Synthesis and Electrochemical Properties of Co₃O₄@Reduced Graphene Oxides Derived from MOF as Anodes for Lithium-Ion Battery Applications

Abstract

In this study, we utilized nano-sized Co₃O₄ and reduced graphene oxides (rGOs) as composite anode materials for Li-ion batteries. The Co₃O₄/C composite anode was derived from ZIF67 (Zeolitic Imidazolate Framework-67) and was wrapped in rGOs through precipitation. X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were used to identify the crystal structure, phase purity, and surface morphology of the composite. The composition-optimized Co₃O₄/rGO/C composite anode exhibited a reversible capacity of 1326 mAh/g in the first cycle, which was higher than that of the Co₃O₄/C composite anode with a capacity of 900 mAh/g at a current density of 200 mA/g. Moreover, after 80 cycles, Co₃O₄/rGO/C maintained a capacity of 1251 mAh/g at the same current density, which was also higher than the bare Co₃O₄/C composite (595 mAh/g). Additionally, the Co₃O₄/rGO/C composite exhibited a good capacity retention of 98% after 90 cycles, indicating its excellent cycling stability and high capacity. Therefore, the Co₃O₄/rGO/C electrode has great potential as a promising anode material for Li-ion batteries.

1. Introduction

At present, the technology industry is booming and people’s living habits have changed. 3C products are must-have items, and intelligent functions, such as mobile Internet access, video and audio playback, and cloud information reception, make these electronic products have a large capacity for power supply. With the progress of society and the economy, the development of lithium-ion batteries (LIBs) has played an important role [1,2]. LIBs have the advantages of high-energy density, long-term cycling life, and no memory, and are widely applied in commercial energy storage devices [1,2,3,4,5,6,7,8]. Recently, graphite has been used as a commercial anode because of its easy preparation and low cost [9]. However, due to its easy exfoliation, poor rate capacity and cycling stability, low theoretical capacity (372 mAh/g), and inherent safety risk, commercial graphite materials cannot achieve the high power density goal of the new type LIBs. In order to solve these problems, transition-metal oxides (TMOs) [5,7,8], such as Co₃O₄ [7,8,10,11,12], ZnO [13,14], and Fe₂O₃, with a moderate price, have received considerable attention because of their higher reversible capacity than that of graphite-based anode materials for Li-ion batteries. Nevertheless, the disadvantages of excessive expansion and low electronic conductivity limit their applications. The over-expansion or contraction leads to the materials being easily damaged. Therefore, a higher electronic conductivity and inhibiting volume expansion and contraction are important goals in anode materials made with transition-metal oxides.

Metal–organic frameworks (MOFs) are a class of porous materials composed of metal ions or clusters linked together by organic ligands. They are typically crystalline solids with a high surface area, allowing them to adsorb gases and liquids with a high selectivity and capacity [15]. MOFs have several advantages that make them attractive for various applications, including high surface area, tunable properties, selectivity, stability, and sustainability. MOFs have an exceptionally high surface area, which allows them to adsorb a large amount of gas or liquid molecules. This property is particularly useful for gas storage, separation, and catalysis. MOFs can be synthesized with a wide range of properties, such as pore size, shape, and functionality, by varying the metal ions, ligands, and synthesis conditions. This makes them highly tunable and adaptable to different applications. MOFs can selectively adsorb specific molecules based on their size, shape, and chemical properties. This selectivity can be further enhanced by functionalizing the MOFs with specific chemical groups. MOFs are generally stable under a wide range of conditions, including high temperatures, acidic or basic environments, and exposure to moisture. This stability makes them suitable for long-term storage and use in harsh conditions. MOFs can be synthesized using sustainable and environmentally friendly processes, such as solvothermal or microwave-assisted synthesis. They can also be recycled and reused, making them a potentially eco-friendly alternative to traditional materials. These advantages make MOFs promising for a wide range of applications, including hydrogen storage [15,16], adsorption [15,17,18,19,20,21,22], separation [15,16,20], electromagnetic wave absorption [11,18,23], catalysts [15,24,25,26,27], sensing [18,28], and supercapacitors [12,29,30]. Zeolitic imidazolate frameworks (ZIFs) [31,32], a new subclass of metal–organic frameworks (MOFs) that have exceptional chemical and thermal stability, are promising and widely used MOFs for heterogeneous catalysis due to their uniform pore size and well-defined morphology.

In recent years, graphene has been described as having a large theoretical specific surface area (2630 m²/g) and high intrinsic mobility (200,000 cm²/V·s) [33], and has been used in many applications, including LIBs, supercapacitors, and computer chips. Graphene [34,35] is flexibile and has stable sites located inside individual nanotubes interconnected or with sidewall openings with curved multi-layers that facilitate Li⁺ diffusion. Jin et al. used a Fe-MOF/RGO composite as new promising anode materials for LIBs. The as-synthesized Fe-MOF/RGO composite anode showed superior Li storage with a reversible capacity of 1010 mAh/g after 200 cycles [36]. Yin et al. proposed an rGO coating/sandwiching Co₃O₄ composites fabricated by employing a conventional coprecipitation method with ZIF-67, using a rhombic dodecahedron as a template and GO as a substrate. The as-prepared rGO@Co₃O₄ and Co₃O₄-rGO-Co₃O₄ composites exhibited excellent electrochemical performance with high initial discharge specific capacities of 1451 and 1344 mAh/g at a current density of 100 mA/g, respectively [37]. Gao et al. successfully synthesized an Al-MOF/graphene composite by the hydrothermal method. The Al-MOF/graphene composite exhibited significantly enhanced electrochemical performances. Its capacity increased continuously from 60 to 400 mAh/g at the current density of 100 mA/g [38].

In this study, we propose a novel composite anode material for Li-ion batteries, namely Co₃O₄/rGO/C, which was derived from ZIF-67. The main aim of our research is to address the low conductivity and poor electrical stability issues commonly observed in transition metal oxides (TMOs). To overcome these challenges, we employ reduced graphene oxide (rGO) as a conductive and stabilizing agent, which effectively enhanced the efficiency of lithium-ion diffusion and the overall electrochemical performance of the anode material. Additionally, we believe that combining metal–organic frameworks (MOFs) with carbon-based materials holds great potential for future research in the field of battery materials.

2. Materials and Methods

2.1. Synthesis of ZIF-67 and ZIF-67/GO

A total of 1.35 g of cobalt dinitrate hexahydrate (Co(NO₃)₂·6H₂O, 98%, SHOWA) was dissolved in 10 mL deionized water. Then, 16.5 g of 2-methylimidazole (CH₃C₃H₂N₂H, 99% ACROS) was dissolved in 70 mL deionized water under vigorous stirring at room temperature overnight. The product was collected by centrifugation, washed three times with methyl alcohol, and finally dried at 80 °C for 12 h.

2.2. Synthesis of ZIF-67/GO

A total of 0.25 g of graphene oxide was placed in 250 mL ethanol with 0.25 g of CTAB (hexadecyl-trimethyl-ammonium bromide) and stirred for 10 min. The above-mentioned solution and 0.75 g of ZIF-67 were stirred vigorously on a hotplate at 280 °C and 280 rpm for 1 h. The powder was collected by centrifugation, washed several times with alcohol, and finally dried at 80 °C for 12 h.

2.3. Synthesis of Co₃O₄/C and Co₃O₄/rGO/C Composite

Alumina crucibles loaded with ZIF-67 and ZIF-67/GO powders were placed in a tube furnace and heated to 500 °C at a rate of 5 °C min⁻¹ and maintained for 4 h under an argon atmosphere. Next, three samples in different alumina crucibles were placed in a box furnace and heated to 350 °C at a heating rate of 5 °C min⁻¹ and maintained for 4 h under air atmosphere.

2.4. Characterizations

All the synthesized samples were examined using powder X-ray diffraction (XRD, Bruker D8 Advance Eco) with Cu Kα radiation (λ = 1.5418 Å). The surface morphology of Co₃O₄/C and Co₃O₄/rGO/C was analyzed by scanning electron microscopy (SEM, Hitachi H-7100). High-resolution transmission electron microscopy (HR-TEM) images were taken on JEOL JEM2000FX with the acceleration voltage of 200 kV.

2.5. Electrochemical Measurements

Electrochemical measurements were characterized by the CR2032-type coin cells. The electrodes were prepared by coating a slurry containing 70 wt.% active material, 20 wt.% Super P (Carbon black from Taipei City, Taiwan, Maxwave Co., Ltd., 40 nm), and 10 wt %. PVDF (Polyvinylidene fluoride). The slurry was coated onto a 10 μm copper foil and dried at 80 °C in an oven. The resulting electrodes were then pressed, punched (14 mm in diameter), and dried at 120 °C for 8 h in a vacuum system to remove any residual water. Coin-type cell batteries were assembled in an Ar-gas-filled glove box with H₂O and O₂ content at <0.5 ppm using lithium foil as the counter electrode, 1 M LiPF₆ in a 30 wt.% ethylene carbonate (EC) and 58 wt.% diethyl carbonate (DEC) as the electrolyte solution with 2 wt.% vinylene carbonate and 10 wt.% fluoroethylene carbonates (FEC) as additives. The separators used were Celgard 2325^®. Cycling stability tests were conducted using AcuTech System in a voltage window of 0.05 V to 1.5 V at room temperature. The electrochemical performance of the working electrodes was investigated using cyclic voltammetry (CV) (CH Instruments Analyzer CHI 6273E) in a voltage window of 0.05 to 1.5 V with a scan rate of 0.1 mV·s⁻¹. Electrochemical impedance spectroscopy (EIS) measurements were taken using a CH Instruments Analyzer (CHI 6273E) with a perturbation amplitude of 5 mV and a frequency range between 105 Hz and 10 mHz.

3. Results and Discussion

Our strategy for preparing the Co₃O₄/rGO/C composite is schematically described in Figure 1. Firstly, we synthesized the ZIF-67 crystals in an aqueous solution and the graphene oxide was dispersed in the aqueous solution with CTAB as the dispersant. Subsequently, both were placed in a container. Eventually, the as-synthesized ZIF-67/GO was heated to 500 °C and 350 °C by annealing of ZIF-67/GO under an Ar atmosphere for 4 h at the heating rate of 5 °C min⁻¹ and, then, at an air atmosphere for 4 h at the heating rate of 5 °C min⁻¹.

Figure 2 presents the XRD patterns of the as-synthesized GO, ZIF-67, ZIF-67/GO, and Co₃O₄/rGO/C nanocomposites to understand the crystal structure and phase purity of the as-synthesized ZIF-67 nanocomposites with different contents of GO. The ZIF-67/GO nanocomposites exhibited similar diffraction peaks to those of ZIF-67. The XRD patterns of the as-synthesized ZIF-67, ZIF-67/GO and ZIF-67/GO with weight ratios of 3:1 and 5:1, respectively, showed sharp and strong diffraction peaks at 2Θ of 7.2°, 10.4°, 12.8°, 14.7°, 16.43°, and 18.02°, which could be observed for all samples, corresponding to the (100), (002), (112), (022) (013), and (222) planes of ZIF-67 (CCDC-671073), respectively. The broad peak of GO in ZIF-67/GO was not observed at 2 of 11°. After annealing in air, the ZIF-67/GO composite became the Co₃O₄/rGO/C composite successfully and the XRD pattern of Co₃O₄/rGO/C displayed strong diffraction peaks of Co₃O₄ (JCPDS-421467) at 2 of 19°, 36.84°, 38.54°, 44.8°, and 49.07°, corresponding to (111), (220), (311), (222), and (400), respectively.

The thermal behaviors of the Co₃O₄/C and Co₃O₄/rGO/C composites are shown in Figure 3. We performed a thermogravimetric analysis by heating the samples at a rate of 10 °C min⁻¹ in an air atmosphere. The first weight loss during ~130 °C was attributed to the evaporation of the adsorbed moisture in the air. Subsequently, the second weight loss, of about 50.1% between 356.4 °C and 375.8 °C, corresponded to the consumption of carbon dioxide. The reduced graphene oxide contents of Co₃O₄/rGO/C composite could be estimated at about 8.9 wt.% by weight loss.

Figure 4 shows the SEM images of the Co₃O₄/C and Co₃O₄/rGO/C samples. As shown in Figure 4a, the as-synthesized Co₃O₄/C exhibited a particle structure with a particle size of 100–200 nm. The Co₃O₄/C sample was homogeneously coated by two-dimensional rGO nanosheets in Figure 4b. Flake-liked rGO nanosheets were closely wrapped on the surface of the Co₃O₄/C particles. The SEM image of Co₃O₄/rGO/C revealed that the rGO was dispersed on the Co₃O₄/C particles.

In order to further explore the structural information of the Co₃O₄/C and Co₃O₄/rGO/C composites, the as-prepared materials were identified by TEM characterizations. Figure 5a shows that the Co₃O₄ has a diameter of around 60 nm. Meanwhile, Co₃O₄/rGO/C was further characterized so as to observe the structure and morphology of the composite material more intuitively, as shown in Figure 5b. Additionally, these nanoparticles were dispersed on reduced graphene nanosheets. It can be seen that the structure of Co₃O₄ is coated with layers of carbon and was dispersed on the rGO sheets successfully. The electron diffraction rings of Co₃O₄/C and Co₃O₄/rGO/C are indexed and displayed in the inset of Figure 5a,b. The results further indicate that the as-prepared Co₃O₄ in these two samples was in the pure phase with a polycrystalline structure and confirm that these series of diffraction rings are indexed to the (111), (220), (311), (400), (422), (511), and (440) planes of Co₃O₄ in the selected-area electron diffraction (SAED) patterns.

The specific surface area and the corresponding pore size distribution of Co₃O₄/C and Co₃O₄/rGO/C were determined from the BET measurements with N₂ adsorption/desorption isotherms, as shown in Figure 6a,b. As shown in Figure 6a, the specific surface area of the Co₃O₄/C powder was about 61.2 m² g⁻¹. Additionally, the corresponding pore size distribution curve, shown in the inset of Figure 6a, exhibited an average pore size of 11.1 nm by the Barrett–Joyner–Halenda (BJH) method. The results indicate that the specific surface area of the Co₃O₄/rGO/C composite was 28.5 m² g⁻¹. The BET surface area of the Co₃O₄/rGO/C composite was much smaller than that of Co₃O₄/C. The results revel that Co₃O₄/C was wrapped by rGO efficiently. The pore size distribution of the Co₃O₄/rGO/C composite presented an average pore size of 24.15 nm, which is in the range of mesopores.

The long-term cyclic performances of the Co₃O₄/C and Co₃O₄/rGO/C samples, shown in Figure 7a,b, were conducted at the rate of 0.2 A g⁻¹ at the voltage window of 0.05–3 V. Figure 7a depicts the discharge capacity of the Co₃O₄/C and Co₃O₄/rGO/C samples as a function of the cycle number at the current density of 0.2 A g⁻¹ within 95 cycles. Co₃O₄/rGO/C electrodes have the high specific capacity of 1321 mAh g⁻¹ at the beginning of the cycle. The capacity of the Co₃O₄/rGO/C electrode increased considerably that was obviously observed. The reasons for this may be due to: (1) The activation process improved LiB diffusion kinetics, causing a high accessibility for LiB insertion/extraction from materials; and (2) the adjustment action and improvement of Li⁺ accessibility in the electrode, and the conversion of crystalline construction to amorphous structure of Co₃O₄/rGO/C. Figure 7b displays the rate capability of Co₃O₄/C and Co₃O₄/rGO/C electrodes at different current density. The Co₃O₄/rGO/C electrode exhibited the reversible specific capacities of 1333, 1377, 1370, and 1283 mAh g⁻¹ at the current densities of 0.2, 0.5, 1, and 2 A g⁻¹, respectively. As shown in Figure 7c, the Co₃O₄/rGO/C electrode exhibited a more stable Coulombic retention than that of Co₃O₄/C during 90 cycles. Figure 7d displays the galvanostatic discharge–charge profiles of the Co₃O₄/rGO/C composite at the current density of 0.2 A g⁻¹ in the voltage window between 0.01 V and 3.0 V. The initial charges and discharges capacities of the Co₃O₄/rGO/C electrodes were 1273 and 1680 mAh g⁻¹, respectively. The Coulombic efficiency of the Co₃O₄/rGO/C electrodes in the first charge/discharge was maintained at 75.9%. The slightly capacity decline in the Co₃O₄/rGO/C electrodes might result from the formation of SEI between the electrolyte and the electrode material. The obvious discharge plateau of Co₃O₄/rGO/C at around 1.0 V during the discharge process was due to the irreversible conversion reaction of Co₃O₄. After 80 cycles, Co₃O₄/rGO/C electrodes maintained a high capacity of 1260 mAh g⁻¹.

Figure 8a,b shows the typical CV curves of the Co₃O₄/C and Co₃O₄/rGO/C electrodes at a scan rate of 0.1 mV s⁻¹ in the voltage range of 0.01–3.0 V for the first three cycles. During the first discharge process, shown in Figure 8a, the broad peak in ~0.79 V corresponded to the initial reduction of Co₃O₄ to metallic Co accompanied by the formation of Li₂O and solid electrolyte interface (SEI). In the charge process, the peak at 2.1 V was ascribed to the reoxidation of metallic Co to Co₃O₄ and the decomposition of Li₂O. The small peak located at 1.25V in the first cycle for Co₃O₄/rGO/C might be due to SEI formation, to which rGO contributed. The functional groups in rGO could also contribute to SEI formation during the first charging process. After two and three cycles, the charge peak shifted from 2.1 V to a higher voltage, indicating that the Co₃O₄/C electrodes are not sufficiently stable. As depicted in Figure 8b, the reduction peak at 0.79 V in the first anode sweep resulted from the reduction of cobalt ion and the generation of the irreversible formation of the solid electrolyte interface (SEI) film [39]. Meanwhile, there was an intense peak at around 2.1 V in the charge process, corresponding to the initial oxidation of metallic Co to Co₃O₄ accompanied by the formation of metallic Li in the first cycle. From the second and third cycles, the reduction peaks shifted from 0.79 V to 1.17 V, when the oxidation peaks remained unchanged. The oxidation and reduction peaks of the Co₃O₄/rGO/C composites during the second to third cycles presented similar voltages, indicating a high reversibility and low polarization, after introducing rGOs, in improving cycling and structure stability.

As shown in Figure 9a,b, the electrochemical impedance spectral (EIS) measurements of Co₃O₄/C and Co₃O₄/rGO/C involved the Nyquist plots and Randles plots of the electrodes. AC impedances are exhibited in Nyquist plots for Co₃O₄/C and Co₃O₄/rGO/C in Figure 9a. All of the Nyquist plots exhibited a depressed semicircle in the middle- and high-frequency regions and an oblique line in the low-frequency region. Furthermore, a straight line in the low-frequency region indicates ion diffusion in the solid electrode, as previously reported [40]. The equivalent circuit for the Nyquist plots is shown in the inset of Figure 9a, where R_SEI, R_ct, $\sigma$, and D represent the SEI resistance, charge transfer resistance, the slope of Randles plots, and diffusion impedance of the electrode systems, shown in

Table 1. The R_SEI, R_CT, $\sigma$, and diffusion coefficient of Li⁺ in the Co₃O₄/C and Co₃O₄/rGO/C electrodes.

Components	Co3O4/C	Co3O4/rGO/C
RSEI (Ω)	201.1	98.9
RCT (Ω)	21.4	12.1
$\sigma$ (Ω /s0.5)	13.15	19.28
D (cm2/s)	9.83 × 10−15	2.11 × 10−14

, respectively. The R_SEI of the Co₃O₄/C electrodes was 201.1 Ω and that of the Co₃O₄/rGO/C electrodes was 98.9 Ω The impedance of the SEI layer of the Co₃O₄/C electrodes had an about two-times higher value than that of the Co₃O₄/rGO/C electrodes. The rGO coating may help to stabilize the formation of SEI on the surface of the electrode during the charge and discharge processes. The R_CT of Co₃O₄/C was 21.4 Ω and that of Co₃O₄/rGO/C electrodes was 12.1 Ω. The relative lower R_ct of the Co₃O₄/rGO/C electrodes could be ascribed to the higher conductivity of the Co₃O₄/rGO/C electrodes. As can be seen, the slopes of the Randles plots ($\sigma$) of the Co₃O₄/rGO/C and Co₃O₄/C electrodes were 19.28 Ω and 13.15 Ω, respectively. Finally, the diffusion coefficient (D_Li⁺) of the Co₃O₄/rGO/C electrodes was 2.11 × 10⁻¹⁴ cm² s⁻¹ and that of the Co₃O₄/C electrodes was 9.83 × 10⁻¹⁵ cm² s⁻¹. The results reveal that the Co₃O₄/rGO/C electrodes had higher values than those of Co₃O₄/C electrodes by 9.83 × 10⁻¹⁵ cm² s⁻¹ due to the rGO coating.

The Randles plots of Z′ versus ω^−1/2 of the Co₃O₄/C and Co₃O₄/rGO/C electrodes are shown in Figure 9b and can be calculated using the following equation [5]:

$$D_{L{i}^{+}}=\frac{R^2{T}^2}{2A^2{n}^4{F}^4{C}^2{\sigma }^2\omega }$$

(1)

In Equation (1), R, T, A, n, F, and C represent the gas constant (8.314 J mol⁻¹ K⁻¹), absolute temperature (298.5 K), the apparent surface area of the electrode (3.14 cm²), the number of electrons participating in the redox reaction per molecule (one electron), the Faraday constant (96,486 C/mol⁻¹), and the concentration of lithium ions in the shuttle (10⁻³ mol cm⁻³), respectively. Each of these parameters has a specific value. The final parameter, ω, is the Warburg coefficient, which is equal to the slope of the line fitted to the plot of Z′ versus ω^−1/2 at a low frequency, as shown in Figure 9b. The results indicate that the smaller slope was obtained from the Co₃O₄/rGO/C electrodes, which shows the greater Li-ion diffusivity in the Co₃O₄/rGO/C electrodes. The diffusion coefficient of the Co₃O₄/rGO/C electrodes had an about two-times higher value than that of the Co₃O₄/C electrodes, corresponding to the Nyquist plots for the Co₃O₄/C and Co₃O₄/rGO/C electrodes. This result of the AC impedance is consistent with the excellent cycling stability and ionic conductivity of the Co₃O₄/rGO/C composite.

The Galvanostatic Intermittent Titration Technique (GITT) measurements of the as-prepared Co₃O₄/C and Co₃O₄/rGO/C electrodes are shown in Figure 10. Figure 10a,b presents the GITT profiles of the Co₃O₄/C and Co₃O₄/rGO/C electrodes at the current density of 0.2 A g⁻¹. The diffusion coefficient of Co₃O₄/C and Co₃O₄/rGO/C can be calculated using the following equation [41,42,43,44]:

$$D=\frac{4}{\pi }{\left(\frac{n_m{V}_m}{S}\right)}^2{\left. [\frac{\Delta E_S}{\Delta E_t}\right]}^2$$

(2)

In Equation (2), the symbol τ represents the duration of the current pulse in seconds (s), nm represents the number of moles in the system (mol), Vm represents the molar volume of the electrode in cubic centimeters per mole (cm³/mol), and S represents the contact area between the electrode and electrolyte in square centimeters (cm²). ΔE_s refers to the steady-state voltage change due to the current pulse, while ΔE_t represents the voltage change during the constant current pulse, with the IR drop eliminated. The discharge time (t) was 600 s (s) and the rest time (s) was 2400. The entire discharging time was regarded as a discharging state from 0 to 1. The diffusion of the Co₃O₄/rGO/C electrodes at 8%–17% and 20%–55 % discharge states was slightly lower than that of Co₃O₄/C because the electrolyte of Co₃O₄/C, infiltrated in the electrodes, is faster than the electrolyte of Co₃O₄/rGO/C initially, indicating that Li⁺-ion diffusion of the Co₃O₄/C electrodes were higher than those of Co₃O₄/rGO/C before 57.6 % of the discharge state. These results are shown in Figure 6a. In the cycling performance of the as-synthesized Co₃O₄/C and Co₃O₄/rGO/C electrodes at 0.2 A/g, the increase trend of the capacity of the Co₃O₄/C electrode was higher than that of the Co₃O₄/rGO/C electrode within 23 cycles. However, after 60 cycles, the diffusion coefficient of lithium ions in the Co₃O₄/rGO/C electrodes was higher than that of the Co₃O₄/C electrodes after 57.6% of the discharge state. The dense rGO layer might be penetrated and activated after 60 cycles; thus, the reversible capacity and diffusivity of the Li-ion in the Co₃O₄/rGO/C electrodes were dramatically enhanced.

We believe that rGO can significantly enhance the electrochemical properties in Co₃O₄/C composite materials due to three main reasons: (1) The nature of the elasticity of rGO can alleviate the severe volume expansion and contraction of the Co₃O₄ active material throughout the charging and discharging processes, stabilizing the electrode structure of Co₃O₄; (2) By adding GO during the synthesis of Co₃O₄/C, Co₃O₄ aggregation during the nucleation process can be suppressed, enabling the formation of nano-sized dispersion and resulting in a more uniform distribution of Co₃O₄ and conductive carbon materials; and (3) the rGO itself can enhance the three-dimensional conductive network, allowing for smoother conductive pathways during the charging and discharging process and resulting in a significant improvement in the rate performance, reversible capacity, and cycle life of the Co₃O₄/C composite anode.

4. Conclusions

In this study, the Co₃O₄/rGO/C composite anode material was synthesized using a sedimentation method and tested for its electrochemical performance in lithium-ion batteries. The composite material exhibited a higher capacity of 1244 mA/g compared to Co₃O₄/C (584 mAh g⁻¹) after 90 cycles at 2 A/g. The stability of the composite was also tested at different current rates, showing superior stability at 5 A/g and 10 A/g. The Randles plots and GITT analyses suggested that the rGO in the composite facilitated the diffusion of Li⁺ and improved insertion/desertion in lithium ions. Overall, the Co₃O₄/rGO/C composite exhibited a better stability and conductivity and has potential as an anode material in LiBs.