Reduced Graphene Oxide Composites with

Ultrathin Hexagonal 2D Co2GeO4 Nanosheets: Excellent Li-Storage Performance ..... flowers using Cu 2 O cube as sacrificial template for good lithium s...
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CoO Hollow Cube/Reduced Graphene Oxide Composites with Enhanced Lithium Storage Capability Xin Guan,† Jianwei Nai,*,† Yuping Zhang,† Pengxi Wang,† Jie Yang,† Lirong Zheng,‡ Jing Zhang,‡ and Lin Guo*,† †

School of Chemistry and Environment, Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, Beihang University, Beijing, 100191, P.R. China ‡ Beijing Synchrotron Radiation Laboratory, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, P.R. China S Supporting Information *

ABSTRACT: Hollow hierarchical CoO nanocube/reduced graphene oxide (COG) composite has been fabricated with the sacrificial-template method and the subsequent thermal treatment. Hollow/porous architectures supply high specific surface area and buffer the volume change during the lithium uptake/release processes, while rGO matrix ensures the system conductivity and further reinforces the structure. Serving as the anode material of lithium ion battery, COG demonstrates high lithium storage capacity, reaching 1170 mA h g−1 at a current density of 150 mA g−1, which is much higher than the capacity of rGO-free hollow CoO nanocubes. Ninety-four percent retention after 60 cycles further proves its stable cyclability. The combination of the advantages of the as-prepared befitting nanostructure and the rGO should be responsible for the durable rate behavior and the high capacity. Moreover, unfully reduced graphene oxide was achieved with the assistance of the multifunctional Na2S2O3, leading to more disorders and defects left in the composite and should also afford a positive influence on the lithium storage performance of the COG.

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INTRODUCTION Li-ion batteries (LIBs) have become the most favorite rechargeable batteries among all the existing battery systems during the past 2 decades.1−4 To thoroughly develop LIBs, researchers face a daunting challenge: the expending need to increase their energy output.5 One main reason why the energy output of LIBs is restricted is that the commercial graphite anode used currently has a low gravimetric capacity of only 372 mA h g−1.6 Substituting the graphite anodes eventually, in other words, devising electrode materials with higher capacity is imperative. In recent decades, many endeavors have been made to exploit metal oxides (MOx, M: Mn, Fe, Co, Ni, Cu, etc.) as the alternative electrode materials for LIBs because of their high specific capacity, typically 2−3 times higher than that of the carbon/graphite-based materials.7−13 One of the MOxs, cobalt monoxide, with a cubic rock-salt structure, has undergone extensive investigation in view of its wide-range promising applications, such as magnetic materials,14 supercapacitor,15 and lithium-ion battery electrode active materials.16,17 The theoretical specific capacity of CoO is as high as 716 mA h g−1. As a 3d transition-metal oxide, its electrochemical reaction appears completely reversible (CoO + 2Li+ + 2e ⇌ Co + Li2O), which also makes it a promising anode electrode material in LIBs. To optimize the lithium storage capacity of electrode materials, one effective method is to construct appropriate nanostructures.18−20 Among the proposed candidates, hierarchical and hollow/porous structures have received significant © XXXX American Chemical Society

attention in materials research. For hierarchical structure, the introduced multiscale architectures provide both extraordinarily high activated surface and robust stability.21,22 The small-size building block also positively impacts the LIB performance.23 For hollow/porous materials, they are much more competitive than solid ones due to their low density and the ability to interact with the ions, atoms, and molecules, not only on the surface or at the interface but also throughout the bulk phase of materials.24 Pores and hollow interior also mitigate the tensile stress and accommodate the large volume variation during Li+ insertion/desertion, preventing electrode polarization. Both of the two architecture features mentioned above can effectively enlarge the surface-to-volume ratio and reduce diffusion distance for lithium ions, allowing better reaction kinetics at the electrode surface.25,26 As a result, fabricating hierarchical and hollow/porous structures is generally considered as an effective method to enhance the performance of the materials for LIBs. Apart from the rational design of the specific nanostructures, improving the conductivity of metal oxides is the next step that should be taken to break the restriction of its practical application. In recent years, many researchers have proposed synthesizing hybridizing nanostructures with conducting carbon Received: July 21, 2014 Revised: September 21, 2014

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then calcined in a muffle furnace at 300 °C for 3 h at a ramping rate of 4 °C min−1 to transform it into black powder. Characterization. The structures and compositions of the as-prepared products were characterized by X-ray powder diffraction (XRD) using a Rigaku Dmax 2200 X-ray diffractometer with Cu Kα radiation (λ = 1.5416 Å) and Thermo Fisher ESCALAB250Xi X-ray photoelectron spectroscopy. The X-ray absorption spectra were collected at the 1W1B beamlines at the Beijing Synchrotron Radiation Facility, running at the electron energy of 2.5 GeV with an electric current between 160 and 250 mA. Estimations of the structural parameters and the Debye−Waller factor (σ2) were performed by curve-fitting analysis using the ARTEMIS (version 0.8.011) module implemented in the IFEFFIT package. The morphologies of the synthesized samples were studied by a JEOL JSM7500F cold-field emission scanning electron microscope. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and selected area electron diffraction (SAED) investigations were carried out by a JEOL JEM2100F microscope. Specific surface areas were measured at 77 K by Brunauer−Emmett−Teller (BET) nitrogen adsorption− desorption (ASAP 2420, Micromeritics, USA). Electrochemical Measurements. The electrochemical properties of the COG composites and CoO nanocrystals as anode materials in lithium ion cells were evaluated on a LAND battery test system (CT2001A model, Wuhan Jinnuo Electronics Ltd., China). The test electrodes were fabricated by mixing 80 wt % active material with 10 wt % carbon black and 10 wt % poly(vinylidene fluoride) dissolved in N-methyl-2pyrrolidone to form a slurry, which was then coated onto a copper foil (current collector, 1 cm × 1 cm, ca. 1.8 mg cm−2), dried at 80 °C for 10 h, and finally pressed under the pressure of 10 MPa. Afterward, CR2032-type coin cells were assembled in a highly pure argon-filled glovebox using the test electrodes, a metallic lithium counter/reference electrode, a polypropylene separator (Celgard 2400), an electrolyte of 1 mol/L LiPF6 in ethylene carbonate and diethyl carbonate (1:1 vol) (Tianjin Jinniu Power Sources Material Co., Ltd., China). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were performed on CHI660D electrochemical workstation (Shanghai Chenhua Co., Ltd., China) at an alternating current voltage of 5 mV amplitude in the 100 kHz to 0.01 Hz.

supports (carbon shell,27 nanotube,28−30 and fiber31) to compensate for the poor intrinsic conductivity of MO x materials, which is also suitable for CoO. Graphene sheet is the most appealing matrix at present. As a remarkable form of conductive carbon, it unites high specific surface area (over 2600 m2 g−1) and high theoretical lithium storage (744 mA h g−1). Thanks to pragmatic Hummers’ method, graphene oxide (GO) is easy to obtain on the large scale under laboratorial conditions, stimulating thorough study of the reduced graphene oxide (rGO) anchored with metal oxides with unique nanostructures. CoO nanoparticle/graphene composites were fabricated via different approaches.32,33 The rGO in these cases ensures the electrical conductivity of active materials and prevents losing active sites ascribing to particle aggregation, resulting in good capacity and cycle ability. However, these approaches either contain complicated processes or involve toxic reagents or organic solvents. Therefore, facile routes to construct MOx/rGO hybrid are highly desired. Herein, we report the achievement of the controllable fabrication of hollow hierarchical CoO nanocube/graphene composite. In the CoO/rGO hybrids that have been reported, the type of structure of CoO material is quite monotonous, focusing on solid nanoparticles.34,35 The novel composite explored in this work not only pushes through the morphological limitation of the CoO nanomaterials but also simplifies the procedure of the reduction of GO. The rGO matrix improves the electrical conductivity, as well as reinforces hollow/porous structures considering its cushion effect against the volume strain during the lithium uptake/release processes. Combining the unique properties of hollow hierarchical CoO nanocube and rGO, this hybrid composite shows a large reversible Li+ storage capacity, durable high-rate performance, and high stability for long-term cycles.

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EXPERIMENTAL SECTION

Synthesis of the Cu2O Precursor. CuCl2·2H2O (0.17 g) was first dissolved in deionized water (100 mL) at room temperature, yielding a light-blue transparent solution. Then 10 mL of 2 M NaOH was added to the solution until it was heated to 55 °C. After 30 min of stirring, 10 mL of 0.6 M ascorbic acid was introduced dropwise into the system. With another 3 h of agitation, a brick-red precipitate was obtained. The precipitate was collected and washed with deionized water and ethanol several times by centrifugation and then dried at 60 °C overnight. Synthesis of the Cobalt Hydroxide/Reduced Graphene Oxide (CHG) Composite. GO was synthesized from natural graphite powder by a modified Hummer’s method.36,37 GO was subjected to dialysis for 7 days to completely remove metal ions and acids. Then it was freeze-dried and kept at room temperature. In a typical procedure, 1.1 mg of GO and 0.017 g of CoCl2·6H2O were dispersed in a mixed solution of 50 mL of deionized water and 50 mL of ethanol, by sonication for 1 h. Then 0.05 g of Cu2O, 0.017 g of CoCl2·6H2O, and 3.3 g of poly(vinyl pyrrolidone) (PVP; K30) were added into the suspension successively, with another 0.5 h of ultrasonic treatment. One molar Na2S2O3 (50 mL) was dropped into the system. After 3 h of agitation, a black precipitate was obtained eventually and rinsed with ethanol and deionized water a couple of times before being dried at 60 °C overnight. Synthesis of the Cobalt Monoxide/Reduced Graphene Oxide (COG) Composite. The COG product was

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RESULTS AND DISCUSSION As shown in Figure 1, the preparation of COG is based on the synthesis of the sacrificial template Cu2O and the CHG precursor. Utilizing the hard template method, we first fabricated cobalt hydroxide/reduced graphene oxide (CHG), the composite of rGO, and hierarchical hollow Co(OH)2 cubes. This novel architecture is taking the good advantages of the Pearson’s hard and soft acid−base (HSAB) principle38 and “coordinating etching and precipitating” route.39 The general chemical route could be described as follows: Cu 2O + xS2 O32 − + H 2O → [Cu 2(S2O3)x ]2 − 2x + 2OH− (1)

Co2 + + 2OH− → Co(OH)2

(2)

S2 O32 − + H 2O ↔ HS2 O3− + OH−

(3)

2−

S2O3 , dissociating from Na2S2O3, is a typical soft base in view of HSAB principle, while Cu+ within the Cu2O template B

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Figure 1. Schematic illustration of the fabrication of Co(OH)2 hollow cube by synchronous coordinating etching of Cu2O nanocrystals and the fabrication of CoO by thermal treating of Co(OH)2. I, coordinating etching and precipitating; II, coordinating etching; III, thermal treatment; NP, nanoparticle.

displays the soft acid feature. [Cu2(S2O3)x]2−2x, a more stable and soluble complex ion, emerges as the result of the interaction between Na2S2O3 and Cu2O at the cost of the disappearance of Cu2O (eq 1). The exhaustion of OH− for the yield of Co(OH)2 (eq 2) provides a strong driving force for further dissolution of Cu2O and the hydrolysis of S2O32− (eq 3). In this room-temperature synthesis process, Co(OH)2 nanoparticles (NPs), the basic building blocks, precipitate around the external surfaces of original Cu2O cubes and construct the shell material. The subsequent thermal treatment strengthens the hollow cubes slightly through crystallizing CoO NPs. At the same time, more pores are created owing to the substances loss, that is, constitution water. Moreover, in our case, we treat sodium thiosulfate as a multifunctional additive, not only a perfect etchant but also an appropriate reductant. Excessive use of Na2S2O3 could reduce the GO in a moderate way, keeping the hierarchical Co(OH)2 shells integrated, avoiding the oxidation of unstable Co2+, and thus achieving the in situ synchronous fabrication and compositing of Co(OH)2 and rGO at room temperature. Compared to the approaches involving hydrazine hydrate32 and sodium sulfide as the reducer for GO,40 this strategy is nontoxic and more environmentally friendly. Taking the relatively weak reducing capacity of Na2S2O3 into consideration, GO might be only partially reduced, which can be confirmed by XRD, XPS techniques discussed later. This kind of incomplete-reduced GO preserves a lot of oxygen-containing functional groups, such as C−O and CO. These residual groups lead to an improved capability of capturing lithium ions via a fast surface redox reaction, which might improve the performance of the CoO-based LIB.41,42 Scanning electron microscopy (SEM) images (Figure 2a,b) of the as-obtained CHG hybrid show that Co(OH)2 cubes are uniform in size (ca. 600 nm). It is the utilization of pregrown templates, Cu2O, contributing to the improvement of the monodispersity and size distribution of Co(OH)2. Flakelike secondary structures on the surface of the nanocages are clearly shown in Figure 2c,d. The delicate structure contributes to the increase of the specific surface area of CHG composite, which can be confirmed by the N2 adsorption−desorption isotherms

Figure 2. SEM images of CHG (a), (b); TEM images of CHG (c), (d); TEM image of unfully reduced GO (e).

of CHG (Figure S1, Supporting Information). The BET specific surface area of CHG composite is as high as 88.9 m2 g−1. In addition to the increase of specific surface area, these ultrathin nanoflakes also provide rough surface texture, which is beneficial to the dispersion of Co(OH)2 within CHG composites.43 Because of the interaction between the residual hydrophilic functional groups of rGO and the hydroxyl groups of Co(OH)2, Co(OH)2 cubes are well-wrapped by rGO sheets. The perfect combination of the two materials is well-illustrated by comparing CHG composites (Figure 2b,d) to the pristine Co(OH)2 hollow cube (Figure S2, Supporting Information). Besides, it is also clearly demonstrated that GO has no significant effect on the final morphology of the Co(OH)2. The rippled and crumpled structure indicates that there are only a few carbon layers within the nearly transparent rGO (Figure 2e). XRD pattern of CHG (Figure S3, Supporting Information) shows no specific peak can be ascribed to crystalline Co(OH)2 (JCPDS No. 30-0443), proving its amorphous property. To testify to the reducing capacity of Na2S2O3, only GO and Na2S2O3 were mixed together in deionized water. The XRD pattern of the product of this reaction, rGO, is shown in Figure S3, Supporting Information. Comparing to the result of GO (Figure S3, Supporting Information), two weak peaks at around 10° and 22° are detected. The former one is the characteristic peak of GO, verifying the unfull reduction of GO as the deduction discussed earlier. In other words, the result reveals the mild-reducer feature of Na2S2O3. The latter peak should be attributed to the (002) faces of graphitic structure caused by the stacking of graphene sheets.44−46 In addition, the XRD pattern of CHG is similar to that of rGO, which means that other reactants or solvents in the reaction system, for example, Cu2O, PVP, and ethanol, should have no contribution to the reduction of GO. After calcination with the protection of argon gas, COG is obtained eventually. Hollow structures can bear the thermal C

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treatment so that the shell preserves the original shape very well (Figure 3a−c and Figure S4, Supporting Information).

Meanwhile, rGO sheets shrink a lot as the consequence of losing additional hydrophilic functional groups (Figure 3a,b). CoO crystal structure is verified via the SAED pattern (the inset in Figure 3d). Diffraction rings, from inside out, can be indexed to the (111), (200), and (220) facets of CoO. With a closer examination of the single CoO nanocube (Figure 3c,d), nanoparticles (ca. 5 nm) are found as the building units of the whole structure, while a porous configuration is manifested by Figure 3e. The HRTEM image in Figure 3f, selected from the area marked with a black frame in Figure 3e, gives two sets of lattice fringes, corresponding with (111) and (200) crystal planes of CoO. The XRD pattern (Figure S5, Supporting Information) is obtained to testify the phase transition from CHG to COG. (001) diffraction of GO can be detected, illustrating that GO should still not be fully reduced even after calcination. The broadened peak at 22°−28° can be ascribed to (002) plane refraction of a graphitic structure. Other small peaks at 36.5°, 42.4°, and 61.5° are corresponding to (111), (200), and (220) facets of CoO (JCPDS No. 48-1719). The low intensity of these peaks is caused by extreme small size of grains and the porous structures.47 These grains and porous structures could effectively enlarge the specific surface area and reduce diffusion distance for lithium ions, endowing the COG with features of a promising electrode for LIBs. X-ray photoelectron spectroscopy (XPS) analysis is used to provide more information on chemical composition and the oxidation state of the atom within COG. All the binding energies of the results are corrected for specimen charging by referencing them to the C 1s peak (set at 284.6 eV). Figure 4a shows the spectrum of Co 2p of the composite. The two peaks centered at 780.8 and 796.7 eV are corresponding to the Co 2p3/2 and 2p1/2.48 Prominent shake-up of satellite peaks

Figure 3. SEM image (a) and TEM (b) image of COG. TEM images at different magnifications (c), (d), and (e) and HRTEM image (f) of hierarchical CoO hollow nanocube. The inset in panel (d) is the corresponding SAED pattern of as-prepared CoO.

Figure 4. XPS spectra of COG: Co 2p (a) and C 1s (b); (c) Co K-edge XANES spectra of COG, commercial CoO, and commercial Co3O4; (d) the Fourier transform (FT) of the Co K-edge k3χ(k) functions of COG and commercial CoO. D

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Table 1. Co K-Edge Extended X-ray Absorption Fine Structure Parameters of COG and the Commercial CoO Material R-factor

path

COG

0.0076

commercial CoO

0.0124

Co−O Co−Co Co−O Co−Co

material

coordination number N 6 8 6 12

± ± ± ±

0.2 0.5 0.3 0.4

Debye−Waller factor σ2 (10−3 Å2) 9.8 10.0 7.3 7.4

± ± ± ±

0.35 0.39 0.34 0.36

bond length r (Å) 2.12 3.01 2.08 3.02

± ± ± ±

0.01 0.02 0.01 0.02

Figure 5. CV curves of (a) COG electrode and (b) pure CoO hollow cubes electrode. Nyquist plots of COG and CoO electrodes (c). Cycling performance (d), galvanostatic charge−discharge curves cycled at the 1st, 2nd, 10th, 20th, 30th, 40th, 50th, and 60th cycle (e) and rate capability (f) of COG. Cycling performance (g), galvanostatic charge−discharge curves (h), and rate capability (i) of CoO.

comparison with the commercial CoO and Co3O4 material, which are well-crystallized. As shown in Figure 4c, X-ray absorption near-edge structures (XANES) of Co within COG is quite similar to that of Co within the commercial CoO material except for a slight amplitude reduction, which is obviously different from the spectrum of Co within commercial Co3O4 material. Therefore, this result confirms the bivalence of Co atoms within COG. Extended X-ray absorption fine structure (EXAFS) in Figure 4d reveals the structural information on each shell at different bond lengths from the core Co atom. The first peak is ascribed to the Co−O bond while the second is attributed to Co−Co in the second coordination shell. Co−O peak of COG is similar to that of CoO; however, the Co−Co peak of COG shows a distinct amplitude reduction, compared with that of the CoO, which is probably caused by the decrease of Co−Co coordination number within COG (Table 1). In addition, the decrease of the coordination number might be ascribed to the shrink of particle size, which proves again that

(located at 786.0 and 802.5 eV) distinctly verify the presence of the CoO phase. When the XPS spectra of C 1s of GO, CHG, and CoO (Figure S6, Supporting Information) are compared, severe loss of oxygen-containing functional groups is observed. Nevertheless, the peaks that represent the residual of C−O, CO, and so forth still can be detected, indicating that GO was not fully reduced, which is very consistent with the XRD result. The distinct difference between the XPS C 1s spectrum of GO and CHG (Figure S6a,b, Supporting Information) can further demonstrate the reducibility of Na2S2O3. The intensity of peaks related to oxygen-containing functional groups in the XPS C 1s spectrum of COG declines much more (Figure 4b) compared with those of CHG (Figure S6b, Supporting Information), suggesting GO can be further reduced after thermal treatment. However, in consideration of the low annealing temperature, this procedure shows limited reducing capacity to GO. To investigate the local structure around Co atoms, Co Kedge X-ray absorption spectra (XAS) of COG was performed in E

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flexible strengthening agent, can precisely buffer the volume change and make up for the inadequacy of the hollow/porous architecture. When compared to many other cobalt oxide-based materials (Table S1), the lithium storage performance of COG presented in this work is effectively enhanced. Figure 5e,h shows the charge/discharge profiles of COG and CoO electrodes in the 1st, 2nd, 10th, 20th, 30th, 40th, 50th, and 60th cycles at a constant current density (150 mA g−1) within a cutoff voltage window from 0.001 to 3 V. For COG, the four plateaus are very consistent with the peaks in the CV test. Except the first discharge curve, the capacity value is maximized in the 30th cycle and declines in the following processes. However, except the first several cycles, no obvious plateau is found in the case of CoO electrode. The result is also consistent with its CV curve (Figure 5b). Figure 5f shows the durable rate behavior of COG. Its reversible capacity can reach 1100 mA h g−1 in the first 10 cycles at a current density of 75 mA g−1, then 967 mA h g−1 in the second 10 cycles at 150 mA g−1, 782 mA h g−1 in the third 10 cycles at 350 mA g−1, and 654 mA h g−1 in the fourth 10 cycles at 750 mA g−1. CoO rate behavior was also examined under identical testing conditions (Figure 5i), reaching 666, 253, 117, and 40 mA h g−1, respectively. As shown in Figure 5f, when the current density returns from 750 to 75 mA g−1, capacity as high as 1108 mA h g−1 is obtained, slightly higher than the initial value. In the case of CoO, however, it suffers poor retention at a total of 41% (271 mA h g−1). It is speculated that the rGO causes the different rate performances of the two materials: CoO hollow nanocubes can not maintain their integrity for long without the assistance of rGO.

the small grains construct the hierarchical/hollow CoO nanostructure. Figure 5a reveals the electrochemical performance of the asprepared COG composites. CV test was applied to investigate the detailed electrochemical progress at a scan rate of 0.1 mV s−1 within the voltage window of 0.001−3 V. The first three cycles are shown in Figure 5a. There are two distinctive reduction peaks at 0.57 and 1.18 V in the first cathodic scan, with obvious decline and shift to higher voltage area (0.8 and 1.5 V, respectively) in the following cycles interfered by the irreversible formation of a solid electrolyte interphase (SEI) and the decomposition of electrolyte.49 Moreover, the Libearing solid−electrolyte interface and possibly interfacial lithium storage contribute to the high capacity beyond the theoretical value (720 mA h g−1).11,18,50 The cathodic peak at 1.5 V and the anodic peak at 2.2 V are derived from the reduction of Li and the regeneration of CoO. Reduction peaks at 0.02 and 0.8 V are partially caused by the addition of Li+ inserting into rGO,51 highlighting the contribution of the rGO to the capacity of electrode materials. From the second cycle onward, the peak potential and current intensity of CV curves are tending toward stability, implying good cycling property. CV test of CoO derived from pure Co(OH)2 hollow cube was also performed (Figure 5b). It is in good agreement with the previously reported CoO anodes.50,52−55 Cathodic peaks at 0.57 and 0.95 V also shift to higher potential position, due to the pulverization of the CoO nanoparticles and the formation of SEI film.53 Anodic peaks, as well as cathodic peaks, fade out gradually during the following cycles, indicating inferior cyclability and stability of the pure CoO electrode. EIS measurements (Figure 5c) were carried out to understand the conductivity of COG composites and CoO nanocubes. The typical characteristics of the two Nyquist plots are one semicircle in the high-frequency range attributed to a charge-transfer phenomenon, and a sloping straight line in the low-frequency range ascribed to the mass-transfer process. The radius of the semicircles of COG electrode is conspicuously smaller than that of CoO electrode, which indicates that the charge-transfer resistance of COG is lower than that of CoO, the rGO-free one. It also implies that the diffusion of Li+ ions inside the COG is much more facile for introducing rGO matrixes into the system to compensate for the poor conduceivity of CoO.24 The cycling performance of COG is shown in Figure 5d. The first discharge and charge capacities are 1496.4 and 1007.1 mA h g−1, respectively. This poor initial Coulombic efficiency (67.3%) agrees well with the CV result that the reduction peaks are present in the first cathodic scan but disappear afterward. It is the formation of SEI layer that results in the large irreversible capacity loss during the first discharge/charge process. From the 2nd to 60th cycle, the capacity increases steadily and then falls gradually, peaking in the 30th cycle (1170 mA h g−1). Despite the first cycle, the Coulombic efficiency is maintained around 97% in the subsequent process. To evaluate the advantage of the introduction of unfully reduced graphene oxide, we compare the cycle performance of COG with CoO (Figure 5g). It shows a relatively high first discharge capacity which verifies that the specific hollow/porous structure has a positive effect on the lithium storage, whereas the rapidly dramatic drop of capacity occurs right after the first cycle, apparently indicating the structure is not good at withstanding the huge volume change during the lithium uptake/release process. On the basis of this comparison, rGO, serving as a

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CONCLUSION In summary, we have successfully designed a novel strategy for making full use of the multifunctional Na2S2O3 to assist with the integration of the hierarchical, hollow, porous structures of CoO nanocubes, and the rGO with excellent properties. Na2S2O3 acts as a coordinating etchant as well as a mild reducer: eliminating Cu2O templates, promoting the precipitation of hierarchical Co(OH)2 shells, achieving synchronous reduction of GO at room temperature, and introducing more disorders and defects into rGO. CoO with specific construction serves as the high-capacity host, producing more valid active sites, enhancing the interaction between matters, and accommodating the volume change to some extent. rGO matrix improves the composite conductivity significantly, as well as consolidates the hollow structure during the lithium insertion/extraction processes. Hence, the COG composite exhibits durable cycle performance and high-rate capability. A specific capacity of 1170 mA h g−1 is obtained, tested under a current density of 150 mA g−1. And it retains 94% of the initial capacity after 60 cycles. These results all testify that the COG we report here could be a promising anode material for LIBs.

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ASSOCIATED CONTENT

S Supporting Information *

SEM and TEM images of pure Co(OH)2 and CoO; XRD patterns of GO, CHG, and COG; XPS spectra of GO, CHG, and CoO; CV and EIS results of COG and CoO. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. F

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*E-mail: [email protected].

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the funding support from the National Basic Research Program of China (2010CB934700) and National Natural Science Foundation of China (11079002 and 51272012).

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dx.doi.org/10.1021/cm502690u | Chem. Mater. XXXX, XXX, XXX−XXX