Letter pubs.acs.org/NanoLett
An Electrochemical Capacitor with Applicable Energy Density of 7.4 Wh/kg at Average Power Density of 3000 W/kg Teng Zhai,†,‡ Xihong Lu,† Hanyu Wang,‡ Gongming Wang,‡ Tyler Mathis,‡ Tianyu Liu,‡ Cheng Li,† Yexiang Tong,*,† and Yat Li*,‡ †
KLGHEI of Environment and Energy Chemistry, MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, Instrumental Analysis and Research Centre, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, P.R. China ‡ Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, United States S Supporting Information *
ABSTRACT: Electrochemical capacitors represent a new class of charge storage devices that can simultaneously achieve high energy density and high power density. Previous reports have been primarily focused on the development of high performance capacitor electrodes. Although these electrodes have achieved excellent specific capacitance based on per unit mass of active materials, the gravimetric energy densities calculated based on the weight of entire capacitor device were fairly small. This is mainly due to the large mass ratio between current collector and active material. We aimed to address this issue by a 2-fold approach of minimizing the mass of current collector and increasing the electrode performance. Here we report an electrochemical capacitor using 3D graphene hollow structure as current collector, vanadium sulfide and manganese oxide as anode and cathode materials, respectively. 3D graphene hollow structure provides a lightweight and highly conductive scaffold for deposition of pseudocapacitive materials. The device achieves an excellent active material ratio of 24%. Significantly, it delivers a remarkable energy density of 7.4 Wh/kg (based on the weight of entire device) at the average power density of 3000 W/kg. This is the highest gravimetric energy density reported for asymmetric electrochemical capacitors at such a high power density. KEYWORDS: 3D hollow structure, graphene, vanadium sulfide, electrochemical capacitor, gravimetric energy density
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s.6 The specific capacitances of many pseudocapacitive electrodes are apparently approaching their theoretical values.3,6 Yet, the solid-state AECs assembled with these electrodes still suffer from poor gravimetric energy density (based on the weight of entire device). This is mainly due to two reasons. First, pseudocapacitive materials have been employed as electrode to increase the device energy density; however, most of them are not very electrically conductive. Therefore, a number of conductive materials such as carbon cloth (∼12 mg/ cm2), nickel foam (∼31−42 mg/cm2), and poly(ethylene terephthalate) (PET, ∼18 mg/cm2) have been added as current collector.9,10 While the addition of current collector improves
ncreasing power and energy demand for next-generation portable and flexible electronics has stimulated intensive efforts to explore new charge storage devices. Solid-state asymmetric electrochemical capacitors (AECs) are an attractive possibility due to their potentials for delivering high energy density at high power density.1−3 By taking the advantage of different potential windows of the anode and cathode, AECs can increase the device operating voltage in aqueous electrolytes and, thus, the energy density.4 Previous studies have been primarily focused on the development of high performance pseudocapacitive electrodes.5−8 For example, sulfur-doped, oxygen-deficient V6O13−x has reached a specific capacitance of 1353 F/g at a current density of 1.9 A/g for an AEC anode.5 Nanostructured MnO2-carbon nanotube (CNT)-sponge hybrid electrode has demonstrated to be excellent cathode material with a specific capacitance of 1230 F/g at a scan rate of 1 mV/ © XXXX American Chemical Society
Received: January 26, 2015 Revised: March 20, 2015
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DOI: 10.1021/acs.nanolett.5b00321 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 1. Structural and electrochemical characterization of 3DGH substrate. (a) Digital micrograph of a 3 cm × 5 cm 3DGH substrate with a mass density of 0.82 mg/cm2. (b) SEM image of a 3DGH structure (c) Magnified SEM image of the 3DGH structure highlighted by the dashed box in (b) shows the hollow structure. (d) CV curves of 3DGH collected in 5 M LiCl solution at various scan rates from 1 to 20 V/s. (e) Nyquist plots of the 3DGH and 3DG/Ni foam electrodes collected at frequencies in a range from 0.1 to 100000 Hz. (f) Bode phase plots collected for the 3DGH and 3DG/Ni foam electrodes. The dashed line highlights the characteristic frequency f 0 (1/τ) at the phase angle of −45°.
scaffold is constructed by hollow graphene skeleton with an average diameter of 100 μm and thickness of ∼2−10 nm (Figure 1b and c). Powder X-ray diffraction (XRD) studies confirmed the complete removal of Ni foam (Figure S1, Supporting Information). The 3DGH substrates were annealed in a reducing atmosphere (hydrogen and nitrogen gas mixture) at 900 °C to increase its crystallinity and electrical conductivity (Figure S2, Supporting Information). The Brunauer−Emmett− Teller surface area of 3DGH was determined to be 219 m2/g, which is higher than the reported values for restacked graphene electrode (44 m2/g)16 and graphene paper electrode (25 m2/ g).17 To test the performance of 3DGH as current collector, we measured its electrochemical properties in a three-electrode cell in 5 M LiCl electrolyte, using a Pt plate as a counter-electrode and a SCE as a reference electrode. Cyclic voltammetry (CV) curves collected for 3DGH electrode showed characteristic quasi-rectangular shapes for double-layer capacitor at all scan rates between 1 and 20 V/s (Figure 1d). Significantly, 3DGH electrode achieved an outstanding capacitance retention rate of 31% when the scan rate increased from 10 to 20000 mV/s (Figure S4, Supporting Information), which is comparable to the best values reported for 3D graphene structures.13,14,18 This retention rate is even comparable to the value reported for carbon onion that prepared at 1800 °C.19 The excellent capacitive behavior at ultrahigh scan rate and the remarkable rate capability indicate that 3DGH electrode has outstanding electrical conductivity even without the support of Ni foam. Furthermore, we collected and compared the electrochemical impedance spectroscopy (EIS) data of 3DGH and 3DG/Ni foam electrodes (Figure 1e). As shown in the Nyquist plots, both electrodes display a steep curve at low frequency region, suggesting they have excellent capacitive behavior and fast ion transport. In the high-frequency region, a transition from
the device rate capability, it substantially increases the total weight of AEC device. Second, in most cases the mass loadings of active pseudocapacitive materials are fairly low (0.1−2 mg/ cm2) compared to the current collectors.11,12 As a result, the gravimetric energy densities of these devices were severely limited by the small mass ratio between active and nonactive materials. The practicality of these devices is therefore also questionable. Here we demonstrate a 2-fold approach to address this limitation by minimizing the weight of current collector and increasing the specific capacitance of electrode with high mass loading. A good current collector should be a lightweight material with extremely large surface area, excellent electrical conductivity, and chemical and mechanical stability. Graphene satisfies all of these requirements; however, the stacking of graphene sheets due to strong π−π interaction can reduce its effective surface area. To increase the surface area and prevent graphene sheets from aggregation, we managed to use threedimensional graphene hollow (3DGH) structure as a current collector. The 3DGH structure can be obtained by depositing graphene sheets onto a template such as nickel foam, followed by dissolving the template.13,14 The challenge was to retain the 3DGH structure during the fabrication processes. Here we demonstrate a new fabrication method of 3DGH structure without the need of adding poly(methyl methacrylate) protective layer to retain the scaffold. 3DGH was obtained by depositing graphene sheets onto nickel foams (denoted as 3DGH/Ni) via a hydrothermal process.15 We found that the 3D structure was reserved in the solution after dissolving nickel foam in HCl, and the collapse of the 3D structure can be avoided by vacuum-dry the sample. The 3DGH structure achieved a fairly small mass density of 0.82 mg/cm2 and still exhibits certain degree of flexibility (Figure 1a). Scanning electron microscopy (SEM) studies revealed that the 3D B
DOI: 10.1021/acs.nanolett.5b00321 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 2. Structural and electrochemical properties of V3S4/3DGH electrode. (a) SEM image collected at the edge of V3S4/3DGH electrode showing both sides of 3DGH substrate are covered with V3S4 nanomaterials. Dashed lines highlight the thickness of 3DGH substrate. (b) XRD spectra of V3S4/3DGH and 3DGH electrodes. Dashed lines highlight the diffraction peaks monoclinic V3S4 (JCPDS: 73-2026). Solid lines indicate the diffraction peaks due to Al substrate holder. (c) CV curves of V3S4/3DGH and 3DGH electrodes collected at the scan rate of 100 mV/s. (d) Specific capacitance of V3S4/3DGH electrode calculated based on the mass of entire electrode as a function of scan rate. The electrode has a mass density of 3.6 mg/cm2. (e) Cycling performance of V3S4/3DGH collected at 100 mV/s for 5000 cycles.
studies have unambiguously proved that the 3DGH is an extremely promising current collector for pseudocapacitors. With the success of making ultralight and high performance 3DGH current collector, the next step was to incorporate different pseudocapacitive materials with 3DGH to increase the specific capacitance of both positive and negative electrodes and, thus, the energy density of AECs. In the past few years, tremendous efforts have been devoted to develop high performance positive (cathode) electrode materials such as MnO2, Ni(OH)2 and CoO.22,23 We decided to use MnO2 decorated 3DGH (denoted as MnO2/3DGH) as positive (cathode) material, because MnO2 is an abundant, low-cost and environmentally benign material with outstanding capacitive performance.6 MnO2 was uniformly deposited onto 3DGH using anodic electrodeposition method (Figure S6−8, Supporting Information).24 The 3D skeleton was well-reserved during the process (Figure S9, Supporting Information). Significantly, the MnO2/3DGH electrode with a mass loading of 1.5 mg/cm2 of MnO2 achieved an outstanding specific capacitance of 345 F/g (at 10 mV/s) and an excellent capacitance retention of 57% when the scan rate increase from 10 to 200 mV/s (Figure
vertical line to the 45° Warburg region followed by a semicircle was observed (Figure 1e, inset). Notably, the charge-transfer resistance (Rct) of 3DGH electrode was determined to be 1.0 Ω, which is just slightly larger than the Rct of 3DG/Ni foam electrode (0.3Ω). It shows that the removal of Ni foam has a minor effect on the electrode resistance. Bode phase plots were also collected for 3DGH and 3DG/Ni foam electrodes (Figure 1f). The characteristic frequency f 0 (1/τ) at a phase angle of −45° marks the point where the resistive and capacitive impedances are equal.20 3DGH and 3DG/Ni foam deliver a f 0 of 2.2 and 1.9 Hz and corresponding time constant τ (1/f 0) of 0.46 and 0.53 s, respectively, which are substantially smaller than the values reported for activated carbon-based electrochemical capacitors (10 s).21 The fast frequency response again confirms that the ion transport capability of 3DGH is outstanding. Moreover, 3DGH electrode also exhibits remarkable electrochemical stability, as expected for carbon based materials. It achieved a capacitance retention rate of 98.3% after 20000 cycles (Figure S5, Supporting Information) at a current density of 1 mA/cm2. Taken together, these electrochemical C
DOI: 10.1021/acs.nanolett.5b00321 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 3. Capacitive performance of quasi-solid-state V3S4/3DGH//MnO2/3DGH device. (a) A schematic illustration of V3S4/3DGH//MnO2/ 3DGH device. (b) Plots of specific capacitance and coulumbic efficiency of the device, obtained at the current density of 6 mA/cm2, as a function of voltage window. (c) Charge/discharge cycling performance of the device collected at 30 mA/cm2 for 5000 cycles. The inset shows the charge/ discharge curves collected at the 1st and the 5000th cycle. (d) A plot of specific capacitance of the device as a function of current density.
electrical conductivity of V3S4.31 We deposited V3S4 onto 3DGH substrate by a hydrothermal method, followed by annealing in hydrogen atmosphere at 450 °C for 5 min (Figure S12, Supporting Information). The electrode is denoted as V3S4/3DGH. SEM image collected at the edge of V3S4/3DGH electrode revealed that both sides of the hollow substrate were uniformly covered by nanosheets (Figure 2a, Figure S12−13, Supporting Information). Powder X-ray diffraction (XRD) studies confirmed that the nanosheets are V3S4 (JCPDS: 732026) (Figure 2b). Furthermore, V 2p core level X-ray photoelectron spectroscopy spectrum shows the coexistence of V2+ and V3+ states in a 1:2 ratio, as expected for V3S4 (Figure S14, Supporting Information). Notably, the mass loading of V3S4 was about 2.8 mg/cm2, which is substantially higher than the loading of 3DGH (0.82 mg/cm2). The mass ratio between active material and current collector in V3S4/3DGH electrode reached an excellent value of 78%. The capacitive performance of V3S4/3DGH electrode was investigated in a three-electrode cell in 5 M LiCl electrolyte, with a Pt counter-electrode and a SCE reference electrode. V3S4/3DGH electrode delivered a substantially larger current density compared to 3DGH electrode at the same scan rate in the same potential window between 0 and −0.8 V vs SCE (Figure 2c). The enhanced capacitance is due to the pseudocapacitance of V3S4 as obvious redox peaks are identified
S10, Supporting Information). They are comparable to the best values reported for MnO2 based electrode with similar mass loading.25 The charge storage capability of electrochemical capacitor is determined by both cathode and anode, as the amount of charge transfer on both sides should be matched. Therefore, to obtain a high performance anode is equally important. In comparison to cathode materials, negative (anode) materials have been less explored. Carbonaceous materials are most commonly used anode materials, yet they suffer from a relatively low capacitance that seriously limits the overall energy density of AECs. Recently, vanadium based materials have received great attention for serving as pseudocapacitive anode due to the existence of multiple oxidation states (V2+, V3+, V4+, V5+), which allows storage of multiple charges.5,26,27 For example, vanadium oxide anode has shown reversible pseudocapacitive behavior between V3+, V4+, and V5+ states, realizing a high specific capacitance of 1350 F/g.5 Furthermore, it is known that vanadium sulfides have an even higher electrical conductivity and Li ion diffusion rate than vanadium oxides.28,29 Therefore, we hypothesized that vanadium sulfides are promising anode materials for AECs. Among vanadium sulfides, V3S4 has a unique distorted Ni−As type structure (Figure S11, Supporting Information) with ordered metal vacancies in alternate metal layers,30 which enhances the D
DOI: 10.1021/acs.nanolett.5b00321 Nano Lett. XXXX, XXX, XXX−XXX
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values reported for other solid-state AECs (Figure 4). As shown in the Ragone plot, the V3S4/3DGH//MnO2/3DGH device
in the CV curve of V3S4/3DGH electrode. Additionally, the electrode still retains the pseudocapacitive behavior even when the scan rate increased from 10 to 200 mV/s (Figure S15, Supporting Information), suggesting the charge transfer and ion diffusion in the electrode is efficient. The specific capacitance of V3S4/3DGH electrode was calculated based on the total mass of the electrode (3.6 mg/cm2). As shown in Figure 2d, the electrode yields an outstanding specific capacitance of 225 F/g (0.81 F/cm2) at 10 mV/s and retains 38.2% of the specific capacitance when the scan rate increased from 10 to 200 mV/s. The excellent rate capability can be attributed to two reasons. First, 3DGH provides a highly conductive 3D scaffold with large electrolyte accessible surface area that allows efficient charge transfer. Second, V3S4 exhibits excellent pseudocapacitive behavior at a fast charging and discharging process. Furthermore, V3S4/3DGH electrode retains 98.3% of its initial capacitance in a cycling stability test at 100 mV/s for 5000 cycles. This is among the best reported cycling perfromance for pseudocapacitive electrodes.32 The outstanding pseudocapacitive performance and the high ratio of active mateiral make V3S4/3DGH to be an highly applicable anode for AECs. With both anode and cathode in hand, we managed to assemble them for an AEC. Figure 3a illustrates the device architecture. Both V3S4/3DGH anode and MnO2/3DGH cathode are soaked with PVA/LiCl gel electrolyte33 and then assembled together by putting a separator in between. The quasi-solid state device is denoted as V3S4/3DGH//MnO2/ 3DGH. The weight of the entire device, including electrodes, separator, and gel electrolyte, was about 21 mg/cm2. The mass ratio of active electrode materials (3DGH, V3S4 and MnO2) to the device is about 24%. The dimension of the device was 2 cm × 1 cm × 0.15 cm. Specific capacitance, energy density, and power density of the device presented in this work are all calcuated based on the weight of the entire device. We tested the capacitive performance of the device at different voltage windows (Figure S16, Supporting Information). When the voltage window was extended to 1.8 V, the device still retains an excellent Coulombic efficiency of 95.2% (Figure 3b). Significantly, at this voltage window, specific capacitance of the device reached 22.2 F/g at the current density of 6 mA/ cm2. Furthermore, the device retains 96.4% of its initial capacitance after testing at the current density of 30 mA/cm2 for 5000 cycles, confirming the device is stable in the voltage window of 1.8 V (Figure 3c). The scan rate dependent CV curves and charge−discharge curves collected at different densities suggest the device has excellent capacitive behavior and fast charge−discharge properties (Figure S17, Supporting Information). Figure 3d shows the specific capacitance and volumetric capacitance of the device as a function of current densities. The device achieved a remarkable specific capacitance of 24.2 F/g at 4 mA/cm2. To our knowledge, these are the best values reported for solid state electrochemical capacitors (ECs), which are substantially higher than the values reported for graphene hydrogel based ECs (4.4 F/g) 34, CNTs coated paper based ECs (0.86 F/g),35 and graphene films based ECs (6.4 F/ g).36 More importantly, the device shows an exceptional rate capability of 68.8% when the current density increased from 4 to 30 mA/cm2, which is comparable to the devices with metal based current collectors.4,24 To evaluate the performance of V3S4/3DGH//MnO2/ 3DGH device for potential applications, we compare the gravimetric power and energy densities of the device to the
Figure 4. Ragone plot of V3S4/3DGH//MnO2/3DGH device. Gravimetric energy densities and power densities of V3S4/3DGH// MnO2/3DGH device were calculated based on the weight of the entire device (21 mg/cm2). The gravimetric energy densities of V3S4/ 3DGH//MnO2/3DGH obtained at different average power densities are marked with “+” in the Ragone plot. The values of other reported devices are added for comparison.34−36 The shadow area mark the specific P againest E for capacitors, electrochemical capacitors, and Li ion batteries reported by Simon et al. in 2008.10
delivers a remarkable gravimetric energy density of 7.4 Wh/kg at the average power density of 3000 W/kg. To our knowledge, this is the best energy density reported for AECs at such a high average power density. The device current densities obtained at lower power densities are also substantially higher than the values of other solid-state ECs, such as graphene hydrogel based ECs (0.61 Wh/kg, 670 W/kg),34 CNTs coated paper based ECs (0.12 Wh/kg, 460 W/kg),35 and graphene films based ECs (2.9 Wh/kg, 1810 W/kg)36 obtained at the same power density. In summary, we have demonstrated a quasi-solid state AEC with applicable energy density of 7.4 Wh/kg at average power density of 3000 W/kg. All these values were calculated based on the weight of the entire device. The ultralight, highly conductive 3DGH plays a critical role in enhancing the effective surface area and charge transport of electrodes, as well as increasing the mass ratio of active materials to current collector. Moreover, we also demonstrated that V3S4 is an exceptional pseudocapacitive anode material, which serves not only as a good power source (high electrical conductivity) but also a good energy source (high capacitance) for the assembled AEC device. By coupling the V3S4/3DGH anode and MnO2/ 3DGH cathode, the device achieved a benchmark gravimetric energy density at the high average power density of 3000 W/kg. This work represents a critical step of moving pseudocapacitive electrochemical capacitors toward applicable region and advancing the technology for high-performance solid-state asymmetric electrochemical capacitors.
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ASSOCIATED CONTENT
S Supporting Information *
Synthetic and analytical methods, capacitive equations, nitrogen (77 K) adsorption−desorption isotherms, XRD, XPS and Raman spectra, SEM images and TEM images, crystal structure, E
DOI: 10.1021/acs.nanolett.5b00321 Nano Lett. XXXX, XXX, XXX−XXX
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(17) Zhao, X.; Hayner, C. M.; Kung, M. C.; Kung, H. H. ACS Nano 2011, 5 (11), 8739−8749. (18) Huang, H.; Tang, Y.; Xu, L.; Tang, S.; Du, Y. ACS Appl. Mater. Interface 2014, 6 (13), 10248−10257. (19) McDonough, J. K.; Frolov, A. I.; Presser, V.; Niu, J.; Miller, C. H.; Ubieto, T.; Fedorov, M. V.; Gogotsi, Y. Carbon 2012, 50 (9), 3298−3309. (20) Xu, Y.; Lin, Z.; Zhong, X.; Huang, X.; Weiss, N. O.; Huang, Y.; Duan, X. Nat. Commun. 2014, 5, 4554. (21) El-Kady, M. F.; Strong, V.; Dubin, S.; Kaner, R. B. Science 2012, 335 (6074), 1326−1330. (22) Cao, G.; Liu, J. P.; Cheng, C. W.; Li, H. X.; Li, X. L.; Zhou, W. W.; Zhang, H.; Fan, H. J. Energy Environ. Sci. 2011, 4, 4496−4499. (23) Xia, X.; Chao, D.; Fan, Z.; Guan, C.; Cao, X.; Zhang, H.; Fan, H. J. Nano Lett. 2014, 14 (3), 1651−1658. (24) Lu, X.; Yu, M.; Wang, G.; Zhai, T.; Xie, S.; Ling, Y.; Tong, Y.; Li, Y. Adv. Mater. 2013, 25 (2), 267−272. (25) Cheng, Y.; Lu, S.; Zhang, H.; Varanasi, C. V.; Liu, J. Nano Lett. 2012, 12 (8), 4206−4211. (26) Wu, C.; Lu, X.; Peng, L.; Xu, K.; Peng, X.; Huang, J.; Yu, G.; Xie, Y. Nat. Commun. 2013, 4, 2431. (27) Wu, C.; Feng, F.; Xie, Y. Chem. Soc. Rev. 2013, 42 (12), 5157− 5183. (28) Taniguchi, M.; Wakihara, M.; Shirai, Y. Z. Anorg. Allg. Chem. 1980, 461 (1), 234−240. (29) Jing, Y.; Zhou, Z.; Cabrera, C. R.; Chen, Z. J. Phys. Chem. C 2013, 117 (48), 25409−25413. (30) Ishii, M.; Wada, H.; Nozaki, H.; Kawada, I. Solid State Commun. 1982, 42 (8), 605−608. (31) Mizusaki, J.; Yonemura, Y.; Kamata, H.; Ohyama, K.; Mori, N.; Takai, H.; Tagawa, H.; Dokiya, M.; Naraya, K.; Sasamoto, T. Solid State Ionics 2000, 132 (3), 167−180. (32) Yu, G.; Xie, X.; Pan, L.; Bao, Z.; Cui, Y. Nano Energy 2013, 2 (2), 213−234. (33) Wang, G.; Lu, X.; Ling, Y.; Zhai, T.; Wang, H.; Tong, Y.; Li, Y. ACS Nano 2012, 6 (11), 10296−10302. (34) Xu, Y.; Lin, Z.; Huang, X.; Liu, Y.; Huang, Y.; Duan, X. ACS Nano 2013, 7 (5), 4042−4049. (35) Kang, Y. J.; Chung, H.; Han, C. H.; Kim, W. Nanotechnology 2012, 23 (6), 065401. (36) Choi, B. G.; Chang, S. J.; Kang, H. W.; Park, C. P.; Kim, H. J.; Hong, W. H.; Lee, S.; Huh, Y. S. Nanoscale 2012, 4 (16), 4983−4988.
rate capability, cycling stability, CV and charge/discharge curves. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*Y.T. 135 Xingang West Road, Chemical North Building 325, Guangzhou, 510275 P.R. China. Phone: 86-20-84110071. Fax: 86-20-84112245. E-mail: [email protected]. *Y.L. 1156 High Street, Physical Science Building 160, Santa Cruz, California 95064, U.S.A. Phone: 1-831-459-1952. Fax: 1831-459-2935. E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Y.L. is thankful for the support of this work by UCSC faculty startup funds. Y.X.T. acknowledges the financial support of this work by the Natural Science Foundations of China (21273290 and 91323101), the Natural Science Foundations of Guangdong Province (S2013030013474), and the Research Fund for the Doctoral Program of Higher Education of China (20120171110043). T.Z. thanks the China Scholarship Council for financial support. We thank Dr. Tom Yuzvinsky for image acquisition and the W. M. Keck Center for Nanoscale Optofluidics for use of the FEI Quanta 3D Dual-beam microscope. We acknowledge Jesse Hauser for XRD characterization and the support of UCSC XRD (Rigaku Americas Miniflex Plus powder diffractometer) facility supported by the U.S. NSF MRI grant (MRI-1126845).
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REFERENCES
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DOI: 10.1021/acs.nanolett.5b00321 Nano Lett. XXXX, XXX, XXX−XXX
An Electrochemical Capacitor with Applicable Energy Density of 7.4 Wh/kg at Average Power Density of 3000 W/kg Teng Zhai,†,‡ Xihong Lu,† Hanyu Wang,‡ Gongming Wang,‡ Tyler Mathis,‡ Tianyu Liu,‡ Cheng Li,† Yexiang Tong,*,† and Yat Li*,‡ †
KLGHEI of Environment and Energy Chemistry, MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, Instrumental Analysis and Research Centre, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, P.R. China ‡ Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, United States S Supporting Information *
ABSTRACT: Electrochemical capacitors represent a new class of charge storage devices that can simultaneously achieve high energy density and high power density. Previous reports have been primarily focused on the development of high performance capacitor electrodes. Although these electrodes have achieved excellent specific capacitance based on per unit mass of active materials, the gravimetric energy densities calculated based on the weight of entire capacitor device were fairly small. This is mainly due to the large mass ratio between current collector and active material. We aimed to address this issue by a 2-fold approach of minimizing the mass of current collector and increasing the electrode performance. Here we report an electrochemical capacitor using 3D graphene hollow structure as current collector, vanadium sulfide and manganese oxide as anode and cathode materials, respectively. 3D graphene hollow structure provides a lightweight and highly conductive scaffold for deposition of pseudocapacitive materials. The device achieves an excellent active material ratio of 24%. Significantly, it delivers a remarkable energy density of 7.4 Wh/kg (based on the weight of entire device) at the average power density of 3000 W/kg. This is the highest gravimetric energy density reported for asymmetric electrochemical capacitors at such a high power density. KEYWORDS: 3D hollow structure, graphene, vanadium sulfide, electrochemical capacitor, gravimetric energy density
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s.6 The specific capacitances of many pseudocapacitive electrodes are apparently approaching their theoretical values.3,6 Yet, the solid-state AECs assembled with these electrodes still suffer from poor gravimetric energy density (based on the weight of entire device). This is mainly due to two reasons. First, pseudocapacitive materials have been employed as electrode to increase the device energy density; however, most of them are not very electrically conductive. Therefore, a number of conductive materials such as carbon cloth (∼12 mg/ cm2), nickel foam (∼31−42 mg/cm2), and poly(ethylene terephthalate) (PET, ∼18 mg/cm2) have been added as current collector.9,10 While the addition of current collector improves
ncreasing power and energy demand for next-generation portable and flexible electronics has stimulated intensive efforts to explore new charge storage devices. Solid-state asymmetric electrochemical capacitors (AECs) are an attractive possibility due to their potentials for delivering high energy density at high power density.1−3 By taking the advantage of different potential windows of the anode and cathode, AECs can increase the device operating voltage in aqueous electrolytes and, thus, the energy density.4 Previous studies have been primarily focused on the development of high performance pseudocapacitive electrodes.5−8 For example, sulfur-doped, oxygen-deficient V6O13−x has reached a specific capacitance of 1353 F/g at a current density of 1.9 A/g for an AEC anode.5 Nanostructured MnO2-carbon nanotube (CNT)-sponge hybrid electrode has demonstrated to be excellent cathode material with a specific capacitance of 1230 F/g at a scan rate of 1 mV/ © XXXX American Chemical Society
Received: January 26, 2015 Revised: March 20, 2015
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Figure 1. Structural and electrochemical characterization of 3DGH substrate. (a) Digital micrograph of a 3 cm × 5 cm 3DGH substrate with a mass density of 0.82 mg/cm2. (b) SEM image of a 3DGH structure (c) Magnified SEM image of the 3DGH structure highlighted by the dashed box in (b) shows the hollow structure. (d) CV curves of 3DGH collected in 5 M LiCl solution at various scan rates from 1 to 20 V/s. (e) Nyquist plots of the 3DGH and 3DG/Ni foam electrodes collected at frequencies in a range from 0.1 to 100000 Hz. (f) Bode phase plots collected for the 3DGH and 3DG/Ni foam electrodes. The dashed line highlights the characteristic frequency f 0 (1/τ) at the phase angle of −45°.
scaffold is constructed by hollow graphene skeleton with an average diameter of 100 μm and thickness of ∼2−10 nm (Figure 1b and c). Powder X-ray diffraction (XRD) studies confirmed the complete removal of Ni foam (Figure S1, Supporting Information). The 3DGH substrates were annealed in a reducing atmosphere (hydrogen and nitrogen gas mixture) at 900 °C to increase its crystallinity and electrical conductivity (Figure S2, Supporting Information). The Brunauer−Emmett− Teller surface area of 3DGH was determined to be 219 m2/g, which is higher than the reported values for restacked graphene electrode (44 m2/g)16 and graphene paper electrode (25 m2/ g).17 To test the performance of 3DGH as current collector, we measured its electrochemical properties in a three-electrode cell in 5 M LiCl electrolyte, using a Pt plate as a counter-electrode and a SCE as a reference electrode. Cyclic voltammetry (CV) curves collected for 3DGH electrode showed characteristic quasi-rectangular shapes for double-layer capacitor at all scan rates between 1 and 20 V/s (Figure 1d). Significantly, 3DGH electrode achieved an outstanding capacitance retention rate of 31% when the scan rate increased from 10 to 20000 mV/s (Figure S4, Supporting Information), which is comparable to the best values reported for 3D graphene structures.13,14,18 This retention rate is even comparable to the value reported for carbon onion that prepared at 1800 °C.19 The excellent capacitive behavior at ultrahigh scan rate and the remarkable rate capability indicate that 3DGH electrode has outstanding electrical conductivity even without the support of Ni foam. Furthermore, we collected and compared the electrochemical impedance spectroscopy (EIS) data of 3DGH and 3DG/Ni foam electrodes (Figure 1e). As shown in the Nyquist plots, both electrodes display a steep curve at low frequency region, suggesting they have excellent capacitive behavior and fast ion transport. In the high-frequency region, a transition from
the device rate capability, it substantially increases the total weight of AEC device. Second, in most cases the mass loadings of active pseudocapacitive materials are fairly low (0.1−2 mg/ cm2) compared to the current collectors.11,12 As a result, the gravimetric energy densities of these devices were severely limited by the small mass ratio between active and nonactive materials. The practicality of these devices is therefore also questionable. Here we demonstrate a 2-fold approach to address this limitation by minimizing the weight of current collector and increasing the specific capacitance of electrode with high mass loading. A good current collector should be a lightweight material with extremely large surface area, excellent electrical conductivity, and chemical and mechanical stability. Graphene satisfies all of these requirements; however, the stacking of graphene sheets due to strong π−π interaction can reduce its effective surface area. To increase the surface area and prevent graphene sheets from aggregation, we managed to use threedimensional graphene hollow (3DGH) structure as a current collector. The 3DGH structure can be obtained by depositing graphene sheets onto a template such as nickel foam, followed by dissolving the template.13,14 The challenge was to retain the 3DGH structure during the fabrication processes. Here we demonstrate a new fabrication method of 3DGH structure without the need of adding poly(methyl methacrylate) protective layer to retain the scaffold. 3DGH was obtained by depositing graphene sheets onto nickel foams (denoted as 3DGH/Ni) via a hydrothermal process.15 We found that the 3D structure was reserved in the solution after dissolving nickel foam in HCl, and the collapse of the 3D structure can be avoided by vacuum-dry the sample. The 3DGH structure achieved a fairly small mass density of 0.82 mg/cm2 and still exhibits certain degree of flexibility (Figure 1a). Scanning electron microscopy (SEM) studies revealed that the 3D B
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Figure 2. Structural and electrochemical properties of V3S4/3DGH electrode. (a) SEM image collected at the edge of V3S4/3DGH electrode showing both sides of 3DGH substrate are covered with V3S4 nanomaterials. Dashed lines highlight the thickness of 3DGH substrate. (b) XRD spectra of V3S4/3DGH and 3DGH electrodes. Dashed lines highlight the diffraction peaks monoclinic V3S4 (JCPDS: 73-2026). Solid lines indicate the diffraction peaks due to Al substrate holder. (c) CV curves of V3S4/3DGH and 3DGH electrodes collected at the scan rate of 100 mV/s. (d) Specific capacitance of V3S4/3DGH electrode calculated based on the mass of entire electrode as a function of scan rate. The electrode has a mass density of 3.6 mg/cm2. (e) Cycling performance of V3S4/3DGH collected at 100 mV/s for 5000 cycles.
studies have unambiguously proved that the 3DGH is an extremely promising current collector for pseudocapacitors. With the success of making ultralight and high performance 3DGH current collector, the next step was to incorporate different pseudocapacitive materials with 3DGH to increase the specific capacitance of both positive and negative electrodes and, thus, the energy density of AECs. In the past few years, tremendous efforts have been devoted to develop high performance positive (cathode) electrode materials such as MnO2, Ni(OH)2 and CoO.22,23 We decided to use MnO2 decorated 3DGH (denoted as MnO2/3DGH) as positive (cathode) material, because MnO2 is an abundant, low-cost and environmentally benign material with outstanding capacitive performance.6 MnO2 was uniformly deposited onto 3DGH using anodic electrodeposition method (Figure S6−8, Supporting Information).24 The 3D skeleton was well-reserved during the process (Figure S9, Supporting Information). Significantly, the MnO2/3DGH electrode with a mass loading of 1.5 mg/cm2 of MnO2 achieved an outstanding specific capacitance of 345 F/g (at 10 mV/s) and an excellent capacitance retention of 57% when the scan rate increase from 10 to 200 mV/s (Figure
vertical line to the 45° Warburg region followed by a semicircle was observed (Figure 1e, inset). Notably, the charge-transfer resistance (Rct) of 3DGH electrode was determined to be 1.0 Ω, which is just slightly larger than the Rct of 3DG/Ni foam electrode (0.3Ω). It shows that the removal of Ni foam has a minor effect on the electrode resistance. Bode phase plots were also collected for 3DGH and 3DG/Ni foam electrodes (Figure 1f). The characteristic frequency f 0 (1/τ) at a phase angle of −45° marks the point where the resistive and capacitive impedances are equal.20 3DGH and 3DG/Ni foam deliver a f 0 of 2.2 and 1.9 Hz and corresponding time constant τ (1/f 0) of 0.46 and 0.53 s, respectively, which are substantially smaller than the values reported for activated carbon-based electrochemical capacitors (10 s).21 The fast frequency response again confirms that the ion transport capability of 3DGH is outstanding. Moreover, 3DGH electrode also exhibits remarkable electrochemical stability, as expected for carbon based materials. It achieved a capacitance retention rate of 98.3% after 20000 cycles (Figure S5, Supporting Information) at a current density of 1 mA/cm2. Taken together, these electrochemical C
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Figure 3. Capacitive performance of quasi-solid-state V3S4/3DGH//MnO2/3DGH device. (a) A schematic illustration of V3S4/3DGH//MnO2/ 3DGH device. (b) Plots of specific capacitance and coulumbic efficiency of the device, obtained at the current density of 6 mA/cm2, as a function of voltage window. (c) Charge/discharge cycling performance of the device collected at 30 mA/cm2 for 5000 cycles. The inset shows the charge/ discharge curves collected at the 1st and the 5000th cycle. (d) A plot of specific capacitance of the device as a function of current density.
electrical conductivity of V3S4.31 We deposited V3S4 onto 3DGH substrate by a hydrothermal method, followed by annealing in hydrogen atmosphere at 450 °C for 5 min (Figure S12, Supporting Information). The electrode is denoted as V3S4/3DGH. SEM image collected at the edge of V3S4/3DGH electrode revealed that both sides of the hollow substrate were uniformly covered by nanosheets (Figure 2a, Figure S12−13, Supporting Information). Powder X-ray diffraction (XRD) studies confirmed that the nanosheets are V3S4 (JCPDS: 732026) (Figure 2b). Furthermore, V 2p core level X-ray photoelectron spectroscopy spectrum shows the coexistence of V2+ and V3+ states in a 1:2 ratio, as expected for V3S4 (Figure S14, Supporting Information). Notably, the mass loading of V3S4 was about 2.8 mg/cm2, which is substantially higher than the loading of 3DGH (0.82 mg/cm2). The mass ratio between active material and current collector in V3S4/3DGH electrode reached an excellent value of 78%. The capacitive performance of V3S4/3DGH electrode was investigated in a three-electrode cell in 5 M LiCl electrolyte, with a Pt counter-electrode and a SCE reference electrode. V3S4/3DGH electrode delivered a substantially larger current density compared to 3DGH electrode at the same scan rate in the same potential window between 0 and −0.8 V vs SCE (Figure 2c). The enhanced capacitance is due to the pseudocapacitance of V3S4 as obvious redox peaks are identified
S10, Supporting Information). They are comparable to the best values reported for MnO2 based electrode with similar mass loading.25 The charge storage capability of electrochemical capacitor is determined by both cathode and anode, as the amount of charge transfer on both sides should be matched. Therefore, to obtain a high performance anode is equally important. In comparison to cathode materials, negative (anode) materials have been less explored. Carbonaceous materials are most commonly used anode materials, yet they suffer from a relatively low capacitance that seriously limits the overall energy density of AECs. Recently, vanadium based materials have received great attention for serving as pseudocapacitive anode due to the existence of multiple oxidation states (V2+, V3+, V4+, V5+), which allows storage of multiple charges.5,26,27 For example, vanadium oxide anode has shown reversible pseudocapacitive behavior between V3+, V4+, and V5+ states, realizing a high specific capacitance of 1350 F/g.5 Furthermore, it is known that vanadium sulfides have an even higher electrical conductivity and Li ion diffusion rate than vanadium oxides.28,29 Therefore, we hypothesized that vanadium sulfides are promising anode materials for AECs. Among vanadium sulfides, V3S4 has a unique distorted Ni−As type structure (Figure S11, Supporting Information) with ordered metal vacancies in alternate metal layers,30 which enhances the D
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values reported for other solid-state AECs (Figure 4). As shown in the Ragone plot, the V3S4/3DGH//MnO2/3DGH device
in the CV curve of V3S4/3DGH electrode. Additionally, the electrode still retains the pseudocapacitive behavior even when the scan rate increased from 10 to 200 mV/s (Figure S15, Supporting Information), suggesting the charge transfer and ion diffusion in the electrode is efficient. The specific capacitance of V3S4/3DGH electrode was calculated based on the total mass of the electrode (3.6 mg/cm2). As shown in Figure 2d, the electrode yields an outstanding specific capacitance of 225 F/g (0.81 F/cm2) at 10 mV/s and retains 38.2% of the specific capacitance when the scan rate increased from 10 to 200 mV/s. The excellent rate capability can be attributed to two reasons. First, 3DGH provides a highly conductive 3D scaffold with large electrolyte accessible surface area that allows efficient charge transfer. Second, V3S4 exhibits excellent pseudocapacitive behavior at a fast charging and discharging process. Furthermore, V3S4/3DGH electrode retains 98.3% of its initial capacitance in a cycling stability test at 100 mV/s for 5000 cycles. This is among the best reported cycling perfromance for pseudocapacitive electrodes.32 The outstanding pseudocapacitive performance and the high ratio of active mateiral make V3S4/3DGH to be an highly applicable anode for AECs. With both anode and cathode in hand, we managed to assemble them for an AEC. Figure 3a illustrates the device architecture. Both V3S4/3DGH anode and MnO2/3DGH cathode are soaked with PVA/LiCl gel electrolyte33 and then assembled together by putting a separator in between. The quasi-solid state device is denoted as V3S4/3DGH//MnO2/ 3DGH. The weight of the entire device, including electrodes, separator, and gel electrolyte, was about 21 mg/cm2. The mass ratio of active electrode materials (3DGH, V3S4 and MnO2) to the device is about 24%. The dimension of the device was 2 cm × 1 cm × 0.15 cm. Specific capacitance, energy density, and power density of the device presented in this work are all calcuated based on the weight of the entire device. We tested the capacitive performance of the device at different voltage windows (Figure S16, Supporting Information). When the voltage window was extended to 1.8 V, the device still retains an excellent Coulombic efficiency of 95.2% (Figure 3b). Significantly, at this voltage window, specific capacitance of the device reached 22.2 F/g at the current density of 6 mA/ cm2. Furthermore, the device retains 96.4% of its initial capacitance after testing at the current density of 30 mA/cm2 for 5000 cycles, confirming the device is stable in the voltage window of 1.8 V (Figure 3c). The scan rate dependent CV curves and charge−discharge curves collected at different densities suggest the device has excellent capacitive behavior and fast charge−discharge properties (Figure S17, Supporting Information). Figure 3d shows the specific capacitance and volumetric capacitance of the device as a function of current densities. The device achieved a remarkable specific capacitance of 24.2 F/g at 4 mA/cm2. To our knowledge, these are the best values reported for solid state electrochemical capacitors (ECs), which are substantially higher than the values reported for graphene hydrogel based ECs (4.4 F/g) 34, CNTs coated paper based ECs (0.86 F/g),35 and graphene films based ECs (6.4 F/ g).36 More importantly, the device shows an exceptional rate capability of 68.8% when the current density increased from 4 to 30 mA/cm2, which is comparable to the devices with metal based current collectors.4,24 To evaluate the performance of V3S4/3DGH//MnO2/ 3DGH device for potential applications, we compare the gravimetric power and energy densities of the device to the
Figure 4. Ragone plot of V3S4/3DGH//MnO2/3DGH device. Gravimetric energy densities and power densities of V3S4/3DGH// MnO2/3DGH device were calculated based on the weight of the entire device (21 mg/cm2). The gravimetric energy densities of V3S4/ 3DGH//MnO2/3DGH obtained at different average power densities are marked with “+” in the Ragone plot. The values of other reported devices are added for comparison.34−36 The shadow area mark the specific P againest E for capacitors, electrochemical capacitors, and Li ion batteries reported by Simon et al. in 2008.10
delivers a remarkable gravimetric energy density of 7.4 Wh/kg at the average power density of 3000 W/kg. To our knowledge, this is the best energy density reported for AECs at such a high average power density. The device current densities obtained at lower power densities are also substantially higher than the values of other solid-state ECs, such as graphene hydrogel based ECs (0.61 Wh/kg, 670 W/kg),34 CNTs coated paper based ECs (0.12 Wh/kg, 460 W/kg),35 and graphene films based ECs (2.9 Wh/kg, 1810 W/kg)36 obtained at the same power density. In summary, we have demonstrated a quasi-solid state AEC with applicable energy density of 7.4 Wh/kg at average power density of 3000 W/kg. All these values were calculated based on the weight of the entire device. The ultralight, highly conductive 3DGH plays a critical role in enhancing the effective surface area and charge transport of electrodes, as well as increasing the mass ratio of active materials to current collector. Moreover, we also demonstrated that V3S4 is an exceptional pseudocapacitive anode material, which serves not only as a good power source (high electrical conductivity) but also a good energy source (high capacitance) for the assembled AEC device. By coupling the V3S4/3DGH anode and MnO2/ 3DGH cathode, the device achieved a benchmark gravimetric energy density at the high average power density of 3000 W/kg. This work represents a critical step of moving pseudocapacitive electrochemical capacitors toward applicable region and advancing the technology for high-performance solid-state asymmetric electrochemical capacitors.
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ASSOCIATED CONTENT
S Supporting Information *
Synthetic and analytical methods, capacitive equations, nitrogen (77 K) adsorption−desorption isotherms, XRD, XPS and Raman spectra, SEM images and TEM images, crystal structure, E
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(17) Zhao, X.; Hayner, C. M.; Kung, M. C.; Kung, H. H. ACS Nano 2011, 5 (11), 8739−8749. (18) Huang, H.; Tang, Y.; Xu, L.; Tang, S.; Du, Y. ACS Appl. Mater. Interface 2014, 6 (13), 10248−10257. (19) McDonough, J. K.; Frolov, A. I.; Presser, V.; Niu, J.; Miller, C. H.; Ubieto, T.; Fedorov, M. V.; Gogotsi, Y. Carbon 2012, 50 (9), 3298−3309. (20) Xu, Y.; Lin, Z.; Zhong, X.; Huang, X.; Weiss, N. O.; Huang, Y.; Duan, X. Nat. Commun. 2014, 5, 4554. (21) El-Kady, M. F.; Strong, V.; Dubin, S.; Kaner, R. B. Science 2012, 335 (6074), 1326−1330. (22) Cao, G.; Liu, J. P.; Cheng, C. W.; Li, H. X.; Li, X. L.; Zhou, W. W.; Zhang, H.; Fan, H. J. Energy Environ. Sci. 2011, 4, 4496−4499. (23) Xia, X.; Chao, D.; Fan, Z.; Guan, C.; Cao, X.; Zhang, H.; Fan, H. J. Nano Lett. 2014, 14 (3), 1651−1658. (24) Lu, X.; Yu, M.; Wang, G.; Zhai, T.; Xie, S.; Ling, Y.; Tong, Y.; Li, Y. Adv. Mater. 2013, 25 (2), 267−272. (25) Cheng, Y.; Lu, S.; Zhang, H.; Varanasi, C. V.; Liu, J. Nano Lett. 2012, 12 (8), 4206−4211. (26) Wu, C.; Lu, X.; Peng, L.; Xu, K.; Peng, X.; Huang, J.; Yu, G.; Xie, Y. Nat. Commun. 2013, 4, 2431. (27) Wu, C.; Feng, F.; Xie, Y. Chem. Soc. Rev. 2013, 42 (12), 5157− 5183. (28) Taniguchi, M.; Wakihara, M.; Shirai, Y. Z. Anorg. Allg. Chem. 1980, 461 (1), 234−240. (29) Jing, Y.; Zhou, Z.; Cabrera, C. R.; Chen, Z. J. Phys. Chem. C 2013, 117 (48), 25409−25413. (30) Ishii, M.; Wada, H.; Nozaki, H.; Kawada, I. Solid State Commun. 1982, 42 (8), 605−608. (31) Mizusaki, J.; Yonemura, Y.; Kamata, H.; Ohyama, K.; Mori, N.; Takai, H.; Tagawa, H.; Dokiya, M.; Naraya, K.; Sasamoto, T. Solid State Ionics 2000, 132 (3), 167−180. (32) Yu, G.; Xie, X.; Pan, L.; Bao, Z.; Cui, Y. Nano Energy 2013, 2 (2), 213−234. (33) Wang, G.; Lu, X.; Ling, Y.; Zhai, T.; Wang, H.; Tong, Y.; Li, Y. ACS Nano 2012, 6 (11), 10296−10302. (34) Xu, Y.; Lin, Z.; Huang, X.; Liu, Y.; Huang, Y.; Duan, X. ACS Nano 2013, 7 (5), 4042−4049. (35) Kang, Y. J.; Chung, H.; Han, C. H.; Kim, W. Nanotechnology 2012, 23 (6), 065401. (36) Choi, B. G.; Chang, S. J.; Kang, H. W.; Park, C. P.; Kim, H. J.; Hong, W. H.; Lee, S.; Huh, Y. S. Nanoscale 2012, 4 (16), 4983−4988.
rate capability, cycling stability, CV and charge/discharge curves. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*Y.T. 135 Xingang West Road, Chemical North Building 325, Guangzhou, 510275 P.R. China. Phone: 86-20-84110071. Fax: 86-20-84112245. E-mail: [email protected]. *Y.L. 1156 High Street, Physical Science Building 160, Santa Cruz, California 95064, U.S.A. Phone: 1-831-459-1952. Fax: 1831-459-2935. E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Y.L. is thankful for the support of this work by UCSC faculty startup funds. Y.X.T. acknowledges the financial support of this work by the Natural Science Foundations of China (21273290 and 91323101), the Natural Science Foundations of Guangdong Province (S2013030013474), and the Research Fund for the Doctoral Program of Higher Education of China (20120171110043). T.Z. thanks the China Scholarship Council for financial support. We thank Dr. Tom Yuzvinsky for image acquisition and the W. M. Keck Center for Nanoscale Optofluidics for use of the FEI Quanta 3D Dual-beam microscope. We acknowledge Jesse Hauser for XRD characterization and the support of UCSC XRD (Rigaku Americas Miniflex Plus powder diffractometer) facility supported by the U.S. NSF MRI grant (MRI-1126845).
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