Article pubs.acs.org/JPCC
Graphene-Wrapped Mesoporous Cobalt Oxide Hollow Spheres Anode for High-Rate and Long-Life Lithium Ion Batteries Hongtao Sun, Xiang Sun, Tao Hu, Mingpeng Yu, Fengyuan Lu, and Jie Lian* Department of Mechanical, Aerospace & Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, United States S Supporting Information *
ABSTRACT: Transition metal oxides, used as LIB anodes, typically experience significant capacity fading at high rates and long cycles due to chemical and mechanical degradations upon cycling. In this work, an effective strategy is implemented to mitigate capacity fading of Co3O4 at high rates by use of hollow and mesoporous Co3O4 spheres and graphene sheets in a core−shell geometry. The core−shell structure exhibits a high reversible capacity of 1076 mAh g−1 at a current density of 0.1 A g−1, and excellent rate performance from 0.1 to 5.0 A g−1. The graphene/Co3O4 nanosphere composite electrode also displays an exceptional cyclic stability with an extraordinarily high reversible capacity over 600 mAh g−1 after 500 cycles at a high current density of 1.0 A g−1 without signs of further degradation. The highly conductive graphene nanosheets wrapping up on surfaces and interfaces of metal oxide nanospheres provide conductive pathways for effective charge transfer. The mesoporous features of graphene and hollow metal oxide nanosphere also enable fast diffusion of lithium ions for the charge/discharge process. The highly flexible and mechanically robust graphene nanosheets prevent particle agglomeration and buffer volume expansion of Co3O4 upon cycling. The unique nanostructure of Co3O4 wrapped up with highly flexible and conductive graphene nanosheets represents an effective strategy that may be applied for various metal oxide electrodes to mitigate the mechanical degradation and capacity fading, critical for developing advanced electrochemical energy storage systems with long cycle life and high rate performance.
1. INTRODUCTION
Various strategies have been utilized to mitigate mechanical degradation and capacity fading including the use of various metal oxide nanostructures such as nanoparticles,6,12,15,22,28,29 nanowires,30−33 nanorods,34 and nanotubes11,35 which are propitious to allowing enhancement of Li ions diffusion into the nanostructure. By using these nanostructured metal oxide electrodes, the large inner lattice stress can be effectively alleviated, thus reducing the exfoliation of the electrode material and a rapid capacity fading upon cycling due to their small grain size and large portion of surface atoms. However, the large surface area of nanostructured materials may lead to the formation of a large amount of unstable SEI resulting in a large first cycle irreversible capacity loss, and low initial CE.36−38 Metal oxide-based nanostructured electrodes still suffer from fast capacity fading at high current density that fails to meet the requirements for the applications of electric vehicles and the storage of renewable energy. None of these nanostructured metal oxide-based electrodes demonstrate a stable cycling performance (>100 cycles) at a current density more than 0.5 A g−1.
The development of rechargeable Li-ion batteries (LIBs) with high specific capacity, long cycle life, and high rate capability is crucial for portable electronics, electric vehicles, and the storage of renewable energy.1−5 Transition metal oxides such as Co3O4,6−14 CoO,15−17 MnO2,18−20 NiO,21 and Fe3O4,22,23 are promising anode candidates with high theoretical capacities typically two times higher than that of commercial graphitic carbon anodes (∼372 mAh g−1). Among them, Co3O4 has attracted extensive attention for LIBs due to its high theoretical capacity of 890 mAh g−1. However, similar to other anodes such as Si, metal oxide electrodes experience significant capacity fading at high rates and long cycles due to chemical and mechanical degradations upon cycling. The mechanical degradation mainly results from large volume expansion/contraction that may lead to electrode pulverization, movement of electrode materials, and the detachment from the conducting environment during long cycling, leading to a large irreversible capacity, low initial Coulombic efficiency (CE), and poor rate capability and cycling stability.24−27 The other challenges for these metal oxide electrodes are the severe aggregation, unstable solid electrolyte interface (SEI) formation due to the repetitive volume changes during cycling, and poor electrical conductivity. © 2014 American Chemical Society
Received: August 10, 2013 Revised: January 11, 2014 Published: January 15, 2014 2263
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Figure 1. Scheme of the fabrication process of Co3O4@G core−shell composites.
min. After that, an additional 355 mL of deionized water containing 5 mL of H2O2 (3 wt %) was added. The solid obtained from centrifugation (3000 rpm, 5 min) was washed with excess deionized water, 20 vol % HCl, and ethanol, and the washing process was repeated three times. The final yellowbrown GO powder was dried under vacuum at 40 °C for 12 h. As-prepared GO powders (50 mg) were dispersed in 100 mL of deionized water followed by ultrasonication in a homogenizer (Cole Parmer, 750 w, 66% amplitude) for 30 min. The obtained suspension was centrifuged (Thermo Scientific, CL2) for 30 min at 3000 rpm to remove nonexfoliated GO. 2.3. Preparation of Hollow and Mesoporous Cobalt Oxide@Graphene Core−Shell Nanostructures. The Co3O4@G core−shell structure was achieved by a three-step fabrication procedure:23 the hollow and mesoporous cobalt oxide spheres were sequentially immersed into 1 g L−1 poly(allylamine hydrochloride) (PAH) solution for 1.0 h, 0.2 g L−1 GO solution for 12.0 h, and 98% hydrazine (N2H4) for 0.5 h. Each step was assisted with magnetic stirring to ensure a homogeneous mixture and centrifuging to separate the nanospheres. The solid was dried at 90 °C for 6 h, and the graphene oxide-wrapped Co3O4 sphere was obtained. Chemical reduction by hydrazine N2H4 was performed on the hydrid composite in order to reduce the graphene oxide to graphene nanosheets and obtain graphene-wrapped Co3O4 hollow spheres. 2.4. Material Characterization. The X-ray diffraction (XRD) was performed by using a PANanalytical X-ray diffraction system with the Cu source wavelength of 1.542 Å at room temperature. X-ray photoelectron spectroscopy (XPS) was carried out on a PHI 5000 Versa Probe system. Thermogravimetric analysis (TGA, Q50) was carried out in air atmosphere from room temperature to 800 °C at a heating rate of 10 deg min−1. The surface area of Co3O4@G composite was measured with use of Quantachrome Autosorb-1 instruments. The morphology and microstructure of materials were obtained by using a Carl Zeiss Supra field-emission scanning electron microscopy (FESEM) and a JEOL 2010 transmission electron microscopy (TEM) with 200 keV electron beam. Zeta potential (ζ) was analyzed by using NiComp 380 ZLS. 2.5. Electrochemical Measurements. The working electrodes were prepared by mixing 80 wt % active materials (Co3O4 hollow spheres), Co3O4@G-1 (hollow spheres wrapped with 23.8 wt % graphene) or Co3O4@G-2 (hollow spheres wrapped with 3.6 wt % graphene), 10 wt % acetylene black (super-Li), and 10 wt % polyvinylidene fluoride (PVDF) binder dissolved in N-methyl-2-pyrrolidinone (NMP). A control sample was prepared by mixing Co3O4 solid sphere with graphene (Co3O4/G mixture) with a comparable graphene content (21.9 wt %) for comparison. After the above mixture
Herein, we report a facile strategy to mitigate the mechanical/chemical degradation and rapid capacity fading by wrapping up highly conductive, flexible, mechanically robust, and chemically durable graphene nanosheets39−43 onto the hollow and mesoporous cobalt oxide (Co3O4) to form a cobalt oxide−graphene core−shell (Co3O4@G) nanostructure. The graphene wrapped up nanosphere core−shell structure displays distinct advantages for use as high-performance LIB anodes: (1) greatly enhanced charge transfer efficiency as a result of an effective conductive pathway of graphene interconnecting active materials, preventing particle aggregation and effectively alleviating the formation of unstable SEI upon repetitive volume changes44 and chemical interaction between electrolyte and electrodes; (2) the surface of mesoporous graphene and metal oxide shell providing extra space for lithium ion storage, enabling fast lithium diffusion; and (3) the free volume in the hollow structure interior accommodating volume expansion during cycling. Therefore, the graphene wrapped up metal oxide nanosphere in the core−shell geometry displays exceptional cyclic stability and high rate performance, enabling the design of long-lived and stable electrochemical energy storage systems.
2. EXPERIMENTAL METHODS 2.1. Preparation of Hollow and Mesoporous Co3O4 Spheres. The hollow Co3O4 spheres were synthesized by using a surfactant-assisted solvothermal method.45 Specifically, 2.4 mmol of Co(NO3)2·6H2O and 0.8 mL of deionized water were dissolved in 22 mL of absolute methanol to form a red solution. Next 5.7 mmol of sodium dodecylbenzenesulfonate (SDBS) was added into the red solution in an ultrasonic bath for 30 min to form a suspension. Solid spheres of Co3O4 were also synthesized (using 2.0 mL of deionized water instead of 0.8 mL) and mixed with graphene (see next section) as the control sample. Subsequently, the suspension was sealed in a 50-mL Teflon-lined autoclave and maintained at 200 °C for 4 h. After the suspension was naturally cooled to room temperature, the precipitates were collected and washed with deionized water and ethanol ultrasonically and then separated by centrifuging. The precipitates went through vacuum drying at 90 °C for 2 h for the subsequent experiments. 2.2. Preparation of Graphene Oxide (GO). Graphene oxide (GO) was synthesized by a modified Hummers method.46 First, 5 g of graphite powders and 2.5 g of NaNO3 were added to 115 mL of 98% concentrated H2SO4 and the mixture was stirred in an ice bath (0 °C). Fifteen grams of KMnO4 was then added carefully to the solution and the temperature was maintained for 30 min at 35 °C followed by the addition of 230 mL of deionized water dropwise. The temperature of the reaction was maintained at 98 °C for 15 2264
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Figure 2. (a) Thermogravimetric analysis of the Co3O4@G-1, Co3O4@G-2, and Co3O4/G hybrid composites; (b) XRD patterns of the Co3O4 and Co3O4@G-1 hybrid; and (c) Co 2p and (d) C 1s XPS spectra of the Co3O4@G-1 hybrid.
was coated on Cu foils, electrodes were dried at 120 °C under vacuum for 12 h to remove the solvent, and then punched into a disk and pressed. Electrochemical measurements were performed by using two-electrode CR 2032 coin-type cells with lithium foil as counter/reference electrode, Celgard 2340 membrane separator, and 1 M LiPF6 electrolyte solution dissolved in a 1:1 (v/v) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The cells were assembled in an argon-filled glovebox with both moisture and oxygen contents below 1.0 ppm. Galvanostatic charge−discharge cycles were tested with use of a Arbin Instrument at various current densities of 0.1−5.0 A g−1 and a voltage window of 0.01−3.00 V. Cyclic voltametry (CV, 0.01− 3.00 V, 0.5 mV s−1) and electrochemical impedance spectroscopy (EIS, 0.1 Hz−100 kHz, 10 mV) were measured with use of an electrochemical workstation (Ametek, Princeton Applied Research, Versa STAT 4) at room temperature.
XPS spectrum (Figure S1b, Supporting Information).48,49 Subsequently the GO sheets were spontaneously assembled onto the surface of PAH-modified Co3O4 by two interactions which are the ring-opening reaction between the amine groups in the modified Co3O4 and the epoxy groups in the GO sheets50−52 and electrostatic interaction between the positively charged amine-modified Co3O4 and the negatively charged GO sheets.8,53 The oppositely charged PAH modified Co3O4 and GO can be testified by zeta potential measurements (Figure S2, Supporting Information). Their assembly processes (with/ without PAH) were comparably demonstrated in the control experiments in Figure S3 (Supporting Information). The interactions eventually result in the core−shell assembly between the Co3O4 and GO sheets. After that, the GOwrapped Co3O4 was reduced by N2H4 transforming GO into graphene. To optimize the effect of graphene in this hybrid electrode, two different weight percentages of graphene measured from thermogravimetric analysis (TGA) in Figure 2a were investigated in this hybrid system: Co3O4@G-1 (23.8 wt % graphene) and Co3O4@G-2 (3.6 wt % graphene) hybrids. A Co3O4/G mixture electrode with a comparable weight percentage (21.9 wt % graphene) to the Co3O4@G-1 hybrid was prepared as the control sample. The typical X-ray diffraction (XRD) patterns of the as-prepared Co3O4 and Co3O4@G-1 hybrid are illustrated in Figure 2b. The diffraction patterns from both Co3O4 and Co3O4@G-1 hybrid could be perfectly indexed to the face-centered cubic (fcc, Fd3m (277), a = 0.808 nm) structure (JCPDS No. 42−1467). In addition, no conventional stacking peak of graphene sheets at 2θ = 22−27° was detected in the Co3O4@G-1 hybrid thus suggesting that the graphene sheets are homogeneously wrapped onto the surface of Co3O4 nanospheres. The above investigations
3. RESULTS AND DISCUSSION Figure 1 shows a typical route to synthesize the hybrid core− shell structure and proposed formation mechanism. The hollow and mesoporous Co3O4 nanospheres were synthesized by using surfactant-assisted solvothermal decomposition of cobalt nitrate.45 To obtain the shell coating of graphene sheet, asprepared Co3O4 spheres were immersed sequentially into poly(allylamine hydrochloride) (PAH) solution, GO solution, and hydrazine (N2H4) solution for 1.0, 12.0, and 0.5 h, respectively.23,47 PAH solution here was used to modify the surface of Co3O4 spheres with amine end groups, as evidenced by N 1s peak in the XPS survey spectra (Figure S1a, Supporting Information) and primary amines (NH2, 399.5 eV) and secondary amines (NC, 398.5 eV) along with oxidized species as amides (NHCO, 400.4 eV) in a high-resolution 2265
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Figure 3. Microstructure characterization of the mesoporous hollow Co3O4 nanospheres and the core−shell composites: SEM images of as-prepared Co3O4 nanospheres (a) before and (e) after self-assembled wrapping of graphene sheets; a low-magnification (b) and a high-magnification TEM image (c) showing a hollow interior and porous shell of a single Co3O4 nanosphere; (d) a close-up TEM image at the edge of the porous shell (the inset is the SAED pattern viewed along the [1̅14] direction, and the red arrow in panel d indicates the nanocrystals); (f) TEM images of as-prepared graphene-wrapped Co3O4 nanospheres; (g) high-resolution TEM (HRTEM) images of the interface of graphene and Co3O4 nanospheres; and (h) HRTEM images of the nanoporous structure at the edge of the Co3O4 nanospheres (red circles indicate nanopores, and the inset displays crystal lattice with a spacing of 2.86 Å corresponding to the (220) plane). The morphology and microstructure characterizations were performed on the Co3O4@G-1 hybrid.
Figure 2c.54 The peak of the C 1s XPS spectrum (Figure 2d) is split into four functional groups: the nonoxygenated C−C bond at 284.6 eV (red curve), the carbon in the C−O bond at 285.4 eV (blue curve), the carbonyl carbon in the CO bond at 286.8 eV (green curve), and the carboxylate carbon in the O−CO bond at 288.8 eV (yellow curve). The nonoxygenated C−C bond dominates the C 1s peak, indicating a
indicate that the hybrid material consists of homogeneously dispersed graphene sheets and well-crystallized Co3O4. To determine the chemical composition and valence state of Co3O4@G-1 hybrid, X-ray photoelectron spectroscopy (XPS) measurements were carried out. Two peaks in the Co 2p XPS spectra of the hybrid core−shell structure are identified locating at 795.6 and 780.2 eV, corresponding to the Co 2p1/2 and Co 2p3/2 spin−orbit peaks of Co3O4, respectively, displayed in 2266
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significant decrease in oxygen content of GO through chemical reduction. The morphology and structure of as-prepared Co3O4 nanospheres before and after wrapping graphene sheets were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM images (Figure S4a,b, Supporting Information) show that the as-prepared Co3O4 are spherical in shape and uniform in size with an average diameter of around 270 nm. The high-magnification SEM image (Figure S4b, Supporting Information) reveals a rough surface in texture consisting of tiny nanocrystals. Some nanospheres are broken due to the ultrasonic wash, as illustrated in Figures 3a and S4c,d (Supporting Information), confirming the hollow structure of Co3O4 spheres. In Figure 3e, the graphene-wrapped hollow Co3O4 hybrid (Co3O4@G-1) shows ripple textures, which are consistent with the characteristics of highly flexible and ultrathin graphene sheets. The continuous graphene sheets not only wrap around the surface of the Co3O4 spheres, but also interconnect neighboring spheres. The hollow interior of Co3O4 nanospheres is further confirmed by TEM images of Co3O4 nanospheres before (Figure 3b,c) and after graphene self-assembly (Figure 3f) in which a strong contrast can be observed between the paler center and darker edge of Co3O4 nanospheres. The close-up high-magnification TEM image of the edge of the sphere (Figure 3d) demonstrates that the nanopores and nanocrystals are smaller than 5 nm. The selected area electron diffraction (SAED) patterns (inset in Figure 3d) clearly show a singlecrystal-like nanostructure along the [1̅14] direction of the facecentered cubic lattice system resulting from the oriented aggregation of nanocrystals in a surfactant-assisted solvothermal process to form hollow and mesoporous Co3O4 nanospheres.45 The lattice fringes in high-resolution TEM (HRTEM) image (Figure 3h) further confirm a well-textured nanocrystal with a lattice interplane spacing of 2.86 Å in the (220) plane. The nanopores are indicated in the red circles, whose sizes are only around 2 nm. The HRTEM image of the interface (Figure 3g) demonstrates closely wrapped graphene sheets on the surface of Co3O4 nanospheres. Note that the porous surface in the shell may provide extra space for the storage of Li ions, resulting in the enhancement of specific capacity, and the free volume in hollow structure interior can buffer against the local volume change during cycling, beneficial for improving cycling stability. Its unique structure may also shorten the diffusion length for Li ions, leading to a better rate capability. The continuous graphene sheet not only provides a two-dimensional conductive support to interconnect the neighboring nanospheres, but also effectively buffers the volume change and particle agglomeration during cycling. In addition, it may also prevent the direct contact of Co3O4 nanospheres and the electrolyte, thus alleviating the unstable SEI growth during electrochemical cycling. The TEM and SEM images of the Co3O4/G mixture in Figure S5 (Supporting Information) show the solid Co3O4 spheres mixed with graphene as a control sample. The porosity of the pristine Co3O4 and Co3O4@G-1 hybrid was characterized by nitrogen adsorption−desorption isotherm measurements in Figure 4, parts a and b, respectively. The hysteresis loops imply that both of them possess a typical mesoporous structure, and the main size of mesopores as indicated in the insets of Figure 4 is 4.2 nm for pristine Co3O4 (Figure 4a) and 3.9 nm for Co3O4@G-1 hybrid (Figure 4b). The Brunauer−Emmet−Teller (BET) specific surface area of
Figure 4. N2 adsorption/desorption isotherms and the corresponding pore size distribution of the pristine Co3O4 (a) and Co3O4@G-1 hybrid (b).
Co3O4@G-1 hybrid is 66.2 m2 g−1, significantly larger than that of the pure Co3O4 (32.2 m2 g−1). The electrochemical performance of the graphene-wrapped nanosphere core−shell structure was tested based on coin cells built on both as-prepared Co3O4 and Co3O4/graphene hybrids. The mass loading of the samples is between 0.98 and 1.26 mg cm−2. All the specific capacity values in this paper are reported with use of the total mass of cobalt oxide and graphene. As illustrated in Figure 5a, a cyclic voltammograms (CV) test of the Co3O4@G-1 hybrid with 23.8 wt % graphene and pure Co3O4 was evaluated at a scan rate of 0.5 mV s−1 within the voltage window of 0.01−3.00 V. In the first cycle, the main cathodic peaks at potentials of 0.38 and 0.66 V are observed for Co3O4@G-1 hybrid and pure Co3O4, respectively, ascribing to an electrochemical reduction (lithiation) reaction of Co3O4 with Li.10 The cathodic peaks disappear in the following cycle as a result of irreversible formation of a SEI and the decomposition of electrolyte.55 Both main cathodic peaks positively shift to 1.02 V, which can be ascribed to polarization of the electrode and tend to be stable indicating that the electrochemical reversibility is gradually built after the initial cycle. The observed main anodic peak at 2.18 V is ascribed to the oxidation (delithiation) reaction of Co3O4, which remains at 2.18 V in the subsequent cycles for both pure and hybrid electrodes. Hence, these cathodic and anodic peaks at 1.02 and 2.18 V represent the electrochemical reduction/oxidation reactions accompanying Li ions insertion (lithiation) and extraction (delithiation).56 The formation of Co and Li2O and the reformation of Co3O4 can be described by the following electrochemical conversion reaction:10 Co3O4 + 8Li + 8e ↔ 4Li 2O + 3Co 2267
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Figure 5. Electrochemical performance of the hollow and mesoporous Co3O4 nanosphere and the core−shell hybrids: (a) cyclic voltammograms of the Co3O4 and Co3O4@G-1 hybrid in a voltage range of 0.01−3.00 V at a scanning rate of 0.5 mV s−1; (b) galvanostatic charge−discharge curves of the hollow Co3O4 nanosphere (black), Co3O4@G-1 hybrid (red), Co3O4@G-2 hybrid (blue), and Co3O4/G mixture (green) cycled at the 1st (solid line) and 10th (dash dot) between 3.00 and 0.01 V at a current density of 0.1 A g−1; (c) comparison of the cycling performance of Co3O4, Co3O4@ G-1, Co3O4@G-2 hybrid electrodes, and Co3O4/G mixture as the control sample at a current density of 1.0 A g−1; (d) rate capability of the Co3O4, Co3O4@G-1, and Co3O4/G electrodes at various current densities between 0.1 and 5.0 A g−1; and (e) long cycling performance and Coulombic efficiency of the Co3O4@G-1 electrode at a current density of 1.0 A g−1.
the main sloping plateau at 2.18 V indicates the oxidation reaction of Co with Li2O. CE is another important consideration. The first discharge and charge capacities are 1170 and 929 mAh g−1 for Co3O4 with an initial CE of 79.4%, 1609 and 1115 mAh g−1 for Co3O4@G-1 with an initial CE of 69.3%, 1261 and 981 mAh g−1 for Co3O4@G-2 with an initial CE of 77.8%, and 1174 and 784 mAh g−1 for Co3O4/G with an initial CE of 67.1% (see Figure 5b). The initial capacity loss may result from the formation of the SEI layer.57 As compared to the theoretical capacity of bulk Co3O4 (890 mAh g−1) and graphite (372 mAh g−1), the extra capacity of the Co3O4@G hybrids may be attributed to the positively synergistic effect between Co3O4 and graphene, and larger electrochemical active surface area of graphene and unique nanostructured Co3O4.37,58 Specifically, the mesoporous shell surface of Co3O4 may provide extra space for the storage of Li ions and enable the fast Li ion diffusion. At the 10th discharge/charge cycle, a high reversible capacity of ∼907 mAh g−1 is obtained for Co3O4 with a CE rising to 97.9%,
The galvanostatic charge/discharge profiles of four different Co3O4 electrodes [pristine hollow and mesoporous Co3O4 (black), Co3O4@G-1 hybrid (red), Co3O4@G-2 hybrid (blue), and Co3O4/G mixture as the control sample (green)] were evaluated at the same current density of 0.1 A g−1 (Figure 5b) in the 1st (solid line) and 10th (dash dot) cycles. In the first discharge step, Co3O4@G-1 displays two voltage plateaus at 0.9 and 0.8 V, followed by a sloping curve down to the cutoff voltage of 0.01 V. Co3O4, Co3O4@G-2, and Co3O4/G electrodes show a long voltage plateau at 1.0 V. In the subsequent discharge step (10th cycle), Co3O4@G-1 has a sloping discharge plateau at 1.4−1.1 V, followed by a plateau at 1.1 V and then a curve down to the cutoff voltage sequentially, while the other electrodes reveal a sloping discharge plateau at 1.4−0.9 V, followed by a curve down to the cutoff voltage. All the discharge plateaus correspond to the generation of Li2O accompanying the reduction of Co3O4 into metal Co. While in the 1st and 10th cycles in the charge steps, the occurrence of 2268
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Table 1. Comparison of Electrochemical Performance Co3O4/graphene
nanostructures
1
graphene-wrapped mesoporous Co3O4 hollow spheres
2 3 4 5
graphene anchored with 10−30 nm Co3O4 NPs 5 nm Co3O4 NPs dispersed on graphene 15 nm Co3O4 NPs dispersed on graphene graphene-encapsulated mesoporous Co3O4 microspheres
∼1076 mAh g−1 for Co3O4@G-1 with a CE rising to 98.4%, ∼982 mAh g−1 for Co3O4@G-2 with a CE rising to 99.1%, and ∼750 mAh g−1 for Co3O4/G with a CE rising to 98.6%. The increased CE may be due to the relatively stable SEI formed during the cycling, which can be further demonstrated in the long cycling performance. The cycling performance of pristine Co3O4, Co3O4@G-1, Co3O4@G-2 hybrid electrodes, and the Co3O4/G mixture as the control experiment is compared by using the same current density of 0.1 A g−1 for the first 5 cycles and then at a high current density of 1.0 A g−1 for the rest of the hundreds of cycles as shown in Figure 5c. Under the deep charge/discharge cycling, a fast and large capacity fading for pristine Co3O4 electrode is observed due to the self-aggregation of Co3O4 nanospheres. The reversible capacity decreases dramatically from 785 mAh g−1 for the initial cycle at the high current density of 1.0 A g−1 to 234 mAh g−1 for the 80th cycle with a low capacity retention of 29.6%. By wrapping low weight percentage (3.6 wt %) graphene nanosheets on Co3O4 spheres (the Co3O4@G-2 hybrid electrode), the cycling stability illustrates a great improvement due to the enhanced electrical conductivity with the capacity retention capacity up to 60% after 80 cycles. With a further increase in the graphene amount to 23.8 wt %, the Co3O4@G-1 hybrid electrode displays a significantly enhanced cycling performance with only less than 20% reversible capacity lost after 80 cycles. A high reversible capacity of over 700 mAh g−1 is maintained after 200 cycles. Although the Co3O4/G mixture electrode with the similar graphene percentage as the Co3O4@G-1 hybrid electrode display a comparable capacity retention of 73.3% after 80 cycles at a rate of 1.0 A g−1, it delivers a much lower reversible capacity. The exceptionally improved cycling stability of the Co3O4@G-1 hybrid electrode may be attributed to the fact that the mechanically robust graphene nanosheets wrapped up on and between Co3O4 hollow spheres greatly reduce the selfaggregation during cycling, which is the major cause of the capacity fading for cobalt oxide. The free space in hollow and mesoporous Co3O4 allows it to expand freely without mechanical constrain during cycling. The cycled electrode of the Co3O4@G-1 hybrid was opened in a glovebox and characterized in detail in order to investigated structural evolution of the electrode materials upon cycling. The electrode was soaked in acetonitrile overnight to remove the remaining electrolytes, and then washed with ethanol and further dipped in water for about 48 h to remove the SEI layer. The amorphous Co3O4@G-1 hybrid (XRD peaks are from the Cu substrate) maintained its initial sphere geometry but displays a rougher surface and more complex morphology with the more wrinkled graphene conformally wrapped outside the Co3O4 hollow spheres (Figure S7, Supporting Information). Therefore, this unique nanostructured electrode may effectively alleviate stress damage to the electrode and graphene sheet,44 leading to more stable SEI during cycling. To the best of our
capacity (mAh g−1)
current density (A g−1)
long cycling
ref
700 600 484 600 500 390
1.0 1.0 0.5 1.0 1.9 1.0
200 500
our work ref ref ref ref
6 12 13 14
knowledge, the reversible capacity of this optimized hybrid electrode is the highest reported capacity for long cycling test (>150 cycles) at a high charge rate of 1.0 A g−1 for the Co3O4based LIBs anode materials. In addition, the Co3O4@G-1 hybrid electrode also exhibits better rate capability compared to the pristine Co3O4 (Figure 5d), Co3O4@G-2 (Figure S6, Supporting Information), and Co3O4/G (Figure 5d) electrodes at various rates between 0.1 and 5.0 A g−1. All four electrodes deliver stable cycling performance at low rate (0.1−0.5 A g−1) due to their short diffusion length for Li ions based on the unique nanostructured Co3O4 or the merit of graphene. However, the reversible capacity of the pure Co3O4 electrodes rapidly drops from 628 to 111 mAh g−1 when the rate increased from 1.0 to 5.0 A g−1. Although the pure Co3O4 does not have a stable cycling performance at high rates of 1.0−5.0 A g−1 due to the relatively low electrical conductivity and unstable SEI formation, the performance of high reversible capacity and good rate capability at low rates of 0.1−0.5 A g−1 are still superior to the other pure Co3O4 with various nanostructures,30,35 and even better than our Co3O4/G hybrid counterpart and many Co3O4/graphene hybrids reported in previous literature.6,7 The Co3O4@G-2 hybrid electrode only shows a slight improvement at high rates (Figure S6, Supporting Information), and the Co3O4/G hybrid electrode shows a stable but low reversible capacity at various rates. In contrast, the Co3O4@G-1 hybrid electrode exhibits a significantly enhanced rate capability from 1.0 to 5.0 A g−1, which is mainly due to the fast electron transfer path provided by the nanostructured cobalt oxide and wrapped graphene layers connecting active materials with the current collector. For example, it delivers a high capacity of 692 mAh g−1 for the 40th cycle at 1.0 A g−1, 482 mAh g−1 for the 50th cycle at 2.5 A g−1, and 259 mAh g−1 for the 60th cycle at 5.0 A g−1. More importantly, the reversible capacity of this hybrid electrode is able to recover its initial capacity value of 1091 mAh g−1 at the rate of 0.1 A g−1 after the high rate measurements, implying its good reversibility. The strong synergistic effect between Co3O4 and graphene sheets in this core−shell composite becomes much more apparent with cycling and high rates. To further investigate the cycling stability of the Co3O4@G core−shell hybrid electrode for long cycling, the reversible capacity and the CE of the Co3O4@G-1 hybrid were evaluated after long cycles (500 cycles) and a high rate of 1.0 A g−1 as shown in Figure 5e. The capacity fading is 18.4% for the first 100 cycles, and only 2.5% degradation per 100 cycles on average occurs after the 100th cycle (0.025% degradation per cycle). The reversible capacity is stable over 600 mAh g−1 after 500 cycles without signs of further degradation indicating its superior cycling stability at high charge rates. The CE rises to 98.4% after 20 cycles and stabilizes between 98.4% and 99.7% due to the stable SEI formed outside graphene sheets. Hence the Co3O4@G-1 hybrid illustrates an enhanced reversible capacity, a superior cycling stability, and long cycle life (the 2269
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mechanically robust graphene nanosheets together, the capacity fading of the hollow Co3O4 nanospheres is greatly mitigated at high rates. The highly electrical conductive graphene nanosheets wrapped on and between metal oxide nanospheres stabilize the Co3O4 spheres against self-agglomeration, provide a conductive channel to improve the charge transfer efficiency, and also alleviate the formation of unstable SEI upon repetitive volume changes during cycling. The mesoporous and hollow Co3O4 nanosphere/graphene electrode in the core/shell geometry displays an excellent rate performance and cycling stability with a large reversible capacity of 1076 mAh g−1 for the 10th cycle at a current density of 0.1 A g−1, as well as 700 mAh g−1 for the 200th cycle and over 600 mAh g−1 after 500 cycles at a high current density of 1.0 A g−1. The strategy of using graphene-wrapped mesoporous and hollow metal oxide to mitigate the capacity fading and performance degradation may be applied to design high-performance electrodes with greatly enhanced rate performance and cycling capability for advanced electrochemical energy storage systems.
longest ever reported for the cobalt oxide-based anode material). Compared with the other Co3O4/graphene hybrids reported before (Table 1), it is worthy to note that our hybrid material (Co3O4@G-1) is the only work that demonstrated a superior long-lived cycling performance at the high rate (1.0 A g−1), as well as the enhanced reversible capacity. The improved electrical conductivity of the hybrid electrodes and enhanced charge transfer efficiency can be investigated by the comparison of the Nyquist plots as measured by electrochemical impedance spectroscopy (EIS) (Figure 6).
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ASSOCIATED CONTENT
* Supporting Information S
XPS spectra and zeta potential measurements of Co3O4 before and after PAH modifying, photographs of the self-assembly process, SEM and TEM images of as-prepared Co3O4, Co3O4@ G-1 hybrid, and Co3O4/G mixture, rate performance of Co3O4@G-2 hybrid, randles equivalent circuit and corresponding kinetic parameters of Co3O4, Co3O4@G-1, Co3O4@G-2, and Co3O4/G hybrid electrodes, and XRD and SEM image of Co3O4@G-1 electrode after cycling. This material is available free of charge via the Internet at http://pubs.acs.org.
Figure 6. Nyquist plots of the electrodes of Co3O4, Co3O4@G-1, Co3O4@G-2, and Co3O4/G.
Apparently, the diameters of the semicircles for the Co3O4@G1, Co3O4@G-2, and Co3O4/G hybrid electrodes are much smaller than that of the pristine Co3O4 electrode, thus possessing much lower contact and charge-transfer impedances.8,23 The kinetic properties were further studied by modeling AC impedance spectra in terms of the modified Randles equivalent circuit (Figure S8 in the Supporting Information).8,59 The values of the surface film resistance Rs and charge-transfer resistance Rct are listed in Table S1 in the Supporting Information. Both resistances of the hybrids are significantly lower than those of pristine Co3O4, therein the resistance of the Co3O4@G-1 electrode is lower than that of Co3O4@G-2 and Co3O4/G electrodes. Hence, Li ion diffusion and electron transfer are expedited for the graphene-wrapped Co3O4 hybrid electrodes, especially at high charging rates. This result confirms that the wrapped graphene sheets play an essential role in the enhancement of the conductivity of the overall electrode and improvement of the electrochemical activity of Co3O4 during the cycling.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +1 5182766081. Fax: +1 5182766025. E-mail: LIANJ@ rpi.edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the NSF DMR Ceramic program under a NSF career award of DMR 1151028. REFERENCES
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4. CONCLUSIONS In summary, graphene integration with the mesoporous and hollow nanosphere in a core−shell geometry was demonstrated as an effective strategy to mitigate the capacity fading experienced at high rates for metal oxide electrodes upon the mitigation of the mechanical degradation and promotion of the effective charge transfer. Particularly, the surface of unique hollow and mesoporous Co3O4 nanospheres is propitious to providing extra space for the storage of Li ions, beneficial for enhancing specific capacity, and behave as ideal platforms for long-lived electrodes in which local volume change during the charge/discharge process can be accommodated. By further synergizing the Co3O4 with highly electrical conductive and 2270
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