Graphene Doped with Transition Metal Oxides: Enhancement of Anode Performance in Lithium-Ion Batteries

Abstract

In recent years, transition metal oxides (TMOs) have emerged as promising candidates for anode materials in lithium-ion batteries (LIBs) owing to their high theoretical capacities. Regrettably, most TMOs exhibit poor electronic/ionic conductivity and undergo substantial volume expansion during the lithiation/delithiation processes. In this study, an electrostatic spinning method using polyacrylonitrile, graphene, and iron(III) acetylacetonate as precursors was employed to synthesize the Fe₃O₄@G/C composite through carbon coating and graphene doping. The composition, phase structure, and morphology of the Fe₃O₄@G/C composite were thoroughly investigated. The electrochemical performance of the Fe₃O₄@G/C composite as a lithium-ion battery anode was evaluated through a continuous charge–discharge cycling test. After 100 cycles at a current density of 0.1 A/g, the specific capacity of the Fe₃O₄@G/C material remained at 595.8 mAh/g. Additionally, the incorporation of graphene leads to a reduction in the electron orbital energy of Fe, which was verified by comparing the density of states (DOS) before and after the doping. Simultaneously, the electrochemical performance of CoO@G/C and NiO@G/C composites further demonstrates that doping transition metal oxides with graphene can enhance their performance as anodes for lithium-ion batteries. We anticipate that this design concept will open new avenues for the development of transition metal oxides (TMOs) and propel their adoption in practical applications.

1. Introduction

Lithium-ion batteries (LIBs) play a pivotal role in the realms of electronic communication devices, energy storage facilities, and electric vehicles (EVs) [1]. As the core component, electrode materials have emerged as crucial factors in the design of LIB products with significant consideration given to their performance enhancement. Nevertheless, the development of electrode materials, particularly anode materials, with superior performance poses a formidable challenge [2,3,4]. Conventional graphite anode materials, characterized by a low lithium intercalation voltage platform and excellent cycling stability, exhibit a theoretical specific capacity of 372 mAh/g. However, this relatively low specific capacity is insufficient to fully meet the current market demands of anode materials [5]. Compared to traditional graphite anodes, transition metal oxides (TMOs) have garnered increasing attention due to their higher reversible specific capacities, abundant resources, and cost-effectiveness. Nonetheless, some TMOs exhibit relatively low electrical conductivity and significant volume changes during lithium ion insertion and extraction processes, leading to pulverization, poor electrode material stability, and rapid capacity degradation during charge–discharge cycles [6,7].

Addressing the challenges encountered in the application of TMOs as LIB anodes, researchers have adopted a innovation nanostructuring strategy [8]. This strategy involves fabricating TMOs into nanoscale materials to increase their specific surface area, while the transmission pathways for lithium ions and electrons are significantly shortened, thereby enhancing the reactivity and rate performance. Furthermore, nanostructuring effectively mitigates the volumetric expansion effect, preserving the structural integrity of the electrode. Tomar et al. successfully synthesized one-dimensional nanowires of NiCo₂O₄/Co₃O₄ nanocomposites via a hydrothermal method, which exhibited a high lithium storage capacity of 1234 mAh/g after 400 cycles at 0.5 C. Notably, even at a higher rate of 1 C, the specific capacity remained impressive, reaching 1305 mAh/g [9]. Thus, the nanostructure strategy demonstrates superior performance characteristics.

The electrospinning method for the fabrication of nanofibers offers significant advantages in terms of simplicity of setup and cost-effectiveness [10,11]. With adjustable parameters, electrospinning facilitates the production of nanofibers with desirable diameters and shapes, which is characterized by their small diameters, and it holds broad application prospects in biomedicine [12], energy [13], and catalysis [14,15]. Additionally, the continuous production capability of electrospinning facilitates its efficient implementation at an industrial scale. With the continuous advancement of renewable energy technologies, electrospinning is poised to play a pivotal role in the preparation of LIBs and supercapacitor materials. Sheng et al. utilized the electrospinning technique to synthesize composite carbon nanofibers (FOCNFs) featuring α-Fe₂O₃ hollow nanosphere structures [16]. This composite exhibits a porous structure and undergoes morphological evolution over time. The nanoscale Kirkendall effect, which is driven by the differential migration rates between oxygen molecules and Fe³⁺ cations, facilitates the formation of α-Fe₂O₃ hollow nanospheres. When integrated with porous carbon nanofibers, these α-Fe₂O₃ hollow nanospheres demonstrate exceptional electrochemical performance for LIBs, which is characterized by enhanced capacity retention, improved rate capability, and increased cycling stability. The optimized sample achieves an initial capacity of 1904 mAh/g at a current density of 100 mA/g and retains a reversible capacity of 601 mAh/g after 300 charge–discharge cycles at a rate of 0.1 C. The combination of α-Fe₂O₃ nanospheres with the porous carbon nanofiber matrix significantly enhances the electrochemical properties. The electrospinning technology possesses extensive application potential and significant research value in the field of electrochemistry. Therefore, how to accurately adjust the composition and structure of nanofibers by continuously optimizing the preparation process can further expand the application range of electrospinning technology in the field of electrochemistry and improve the performance of related devices, which has become an urgent exploration.

The exceptional electrical, mechanical, and thermal properties of graphene, along with its swift transport kinetics, make it advantageous as an anode material for LIBs. Furthermore, the specific capacity for storing lithium ions to form Li₂C₆ reaches 744 mAh/g, which is approximately twice that of graphite [17]. Nevertheless, the direct utilization of graphene as an electrode leads to a significant decrease in its specific capacity during cycling. This decline is attributed to interlayer restacking and agglomeration induced by van der Waals forces, thereby resulting in a reduced lithium storage capacity. The integration of graphene with TMOs effectively synergizes the inherent strengths of both components while mitigating their respective limitations, ensuring a high specific capacity accompanied by excellent electrical conductivity and cycling stability. A recent study reveals a synergistic effect within these composites. Graphene, as an outstanding conductor, significantly facilitates rapid electron and charge transport, thereby enhancing the overall electrical performance. It serves as a platform for the uniform growth and anchoring of TMO nanoparticles, effectively preventing aggregation and consequently enhancing both the high specific capacity and stable cycling performance. The superior mechanical properties of graphene effectively mitigate the volumetric expansion of TMOs during charge–discharge processes, significantly improving the stability of the electrode material [18,19,20]. Kim et al. demonstrated a simple synthesis route combining electrospinning and calcination to fabricate graphene oxide (rGO)-coated Zn₂SnO₄@NiO nanofibers (ZSO@NiO@GNFs), which exhibited remarkable durability, with a high capacity of 1060 mAh/g over 1600 cycles at a current density of 1000 mA/g, highlighting their potential for practical applications [21].

In this study, carbon nanofibers coated with graphene-doped transition metal oxides were synthesized via an electrospinning method. By utilizing polyacrylonitrile (PAN) as the matrix material, the incorporation of graphene was investigated to address the issues of volume expansion and low electrical conductivity associated with transition metal oxides. As a result, Fe₃O₄@G/C composites were successfully synthesized and comprehensively studied through electrochemical performance analysis and DOS calculations. In the unique structure of electrospun TMO-doped graphene, the large specific surface area of the nanofibers, coupled with the small size of the TMO nanoparticles, effectively shortens the diffusion path of lithium ions. This architecture not only mitigates the issue of low capacity retention caused by volume expansion in TMOs during charging and discharging processes but also imparts excellent cycle stability and capacity retention to the composites. After 100 cycles at a current density of 0.1 A/g, the specific capacities of Fe₃O₄@G/C, CoO@G/C, and NiO@G/C materials remained at 595.8, 538.1, and 546.3 mAh/g, respectively, demonstrating a promising trend of continued performance improvement. It is hoped that the systematic description in this study will offer some perspectives on battery components of LIBs as well as the advances in electrospinning technology in LIBs.

2. Materials and Methods

2.1. Material Source

Polyacrylonitrile (PAN, Mw = 80,000) (Aladin Biochemical Technology Co., Ltd., Shanghai, China), iron (III) acetylacetonate (99.0%) (Aladin Biochemical Technology Co., Ltd., Shanghai, China), N,N-dimethylimidazole (Aladin Biochemical Technology Co., Ltd., Shanghai, China), N,N-dimethylformamide (Aladin Biochemical Technology Co., Ltd., Shanghai, China), Graphene (Aladin Biochemical Technology Co., Ltd., Shanghai, China). All reagents were used directly without further purification.

2.2. Materials Synthesis

Fe@G Synthesis of materials: Acetylacetone iron and graphene were added to a predetermined amount of anhydrous ethanol in a fixed molar ratio, which was followed by vigorous stirring. The resulting mixed solution was subsequently placed in a high-pressure reactor and maintained at 120 °C for a duration of 6 h. The obtained solution should be subjected to centrifugation and drying processes to obtain Fe@G.

Synthesis of Fe₃O₄@G/C Materials: Initially, 0.5 g of polyacrylonitrile (PAN) was dissolved in 5 mL of N,N-dimethylformamide (DMF) and stirred for 10 h to obtain a homogeneous PAN solution. Subsequently, the Fe@G solution was added to the PAN solution, and the resulting mixture was sonicated in an ultrasonic cleaning bath for 1 h, which was followed by stirring at room temperature for 24 h. The well-mixed solution was then subjected to electrospinning at a voltage of 16 kV with a flow rate set at 0.5 mL/h, and the distance between the syringe and collector was maintained at 15 cm. The electrospun fibers were subsequently dried in a constant temperature oven and then stabilized in a tube furnace filled with dry air. The temperature was raised from room temperature to 230 °C at a rate of 5 °C/min and maintained for 2 h. Subsequently, argon gas was introduced, and the temperature was further increased to 800 °C at a rate of 5 °C/min under argon atmosphere. The temperature was maintained for 2 h, and then the furnace was cooled to room temperature inside the furnace. To investigate the optimal graphene doping ratio, Fe@G precursors with molar ratios of 1:0.5, 1:0.6, 1:1, and 1:2 (iron (III) acetylacetonate: graphene) were employed to prepare Fe₃O₄@G/C-1, Fe₃O₄@G/C-2, Fe₃O₄@G/C-3, and Fe₃O₄@G/C-4, respectively. For comparison, a sample (Fe₃O₄@G/C-0) was also prepared by directly adding iron (III) acetylacetonate to the PAN solution without graphene while keeping other conditions unchanged. The synthesis process is shown in Figure 1.

2.3. Materials Characterization

The materials were characterized using an XRD diffractometer (Bruker, Ettlingen, Germany), a 5000X scanning electron microscope (Zeiss, Oberkochen, Germany), and a field-emission transmission electron microscope (JEOL, Tokyo, Japan). The samples were sintered in an SG-1.5-1.0 high-temperature tube furnace (Shanghai Zhengfei Electric Furnace Co., Ltd., Shanghai, China). Other equipment included a super-clean glove box (Mikrouna, Shanghai, China) and a Neware battery testing system (Shenzhen Neware Electronic Co., Ltd., Shenzhen, China).

The samples to be tested were prepared by mixing Super P Li conductive carbon black (ultra-high density), PVDF (Arkema polyvinylidene fluoride binder, HSV900 type), and NMP (solvent) in a mass ratio of 8:1:1. The mixture was thoroughly ground and homogenized. The prepared electrode slurry was uniformly coated onto a 3 cm × 10 cm copper foil, vacuum-dried at 60 °C for 12 h, and then punched into electrodes with a diameter of 12 mm. The loading of active material ranged from 0.8 to 1.2 mg/cm². Half-cells were assembled in a glove box (with both water and oxygen content below 0.01 ppm) using CR2032 coin cells. The separator was made of Celgard 2400 polypropylene, and the counter electrode of the CR2032 coin cell was a commercially available circular lithium disc with a diameter of 1.5 cm. The electrolyte consisted of a 1 M LiPF₆ solution mixed with ethylene carbonate (EC) and diethyl carbonate (DEC) in a 1:1 volume ratio. Galvanostatic charge–discharge (GCD) tests were performed on Neware battery test systems with a potential window from 0.01 to 3.0 V. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests were carried out on a CHI760E electrochemical workstation. All CV curves were obtained in a potential range from 0.01 to 3.0 V. EIS measurements were plotted in a frequency range of 0.01 Hz to 100 kHz with a 10 mV voltage amplitude.

3. Results

To understand the micromorphology of the samples, we characterized the electrospun precursors and sintered powders of these two composite materials using scanning electron microscopy (SEM) to elucidate their microstructures. As shown in Figure 2a, the electrospun carbon fibers derived from PAN exhibit a continuous and randomly oriented morphology. The electrospun precursors of the Fe₃O₄@G/C composite material, as depicted in Figure 2b, exhibit an average diameter of 300 nm and a relatively rough surface. During the subsequent two-step heat treatment process, the fibers were pre-sintered at 230 °C to preserve the fibrous structure. After calcination at 800 °C in a nitrogen atmosphere, as shown in Figure 2c, the fibrous structure remained stable after carbonization with a notable reduction in diameter to approximately 180 nm. The electrospun fibers are interwoven with the surrounding fibers, forming an intrinsic network that facilitates efficient electron transport and provides favorable mechanical properties, making them well suited for direct use as an anode. Figure 2d presents the X-ray diffraction (XRD) pattern of the Fe₃O₄@G/C composite material. The XRD pattern confirms the presence of the Fe₃O₄ phase (PDF#99-000-2246), with characteristic peaks observed at 30.8°, 36.2°, 43.2°, 54.0°, 62.5°, and 74.1°, corresponding to the (220), (311), (400), (331), (422), and (511) crystal planes of Fe₃O₄, respectively.

The morphology of the Fe₃O₄@G/C composites was further investigated using Transmission Electron Microscopy (TEM). In Figure 3a,b, black dots representing dark metal oxide nanoparticles of varying sizes can be observed, which are uniformly distributed within the gray carbon fiber matrix. Notably, the majority of the metal oxide nanoparticles are encapsulated by the carbon fiber matrix, while some others are embedded on the surface of the fiber matrix. A clear lattice fringe image of the circled area in Figure 3b, obtained via High-Resolution Transmission Electron Microscopy (HRTEM), is displayed in Figure 3c. The average particle size of the metal oxide nanoparticles is 10 nm. A linterplanar spacing of 2.5 nm corresponding to the (311) plane of Fe₃O₄ is consistent with the X-Ray Diffraction (XRD) pattern shown in Figure 2d, further indicating the high crystallinity of the synthesized material. As evidenced by the Energy-Dispersive Spectroscopy (EDS) mapping in Figure 3d, Fe, O, and C constituents exhibit homogeneous spatial distribution throughout the fibrous architecture of the Fe₃O₄@G/C composites, confirming the successful and uniform decoration of Fe₃O₄ nanoparticles on the carbonaceous fibers substrates. Complementary characterization through TEM imaging and EDS mapping conclusively indicates the successful encapsulation of metal oxide particles within the carbon matrix. The graphitic carbon matrix serves as a dual-function mechanism: (i) effectively prevents the growth and aggregation of Fe₃O₄@G material crystals during calcination, while (ii) simultaneously enhances the conductivity of the electrode and buffers the strain caused by volume changes during charging and discharging processes [22].

XPS technology was employed to investigate the surface elemental composition and chemical states of the Fe₃O₄@G/C composites with the deconvoluted spectra and corresponding binding energy assignments presented in Figure 4. The full-range XPS survey spectrum in Figure 4a exhibits four distinct peaks particularly associated with the elements Fe, C, N and O along with their primary and secondary peaks. To more precisely analyze the energy spectra of these four elements, individual scans of the primary spectra for each element were conducted. As shown in Figure 4b, the C 1s spectrum of the Fe₃O₄@G/C composites indicates that upon curve fitting analysis of the C elemental composition, a higher proportion of C-C bonds [23] and C-N bonds are observed at 284.2 eV and 285.2 eV, respectively. The deconvoluted components at 284.4 eV and 288.4 eV were corresponded to carbonyl(C=O) and carboxyl(C-O) [24], respectively. Notably, the N element originating from the polyacrylonitrile (PAN) precursor during carbonization form the incorporation of C-N bonds that enhances the electronic conductivity of the material, thereby improving its rate performance. Deconvolution of the O 1s spectrum in Figure 4c displays four distinct peaks, corresponding to lattice oxygen (Fe-O, 530.8 eV), hydroxyl groups (O-H, 531.2 eV), adsorbed water (H₂O, 532.6 eV), and carboxylate species (C=O/O-C-O, 533.5 eV) [25], which collectively reflect the hybrid oxygen environment arising from the composite’s interfacial interactions and surface functionalization. The Fe 2p spectral analysis in Figure 4d indicates that the coexistence of mixed oxidation states corresponds to Fe²⁺ and Fe³⁺. The primary peaks observed at 711.08 eV, 713.08 eV, 723.08 eV, and 726.08 eV are attributed to the Fe 2p_3/2 and Fe 2p_1/2 of Fe³⁺ [26], respectively, indicating that the final product is Fe₃O₄ rather than other forms of iron oxides, which aligns with the X-ray diffraction (XRD) data in Figure 2d.

The electrochemical reactions of the Fe₃O₄@G/C composites were systematically evaluated via cyclic voltammetry (CV) at a scan rate of 0.5 mV/s with the results illustrated in Figure 5a. Figure 5a displays the CV curves for the first six cycles of the Fe₃O₄@G/C composites. During the initial cycle, the cathodic peak observed at 0.25 V is associated with the formation of the solid-electrolyte interphase (SEI) film and the lithiation reaction of Fe₃O₄@G/C, as described in Equation (1). The broad anodic peak observed at 1.5 V is attributed to the oxidation of metallic Fe to Fe₃O₄, as described by Equation (2). After the first cathodic-anodic cycle, the area enclosed by the CV curves decreases significantly [27], indicating a substantial irreversible capacity loss during the initial cycle.

The charge–discharge voltage profiles for the Fe₃O₄@G/C-2 composites during the first, second, third, tenth, twentieth, fiftieth, and hundredth cycles within a voltage range of 0.01–3.0 V at a current rate of 100 mA/g are illustrated in Figure 5b. The charge–discharge curves of the Fe₃O₄@G/C-2 electrode show a high initial discharge capacity of 829.6 mAh/g and a charging capacity of 591.7 mAh/g. In the initial discharge curve, the transient voltage plateau observed between 0.2 and 0.8 V corresponds to the lithiation reaction of Fe₃O₄ and the formation of a substantial SEI film (Equation (1)). After the initial cycle, significant capacity loss is observed during charging and discharging processes. This phenomenon can be attributed to the formation of inactive Li₂O, the development of the SEI film, and incomplete conversion reactions. Notably, the composite profiles nearly overlap after the first cycle, suggesting the successful formation of a robust SEI film on both the surface and interface of the carbon layer during the initial cycle process. This SEI film effectively prevents direct contact between the internal Fe₃O₄ nanoparticles and the electrolyte, thereby preserving the structural integrity of the internal Fe₃O₄ during subsequent charge–discharge cycles. Consequently, this protective mechanism not only enhances the reversible specific capacity but also ensures the excellent reversibility and stability of the sample.

$${Fe}_3{O}_4+x{Li}^{+}+x{e}^{-}\to {Li}_x{Fe}_3{O}_4$$

(1)

$$3Fe+4{Li}_{2}O\leftrightarrow {Fe}_3{O}_4+8{Li}^{+}+8e^{-}$$

(2)

The long-cycle performance of Fe₃O₄@G/C composite anodes with varying graphene doping amounts at a current density of 100 mA/g is illustrated in Figure 5c. With the incorporation of different graphene amounts, notable capacity decay is observed during the initial 10 cycles. This phenomenon can primarily be attributed to the rapid lithiation and delithiation processes that disrupt the metal oxide particles, the sluggish activation of active materials, and the formation of irreversible solid-electrolyte interphase (SEI) films. After 40 cycles, it is noteworthy that Fe₃O₄@G/C-2 exhibits the highest specific capacity among the graphene-added samples, achieving a relatively stable reversible capacity of approximately 600 mAh/g. This exceptional stability and remarkable reversibility can be attributed to the stabilized SEI formed between the electrode and electrolyte, resulting in an almost perfect Coulombic efficiency of approaching 100%. This phenomenon is intimately associated with the contribution of graphene coating and the carbon matrix in mitigating the volume expansion of the oxide. However, excessive doping with graphene can lead to the blockage of Li⁺ channels, thereby reducing their transport efficiency and subsequently causing a decline in capacity, as illustrated in Figure 5c.

As shown in Figure 5d, the electrochemical performance of the Fe₃O₄@G/C samples was quantitatively evaluated through rate capability tests conducted at various current densities. When the current rate was incrementally increased from 100 mA/g to 200, 500, 1000, 2000, and 3000 mA/g, a gradual decrease in the discharge capacity of the Fe₃O₄@G/C electrode was observed. For the Fe₃O₄@G/C electrode, as the current rate increased, the discharge capacity decreased. During the initial 30 cycles, the electrode exhibited instability, which can be attributed to the gradual material activation and insufficient electrolyte infiltration. However, starting from the subsequent current density of 1 A/g, the curves exhibited a pronounced tendency toward stabilization with the discharge capacity reaching a notably more consistent and stable state. After 10 cycles at current densities ranging from 0.1 to 3 A/g, the capacities of Fe₃O₄@G/C-2 were measured to be 580.0, 497.5, 421.6, 339.1, 254.3, and 209.4 mAh/g, respectively. When the current density was restored to 100 mA/g, the capacity of Fe₃O₄@G/C-2 recovered to 540.9 mAh/g, indicating significantly enhanced electrochemical performance compared to other samples. To further investigate the excellent electrochemical performance of the Fe₃O₄@G/C material, electrochemical impedance spectroscopy (EIS) tests were conducted within the frequency range of 0.01 Hz to 100 kHz with the corresponding Nyquist plots displayed in Figure 5d. All Nyquist plots exhibited a semicircle from high to medium frequencies and a linear slope in the low-frequency region. This characteristic indicates the charge transfer resistance (Rct) at the electrode/electrolyte interface and the Warburg impedance (Ws) associated with the diffusion of Li⁺ ions within the fiber structure. Additionally, the high-frequency intercept on the real axis represents Rs, which includes both ohmic resistance and solution resistance. The medium-frequency semicircle indicates the charge transfer impedance (Rct) arising from the interface between the electrolyte and the electrode [28]. The fitting results, which are further detailed in

Table 1. Equivalent circuit parameters fitted to the impedance spectra of various Fe₃O₄@G/C electrodes.

Electrode	Rs (Ω)	Rct (Ω)
Fe3O4@G/C-0	1.3	226.6
Fe3O4@G/C-1	1.8	170.5
Fe3O4@G/C-2	4.7	61.7
Fe3O4@G/C-3	1.4	180.4
Fe3O4@G/C-4	1.3	158.4

, Tables S1 and S2, indicated that the Rct value for the Fe₃O₄@G/C-2 electrode was 61.79 Ω, showing a significantly lower resistance compared to other electrodes, such as CoO@G/C-4 (with the lowest impedance among them at 103.6 Ω) and NiO@G/C-3 (at 94.1 Ω).

The electronic conductivity of potential LIB anode materials is a crucial attribute. Therefore, the electronic structures of Fe₃O₄ and GO-Fe₃O₄ were systematically investigated using the density of states (DOS). Initially, the DOS changes for both models were initially calculated with the comparative results illustrated in Figure 6a,b. The red, gray, and blue spheres correspond to iron, carbon, and oxygen atoms, respectively. It is evident that doping with graphene oxide (GO) results in a minimal shift of the Fermi level toward the conduction band. This shift toward the conduction band (CB) is synonymous with bandgap narrowing, indicating electron transfer from the doped GO to Fe₃O₄. This suggests that GO-doped Fe₃O₄ is likely to exhibit increased electrical conductivity [29]. The Fermi level of the Fe₃O₄ model demonstrates semiconducting behavior across the entire conduction band. The oxide and graphene form a better encapsulation structure, leading to a shift in the overall DOS distribution toward lower energy levels, thereby resulting in a more stable structure. This indicates that graphene can better encapsulate the oxide, addressing volume expansion and stabilizing the processes of lithium-ion intercalation and deintercalation [30]. Furthermore, the total density of states (TDOS) near the Fermi level exhibits an increase, which in turn influences the material’s electrical conductivity. This implies that more electronic states are available for electrons to occupy within this energy range. This is equivalent to providing more conduction channels for electrons, thereby significantly increasing the probability of efficient electron conduction. This finding holds significant implications not only for battery applications but also for other electronic reactions/systems, all of which contribute to the kinetics of LIBs anode materials.

Meanwhile, phase characterization and electrochemical performance tests were systematically performed on the CoO@G/C and NiO@G/C composites with comprehensive datasets archived in the supporting documents. As shown in Figure S6b, the specific capacity of the CoO@G/C-4 composite is significantly higher than that of the other composites, stabilizing at approximately 530 mAh/g after 50 cycles. After 70 cycles, the capacity gradually increases due to the complete reaction of the original Co particles to form a stable solid-electrolyte interface (SEI) film, while the CoO particles becomes fully lithiated. As for NiO@G/C, its reversible capacity decreases during the initial 20 cycles due to insufficient electrolyte infiltration. However, as illustrated in Figure S6f, the cycling performance of the NiO@G/C electrode subsequently reaches a stable state with a reversible capacity of 546.3 mAh/g. It is evident that the addition of graphene significantly enhances the specific capacities of both composite anode materials, exhibiting substantially superior performance compared to their graphene-free counterparts. This result corroborates the excellent improvement in capacity performance achieved by the structure of Fe₃O₄@G/C. In this structure, the incorporation of GO modifies the electronic conductivity of semiconducting materials such as Fe₃O₄, making them electrically conductive. The unique structure of the electrospun carbon fiber matrix, encapsulating metal oxide nanoparticles and doped with graphene, significantly boosts their electrochemical performance. This innovative structural design effectively accommodates the volume changes associated with lithium-ion insertion and extraction, thereby exhibiting superior capacity retention.

4. Conclusions

Based on the findings of this study, it can be firmly concluded that doping graphene oxide (GO) is an efficacious approach to modulate its electronic properties and augment the reversible lithium storage capacity. Specifically, the electrospun carbon fiber framework offers ample space to mitigate the volumetric strain induced by the insertion and extraction of metal oxide nanoparticles and lithium ions. The extensive surface area of the nanofibers, in conjunction with the diminutive size of Fe₃O₄, CoO, and NiO nanoparticles, curtails the diffusion pathway for Li⁺ ions. Furthermore, the integration of graphene not only ensures the electrical conductivity of the electrode and enhances electron transport but also forestalls particle pulverization and agglomeration during cycling. The uniform fiber architecture acts as a conduit between transition metal oxides with poor conductivity, and the incorporation of graphene further ameliorates the volume expansion issue associated with transition metal oxides, effectively bolstering the electrical conductivity and structural stability of the composites. Moreover, as anode materials for lithium-ion batteries, the Fe₃O₄@G/C and CoO@G/C fiber electrodes exhibit superior cycling and rate performance compared to their counterparts without graphene. When evaluated at a current density of 100 mA/g over 100 cycles, the Fe₃O₄@G/C, CoO@G/C, and NiO@G/C anodes demonstrated discharge specific capacities of 595.8, 538.1, and 546.3 mAh/g, respectively, which is accompanied by consistently high Coulombic efficiencies. In Nyquist plots, the graphene-doped samples exhibited significantly lower impedance compared to our reference materials. DOS calculations revealed that the addition of graphene reduced the electronic orbital energy of the metal elements, thereby enhancing the consistency of the electrochemical performance when combined with other transition metal oxides. The synthesis method developed in this study showcases exceptional simplicity and facilitates the scalable production of high-performance anodes, offering a novel strategy for the application of transition metal oxides in next-generation LIB anodes.