion reaction mechanism once the composite is discharged to 0 V, so V2O5 can display a maximum theoretical capacity of 1471 mAh g−1, from which we can comprehend that high reversible capacity could be delivered by this composite as an anode material, although the mechanism needs to be further investigated.
Figure 4. Nitrogen adsorption and desorption isotherms (inset: the BJH pore-size distribution) of the sandwich-like V2O5/graphene mesoporous composite.
using the BJH model (inset in Figure 4), allowing better accessibility of the electrolyte to the active material. The BET specific surface area and pore volume of the composite are 74.29 m2 g−1 and 0.38 cm3 g−1, respectively. Therefore, an appropriate graphene content in the composite and the mesoporous property of the composite could contribute to excellent electrochemical performance when the composite is employed as an anode material for LIBs. Even though many works on V2O5-based materials employed as cathode material for LIBs have been reported,54−56 few works focus on V2O5-based materials used as anode materials for LIBs.10 Herein, the electrochemical performance of the V2O5/graphene mesoporous composite as LIB anode was investigated systematically. Figure 5a displays the first five consecutive cyclic voltammograms (CVs) of the composite between 0 and 3.0 V. It can be clearly seen that the first cycle of the CV curve has multiple redox peaks, which is consistent with a previous report.10 Three reduction peaks at about 2.3, 1.9, and 1.5 V correspond to Li+ intercalated into V2O5 layers,
V2O5 + x Li+ + x e− ↔ LixV2O5
(0 < x ≤ 2)
Li 2V2O5 + 8Li+ ↔ 5Li 2O + 2V(0)
(1) (2)
The cycling performance of V2O5/graphene mesoporous composite and pristine V2O5 was investigated at a current
Figure 5. (a) CV curves for the first 5 cycles; (b) the first cycle charge−discharge curves (inset: the second and third cycle charge−discharge curves); (c) the cycling performance at a current density of 0.5 A g−1; and (d) the rate performance of the sandwich-like V2O5/graphene mesoporous composite. E
DOI: 10.1021/acs.inorgchem.5b01914 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry density of 0.5 A g−1. It can be seen that the reversible capacity of pristine V2O5 gradually decreases from 561 to 420 mAh g−1 over 100 cycles (Figure S5). In contrast, after the second cycle, the discharge capacity of the composite slightly increases to 1006 mAh g−1 after 300 cycles, although the capacity then decreases to 843 mAh g−1 at the end of the 500th cycle (Figure 5c). This excellent cyclability is also superior to that of doubleshelled V2O5 nanocapsules, which exhibit a reversible capacity of 673 mAh g−1 at a current rate of 0.25 A g−1 after 50 cycles.10 More importantly, the composite exhibits superior cycling performance at 0.9 A g−1 (Figure S6). The reversible discharge capacity of V2O5/graphene mesoporous composite drops in the initial cycles and then gradually increases to 734 mAh g−1 after 60 cycles at 0.9 A g−1. The increasing trend in the capacity of V2O5/graphene mesoporous composite during the initial 60 cycles is likely to have originated from the reversible formation of the polymeric gel-like SEI film by electrolyte degradation, which closely resembles the SEI film in other metal oxide composites.41,59 Additionally, the composite also manifests an extraordinary cycling performance at a much larger current rate of 1.6 A g−1 (Figure S7). The evolution of the morphology of the sandwich-like V2O5/graphene mesoporous composite after repeated cycling was further investigated in this work. From Figure S8, it is striking that the morphology of the sandwichstructured V2O5/graphene mesoporous composite shows no obvious change compared with the fresh electrode after undergoing 100 cycles at 0.9 A g−1. The composite manifests extremely durable high rate capability (Figure 5d). It shows a high irreversible capacity of 872.2 mAh g−1 after the first five cycles at 0.2 A g−1, and subsequently, its specific capacity decreases to 740.8, 629.0, and 586.5 mAh g−1 when the current rate increases to 0.5, 1.0, and 1.5 A g−1, respectively. Even at the very high current rates of 2.0 and 3.0 A g−1, the reversible capacity of the composite still remains approximately 535.5 and 500.1 mAh g−1, respectively. Remarkably, respective capacity of 944.5, 748.1, 631, 516.7, and 413.9 mAh g−1 is still recoverable when the current density is first returned to its initial value of 0.2, 0.5, 1.0, 2.0, and 3.0 A g−1. Furthermore, a specific capacity of 783.6 mAh g−1 is still possible once the current rate is finally returned to 0.2 A g−1 again, which also suggests the remarkable reversibility of the materials. The outstanding electrochemical performance of the sandwich-like V2O5/graphene mesoporous composite is extraordinarily associated with the unique sandwich nanostructure and the uniformly dispersed V2O5 NPs in graphene layers. First, the 3-dimensional sandwich-like structure of the electrode can provide particularly short transport lengths for both electrons and Li+ ions, and more electrochemically active sites. Second, the highly porous framework with void space offers ample hierarchical porous channels for facile penetration of the electrolyte into the active materials. Finally, the synergistic interactions between the graphene substrate and the uniformly dispersed V2O5 NPs, which are endowed with shortened Li+ diffusion pathways by the robust framework of graphene sheets with large surface area and highly conductive networks, improve the electrochemical kinetics and yield the remarkable electrochemical performance of the composite. EIS is an efficient tool to investigate electrode interfacial kinetics, so we conducted EIS of the sandwich-like V2O5/ graphene mesoporous composite (Figure 6). The Nyquist plots were analyzed and fitted using an equivalent circuit model (shown in the inset of Figure 6) via the software package Z-
Figure 6. Nyquist plots of pristine V2O5 and V2O5/graphene mesoporous composite, with the inset showing the equivalent circuit.
view 2.0. The solution resistance, Rs, and the ohmic resistance of the electrode, Rsf, reflect the Li+ diffusion impedance in the electrolyte solution and at the electrode−electrolyte interface, respectively. Rct is the charge-transfer resistance. CPE is a constant phase element. ZW is the Warburg impedance, representing Li+ diffusion within the electrode. The fitted results identify the physicochemical and electrochemical nature of the as-prepared materials. From Table 1, it can be seen that Table 1. Kinetic Parameters of Pristine V2O5 and Sandwichlike V2O5/Graphene Mesoporous Composite sample
Rs (Ω)
Rsf (Ω)
CPE1 (μF)
Rct (Ω)
CPE2 (μF)
pristine V2O5 V2O5/graphene
3.8 2.5
14.1 2.7
6.0 1.0
125.1 64.0
12.3 10.8
the Rs values are quite small and show no obvious difference between the pristine V2O5 and the composite. The Rsf and Rct of the composite, however, are significantly smaller than those of the pristine V2O5, indicating that the graphene sheets and the mesoporous structure can facilitate electron transfer. The sandwich-like V2O5/graphene mesoporous composite not only ensures a large contact interface between the electrolyte and electrode but also shortens the diffusion pathways for electrons and ions, and thus decreases the resistance. The improvement in electrode kinetics is well corroborated by the electrochemical performance described above.
4. CONCLUSIONS In this work, a novel sandwich-like V2O5/graphene mesoporous composite was synthesized via a facile solvothermal approach, and the composite was explored as an anode material for LIBs. The nanostructured V2O5 particles were anchored on graphene sheets and packed in a sandwich structure. The novel structured composite delivers a high reversible capacity of 1006 mAh g−1 at 0.5 A g−1 after 300 cycles. Moreover, the composite still exhibits a very high reversible capacity of 727 mAh g−1 at 0.9 A g−1 after 60 cycles. Remarkably, after 25 cycles of discharge and charge at different current densities, a reversible discharge capacity of 500 mAh g−1 can still be achieved, even at 3.0 A g−1. The outstanding cycling stability, large reversible specific capacity, and ultrahigh rate performance are very much associated with the unique graphene-based sandwich nanoarchitecture combined with the porous framework, which offers a large surface area, superior conductivity, and a short Li+ diffusion distance. F
DOI: 10.1021/acs.inorgchem.5b01914 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
■
(19) Lou, X. W.; Li, C. M.; Archer, L. A. Adv. Mater. 2009, 21, 2536− 2539. (20) Lin, Y.-M.; Abel, P. R.; Heller, A.; Mullins, C. B. J. Phys. Chem. Lett. 2011, 2, 2885−2891. (21) Wang, J.; Yang, N.; Tang, H.; Dong, Z.; Jin, Q.; Yang, M.; Kisailus, D.; Zhao, H.; Tang, Z.; Wang, D. Angew. Chem., Int. Ed. 2013, 52, 6417−6420. (22) Pan, A.; Zhang, J.-G.; Nie, Z.; Cao, G.; Arey, B. W.; Li, G.; Liang, S.-q.; Liu, J. J. Mater. Chem. 2010, 20, 9193−9199. (23) Chen, X.; Zhu, H.; Chen, Y. C.; Shang, Y.; Cao, A.; Hu, L.; Rubloff, G. W. ACS Nano 2012, 6, 7948−7955. (24) Hu, Y. S.; Liu, X.; Muller, J. O.; Schlogl, R.; Maier, J.; Su, D. S. Angew. Chem., Int. Ed. 2009, 48, 210−214. (25) Yu, D. M.; Chen, C. G.; Xie, S. H.; Liu, Y. Y.; Park, K.; Zhou, X. Y.; Zhang, Q. F.; Li, J. Y.; Cao, G. Z. Energy Environ. Sci. 2011, 4, 858− 861. (26) Perera, S. D.; Patel, B.; Nijem, N.; Roodenko, K.; Seitz, O.; Ferraris, J. P.; Chabal, Y. J.; Balkus, K. J. Adv. Energy Mater. 2011, 1, 936−945. (27) Borghols, W. J.; Wagemaker, M.; Lafont, U.; Kelder, E. M.; Mulder, F. M. J. Am. Chem. Soc. 2009, 131, 17786−17792. (28) Song, H.-K.; Lee, K. T.; Kim, M. G.; Nazar, L. F.; Cho, J. Adv. Funct. Mater. 2010, 20, 3818−3834. (29) Amine, K.; Belharouak, I.; Chen, Z.; Tran, T.; Yumoto, H.; Ota, N.; Myung, S. T.; Sun, Y. K. Adv. Mater. 2010, 22, 3052−3057. (30) Kang, E.; Jung, Y. S.; Kim, G. H.; Chun, J.; Wiesner, U.; Dillon, A. C.; Kim, J. K.; Lee, J. Adv. Funct. Mater. 2011, 21, 4349−4357. (31) Ren, Y.; Armstrong, A. R.; Jiao, F.; Bruce, P. G. J. Am. Chem. Soc. 2010, 132, 996−1004. (32) Zhang, X. F.; Wang, K. X.; Wei, X.; Chen, J. S. Chem. Mater. 2011, 23, 5290−5292. (33) Jia, X. L.; Chen, Z.; Suwarnasarn, A.; Rice, L.; Wang, X. L.; Sohn, H.; Zhang, Q.; Wu, B. M.; Wei, F.; Lu, Y. F. Energy Environ. Sci. 2012, 5, 6845−6849. (34) Su, Y.; Li, S.; Wu, D.; Zhang, F.; Liang, H.; Gao, P.; Cheng, C.; Feng, X. ACS Nano 2012, 6, 8349−8356. (35) Huang, X.; Zeng, Z.; Fan, Z.; Liu, J.; Zhang, H. Adv. Mater. 2012, 24, 5979−6004. (36) Chen, D.; Feng, H.; Li, J. Chem. Rev. 2012, 112, 6027−6053. (37) Ai, W.; Jiang, J.; Zhu, J. H.; Fan, Z. X.; Wang, Y. L.; Zhang, H.; Huang, W.; Yu, T. Adv. Energy Mater. 2015, 5, 1−8. (38) Ai, W.; Luo, Z.; Jiang, J.; Zhu, J.; Du, Z.; Fan, Z.; Xie, L.; Zhang, H.; Huang, W.; Yu, T. Adv. Mater. 2014, 26, 6186−6192. (39) Wang, H.; Cui, L. F.; Yang, Y.; Sanchez Casalongue, H.; Robinson, J. T.; Liang, Y.; Cui, Y.; Dai, H. J. Am. Chem. Soc. 2010, 132, 13978−13980. (40) Wu, Z.-S.; Ren, W.; Wen, L.; Gao, L.; Zhao, J.; Chen, Z.; Zhou, G.; Li, F.; Cheng, H.-M. ACS Nano 2010, 4, 3187−3194. (41) Wei, W.; Yang, S.; Zhou, H.; Lieberwirth, I.; Feng, X.; Mullen, K. Adv. Mater. 2013, 25, 2909−2914. (42) Ai, W.; Zhu, J. H.; Jiang, J.; Chao, D. L.; Wang, Y. L.; Ng, C. F.; Wang, X. L.; Wu, C.; Li, C. M.; Shen, Z. X.; Huang, W.; Yu, T. 2D Mater. 2015, 2, 1−9. (43) Zhou, W.; Tay, Y. Y.; Jia, X.; Yau Wai, D. Y.; Jiang, J.; Hoon, H. H.; Yu, T. Nanoscale 2012, 4, 4459−4463. (44) Lee, J. W.; Lim, S. Y.; Jeong, H. M.; Hwang, T. H.; Kang, J. K.; Choi, J. W. Energy Environ. Sci. 2012, 5, 9889−9894. (45) Rui, X. H.; Zhu, J. X.; Sim, D.; Xu, C.; Zeng, Y.; Hng, H. H.; Lim, T. M.; Yan, Q. Y. Nanoscale 2011, 3, 4752−4758. (46) Chen, D.; Yi, R.; Chen, S. R.; Xu, T.; Gordin, M. L.; Lv, D. P.; Wang, D. H. Mater. Sci. Eng., B 2014, 185, 7−12. (47) Sun, X.; Zhou, C.; Xie, M.; Hu, T.; Sun, H.; Xin, G.; Wang, G.; George, S. M.; Lian, J. Chem. Commun. 2014, 50, 10703−10706. (48) Ai, W.; Xie, L.; Du, Z.; Zeng, Z.; Liu, J.; Zhang, H.; Huang, Y.; Huang, W.; Yu, T. Sci. Rep. 2013, DOI: 10.1038/srep02341. (49) Chae, O. B.; Kim, J.; Park, I.; Jeong, H.; Ku, J. H.; Ryu, J. H.; Kang, K.; Oh, S. M. Chem. Mater. 2014, 26, 5874−5881. (50) Augustyn, V.; Dunn, B. Electrochim. Acta 2013, 88, 530−535.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01914. Crystallographic details, TG analysis, Raman spectroscopy, HRTEM mapping images, SEM images, and cycling performance (PDF)
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. *E-mail: [email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of Xinjiang (2015211C286 and 2013211A004), the Program for New Century Excellent Talents in University (NCET-12-1076), the National Natural Science Foundation of China (21161021, 21466036, 21271151, and 21361024), and the Young Scholar Science Foundation of Xinjiang Educational Institutions (XJEDU2014S013). The authors also would like to thank Prof. Hua Zhang at Nanyang Technological University for his strong support and stimulating discussions.
■
REFERENCES
(1) Tang, Y.; Zhang, Y.; Li, W.; Ma, B.; Chen, X. Chem. Soc. Rev. 2015, 44, 5926−5940. (2) Zhang, G.; Lou, X. W. Angew. Chem., Int. Ed. 2014, 53, 9041− 9044. (3) Liu, J.; Liu, X. W. Adv. Mater. 2012, 24, 4097−4111. (4) Choi, N. S.; Chen, Z.; Freunberger, S. A.; Ji, X.; Sun, Y. K.; Amine, K.; Yushin, G.; Nazar, L. F.; Cho, J.; Bruce, P. G. Angew. Chem., Int. Ed. 2012, 51, 9994−10024. (5) Kaskhedikar, N. A.; Maier, J. Adv. Mater. 2009, 21, 2664−2680. (6) Zeng, L.; Zheng, C.; Deng, C.; Ding, X.; Wei, M. ACS Appl. Mater. Interfaces 2013, 5, 2182−2187. (7) Yuan, C.; Wu, H. B.; Xie, Y.; Lou, X. W. Angew. Chem., Int. Ed. 2014, 53, 1488−1504. (8) Yang, Y.; Albu, S. P.; Kim, D.; Schmuki, P. Angew. Chem., Int. Ed. 2011, 50, 9071−9075. (9) Pan, A.; Wu, H. B.; Yu, L.; Zhu, T.; Lou, X. W. ACS Appl. Mater. Interfaces 2012, 4, 3874−3879. (10) Liu, J.; Xia, H.; Xue, D. F.; Lu, L. J. Am. Chem. Soc. 2009, 131, 12086−12087. (11) Zhang, C. F.; Chen, Z. X.; Guo, Z. P.; Lou, X. W. Energy Environ. Sci. 2013, 6, 974−978. (12) Rui, X.; Lu, Z.; Yu, H.; Yang, D.; Hng, H. H.; Lim, T. M.; Yan, Q. Nanoscale 2013, 5, 556−560. (13) Zhou, B.; Shi, H.; Cao, R.; Zhang, X.; Jiang, Z. Phys. Chem. Chem. Phys. 2014, 16, 18578−18585. (14) Chao, D.; Xia, X.; Liu, J.; Fan, Z.; Ng, C. F.; Lin, J.; Zhang, H.; Shen, Z. X.; Fan, H. J. Adv. Mater. 2014, 26, 5794−5800. (15) Xu, Y.; Dunwell, M.; Fei, L.; Fu, E.; Lin, Q.; Patterson, B.; Yuan, B.; Deng, S.; Andersen, P.; Luo, H.; Zou, G. ACS Appl. Mater. Interfaces 2014, 6, 20408−20413. (16) Tang, Y. X.; Rui, X. H.; Zhang, Y. Y.; Lim, T. M.; Dong, Z. L.; Hng, H. H.; Chen, X. D.; Yan, Q. Y.; Chen, Z. J. Mater. Chem. A 2013, 1, 82−88. (17) An, Q. Y.; Zhang, P. F.; Wei, Q. L.; He, L.; Xiong, F. Y.; Sheng, J. Z.; Wang, Q. Q.; Mai, L. Q. J. Mater. Chem. A 2014, 2, 3297−3302. (18) Reddy, M. V.; Subba Rao, G. V.; Chowdari, B. V. Chem. Rev. 2013, 113, 5364−5457. G
DOI: 10.1021/acs.inorgchem.5b01914 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry (51) Pan, A. Q.; Wu, H. B.; Zhang, L.; Lou, X. W. Energy Environ. Sci. 2013, 6, 1476−1479. (52) Sun, Y. M.; Hu, X. L.; Luo, W.; Huang, Y. H. J. Phys. Chem. C 2012, 116, 20794−20799. (53) Cheng, F. Y.; Chen, J. J. Mater. Chem. 2011, 21, 9841−9848. (54) Liu, Y.; Clark, M.; Zhang, Q.; Yu, D.; Liu, D.; Liu, J.; Cao, G. Adv. Energy Mater. 2011, 1, 194−202. (55) Pan, A.; Wu, H. B.; Yu, L.; Lou, X. D. Angew. Chem., Int. Ed. 2013, 125, 1−6. (56) Lee, M.; Balasingam, S. K.; Jeong, H. Y.; Hong, W. G.; Lee, H. B.; Kim, B. H.; Jun, Y. Sci. Rep. 2015, 5, 8151. (57) Huang, Y.; Dong, Z.; Jia, D.; Guo, Z.; Cho, W. I. Electrochim. Acta 2011, 56, 9233−9239. (58) Huang, Y. D.; Dong, Z. F.; Jia, D. Z.; Guo, Z. P.; Cho, W. I. Solid State Ionics 2011, 201, 54−59. (59) Zhou, G. M.; Wang, D. W.; Li, F.; Zhang, L. L.; Li, N.; Wu, Z. S.; Wen, L.; Lu, G. Q.; Cheng, H. M. Chem. Mater. 2010, 22, 5306− 5313.
H
DOI: 10.1021/acs.inorgchem.5b01914 Inorg. Chem. XXXX, XXX, XXX−XXX
Self-Assembled Sandwich-like Vanadium Oxide/Graphene Mesoporous Composite as High-Capacity Anode Material for Lithium Ion Batteries Xingchao Wang,† Yudai Huang,*,† Dianzeng Jia,*,† Wei Kong Pang,‡,§ Zaiping Guo,‡ Yaping Du,∥ Xincun Tang,†,⊥ and Yali Cao† †
Key Laboratory of Energy Materials Chemistry, Ministry of Education, Key Laboratory of Advanced Functional Materials, Autonomous Region, Institute of Applied Chemistry, Xinjiang University, Urumqi, Xinjiang 830046, China ‡ Institute for Superconducting & Electronic Materials, University of Wollongong, Wollongong, New South Wales 2522, Australia § Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, New South Wales 2232, Australia ∥ Frontier Institute of Science and Technology jointly with College of Science, Xi’an Jiaotong University, 99 Yanxiang Road, Yanta District, Xi’an, Shaanxi Province 710054, China ⊥ School of Chemistry and Chemical Engineering, Central South University, Changsha, Hunan 410083, China S Supporting Information *
ABSTRACT: Sandwich-like V2O5/graphene mesoporous composite has been synthesized by a facile solvothermal approach. The crystalline structure, morphology, and electrochemical performance of the as-prepared materials have been investigated in detail. The results demonstrate that the 30−50 nm V2O5 particles are homogeneously anchored on conducting graphene sheets, which allow the V2O5 nanoparticles to be wired up to a current collector through the underlying conducting graphene layers. As an anode material for lithium ion batteries, the composite exhibits a high reversible capacity of 1006 mAh g−1 at a current density of 0.5 A g−1 after 300 cycles. It also exhibits excellent rate performance with a discharge capacity of 500 mAh g−1 at the current density of 3.0 A g−1, which is superior to the performance of the vanadium-based materials reported previously. The electrochemical properties demonstrate that the sandwich-like V2O5/graphene mesoporous composite could be a promising candidate material for high-capacity anode in lithium ion batteries. ions in its layered structure.8,9 More interestingly, V2O5 can be studied as a potential anode or cathode material for LIBs.10−15 As a cathode material for LIBs, V2O5 can accommodate 2 Li+ per mole of V2O5, offering a high theoretical capacity of ∼296 mAh g−1.16,17 In the voltage window of 0−3.0 V (vs Li/Li+), V2O5 may participate in a conversion reaction. In principle, large capacity in TMOs can be delivered by utilizing all possible oxidation states of a compound via the “conversion reaction” or “displacement redox reaction”, where the TMOs can reversibly react with Li+ to form the corresponding metallic nanoparticles (NPs).18 Therefore, V2O5 may be reduced to metallic V within the potential window of 0−3.0 V when used as an anode material for LIBs, which can yield a theoretical capacity of 1471 mAh g−1, higher than that of other TMOs, such as SnO2, Fe2O3, Co3O4, etc.10,19−21 Unfortunately, the intrinsically low diffusion coefficient of Li+ (D ≈ 10−12 cm2 s−1), poor structural
1. INTRODUCTION Lithium ion batteries (LIBs) are undoubtedly the most promising energy conversion devices and have conquered the market in the field of advanced portable electronic devices such as mobile phones, digital cameras, laptops, etc. The performance of LIBs, however, cannot meet the requirements of plug-in hybrid electric vehicles (PHEVs) or all-electric vehicles (EVs) with respect to high power density, high rate performance, and safety concerns.1−4 Commercial graphite anode with low theoretical capacity (372 mAh g−1) and poor rate capability is one of the main reasons why the LIBs cannot completely meet the above demands.5 Hence, developing new electrode materials with larger reversible capacity and higher energy density is vitally important.6 Electrochemically active transition metal oxides (TMOs) with their large theoretical capacity are considered as promising anode materials for next-generation LIBs.7 Among them, V2O5 is a well-known TMO and has been extensively explored in various fields, due to the ease of accommodating molecules or © XXXX American Chemical Society
Received: August 27, 2015
A
DOI: 10.1021/acs.inorgchem.5b01914 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry stability, and low electronic conductivity (10−2 to 10−3 S cm−1) of V2O5 limit its rate capability as well as its long-term cycling stability.22,23 Extensive research has been conducted to improve the electrochemical kinetics. One widely practiced strategy to effectively mitigate the above problems is to synthesize nanosized V2O5 with various morphologies which endow it with shorter Li+ diffusion pathways and easier Li+/electron transport, leading to marked improvement in electrochemical performance.24−26 Therefore, substantial efforts are being made to fabricate nanoscale materials with better cyclability and superior rate performance.27,28 Nevertheless, the nanostructured materials feature low packing density, resulting in low volumetric density and the formation of insulating layers, as well as unwanted poisonous side reactions.29 In order to mitigate these issues associated with nanomaterials, confining the nanomaterials in a conductive and porous robust framework is a favorable choice. Namely, the mesostructured materials, containing both nanoparticles and porous structure, combining the merits and mitigating the shortcomings of both the components, are ideal candidates. The uniform nanostructure provides a large contact area and short diffusion paths for both ions and electrons, leading to a better rate capability.30 In addition, the porous structure can accommodate the volume changes during Li+ insertion and extraction, contributing to good cyclability.31 Carbonaceous materials, with good electrical conductivity and excellent mechanical/electrochemical stability, are commonly introduced as matrices to buffer the volume changes, and enhance the structural stability, as well as conductivity of electrodes containing TMOs.32−34 Compared to the commonly used graphite, carbon black, and carbon nanotube, graphene exhibits the advantages of maintaining higher electrode conductivity of the overall electrode due to its higher surface area, excellent electronic properties, and good thermal and mechanical stability, which render it a good candidate for hybridizing with metallic or TMO NPs to facilitate the overall conductivity of the anode, prevent agglomeration of particles, and buffer the associated volume changes during cycling.35−38 Co 3 O 4 -graphene, Fe 3 O 4 -graphene, SnO 2 -graphene, and Mn3O4-graphene composites were reported as anode materials for LIBs recently, with better performances compared to those of previously reported Co3O4-, Fe3O4-, SnO2-, and Mn3O4based anodes.39−43 Lee et al.44 synthesized V2O5-nanowiregraphene, Rui et al.45 reported reduced-graphene-oxide supporting porous V2O5 material, and Chen et al.46 synthesized a composite of graphene and V2O5 NPs, which featured better performances compared to bare V2O5. Recently, Xie et al.47 synthesized amorphous V2O5-graphene composite via atomic layer deposition, and the composite displayed capacity of 900 mAh g−1 at 200 mA g−1 when used as an anode material for LIBs. To the best of our knowledge, however, there are no reports on a sandwich-like V2O5/graphene mesoporous composite, especially in terms of systematically exploring the potential of the composite as anode for LIBs. Furthermore, it is still a challenge to develop simple techniques to prepare V2O5/ graphene mesoporous composite for high performance LIBs.48 Herein, a facile solvothermal approach is introduced for growing V2O5 NPs on graphene to form a novel sandwich-like V2O5/graphene mesoporous composite. The composite exhibits many structural advantages. Specifically, the graphene sheets not only act as a highly conductive skeleton to sustain the sandwich structure but also immobilize the V2O5 NPs and suppress particle aggregation, as well as absorb the large volume
changes of V2O5 NPs during cycling. In addition, the wrinkled graphene nanosheets cross-link and form a mesoporous structure, allowing good penetration of the electrolyte into the active materials. On the other hand, the V2O5 NPs are confined to the regions between the graphene layers, which restricts growth during the synthesis process, leading to easily available, well-distributed uniform V2O5 NPs, which promotes good Li+/electron transport. As a result, when evaluated as an anode material for LIBs, the composite demonstrates significantly superior electrochemical performance compared to pristine V2O5 and to the materials in previous reports in terms of specific capacity, cyclability, and rate performance.15,49,50
2. EXPERIMENTAL SECTION 2.1. Material Synthesis. 2.1.1. Synthesis of Vanadium Precursor. 4.0 g of commercial V2O5 was dispersed in 40 mL of deionized water with stirring at 0−5 °C in an ice bath, and then 12 mL of 30% H2O2 was added dropwise to form a reddish brown solution. The solution was adjusted to pH 7.0−8.0 with 1 M NaOH aqueous solution, and 15 mL of acetylacetone was (within 20 min) slowly injected into the above solution. During the whole procedure, the solution was magnetically stirred at 800 rpm. Finally, the resultant blue precipitates were centrifuged, washed several times with ethanol, and dried under vacuum at 80 °C for 24 h, yielding the vanadyl precursor. 2.1.2. Synthesis of V2O5/Graphene Mesoporous Composite. First, 20 mg of graphene (Shenzhen Nano Port Co. Ltd.) was suspended in 30 mL of N,N-dimethylformamide (DMF) solution with dispersion by ultrasound in an ultrasonic cleaner (KQ-100, frequency 40 kHz, output power 100 W) at room temperature for 2 h to form a stable graphene nanosheet dispersion. 0.346 g of vanadyl precursor was diluted in another 30 mL of DMF solution via constant stirring for 1 h, after which the solution was added into the above graphene nanosheet dispersion and ultrasonicated for 2 h. The mixture was transferred into a 100 mL Teflon-lined autoclave and maintained at 200 °C for 20 h. After cooling down naturally, the resulting precipitate was centrifuged, washed several times with ethanol solution, and then dried under vacuum at 80 °C overnight to yield the V2O5/graphene composite precursor. The sandwich-like V2O5/graphene mesoporous composite was obtained after thermal pyrolysis of the as-prepared precursor at 350 °C for 0.5 h, with a heating rate of 1 °C min−1 in air. For comparison, pristine V2O5 was obtained by a similar process, but in the absence of graphene. 2.2. Materials Characterization. The crystal structures of the samples were investigated via a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation. The collected X-ray diffraction (XRD) patterns were analyzed by the Rietveld method using Rietica ver. 1.7.7. In the refinement, the optimized parameters included the background coefficients, zero-shift, peak shape parameters, phase lattice, positional parameters, and isotropic atomic displacement parameters. The figures of merit for the refinement include the Bragg statistical reliability factor (RB) and the weighted profile factor (Rwp). The morphology of the sample was analyzed by field emission scanning electron microscopy (FE-SEM, S-4800, Hitachi, Japan), transmission electron microscopy (TEM, H-600, Hitachi, Japan), and high resolution transmission electron microscopy (HRTEM, 2100F, JEOL, Japan). X-ray photoelectron spectroscopy (XPS) measurements were carried out on a PHI-5300 ESCA1610 SAM instrument equipped with a Mg Kα X-ray excitation source (1253.6 eV) operating at 10 kV and 10 mA. Raman spectroscopy was conducted on a Bruker Senterra R200-L spectrometer (532 nm). The weight percent of graphene in the composite was investigated by a thermogravimetry−differential scanning calorimetry (TG−DSC) instrument (NETZSCH STA 449 F3) in air at a heating rate of 10 °C min−1 from 25 to 800 °C. Brunauer−Emmett−Teller (BET) absorption and the pore size distribution were measured on an Xtended Pressure Sorption Analyzer (ASAP 2020, Micromeritics, USA) at 77 K. The specific surface area B
DOI: 10.1021/acs.inorgchem.5b01914 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry and pore diameter distribution of the as-obtained material were analyzed by BET and Barrett−Joyner−Halenda (BJH) methods. 2.3. Electrochemical Measurements. The electrochemical properties of the as-prepared materials were characterized using 2032 type coin cells, wherein Celgard 2300 was used as separator and lithium foil as the counter/reference electrode. The working electrodes were prepared by mixing the active material, acetylene black, and polyvinylidene fluoride (PVDF) in the weight ratio of 70:20:10 in Nmethyl-2-pyrrolidinone (NMP). The mixture was then pasted onto Cu foil for use as anode in LIBs. The cast electrode was dried at 110 °C in vacuum for 10 h and then cut into individual electrodes ∼1.13 cm2 in area by a punching machine. A 1 M solution of LiPF6 in ethylene carbonate:diethyl carbonate (EC:DEC) in the volume ratio of 1:1 was used as the standard organic electrolyte. Laboratory-built twoelectrode coin-type cells were assembled in an argon-filled glovebox (Mbraun, Germany), in which the moisture and oxygen concentrations were kept below 1 ppm. Charge/discharge tests were conducted on a multichannel battery test system (Land 2100, China) at different current densities. Cyclic voltammogram (CHI660D, Chenhua, China) experiments were performed in the voltage range of 0 to 3.0 V at 0.1 mV s−1. Electrochemical impedance spectroscopy (EIS) was carried out by applying an ac voltage of 5 mV on a Zahner Elektrik electrochemical workstation in the frequency range from 0.01 to 100 kHz. The specific capacities are reported based on the total mass of active materials, including V2O5 and graphene.
Specifically, the sandwich-like V2O5/graphene mesoporous composite was obtained through a one-pot reaction to form the V2O5/graphene precursor, followed by thermal annealing in an air atmosphere. Since graphene sheets have extremely large surface areas and are easy to disperse in solution, a small amount is sufficient to anchor the V2O5 NPs. Furthermore, the V2O5 NPs are well dispersed between/on the graphene sheets, preventing stacking of the graphene sheets and forming a novel sandwich-like structure. The phase purity and crystal structure of the as-synthesized pristine V2O5 and sandwich-like V2O5/graphene mesoporous composite were examined by XRD. With the aid of Rietveld refinement, the crystallographic details of the samples were extracted. It was found that both samples are single-phase and crystalline in the Pmmn space group. The lattice parameters of the orthorhombic structure that were determined are shown in Figure 1. It was found that the V2O5 NPs anchored on the graphene layers exhibit larger lattice parameters (a, b, and c) than those of the pristine V2O5. Scherrer’s equation was also used to determine the crystallite size of the samples, and it was found that the pristine V2O5 has a crystallite size more than double that of the V2O5/graphene. The smaller size of V2O5 in the composite provides a larger surface-to-volume ratio. It is also notable that a higher background is observed in the composite, and no characteristic peak of graphene at 22−28° is observed, testifying that the graphene nanosheets in the composite are in a homogeneous dispersion and successfully covered with V2O5, which is similar to what has been previously reported.41,51,52 In terms of the crystallographic details (i.e., positional parameters, isotropic atomic displacement parameters, and occupancy), the two samples are nearly identical (Table S1 and Figure S1). In other words, there is no significant difference between the structure of the composite and that of the pristine V2O5, except for the lattice parameters and the crystallite sizes, which could mainly be responsible for the different electrochemical performances of the samples. Raman spectroscopy was conducted to characterize the ordered/disordered structure of graphene (Figure S2). Both the G (∼1591.3 cm−1, E2g phonons of C sp2 atoms) and the D (∼1365.6 cm−1, κ-point phonons of A1g symmetry) bands are respectively detected in the Raman spectrum for the sandwichlike V2O5/graphene mesoporous composite, which further demonstrates the presence of graphene in the as-prepared material. The graphene content in the composite was about
3. RESULTS AND DISCUSSION The V2O5/graphene mesoporous composite was prepared by the facile solvothermal approach illustrated in Scheme 1. Scheme 1. Schematic Illustration of the Synthesis of the Sandwich-like V2O5/Graphene Mesoporous Composite
Figure 1. Rietveld-refined fits using XRD data of (a) pristine V2O5 and (b) V2O5/graphene mesoporous composite. The crosses (+) represent the experimental data, the red solid line is the calculated pattern, the green line is the difference between the experimental data and the calculated pattern, and the blue bars at the bottom show the Bragg positions of the orthorhombic V2O5. C
DOI: 10.1021/acs.inorgchem.5b01914 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 2. XPS spectra of the sandwich-like V2O5/graphene mesoporous composite: (a) survey spectrum, (b) V 2p, (c) O 1s, and (d) C 1s.
average diameter of about 30−50 nm are well-dispersed on the surfaces of the graphene sheets and the interfaces of the graphene, which self-assemble into a sandwich structure. Such a sandwich nanostructure gives rise to a high specific surface area. Meanwhile, the intimate contact of the V2O5 NPs with the conductive graphene framework is able to afford rapid electron transfer to the active material during repeated cycling. TEM images (Figure 3b,c) and HRTEM mapping images (Figure S4) further confirm that the V2O5 NPs are homogeneously anchored on the thin graphene nanosheets almost without any aggregation. Collection of HRTEM images was carried out to determine the lattice spacing (Figure 3d). The obvious lattice fringes with spacing of 0.437 nm match very well with the d-spacing of the (001) planes of V2O5. The well-dispersed V2O5 NPs benefit from the unstacked graphene nanosheets, which not only are favorable for preventing the V2O5 NPs from aggregating but also act as a framework to support the sandwich structure. Meanwhile, the V2O5 NPs anchored on the surfaces of the graphene sheets can act as spacers to efficiently prevent the graphene sheets from restacking, achieving a high active surface area. Nitrogen isothermal adsorption−desorption measurements were carried out to determine the BET surface area and the porosity of the materials. As shown in Figure 4, the isotherm curve of the as-synthesized composite is a typical type IV isotherm, indicating a mesoporous structure with a sharp capillary condensation at relative pressure P/P0 of ∼0.85, which proves the presence of large and uniform mesopores in the composite. The formation of the porous structure is probably attributable to the crystalline shrinkage and disconnection in the precursor, as well as graphene decomposition, during the annealing process. The interconnected mesopores will provide rapid transport channels for solvated ions to the nanopores throughout the electrode, which is essential to achieve high rate performance. The average pore size is ∼20.4 nm, calculated
4.1% by thermogravimetric (TG) analysis (Figure S3). To determine the chemical composition of the composite, XPS measurements were conducted (Figure 2). Figure 2b displays the XPS spectra of 2p3/2 at 516.4 and 517.8 eV together with 2p1/2 at 525.0 eV of V 2p, corresponding to the V4+ and V5+ states, respectively. The mixed valence (V4+/5+) is crucial for the higher electric conductivity.26,53 The O 1s excitation spectrum of vanadyl oxygen is characterized by a large peak at 530.6 eV, corresponding to V−O(2)−V bridging oxygen (Figure 2c). The high-resolution C 1s spectrum of the composite presented in Figure 2d can be deconvolved into two peaks, corresponding to two oxygen-containing functional groups: (a) carbon in C− C at 284.5 eV and (b) carbon in C−O at 286.3 eV. The general morphology of the sandwich-like V2O5/ graphene mesoporous composite was characterized by FESEM. As shown in Figure 3a, uniform V2O5 NPs with an
Figure 3. (a) SEM, (b, c) low-magnification TEM, and (d) HRTEM images of the sandwich-like V2O5/graphene mesoporous composite. D
DOI: 10.1021/acs.inorgchem.5b01914 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
thereby forming LixV2O5 phases. Meanwhile, three oxidization peaks centered at about 2.0, 2.4, and 2.6 V can be associated with the Li+ deintercalation from the LixV2O5 phases. Notably, the CV curves almost overlap after the first cycle, suggesting that the composite possesses highly reversible phase transition processes. Figure 5b displays the galvanostatic discharge/charge curves for the composite at a constant current rate of 0.2 A g−1, with a cutoff voltage of 0−3.0 V. For the first discharge curve, a discernible plateau centered at 2.6 V represents the Li+ insertion process, while the other sloping plateau between 0.7 and 0.8 V is related to the formation of the solid electrolyte interphase (SEI) film. It is noteworthy that the first discharge/ charge cycle can deliver capacities of 1903 and 871 mAh g−1, respectively. The relatively low initial Coulombic efficiency may arise from the incomplete conversion reaction and decomposition of the electrolyte, as well as the formation of the SEI and Li2O, which is common to most anode materials.57,58 After the first cycle, however, the second and the third discharge profiles overlap, indicating the highly reversible performance of the V2O5/graphene mesoporous composite. Li+ insertion/ extraction into/from V2O5 may be described by eqs 1 and 2. It is assumed that V2O5 could be reduced to metallic V based on a conversion reaction mechanism once the composite is discharged to 0 V, so V2O5 can display a maximum theoretical capacity of 1471 mAh g−1, from which we can comprehend that high reversible capacity could be delivered by this composite as an anode material, although the mechanism needs to be further investigated.
Figure 4. Nitrogen adsorption and desorption isotherms (inset: the BJH pore-size distribution) of the sandwich-like V2O5/graphene mesoporous composite.
using the BJH model (inset in Figure 4), allowing better accessibility of the electrolyte to the active material. The BET specific surface area and pore volume of the composite are 74.29 m2 g−1 and 0.38 cm3 g−1, respectively. Therefore, an appropriate graphene content in the composite and the mesoporous property of the composite could contribute to excellent electrochemical performance when the composite is employed as an anode material for LIBs. Even though many works on V2O5-based materials employed as cathode material for LIBs have been reported,54−56 few works focus on V2O5-based materials used as anode materials for LIBs.10 Herein, the electrochemical performance of the V2O5/graphene mesoporous composite as LIB anode was investigated systematically. Figure 5a displays the first five consecutive cyclic voltammograms (CVs) of the composite between 0 and 3.0 V. It can be clearly seen that the first cycle of the CV curve has multiple redox peaks, which is consistent with a previous report.10 Three reduction peaks at about 2.3, 1.9, and 1.5 V correspond to Li+ intercalated into V2O5 layers,
V2O5 + x Li+ + x e− ↔ LixV2O5
(0 < x ≤ 2)
Li 2V2O5 + 8Li+ ↔ 5Li 2O + 2V(0)
(1) (2)
The cycling performance of V2O5/graphene mesoporous composite and pristine V2O5 was investigated at a current
Figure 5. (a) CV curves for the first 5 cycles; (b) the first cycle charge−discharge curves (inset: the second and third cycle charge−discharge curves); (c) the cycling performance at a current density of 0.5 A g−1; and (d) the rate performance of the sandwich-like V2O5/graphene mesoporous composite. E
DOI: 10.1021/acs.inorgchem.5b01914 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry density of 0.5 A g−1. It can be seen that the reversible capacity of pristine V2O5 gradually decreases from 561 to 420 mAh g−1 over 100 cycles (Figure S5). In contrast, after the second cycle, the discharge capacity of the composite slightly increases to 1006 mAh g−1 after 300 cycles, although the capacity then decreases to 843 mAh g−1 at the end of the 500th cycle (Figure 5c). This excellent cyclability is also superior to that of doubleshelled V2O5 nanocapsules, which exhibit a reversible capacity of 673 mAh g−1 at a current rate of 0.25 A g−1 after 50 cycles.10 More importantly, the composite exhibits superior cycling performance at 0.9 A g−1 (Figure S6). The reversible discharge capacity of V2O5/graphene mesoporous composite drops in the initial cycles and then gradually increases to 734 mAh g−1 after 60 cycles at 0.9 A g−1. The increasing trend in the capacity of V2O5/graphene mesoporous composite during the initial 60 cycles is likely to have originated from the reversible formation of the polymeric gel-like SEI film by electrolyte degradation, which closely resembles the SEI film in other metal oxide composites.41,59 Additionally, the composite also manifests an extraordinary cycling performance at a much larger current rate of 1.6 A g−1 (Figure S7). The evolution of the morphology of the sandwich-like V2O5/graphene mesoporous composite after repeated cycling was further investigated in this work. From Figure S8, it is striking that the morphology of the sandwichstructured V2O5/graphene mesoporous composite shows no obvious change compared with the fresh electrode after undergoing 100 cycles at 0.9 A g−1. The composite manifests extremely durable high rate capability (Figure 5d). It shows a high irreversible capacity of 872.2 mAh g−1 after the first five cycles at 0.2 A g−1, and subsequently, its specific capacity decreases to 740.8, 629.0, and 586.5 mAh g−1 when the current rate increases to 0.5, 1.0, and 1.5 A g−1, respectively. Even at the very high current rates of 2.0 and 3.0 A g−1, the reversible capacity of the composite still remains approximately 535.5 and 500.1 mAh g−1, respectively. Remarkably, respective capacity of 944.5, 748.1, 631, 516.7, and 413.9 mAh g−1 is still recoverable when the current density is first returned to its initial value of 0.2, 0.5, 1.0, 2.0, and 3.0 A g−1. Furthermore, a specific capacity of 783.6 mAh g−1 is still possible once the current rate is finally returned to 0.2 A g−1 again, which also suggests the remarkable reversibility of the materials. The outstanding electrochemical performance of the sandwich-like V2O5/graphene mesoporous composite is extraordinarily associated with the unique sandwich nanostructure and the uniformly dispersed V2O5 NPs in graphene layers. First, the 3-dimensional sandwich-like structure of the electrode can provide particularly short transport lengths for both electrons and Li+ ions, and more electrochemically active sites. Second, the highly porous framework with void space offers ample hierarchical porous channels for facile penetration of the electrolyte into the active materials. Finally, the synergistic interactions between the graphene substrate and the uniformly dispersed V2O5 NPs, which are endowed with shortened Li+ diffusion pathways by the robust framework of graphene sheets with large surface area and highly conductive networks, improve the electrochemical kinetics and yield the remarkable electrochemical performance of the composite. EIS is an efficient tool to investigate electrode interfacial kinetics, so we conducted EIS of the sandwich-like V2O5/ graphene mesoporous composite (Figure 6). The Nyquist plots were analyzed and fitted using an equivalent circuit model (shown in the inset of Figure 6) via the software package Z-
Figure 6. Nyquist plots of pristine V2O5 and V2O5/graphene mesoporous composite, with the inset showing the equivalent circuit.
view 2.0. The solution resistance, Rs, and the ohmic resistance of the electrode, Rsf, reflect the Li+ diffusion impedance in the electrolyte solution and at the electrode−electrolyte interface, respectively. Rct is the charge-transfer resistance. CPE is a constant phase element. ZW is the Warburg impedance, representing Li+ diffusion within the electrode. The fitted results identify the physicochemical and electrochemical nature of the as-prepared materials. From Table 1, it can be seen that Table 1. Kinetic Parameters of Pristine V2O5 and Sandwichlike V2O5/Graphene Mesoporous Composite sample
Rs (Ω)
Rsf (Ω)
CPE1 (μF)
Rct (Ω)
CPE2 (μF)
pristine V2O5 V2O5/graphene
3.8 2.5
14.1 2.7
6.0 1.0
125.1 64.0
12.3 10.8
the Rs values are quite small and show no obvious difference between the pristine V2O5 and the composite. The Rsf and Rct of the composite, however, are significantly smaller than those of the pristine V2O5, indicating that the graphene sheets and the mesoporous structure can facilitate electron transfer. The sandwich-like V2O5/graphene mesoporous composite not only ensures a large contact interface between the electrolyte and electrode but also shortens the diffusion pathways for electrons and ions, and thus decreases the resistance. The improvement in electrode kinetics is well corroborated by the electrochemical performance described above.
4. CONCLUSIONS In this work, a novel sandwich-like V2O5/graphene mesoporous composite was synthesized via a facile solvothermal approach, and the composite was explored as an anode material for LIBs. The nanostructured V2O5 particles were anchored on graphene sheets and packed in a sandwich structure. The novel structured composite delivers a high reversible capacity of 1006 mAh g−1 at 0.5 A g−1 after 300 cycles. Moreover, the composite still exhibits a very high reversible capacity of 727 mAh g−1 at 0.9 A g−1 after 60 cycles. Remarkably, after 25 cycles of discharge and charge at different current densities, a reversible discharge capacity of 500 mAh g−1 can still be achieved, even at 3.0 A g−1. The outstanding cycling stability, large reversible specific capacity, and ultrahigh rate performance are very much associated with the unique graphene-based sandwich nanoarchitecture combined with the porous framework, which offers a large surface area, superior conductivity, and a short Li+ diffusion distance. F
DOI: 10.1021/acs.inorgchem.5b01914 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
■
(19) Lou, X. W.; Li, C. M.; Archer, L. A. Adv. Mater. 2009, 21, 2536− 2539. (20) Lin, Y.-M.; Abel, P. R.; Heller, A.; Mullins, C. B. J. Phys. Chem. Lett. 2011, 2, 2885−2891. (21) Wang, J.; Yang, N.; Tang, H.; Dong, Z.; Jin, Q.; Yang, M.; Kisailus, D.; Zhao, H.; Tang, Z.; Wang, D. Angew. Chem., Int. Ed. 2013, 52, 6417−6420. (22) Pan, A.; Zhang, J.-G.; Nie, Z.; Cao, G.; Arey, B. W.; Li, G.; Liang, S.-q.; Liu, J. J. Mater. Chem. 2010, 20, 9193−9199. (23) Chen, X.; Zhu, H.; Chen, Y. C.; Shang, Y.; Cao, A.; Hu, L.; Rubloff, G. W. ACS Nano 2012, 6, 7948−7955. (24) Hu, Y. S.; Liu, X.; Muller, J. O.; Schlogl, R.; Maier, J.; Su, D. S. Angew. Chem., Int. Ed. 2009, 48, 210−214. (25) Yu, D. M.; Chen, C. G.; Xie, S. H.; Liu, Y. Y.; Park, K.; Zhou, X. Y.; Zhang, Q. F.; Li, J. Y.; Cao, G. Z. Energy Environ. Sci. 2011, 4, 858− 861. (26) Perera, S. D.; Patel, B.; Nijem, N.; Roodenko, K.; Seitz, O.; Ferraris, J. P.; Chabal, Y. J.; Balkus, K. J. Adv. Energy Mater. 2011, 1, 936−945. (27) Borghols, W. J.; Wagemaker, M.; Lafont, U.; Kelder, E. M.; Mulder, F. M. J. Am. Chem. Soc. 2009, 131, 17786−17792. (28) Song, H.-K.; Lee, K. T.; Kim, M. G.; Nazar, L. F.; Cho, J. Adv. Funct. Mater. 2010, 20, 3818−3834. (29) Amine, K.; Belharouak, I.; Chen, Z.; Tran, T.; Yumoto, H.; Ota, N.; Myung, S. T.; Sun, Y. K. Adv. Mater. 2010, 22, 3052−3057. (30) Kang, E.; Jung, Y. S.; Kim, G. H.; Chun, J.; Wiesner, U.; Dillon, A. C.; Kim, J. K.; Lee, J. Adv. Funct. Mater. 2011, 21, 4349−4357. (31) Ren, Y.; Armstrong, A. R.; Jiao, F.; Bruce, P. G. J. Am. Chem. Soc. 2010, 132, 996−1004. (32) Zhang, X. F.; Wang, K. X.; Wei, X.; Chen, J. S. Chem. Mater. 2011, 23, 5290−5292. (33) Jia, X. L.; Chen, Z.; Suwarnasarn, A.; Rice, L.; Wang, X. L.; Sohn, H.; Zhang, Q.; Wu, B. M.; Wei, F.; Lu, Y. F. Energy Environ. Sci. 2012, 5, 6845−6849. (34) Su, Y.; Li, S.; Wu, D.; Zhang, F.; Liang, H.; Gao, P.; Cheng, C.; Feng, X. ACS Nano 2012, 6, 8349−8356. (35) Huang, X.; Zeng, Z.; Fan, Z.; Liu, J.; Zhang, H. Adv. Mater. 2012, 24, 5979−6004. (36) Chen, D.; Feng, H.; Li, J. Chem. Rev. 2012, 112, 6027−6053. (37) Ai, W.; Jiang, J.; Zhu, J. H.; Fan, Z. X.; Wang, Y. L.; Zhang, H.; Huang, W.; Yu, T. Adv. Energy Mater. 2015, 5, 1−8. (38) Ai, W.; Luo, Z.; Jiang, J.; Zhu, J.; Du, Z.; Fan, Z.; Xie, L.; Zhang, H.; Huang, W.; Yu, T. Adv. Mater. 2014, 26, 6186−6192. (39) Wang, H.; Cui, L. F.; Yang, Y.; Sanchez Casalongue, H.; Robinson, J. T.; Liang, Y.; Cui, Y.; Dai, H. J. Am. Chem. Soc. 2010, 132, 13978−13980. (40) Wu, Z.-S.; Ren, W.; Wen, L.; Gao, L.; Zhao, J.; Chen, Z.; Zhou, G.; Li, F.; Cheng, H.-M. ACS Nano 2010, 4, 3187−3194. (41) Wei, W.; Yang, S.; Zhou, H.; Lieberwirth, I.; Feng, X.; Mullen, K. Adv. Mater. 2013, 25, 2909−2914. (42) Ai, W.; Zhu, J. H.; Jiang, J.; Chao, D. L.; Wang, Y. L.; Ng, C. F.; Wang, X. L.; Wu, C.; Li, C. M.; Shen, Z. X.; Huang, W.; Yu, T. 2D Mater. 2015, 2, 1−9. (43) Zhou, W.; Tay, Y. Y.; Jia, X.; Yau Wai, D. Y.; Jiang, J.; Hoon, H. H.; Yu, T. Nanoscale 2012, 4, 4459−4463. (44) Lee, J. W.; Lim, S. Y.; Jeong, H. M.; Hwang, T. H.; Kang, J. K.; Choi, J. W. Energy Environ. Sci. 2012, 5, 9889−9894. (45) Rui, X. H.; Zhu, J. X.; Sim, D.; Xu, C.; Zeng, Y.; Hng, H. H.; Lim, T. M.; Yan, Q. Y. Nanoscale 2011, 3, 4752−4758. (46) Chen, D.; Yi, R.; Chen, S. R.; Xu, T.; Gordin, M. L.; Lv, D. P.; Wang, D. H. Mater. Sci. Eng., B 2014, 185, 7−12. (47) Sun, X.; Zhou, C.; Xie, M.; Hu, T.; Sun, H.; Xin, G.; Wang, G.; George, S. M.; Lian, J. Chem. Commun. 2014, 50, 10703−10706. (48) Ai, W.; Xie, L.; Du, Z.; Zeng, Z.; Liu, J.; Zhang, H.; Huang, Y.; Huang, W.; Yu, T. Sci. Rep. 2013, DOI: 10.1038/srep02341. (49) Chae, O. B.; Kim, J.; Park, I.; Jeong, H.; Ku, J. H.; Ryu, J. H.; Kang, K.; Oh, S. M. Chem. Mater. 2014, 26, 5874−5881. (50) Augustyn, V.; Dunn, B. Electrochim. Acta 2013, 88, 530−535.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01914. Crystallographic details, TG analysis, Raman spectroscopy, HRTEM mapping images, SEM images, and cycling performance (PDF)
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. *E-mail: [email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of Xinjiang (2015211C286 and 2013211A004), the Program for New Century Excellent Talents in University (NCET-12-1076), the National Natural Science Foundation of China (21161021, 21466036, 21271151, and 21361024), and the Young Scholar Science Foundation of Xinjiang Educational Institutions (XJEDU2014S013). The authors also would like to thank Prof. Hua Zhang at Nanyang Technological University for his strong support and stimulating discussions.
■
REFERENCES
(1) Tang, Y.; Zhang, Y.; Li, W.; Ma, B.; Chen, X. Chem. Soc. Rev. 2015, 44, 5926−5940. (2) Zhang, G.; Lou, X. W. Angew. Chem., Int. Ed. 2014, 53, 9041− 9044. (3) Liu, J.; Liu, X. W. Adv. Mater. 2012, 24, 4097−4111. (4) Choi, N. S.; Chen, Z.; Freunberger, S. A.; Ji, X.; Sun, Y. K.; Amine, K.; Yushin, G.; Nazar, L. F.; Cho, J.; Bruce, P. G. Angew. Chem., Int. Ed. 2012, 51, 9994−10024. (5) Kaskhedikar, N. A.; Maier, J. Adv. Mater. 2009, 21, 2664−2680. (6) Zeng, L.; Zheng, C.; Deng, C.; Ding, X.; Wei, M. ACS Appl. Mater. Interfaces 2013, 5, 2182−2187. (7) Yuan, C.; Wu, H. B.; Xie, Y.; Lou, X. W. Angew. Chem., Int. Ed. 2014, 53, 1488−1504. (8) Yang, Y.; Albu, S. P.; Kim, D.; Schmuki, P. Angew. Chem., Int. Ed. 2011, 50, 9071−9075. (9) Pan, A.; Wu, H. B.; Yu, L.; Zhu, T.; Lou, X. W. ACS Appl. Mater. Interfaces 2012, 4, 3874−3879. (10) Liu, J.; Xia, H.; Xue, D. F.; Lu, L. J. Am. Chem. Soc. 2009, 131, 12086−12087. (11) Zhang, C. F.; Chen, Z. X.; Guo, Z. P.; Lou, X. W. Energy Environ. Sci. 2013, 6, 974−978. (12) Rui, X.; Lu, Z.; Yu, H.; Yang, D.; Hng, H. H.; Lim, T. M.; Yan, Q. Nanoscale 2013, 5, 556−560. (13) Zhou, B.; Shi, H.; Cao, R.; Zhang, X.; Jiang, Z. Phys. Chem. Chem. Phys. 2014, 16, 18578−18585. (14) Chao, D.; Xia, X.; Liu, J.; Fan, Z.; Ng, C. F.; Lin, J.; Zhang, H.; Shen, Z. X.; Fan, H. J. Adv. Mater. 2014, 26, 5794−5800. (15) Xu, Y.; Dunwell, M.; Fei, L.; Fu, E.; Lin, Q.; Patterson, B.; Yuan, B.; Deng, S.; Andersen, P.; Luo, H.; Zou, G. ACS Appl. Mater. Interfaces 2014, 6, 20408−20413. (16) Tang, Y. X.; Rui, X. H.; Zhang, Y. Y.; Lim, T. M.; Dong, Z. L.; Hng, H. H.; Chen, X. D.; Yan, Q. Y.; Chen, Z. J. Mater. Chem. A 2013, 1, 82−88. (17) An, Q. Y.; Zhang, P. F.; Wei, Q. L.; He, L.; Xiong, F. Y.; Sheng, J. Z.; Wang, Q. Q.; Mai, L. Q. J. Mater. Chem. A 2014, 2, 3297−3302. (18) Reddy, M. V.; Subba Rao, G. V.; Chowdari, B. V. Chem. Rev. 2013, 113, 5364−5457. G
DOI: 10.1021/acs.inorgchem.5b01914 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry (51) Pan, A. Q.; Wu, H. B.; Zhang, L.; Lou, X. W. Energy Environ. Sci. 2013, 6, 1476−1479. (52) Sun, Y. M.; Hu, X. L.; Luo, W.; Huang, Y. H. J. Phys. Chem. C 2012, 116, 20794−20799. (53) Cheng, F. Y.; Chen, J. J. Mater. Chem. 2011, 21, 9841−9848. (54) Liu, Y.; Clark, M.; Zhang, Q.; Yu, D.; Liu, D.; Liu, J.; Cao, G. Adv. Energy Mater. 2011, 1, 194−202. (55) Pan, A.; Wu, H. B.; Yu, L.; Lou, X. D. Angew. Chem., Int. Ed. 2013, 125, 1−6. (56) Lee, M.; Balasingam, S. K.; Jeong, H. Y.; Hong, W. G.; Lee, H. B.; Kim, B. H.; Jun, Y. Sci. Rep. 2015, 5, 8151. (57) Huang, Y.; Dong, Z.; Jia, D.; Guo, Z.; Cho, W. I. Electrochim. Acta 2011, 56, 9233−9239. (58) Huang, Y. D.; Dong, Z. F.; Jia, D. Z.; Guo, Z. P.; Cho, W. I. Solid State Ionics 2011, 201, 54−59. (59) Zhou, G. M.; Wang, D. W.; Li, F.; Zhang, L. L.; Li, N.; Wu, Z. S.; Wen, L.; Lu, G. Q.; Cheng, H. M. Chem. Mater. 2010, 22, 5306− 5313.
H
DOI: 10.1021/acs.inorgchem.5b01914 Inorg. Chem. XXXX, XXX, XXX−XXX
