Cobalt Oxide Modified Reduced Graphene Oxide Nanocomposite as Anode Materials for Lithium-Ion Batteries

In this paper, Co3O4 nanoparticles and Co3O4 modified reduced graphene oxide (Co3O4/rGO) are successfully elaborated by hydrothermal method and used as anode materials in lithium-ion batteries. The structure, composition, and morphology of the hydrothermal powders are characterized by XRD, Raman spectroscopy, SEM, and TEM while their electrochemical performance was evaluated using cyclic voltammetry and galvanostatic charge/discharge studies. The Co3O4/rGO anode exhibit improved electrochemical performance in terms of specific capacity, reversibility and stability compared to single-component Co3O4. At 0.1 A g−1, the specific charge/discharge capacity for the pure Co3O4 is 855 mAh g−1 and 850 mAh g−1 respectively, while for Co3O4/rGO composite is about 1198 mAh g−1 and 1285 mAh g−1 respectively. It is found that the conductivity values increase with adding of the rGO from 4.4 × 10−4 Ω cm for the Co3O4 to 4.5 × 105 Ω cm for Co3O4/rGO composite. The improvement in the electrochemical capacity of the composite anode is mainly ascribed to a cooperative effect between the rGO with good electrical conductivity and the unique nano-sized Co3O4 with a short diffusion pathway for lithium ions diffusion.


Introduction
Environmental problems are causing more and more threats to society, including air pollution, climate change, and global warming.These concerns are highly associated with the combustion of fossil fuels.Exploring sustainable and environmentally friendly energy resources is a significant task for the present energy crisis.Solar energy, hydropower, and wind energy are the leading technologies of clean energy.In common, they have unstable and intermittent nature, which increases the cost and the difficulty of energy storage.Rechargeable lithium-ion batteries (RLIBs) are playing an important role in the storage and output of clean energy [1,2].The produced energy can be efficiently stored in rechargeable batteries and be transmitted as needed later on.In addition, rechargeable batteries also dominate in powering electronic devices and plug-in hybrid electric vehicles (PHEVs) [3][4][5][6][7][8].Particularly, PHEVs require highperformance rechargeable batteries with high energy and power densities.LIBs are composed of two electrodes separated by an electrolyte [9].The performance of the batteries depends on the performance of the electrodes (cathode and anode) and the stability of the electrolyte [9].To improve the electrochemical performance of rechargeable batteries, it is necessary to develop new electrode materials with higher capacities.Various materials, for example, graphitic carbons [10][11][12], silicon-or tin-based lithium alloys [13,14], transition-metal oxides [15][16][17], and nitrides [18] have been studied as alternative anode materials for lithium-ion batteries because they can provide higher capacities.Among this category of materials, transition-metal oxides have been considered a promising anode material for lithium-ion batteries (LIBs) thanks to their low cost, abundance, a lamellar structure allowing the intercalation of a considerable amount of lithium, easy synthesis, and high energy density [19].
Among the metal oxide anode, cobalt oxide has attracted tremendous attention and has been widely studied as an anode material for lithium-ion applications [20].However, several problems largely restrict the battery performance of Co 3 O 4 , such as a small Li + diffusion coefficient, low electrical conductivity, irreversible phase transitions, and dissolution of cobalt into the electrolyte.To address these issues, various strategies have been applied, such as incorporating conductive materials along cobalt oxide.For example, Cao et al. [21] have successfully synthesized the Co@2AQ-MnO 2 electrode with improved charge conductivity.Co@2AQ-MnO 2 as an electrode in the battery exhibits good cycling performance with large reversible capacity and excellent rate performance (1534.4mA h g -1 after 200 cycles at 100 mA g -1 and 596.0 mA h g -1 after 1000 cycles at 1000 mA g -1 ).In the same context, Wang et al. [22] have shown that the association of cobalt and covalent organic frameworks leads to a hybrid material with improved electrochemical properties.Recently, rGO, has been considered as potential conductive material due to high electroactive surface area, good chemical stability, high conductivity, and excellent flexibility [23].Several studies have explored the fabrication of graphene-based composite electrode materials for LIBs, such as α-Fe 2 O 3 /reduced graphene oxide [24], α-MnO 2 /graphene [25], and Sn 4 P 3 /rGO [26].All of these materials benefitted from the high electronic conductivity of 2D graphene and exhibited excellent electrochemical performance for lithium-ion batteries.In addition, these different studies show that the association of rGO with transition metal oxide not only improves the electrical conductivities of the oxide anodes but also prevents the agglomeration of the nanostructured active materials during cycling.Mixing the advantages of rGO and oxide nanostructures offer great potential for exploration.Inspired by these materials, we have developed a new composite anode based on the mixture of the rGO and the Co 3 O 4 nanostructured.
In this work, Co 3 O 4 nanospheres with a crystallite size ranging from 50 to 100 nm were synthesized by a facile hydrothermal method.The nano-Co 3 O 4 powders were used to fabricate the new composite Co 3 O 4 /rGO anode.Benefiting from the improved electronic conductivity and preventing the agglomeration of the nanostructured Co 3 O 4 during cycling, Co 3 O 4 /rGO anode delivered a higher specific capacity and rate performance in comparison to the Co 3 O 4 anode.All the results reflect the opportunities of this new composite as advanced anode materials for LIBs.

Preparation of Co 3 O 4 Nanoparticles
Co 3 O 4 nanoparticles were synthesized by hydrothermal process.Commercial Cobalt(II) nitrate hexahydrate (Co(NO 3 ) 2 •6H 2 O) was used as a cobalt source and sodium hydroxide (NaOH) has been used as a template.In a typical synthesis, a mixture of 5.82 g of Co(NO 3 ) 2 •6H 2 O, 0.2 g of NaOH, and 5 mL of distilled water (H 2 O).Precursors were introduced in this order and stirred for 1 h before introducing the resulting suspension in a Teflon-lined steel autoclave and kept at 180 °C for 6 h.At the end of the hydrothermal treatment, a precipitate was obtained.To fabricate the final Co 3 O 4 powder, firstly, the obtained precipitate was washed with distilled water and ethanol and dried at 90 °C under air for 2 h, then finally annealed at 500 °C in the air for 2 h.

Synthesis of the Co 3 O 4 /Graphene Composite
The cobalt oxide powders obtained by the hydrothermal process were used as an initial precursor for the fabrication of the Co 3 O 4 /rGO composite phase.In a typical synthesis, a mixture of Co 3 O 4 (0.1 g), and 0.2 g cetyltrimethylammonium bromide (CTAB), 2 mg of commercial reduced graphene, and distilled water (10 mL) were used.The various precursors are introduced in this order and stirred for 4 h at room temperature.The obtained black-bluish solution was introduced in a Teflon-lined steel autoclave at 200 °C for 24 h.At the end of the reaction, a black suspension was obtained.The precipitate was centrifuged and washed several times with water and ethanol and then calcined for 2 h at 500 °C.

Materials Characterization
The structure of the synthesized material was quantified using X-ray diffraction analysis (Philips PW 1820, PANalyticalX'Pert instrument, 2θ range from 10° to 40° and λ CuKα1 = 1.54056Å).The morphological details of different materials have been examined with a Scanning electron microscope (SEM) and transmission electron microscopy (TEM) images.SEM images were collected using a JEOL JSM-7100 F. TEM images were recorded using a JEOL JEM-2100 F. Raman spectroscopy was performed using a Jobin Yvon T 64,000 spectrometer (blue laser excitation with 488 nm wavelength and < 55 mW power at the sample) the product is in the form of powders.To quantify the specific surface areas, Brunauer-Emmett-Teller (BET) was performed by nitrogen physisorption at 77 K using a BET, ASAT 2020 M+C.

Electrochemical Measurements
The different electrodes have been fabricated from the powders obtained by mixing the active materials (Co 3 O 4 and Co 3 O 4 /rGO composite), acetylene black, and poly(vinylidene fluoride) (PVDF) with a weight ratio of 80:10:10 in N-methyl-2-pyrrolidone (NMP) solvent.The as-prepared slurry was mixed thoroughly by sonication and then pasted onto the aluminum foil thanks to the doctor's blading method.The different coated electrodes were dried at 120 °C for 12 h in a vacuum and have been cut into small disks.The mass loading of the active materials was around 1.9 mg.After the dried treatment, the electrodes were pressed to enhance the contact between the Al foil and the active material.After electrode optimization, coin-type cells (CR2032) were assembled in an Ar-filled glove box.Li foils were used as anodes, microporous polyethylene film was used as a separator and the electrolyte solution was made of 1 M LiPF 6 in ethylene carbonate (EC), diethylene carbonate (DEC), and dimethyl carbonate (DMC) with a volume ratio of 1:1:1.Galvanostatic charge-discharge tests of all cells were carried out on a Netware multi-channel battery with various current densities in the voltage range of 3.0-0.0V at different current densities.Cyclic voltammetry (CVs) was investigated by a BioLogic SP150 within the potential range of 0.0-3.0V.

Structure and Morphology Study
The structure, composition, and crystallinity of the different powders powder synthesized by the hydrothermal process are examined by X-ray diffraction (XRD) (Fig. 1).The XRD result of the powder obtained after the first hydrothermal treatment (Fig. 1a) shows the existence of very fine and intense peaks and these various peaks are indexed to the crystalline phase Co 3 O 4 (JCPDS data file 00-042-1467).The absence of any additional peaks from other phases or impurities indicates that Co 3 O 4 with high purity can be synthesized via hydrothermal synthesis at 180 °C for 6 h.Whereas the XRD analysis of the powder obtained after the second hydrothermal process (Fig. 1b) shows a slight increase in the width of the XRD peaks.The diffractogram highlights the presence of a series of peaks that are perfectly indexed to the crystal phase Co 3 O 4 while a new peak located at 23.9° was detected.This peak could be identified as the rGO phase.This result confirms the presence of composite materials based on Co 3 O 4 and (rGO).
The average crystallite size of the as-synthesized materials was calculated using Scherrer's formula: where L is the average crystallite size, λ = 0.154056 nm, β is the half maximum peak width and θ is the diffraction angle in degrees.The average crystallite size value calculated from XRD patterns of Co 3 O 4 and Co 3 O 4 /rGO composite is about 50 ± 5 nm and 90 ± 10 nm respectively.The structure information of the two materials (Co 3 O 4 and Co 3 O 4 /rGO composite) was studied by Raman spectroscopy (Fig. 2).For the Co 3 O 4 (Fig. 2a) the analysis of the Raman spectrum exhibits well-defined bands centered at around 478, 528, 619 and 693 cm − 1 belong to the characteristics Raman-active E g , 2F 2g , 1F 2g , and A 1g , respectively, which are related to the vibration modes of cobalt oxide.Figure 2b shows the Raman spectrum of the composite materials Co 3 O 4 /rGO.The analysis of the spectrum reveals the presence of the first series of peaks that are attributed to the different vibrations characteristic of the Co 3 O 4 phase and we show also the presence of two broad peaks characteristic of the rGO bands.The two predominant peaks appear at about 1322 and 1592 cm −1 in the spectrum, assigned as the D band originating from L = 0.9 ∕( cos ) the disordered carbon and the G band corresponding to the sp2 hybridized carbon, respectively [9].The G band is associated with the vibrations of sp2 carbon atoms in reduced graphene while the D band is assigned to the vibrations of sp3 carbon atoms.These two bands confirm the presence of rGO in the cobalt oxide nanocomposite.
Figure 3 shows the corresponding SEM and TEM images of the Co 3 O 4 and Co 3 O 4 /rGO composite powders elaborated by the hydrothermal process.The SEM images of the two powders show that there is quite a difference in morphology between Co 3 O 4 and Co 3 O 4 /rGO composite.The SEM images of the Co 3 O 4 nanoparticles (Fig. 3a) show the presence of a homogeneous phase with particles regularly sized and which display a 2D nanosphere morphology, with a radius ranging from 10 to 25 nm.However, the SEM image of the composite material (Fig. 3b) shows that the Co 3 O 4 / rGO composite consists of two types of morphologies: spherical particles due to the presence of Co 3 O 4 and platelets fused between them to form a reduced graphene block.The results observed on the composite material confirm the existence of two morphology distributions (cobalt and rGO).The SEM result confirms the result obtained by XRD and Raman spectroscopy.
The TEM image (Fig. 3c) demonstrates that the spheres consist of numerous interconnected Co 3 O 4 nanocrystals forming the mesoporous structure.The average diameter of spheres can range from 10 to 40 nm.The porosity promotes the diffusion of Li ions in the electrode and enables the achievement of high-rate capability.The TEM images of Co 3 O 4 /rGO composite materials (Fig. 3d) revealed homogeneously dispersed Co 3 O 4 spheres coated by reassembled reduced graphene sheets.rGO sheets are indeed tightly attached to the Co 3 O 4 .It shows that maximum spheres were covered by rGO sheets formed agglomerated nanocomposite system.Besides, some free graphene sheets reconstitute graphite to serve as the highly conductive channels between Co 3 O 4 , which decreases the inner resistance of electrodes and is favorable for stabilizing the electronic conductivity.
The textural properties including specific surface area BET (S BET ), pore volume (Vpor), and average pore size (dpor) of Co 3 O 4 nanospheres and Co 3 O 4 /rGO composite were investigated by the studies of nitrogen adsorption-desorption using the Brunauer-Emmett-Teller method (BET).

Electrochemical Study
A series of electrochemical measurements are made to evaluate the electrochemical properties of the cobalt oxide nanoparticles as anode for LIBs application.To compare the electrochemical performances of the Co 3 O 4 and Co 3 O 4 /rGO composite, CV and galvanostatic charge-discharge (GCD) tests are performed.has an extraordinary effect on its electrochemical performance.The excellent performance of composite Co 3 O 4 /rGO anode is believed to result from the following aspects: (i) the highly conductive rGO sheets facilitate the fast charge transfer within the electrode and to the current collector; (ii) the highly porous structure of Co 3 O 4 nanospheres is beneficial for penetration of the electrolyte into the electrode and creates short diffusion paths of lithium ions; (iii) the well-crystallized nanoparticles ensure the reversible phase transition and the 2D porous structure provide good buffering effects to maintain the electrode structures during the cycling process.For these reasons, in the following, we focus our attention on the study of the Co 3 O 4 /rGO composite as anode materials in LIBs.
Figure 7 shows the CV curves of the composite anode performed at room temperature cycled at a scan rate of 0.1 mV s −1 in the potential range of 3.0 to 0.0 V vs. Li + /Li.The analysis of the CVs highlights the presence of several cathodic and anodic peaks that indicate the different intercalation and de-intercalation processes of lithium ions in the anode, respectively.In the first cycle, a weak shoulder peak Moreover, the weak broad oxidation peak of about 1.21 V was due to the lithium extraction from graphene sheets.In the second cycle, the peak intensity dropped significantly, indicating irreversible reactions occurred in the first cycle.That might be mainly owing to SEI layer formation and electrolyte decomposition.In the third cycle, a pair of redox peaks (0.93/2.1 V) corresponded to the redox couple of cobalt composite/Co.From 2nd cycle to the 100th cycle, the peak location and area integration did not change, suggesting good reversibility and stability.
Figure 8 shows the selected galvanostatic charge/discharge (GCD) profiles of the composite Co 3 O 4 /rGO anode at a current density of 0.5 A g −1 between 3.0 and 0.0 V vs. Li + /Li.The analysis of the discharge curves reveals the presence of several plateaus that highlight the different structural transformation processes of crystalline Co 3 O 4 /rGO composite electrodes.The analysis of the results obtained shows higher electroactivity for the Co 3 O 4 /rGO composite anode associated with a higher initial discharge capacity of 885 mAh g −1 .After 10 cycles, there is a slight decrease in specific capacity of about 3% compared to the initial specific capacity value.
To evaluate the stability and the durability of the various cathodes in terms of the specific capacity, long cycling tests are performed under a constant current density of 0.5 A g −1 between 3.0 and 0.0 V vs. Li + /Li.(Fig. 9).The repetitive cycling of the Co 3 O 4 /rGO anode shows good cyclability and stability in terms of specific capacity up to 100 cycles.However, a discharge capacity of 730 mAh g −1 is delivered during the 100th cycle, which corresponds to a slight decrease of 17% compared to the initial capacity of 885 mAh g −1 .
The reversibility of the process is illustrated by a Coulombic efficiency defined as a Q red /Q ox ratio keeping close to 99% (Fig. 9).This indicates that all the lithium-ion intercalation into the cathode material during the reduction process is expelled after electrode oxidation and that the Li + intercalation/ de-intercalation is reversible in this condition.
Figure 10 shows the rate capabilities of the Co  mAh/g can be recovered as the current density returns to 100 mA/g (0.1 C), suggesting the good structure stability of the sample after a high rate of discharge and charge.This discharge capacity retention is much higher than that previously reported in the literature [27][28][29].In our study, rGO is advantageous compared to other carbonaceous materials.Thus, these results indicate that the addition of the rGO can not only enhance the cycle stability of the anode but also improve the rate capability.Electrochemical impedance spectroscopy (EIS) measurements of of Co This result confirms that the rGO can provide efficient electron conduction of the overall electrode.Therefore, in the composite electrodes, rGO not only prevents aggregation of the intermediate and active materials but also provides good electrical contact with the particles which could reduce the polarization of the electrode.Co 3 O 4 /rGO composite structure not only facilitates the kinetics for Li + ion diffusion and electron transport by shortening the diffusion pathways to the nanoscale but also allows the most freedom for a change in dimension during lithium intercalation/deintercalation.In this composite material, rGO not only serves as a matrix for Co 3 O 4 nanoparticles, but it also acts as a building block and establishes inimitable superstructures with interconnected networks; which is more helpful for the fast transportation of Li + and therefore dramatically improves the performance of electrode materials.

Conclusion
In summary, we have developed a simple, facile, and inexpensive approach to fabricate Co 3 O 4 urchin-like and Co 3 O 4 /rGO nanocomposite.The Co 3 O 4 /rGO composite display excellent capacity and rate performance as an anode material for lithium-ion storage, with a stable reversible capacity of 1285 mAh g −1 at the current of 0.1 A g −1 and 890 mAh g −1 even at 0.3 A g −1 .The porous structure and the enhanced charge conductivity of Co 3 O 4 /rGO particles play a key role in such superior electrochemical properties.The porous structure not only benefits the electrolyte to penetrate the electrode material and significantly improve electron and ion transport, but also effectively alleviates the volume change of the material during the charge and discharge processes to improve the electrochemical properties.This work provides strong evidence that the electrochemical properties of the Co 3 O 4 can be significantly enhanced by the addition of the rGO.
Funding The authors have not disclosed any funding.

Data Availability
The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

Fig. 7 Fig. 8 Fig. 9
Fig. 7 CV curves of Co 3 O 4 /rGO composite at a scan rate of 0.1 mV s −1 in the voltage range between 0 and 3 V (vs.Li + /Li) 3 O 4 nanospheres and Co 3 O 4 /rGO composite were evaluated in the frequency range between 100 kHz and 0.1 Hz.The typical Nyquist plots of of Co 3 O 4 and Co 3 O 4 /rGO are given in Fig. 11.Both Nyquist displays a depressed semicircle in the high-frequency region and a sloped line in the low-frequency, which can be ascribed to the charge transfer resistance and Warburg impedance associated with lithium-ions diffusion in the cathode material, respectively.The straight line observed at low frequency is due to the diffusion processes of cobalt ion through the solution.As shown in Fig. 11, the semicircle diameter of the Co 3 O 4 /rGO nanocomposite is much smaller than the Co 3 O 4 nanospheres in the high-frequency region.

Fig. 10 Fig. 11
Fig. 10 Rate capabilities of V 2 O 5 /rGO composite anode at various current densities