Yingmeng Zhang,Luting Liu,Qingwei Deng,Wanlin Wu,Yongliang Li,Xiangzhong Ren,Peixin Zhang,Lingna Sun

College of Chemistry and Environmental Engineering,Shenzhen University,Shenzhen 518060,China

Keywords:Composites Nanostructure Nanoparticles Reduced graphene oxide Metal oxides Lithium ion batteries

ABSTRACT Hybrid CuO-Co3O4 nanosphere building blocks have been embedded between the layered nanosheets of reduced graphene oxides with a three dimensional(3D)hybrid architecture(CuO-Co3O4-RGO),which are successfully applied as enhanced anodes for lithium-ion batteries (LIBs).The CuO-Co3O4-RGO sandwiched nanostructures exhibit a reversible capacity of～847 mA·h·g-1 after 200 cycles’ cycling at 100 mA·g-1 with a capacity retention of 79%.The CuO-Co3O4-RGO compounds show superior electrochemical properties than the comparative CuO-Co3O4,Co3O4 and CuO anodes,which may be ascribed to the following reasons:the hybridizing multicomponent can probably give the complementary advantages;the mutual benefit of uniformly distributing nanospheres across the layered RGO nanosheets can avoid the agglomeration of both the RGO nanosheets and the CuO-Co3O4 nanospheres;the 3D storage structure as well as the graphene wrapped composite could enhance the electrical conductivity and reduce volume expansion effect associated with the discharge-charge process.

1.Introduction

Li-ion batteries(LIBs)have been substantially utilized to energy storage devices,given their long-term cyclic life,high-efficiency energy density and low maintenance costs [1].Many efforts have been devoted to explore novel electrode materials with high initial coulombic efficiency,large reversible capacity,long-term cycling stability and high energy density [2].Up to date,3d-transition metal oxides are widely exploited and considered as promising materials for LIB anodes with relatively bigger theoretical capacities,which are much larger than that of the commercial graphite with theoretical capacity of 372 mA·h·g-1[3,4].Nevertheless,the inherent deficiencies of 3d-transition metal oxides for the application in LIBs are their essentially low electronic conductivity,huge volume expansion and structure degradation associated with the Li-ion insertion/extraction,which finally cause the low initial coulombic efficiency,poor cycling capacity retention and insufficient rate performance [5].

Generally,many strategies have been made to address the above drawbacks and improve the electrochemical performance,such as preparing mixed or hybrid metal oxides,designing hierarchical or special morphologies and hybridizing with conductive matrix [6].The hybrid metal oxide strategy is hybridizing two or more components to complement and mitigate the shortages of single metal oxides,which helps to achieve the enhanced electrochemical properties with higher electronic conductivity,faster ion transport kinetics and improved electrochemical activities [7,8].Such as Co3O4/Co nanoparticles [9],Fe2O3/CuO nanowires [10],Fe3O4/Co3O4composite [11],and Cu2O/CuO composite [12].These hybrid composites have exhibited better cycling properties than their single component,which might be related to the synergistic and complementary effects of each component during conversion reaction.

Another strategy to overcome these drawbacks is to construct carbon-based metal oxide composites,which can improve the structural stability and electrical conductivity [13-15].Among a variety of carbon-based matrices,graphene is proposed to be a priority because of its unique 2D layered structure with superior thermodynamic,electrical and mechanical properties [3,12].Especially,the reduced graphene oxide (RGO) has mostly been used to prepare graphene-based composites.For instance,Leeet al.reported a SnO2/Fe2O3/RGO showing a capacity of～795 mA·h·g-1after 220 cycles with a retention of 90% for LIBs[16].The RGO incorporation can increase the specific surface area for the final composite and enhance the electronic conductivity of the semiconductor metal oxides,which is benefited for the acceleration of electrochemical kinetics.Furthermore,the good mechanical property of RGO can behave as a buffer to accommodate the volume expansion during the repeated Li insertion/extraction process.Besides,the defects on the RGO can provide more active sites,which can supply an extra capacity.

Among the 3d-transition metal oxides,Co3O4has been considered as one of the promising anode materials with relatively larger theoretical capacity of 890 mA·h·g-1.However,cobalt metal is expensive and cobalt oxides are toxic,therefore partially replacing the cobalt element with cost-effective metals and more environmental-friendly metals will be one of the effective solutions.Among them,copper is considered as one of the inexpensive and eco-friendly candidates.Recently,some hybrid CuO-Co3O4nanostructured materials have been synthesized and investigated[17-19].For example,Shiet al.fabricated Co3O4/CuO composites with different mole ratios of Co to Cu (1:0,1:1,2:1,4:1,6:1,0:1),which showed that the Co3O4/CuO composite (Co:Cu=6:1) anode delivers a highest reversible capacity of 1056 mA·h·g-1after 500 cycles at 0.2 A·g-1,compared to pure Co3O4or CuO electrodes[20].Furthermore,incorporating graphene and CuO-Co3O4will provide a synergistic improvement on lithium storage performance.For instance,Wuet al.prepared yolk-shell Co3O4@CuO microspheres derived from metal-organic frameworks(MOFs),followed by the surface modification of graphene quantum dots(GQDs).The Co3O4@CuO@GQDs anode displayed superior performance for LIBs with a high reversible capacity of 1054 mA·h·g-1after 200 cycles at 0.1 A·g-1,compared to the pristine Co3O4@CuO anode suffering from a severe capacity decline [21].

Herein,a facile solvothermal method was employed to synthesize the CuO-Co3O4nanospheres embedded RGO hybrid (CuOCo3O4-RGO)compounds,which were constructed into sandwiched structures in a self-assembly manner.Based on the 3D hybrid architecture,the CuO-Co3O4-RGO composite takes the advantages of both the wrapped graphene nanosheets and the uniformly distributed CuO-Co3O4nanospheres,which accelerate the electrochemical reaction kinetics with improved conductivity and faster electron transport,and prevent the pulverization and aggregation of the nanoparticles.As a result,after cycling at 100 mA·g-1for 200 cycles,CuO-Co3O4-RGO compound exhibits a reversible capacity of 847 mA·h·g-1.The rate capability of CuO-Co3O4-RGO obtained at 100,200,500,1000,1500,2000 and 100 mA·g-1was about 927,823,702,594,446,291 and 765 mA·h·g-1,respectively.

2.Experimental

All the chemicals used in experiments were purchased from Aladdin (Shanghai,China) which are analytical grade and directly used.

2.1.Synthesis of CuO-Co3O4-RGO compounds

Before the typical synthesis of CuO-Co3O4-RGO compounds,graphene oxide (GO) nanosheets were firstly synthesized via a modified Hummers method followed by the spray-drying.The solvothermal syntheses in the ethanol-water mixtures were conducted as follows:80 mg of GO was ultrasonicately dispersed in a mixture solution with ethanol (20 ml) and DI water (20 ml) for 2 h(A-solution).To prepare B-solution,0.3 g of urea,0.2 g of polyvinyl pyrrolidone (PVP),1 mmol of copper acetate,2 mmol of cobalt(II) acetate,7.5 mmol of sodium hydroxide and 2 ml of 3-aminopropyltriethoxysilane (APTES) were dissolved in 120 ml of ethanol.After that,A-solution was slowly dropped into Bsolution to make them a mixed solution.After ultrasonicating for 2 h,the mixture was transferred to the 100 ml of Teflon-lined stainless steel autoclave,heated at 140°C for 10 h,then cooled down and washed with DI water and ethanol,further purified by filtration and drying in the air.Subsequently,the as-prepared samples were thermally treated at 500 °C in the nitrogen atmosphere for 2 h,then finally calcinated at 180°C in air for 8h to obtain CuO-Co3O4-RGO sample (comparative experiments:Pure CuO,pure Co3O4and CuO-Co3O4samples were synthesized with similar procedures excluding the addition of graphene oxides.A similar solvothermal and thermal treatment was also applied to the pure RGO.).

2.2.Characterization

X-ray diffraction(XRD)on a D8 Advance diffractometer(Bruker AXS) was used to characterize the phase compositions.Fieldemission scanning electron microscopy (FESEM,JEOL-JSM-7800F)is conducted to study the morphology and perform the elemental mapping analysis.Transmission electron microscope (TEM) image was performed on JEM-2100 &X-Max 80 transmission electron microscope.Raman spectra were conducted on the HORIBA Lab-RAMHR 800 Raman system.Brunauer-Emmett-Teller method(BET,NOVA1200e) was used to obtain the specific surface area and pore size distribution data.X-ray photoelectron spectroscopy(XPS) was collected on the Perkin-Elmer ESCALAB 250.

2.3.Electrochemical measurements

CR2032 coin-type cells were assembled to measure the electrochemical performances with metallic lithium foils as counter electrodes by LAND CT2001A tester.The working electrode was usually produced by casting the active-material containing slurry onto the current collector (Cu foil for anodes),and then dried in a vacuum oven at 80℃for several hours.The slurry was mixed by active materials,acetylene black and sodium alginate (85%(mass) :10%(mass) :5%(mass)).The used liquid electrolyte was lithium hexafluorophosphate (LiPF6,1.0 mol·L-1) in a mixture of ethylene carbonate (EC),dimethyl carbonate (DMC) and ethyl methyl carbonate(EMC)with a volume ratio of 1:1:1.Celgard polymer membrane was applied as the separator.Cyclic voltammetry (CV) tests were carried out on an electrochemical workstation(CHI660A)at a scan rate of 0.1 mV·s-1.The electrochemical impedance spectroscopy (EIS) tests were measured on the above workstation in a frequency range of 0.01-105Hz.

Fig.1.Schematic diagram for synthesizing CuO-Co3O4-RGO compounds.

Fig.2.XRD patterns of RGO nanosheets,pure CuO,pure Co3O4 and CuO-Co3O4-RGO compounds.

3.Results and Discussion

The target CuO-Co3O4-RGO compounds were synthesizedviaa solvothermal method,the detailed synthesis process was described in Fig.1.The prepared graphene oxide(GO)nanosheets were firstly dispersed in a mixture solution with ethanol and DI water(1:1;v:v),then slowly dropped into the Co2+and Cu2+ions containing ethanol solution.After mixing by ultrasonication for 2 h,the mixture was transferred into Teflon-lined stainless steel autoclave and heated at 140 °C for 10 h.Finally,the collected samples were calcinated at 500 °C for 2 h in the nitrogen atmosphere,followed by thermally treated at 180 °C for 8 h in air to obtain the CuOCo3O4-RGO sample.

Fig.2 reveals the obtained X-ray diffraction (XRD) patterns of the pure RGO,CuO,Co3O4samples and CuO-Co3O4-RGO compounds.Obviously,it can be observed for the CuO-Co3O4-RGO compounds,two groups of diffraction peaks belong to the CuO and Co3O4phases,respectively,which are overlapped with the pure CuO and Co3O4samples.The peaks indexed to (1 1 0),(0 0 2),(1 1 1),(-2 0 2),(0 2 0),(2 0 2),(-1 1 3),(-3 1 1),(2 2 0),(3 1 1) and (-2 2 2) planes are corresponding well to the monoclinic CuO phase (JCPDS 80-1917),and the other peaks are indexed to the (1 1 1),(2 2 0),(3 1 1),(4 0 0),(4 2 2),(5 1 1),(4 4 0) and (5 3 3) planes of the cubic Co3O4phase (JCPDS 74-2120).By comparing the XRD pattern with the pure RGO nanosheets,it can be concluded that the broad peaks near 26°are derived from the graphitic structures of the RGO.

The chemical compositions of the CuO-Co3O4-RGO are further confirmed by X-ray photoelectron spectroscopy(XPS),as displayed in Fig.3.The elements of Cu,Co,O,and C are clearly identified in the survey spectrum (Fig.3(a)).The Cu 2p core level spectrum depicts four peaks at 934.2,942.5,954.1 and 962.7 eV (Fig.3(b)).The distinct signals located at 934.2 and 954.1 eV are belonged to the Cu 2p3/2and Cu 2p1/2peaks of CuO.Besides,two peaks located at around 942.5 and 962.7 eV are assigned to the CuO satellite peaks(Sat.)[22,23].The Co 2p XPS spectrum(Fig.3(c))reveals two groups of peaks corresponding to the Co 2p1/2and Co 2p3/2peaks of Co3O4,with the satellite peaks (Sat.) observed at 803.9 and 788.4 eV [24,25].The fitted results reveal two contributions of Co ion species,including Co2+ions with binding energies at 798.3 and 784.1 eV,as well as Co3+ions at 797.1 and 781.4 eV.The finely scanned C1 s spectrum (Fig.3(d)) shows three photon energies centered at 284.8,286.1 and 287.8 eV,which are attributed to the chemical groups of C-C,C-O/C=O and O-C=O [26].

Fig.3.XPS spectra of (a) the survey,(b) Cu 2p,(c) Co 2p and (d) C 1s in CuO-Co3O4-RGO compounds.

Fig.4.FESEM images of (a) pure RGO nanosheets,(b) CuO-Co3O4 nanosphere powers,(c) CuO-Co3O4-RGO compounds;(d) Energy dispersive spectrometer (EDS)compositional analysis,and elemental mapping of (e) C,(f) O,(g) Co,(h) Cu from the CuO-Co3O4-RGO compounds.

Fig.5.(a)-(c) TEM images and (d) HRTEM image of CuO-Co3O4-RGO compounds,with the SAED pattern (inset).

Fig.6.(a)First three CV curves of CuO-Co3O4-RGO compounds;(b)Galvanostatic discharging and charging curves of CuO-Co3O4-RGO at 100 mA·g-1;(c)Comparative cycling performances with the corresponding coulombic efficiency data of CuO-Co3O4-RGO,CuO-Co3O4,Co3O4 and CuO at 100 mA·g-1;(d) Rate performances of CuO-Co3O4-RGO,CuO-Co3O4,Co3O4 and CuO at different current densities.

Raman spectroscopic analysis was conducted to characterize the CuO-Co3O4-RGO compounds and the RGO nanosheets (Fig.S1(a)).The vibrations located at 298 and 615 cm-1are derived from the CuO and Co3O4phases[24,27],respectively.The peak observed at 1591.5 cm-1is assigned to the bond stretching of sp2 carbon atoms (G band) [28],and the vibration of 1350.2 cm-1is assigned to the K-point phonons of aromatic rings (D band) [28].Furthermore,the intensity ratio of D band and G band (ID/IG) for CuOCo3O4-RGO compounds is around 1.04,larger than that of the RGO (～0.96),which indicates the disordered graphitic structure is formed during the formation of the CuO-Co3O4-RGO sandwiched structure.

The porosity of the CuO-Co3O4-RGO compounds and CuO-Co3O4nanosphere powers are displayed in Fig.S1(b).The typical pore size of CuO-Co3O4nanosphere powers is around 3.8 nm,while that of CuO-Co3O4-RGO compounds is about 4.7 nm.The Brunauer-Emmet-Teller (BET) specific surface area for the CuO-Co3O4-RGO compounds is 141 m2·g-1,which is larger than that of the pure CuO-Co3O4nanosphere powers (118 m2·g-1).The data indicated that the RGO encapsulation creates a 3D storage structure with more active sites,which is benefit for the effective infiltration of electrolyte through the CuO-Co3O4-RGO electrode during the Liion insertion/extraction process.

Field-emission scanning electron microscopy (FESEM) studies are conducted to exam the morphology of the as-obtained pure RGO nanosheets,CuO-Co3O4nanosphere powers and CuO-Co3O4-RGO compounds.As shown in Fig.4(a),it’s revealed that the RGO nanosheets exhibit a thin layer structure with a lot of wrinkles.Fig.4(b) shows that the CuO-Co3O4nanospheres are monodispersed among the RGO nanosheets with the diameter of 100 nm.The high-magnification FESEM image(Fig.4(c))demonstrates that the building blocks of CuO-Co3O4nanospheres are successfully sandwiched and uniformly distributed among the RGO nanosheet layers.In addition,the surface-scanning element mappings are presented in Fig.4(d)-(h),the four elements of C,O,Co and Cu are dispersed homogenously,indicating the uniformly intercrossed growth of both Co3O4and CuO in the composite product.

Further structure information about the CuO-Co3O4-RGO compounds was obtained from Transmission electron microscope(TEM) analyses.Clearly observed in Fig.5(a) and (b),the CuOCo3O4building blocks are encapsulated in between the RGO nanosheet layers,which have formed a sandwiched structure.It is also be observed that the CuO-Co3O4nanospheres show part of hollow structure (Fig.5(c)),which might be benefit for addressing the volume change issues.The (0 0 2) and (1 1 1) planes of CuO,and the (2 2 0) and (1 1 1) planes of Co3O4are observed in the selected area electron diffraction (SAED) pattern (inset,Fig.5(d)),consistent well with the XRD analysis results (Fig.2).Besides,the High Resolution Transmission Electron Microscope(HRTEM)image(Fig.5(d))reveals obvious lattice spacing of 0.253(CuO:0 0 2)and 0.243 nm (Co3O4:3 1 1),respectively.

As shown in Fig.6(a),cyclic voltammetry (CV) measurements were tested to reveal the reaction mechanisms of CuO-Co3O4-RGO compounds during Li insertion/extraction process.During the first scan,three cathodic peaks situated at 1.05,0.75,0.52 V with an anodic peak at 2.38 V are observed,following reactions can be used to explain the corresponding peaks [17,20]:

Fig.7.SEM images of (a) bare CuO,(b) bare Co3O4 and (c) hybrid CuO-Co3O4 nanosphere powers,and (d) CuO-Co3O4-RGO compounds after cycling performance testing at current density of 100 mA·g-1 for 200 cycles.

The cathodic peak around 1.05 V is ascribed to the redox process of CuO to Cu phases.When the voltage decreases to 0.75 V,metal Co phase can be obtained from the continuous reduction of Co3O4component.The last cathodic peak located at 0.52 V is derived from a solid electrolyte interphase (SEI) film formation.As the positive scan carrying on,the peak around 2.38 V is observed due to the subsequent oxidizations of metallic Cu and Co to the CuO and Co3O4phases,respectively.During the 2nd and 3rd cycles,the intensities of cathodic peaks decrease associated with the integral areas,which can be ascribed to the irreversible capacity loss associated with the SEI formation.The peaks situated about 1.25 and 0.82 V are corresponded to the reduction transformations from CuO and Co3O4to Cu and Co phases,respectively.The electrochemical reactions between the nano-grain metals and Li2O are contributed to the anodic peak located at about 2.51 V.

The curves during the discharge-charge process of the CuOCo3O4-RGO compounds were evaluated at 100 mA·g-1over a potential window of 0.01 to 3.0 V (vs.Li+/Li,Fig.6(b)).The CuOCo3O4-RGO compounds exhibit the initial discharge capacity of～1415.2 mA·h·g-1associated with the corresponding initial efficiency columbic of 75%.The irreversible capacity loss is attributed to the surface formation of SEI film layer,which is ascribed to the decomposition of the organic electrolytes.There is a gradual decrease of capacity when increasing the cycle numbers for the first 50 cycles,after that,the CuO-Co3O4-RGO compounds maintain a reversible specific capacity of～847 mA·h·g-1during the following 50 cycles (Fig.6(b)).The capacity decay is usually ascribed to the volume expansion and structure degradation associated with the Li-ion insertion/extraction for the metal oxide based anodes,which is derived from the conversion reaction mechanism and conductivity-related polarization.That’s why many efforts have been devoted to explore novel electrode materials with improved electrochemical performance by preparing mixed or hybrid metal oxides,designing hierarchical or special morphologies and hybridizing with conductive matrix,etc.

Fig.6(c) reveals the cycling performance of CuO-Co3O4-RGO compounds as well as bare CuO-Co3O4,Co3O4and CuO for comparison.The CuO-Co3O4-RGO compounds show a gradual decrease of the capacity at initial 10 cycles,then the capacity tends to be stable during the following cycles at 100 mA·g-1,which exhibit considerably enhanced cycling stability with capacity retention of 79%.Furthermore,the comparative capacity retention of CuO-Co3O4(～695 mA·h·g-1,68%) is larger than those of individual Co3O4(～502 mA·h·g-1,42%) and CuO (～110 mA·h·g-1,15%) under the same tested conditions.Comparing to the CuO-Co3O4compounds,the individual Co3O4and CuO both show rapidly capacity drop during the increasing cycles.It should be pointed out that the specific capacity of pure Co3O4is higher than that of the CuO-Co3O4-RGO composites during the initial 70 cycles,which might be ascribed to the bigger theoretical capacity of Co3O4(674 mA·h·g-1for CuO and 890 mA·h·g-1for Co3O4).For the CuO-Co3O4-RGO composites,the components of CuO and RGO have lower specific capacities than the Co3O4,which contributes to a smaller initial specific capacity for CuO-Co3O4-RGO composites.

To further estimate the cycling performance,current densities of 500,1000 and 2000 mA·g-1have been applied with the prolonged cycling number of 500 cycles (Fig.S2).At the current density of 500 mA·g-1,the CuO-Co3O4-RGO compounds deliver a capacity of 513.7 mA·g-1after 200 cycles (Fig.S2).Furthermore,the CuO-Co3O4-RGO compounds can still maintain a stable capacity of 278.0 mA·g-1after 500 cycles at current density of 2000 mA·g-1.

Moreover,the rate capabilities of the above four electrodes are also evaluated (Fig.6(d)).It is worth noting that CuO-Co3O4-RGO compounds deliver the high and stable capacities of 927,823,702,594,446,291 and 765 mA·h·g-1after cycling at 0.1,0.2,0.5,1.0,1.5,2.0 and 0.1 A·g-1,respectively.It should be noticed that the capacity of CuO-Co3O4-RGO is regained to 765 mA·h·g-1when recovering to the original 0.1 A·g-1.In contrast,the CuO-Co3O4electrode shows poorer rate capacity because of the inefficient electron transport.Besides,the Co3O4and CuO electrodes show the lowest reversible capacities.

The cycling performance of CuO-Co3O4nanosphere powers with different molar ratios of CuO and Co3O4is also presented in Fig.S3.The CuO-Co3O4electrode (1:2,the molar ratio used for preparing the CuO-Co3O4-RGO compounds) delivers the highest reversible specific capacity of 715 mA·h·g-1and a relatively stable capacity retention during 100 cycles.However,the other electrodes (1:3,1:1,2:1,3:1)exhibit lower capacities and poor capacity retentions.Thus,by optimizing the molar ratios of CuO-Co3O4materials during preparation can play an important role in affecting electrochemical properties.

The electrochemical improvements associated with the increased conductivity of CuO-Co3O4-RGO are further investigated by electrochemical impedance spectroscopy (EIS) analyses,for comparison,CuO-Co3O4,Co3O4and CuO samples are also evaluated.Fig.S4 displays the Nyquist plots with fitted equivalent circuit model (inset,Fig.S4),in whichR1is referred to the solution resistance,R2is referred to the charge-transfer resistance andZwis typical Warburg impedance associated with the linear plots in low frequency region of Nyquist plots.The EIS measurements were tested before(Fig.S4(a))and after(Fig.S4(b))cycling at 100 mA·g-1for 50 cycles.The initial EIS spectra shows the charge-transfer resistance (R2) values of bare CuO (～137 Ω),bare Co3O4(～99 Ω),hybrid CuO-Co3O4(～68 Ω),and CuO-Co3O4-RGO compounds(～31 Ω).After cycling for 50 cycles (Fig.S4(b)),compared to the impedance values of bare CuO (～130 Ω) and bare Co3O4(～125 Ω)as well as hybrid CuO-Co3O4(～95 Ω),CuO-Co3O4-RGO exhibits the lowest charge-transfer resistance (R2) value of～68 Ω.Moreover,theZwvalue of the CuO-Co3O4-RGO (～16 Ω) is smaller than the other electrodes (CuO-Co3O4:～29 Ω,Co3O4:～42 Ω,CuO:～40 Ω.).Therefore,the CuO-Co3O4building blocks can facilitate the Li+ion transference into the inner part and further make full use of the electrode materials.Moreover,benefitted from the conductive and flexible 3D RGO network,CuO-Co3O4-RGO compounds are also demonstrated to allow for faster charge transport efficiency and mitigate the polarization effects during the electrochemical reactions [29].

To further understand the electrochemical performance related to the morphological changes,the bare CuO and Co3O4,hybrid CuO-Co3O4,and CuO-Co3O4-RGO electrodes were collected from the disassembled cells and investigated by the SEM measurements,which had undergone cycling performance tests at the current density of 100 mA·g-1for 200 cycles(Fig.7).For the bare CuO electrode,the morphology of nanosphere powers has been completely crushed (Fig.7(a)).And for the bare Co3O4(Fig.7(b)) and hybrid CuO-Co3O4(Fig.7(c)) nanosphere powers,the spherical morphology has been partially preserved.As can be seen from Fig.7(d),it is obvious that the CuO-Co3O4-RGO electrode can mostly maintain the nanospheres to a great extent,which indicates the CuO-Co3O4-RGO compounds have an enhanced structure integrity.

The improved performances can be attributed to the hybrid CuO-Co3O4nanosphere building blocks and the 3D constructed graphene nanosheet network.On one hand,the highly conductive graphene nanosheets formed as a 3D transport network,can improve the electron transport efficiency,which further accelerate the electrochemical reaction kinetics[28].Furthermore,the robust graphene can serve as a flexible cushion and reduce the volume expansion effect of metal oxides,which is finally maintaining the structure integrity of electrodes [28].On the other hand,the uniformly distributing CuO-Co3O4nanospheres served as a backbone can effectively restrain the agglomeration of graphene nanosheets,while the RGO nanosheets also prevent the agglomeration and the pulverization of the CuO-Co3O4nanospheres [28].Besides,the hybridizing multicomponent of metal oxides can probably give the complementary advantages over each individual component,and might behave like heterostructures with internal electric fields to increase the electrical conductivity of the hybrid metal oxides.

4.Conclusions

Hybrid CuO-Co3O4nanospheres/reduced graphene oxide (CuOCo3O4-RGO) compounds were synthesized by a solvothermal process and constructed into sandwiched structures in a selfassembly manner.The CuO-Co3O4-RGO compounds show a larger reversible capacity,improved cycling stability and enhanced rate capability for LIBs compared with the individual CuO and Co3O4electrodes.The enhanced Li storage performances are mainly attributed to the hybrid CuO-Co3O4nanosphere building blocks as well as the 3D constructed graphene nanosheet network:Firstly,the 3D hybrid architecture can restrain the restacking of RGO nanosheets and the agglomeration of the CuO-Co3O4nanospheres.Secondly,the conductive and flexible graphene nanosheets can provide a unique 3D conductive network and hinder the large volume change of CuO-Co3O4nanospheres.Thirdly,the hybridizing multicomponent of metal oxides gives a complementary advantage over each individual component.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China(21471100,22005199),the Shenzhen Natural Science Fundation(20200813081943001),and the Natural Science Foundation of Guangdong Province,China(2021A1515010241,2021A1515010142).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.06.026.