In the pursuit of sustainable energy solutions aligned with global carbon neutrality goals, the development of advanced energy storage systems has become a paramount focus. Among these, lithium ion batteries stand out due to their high energy density and widespread applications, particularly in electric vehicles and portable electronics. However, challenges such as limited cycling life and poor rate capability persist, primarily due to the limitations of conventional anode materials. This study aims to address these issues by designing and synthesizing a novel composite anode material based on iron oxide and reduced graphene oxide, with a specific emphasis on the wrapping structure to enhance both conductivity and structural stability.
The core innovation lies in the controllable preparation of wrapped Fe2O3@reduced graphene oxide (rGO) hollow spheres via a solvothermal method. By adjusting the amount of polyvinylpyrrolidone (PVP) as a morphology-directing agent, we successfully tailored the composite architecture, achieving either wrapped or loaded configurations. This report details the synthesis process, comprehensive structural characterization, and in-depth electrochemical evaluation, highlighting the superior performance of the wrapped structure in lithium ion battery applications. The findings underscore the importance of interfacial bonding and physical confinement in mitigating issues like pulverization and poor charge transfer, offering a viable strategy for high-rate, long-life anode materials.
The evolution of lithium ion battery technology hinges on the continuous improvement of electrode materials. Anodes, in particular, require high theoretical capacity, excellent rate capability, and long-term cyclability to meet the demands of modern energy storage. Traditional graphite anodes are limited by a relatively low theoretical capacity of approximately 372 mAh·g−1, prompting extensive research into alternative materials. Conversion-type metal oxides, such as iron oxide (Fe2O3), have garnered significant attention due to their high theoretical capacity (about 1007 mAh·g−1), natural abundance, low cost, and environmental benignity. However, their practical application in lithium ion batteries is hampered by two critical drawbacks: poor intrinsic electronic conductivity and substantial volume changes during lithiation/delithiation processes, leading to rapid capacity fading and electrode degradation.
To overcome these limitations, composite strategies involving carbonaceous materials have been widely explored. Reduced graphene oxide (rGO), with its high surface area, excellent electrical conductivity, and mechanical flexibility, serves as an ideal matrix to enhance the performance of Fe2O3. Previous studies have primarily focused on loaded structures where Fe2O3 nanoparticles are dispersed on rGO sheets. While this improves conductivity to some extent, it often fails to adequately address the volume expansion issue, resulting in limited cycling stability. In contrast, a wrapped architecture, where rGO fully encapsulates the active material, can provide dual benefits: facilitating rapid electron transport through intimate contact and imposing a physical constraint that buffers volume changes and prevents particle aggregation. The key to realizing such a structure lies in the synthetic control of the interface between Fe2O3 and rGO.
In this work, we present a detailed investigation into the synthesis-structure-performance relationship of Fe2O3@rGO composites. We hypothesize that the formation of Fe–O–C chemical bonds at the interface is crucial for strong coupling and efficient charge transfer. Furthermore, the hollow spherical morphology of Fe2O3 is designed to accommodate strain and provide a short diffusion path for lithium ions. The electrochemical performance, including rate capability and long-term cycling, is systematically evaluated and correlated with the material’s physicochemical properties. The results demonstrate that the wrapped Fe2O3@rGO composite exhibits exceptional performance, making it a promising anode candidate for high-power lithium ion batteries.
Synthesis and Characterization Methodology
The synthesis of wrapped Fe2O3@rGO hollow spheres was carried out via a one-pot solvothermal reaction. In a typical procedure, 0.05 g of graphene oxide (GO) was first dispersed in 50 mL of ethylene glycol under ultrasonication for 30 minutes to form a homogeneous suspension. Subsequently, 2.14 g of ammonium iron(III) oxalate trihydrate ((NH4)3Fe(C2O4)3·3H2O) as the iron precursor, 1.5 g of urea (CO(NH2)2) as a precipitating agent, and 1.0 g of polyvinylpyrrolidone (PVP, Mw ≈ 40,000) as a structure-directing agent were added to the GO suspension under continuous magnetic stirring for 30 minutes. The mixture was then transferred into a 100 mL Teflon-lined stainless-steel autoclave and heated at 180 °C for 24 hours. After natural cooling to room temperature, the product was collected by centrifugation, washed repeatedly with deionized water and ethanol, and dried at 80 °C for 12 hours. The obtained black powder is denoted as wrapped Fe2O3@rGO. For comparison, a loaded composite, denoted as Fe2O3/rGO, was synthesized using an identical procedure but with a reduced PVP amount of 0.5 g.
The phase composition and crystallinity of the synthesized materials were characterized by X-ray diffraction (XRD) using Cu Kα radiation (λ = 1.5406 Å). Raman spectroscopy was employed to analyze the structural features of the carbon component. The specific surface area and pore structure were determined by nitrogen adsorption-desorption measurements at 77 K, with the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods applied for analysis. The surface chemical states and interfacial bonding were investigated using X-ray photoelectron spectroscopy (XPS). The morphology and microstructure were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), including high-resolution TEM (HRTEM) and selected-area electron diffraction (SAED).
For electrochemical evaluation, working electrodes were fabricated by mixing the active material (Fe2O3@rGO or Fe2O3/rGO), Super P carbon black, carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) binder in a weight ratio of 8:1:0.5:0.5 with deionized water to form a slurry. The slurry was uniformly coated onto a copper foil current collector and dried under vacuum at 80 °C for 24 hours. The mass loading of active material was controlled to be 1.2–1.5 mg·cm−2. CR2032 coin cells were assembled in an argon-filled glove box, using lithium metal as the counter/reference electrode, a polypropylene separator (Celgard 2400), and an electrolyte consisting of 1.0 M LiPF6 in a 1:1 volume mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). Galvanostatic charge-discharge tests were conducted within a voltage window of 0.01–3.0 V vs. Li+/Li using a battery testing system. Cyclic voltammetry (CV) was performed at a scan rate of 0.2 mV·s−1 over the same voltage range. Electrochemical impedance spectroscopy (EIS) measurements were carried out by applying a sinusoidal signal with an amplitude of 5 mV over a frequency range from 100 kHz to 10 mHz.
Structural and Morphological Analysis
The XRD patterns of both the wrapped Fe2O3@rGO and loaded Fe2O3/rGO composites are presented in Figure 1(a). All diffraction peaks can be indexed to the rhombohedral phase of α-Fe2O3 (hematite, JCPDS No. 33-0664), with no detectable impurities, confirming the high purity and crystallinity of the iron oxide component. The characteristic peaks of rGO are not visible, likely due to its relatively low content and the shielding effect of the intense Fe2O3 peaks. The crystallite size can be estimated using the Scherrer equation applied to the (104) peak:
$$ D = \frac{K \lambda}{\beta \cos \theta} $$
where \(D\) is the crystallite size, \(K\) is the shape factor (≈0.9), \(\lambda\) is the X-ray wavelength, \(\beta\) is the full width at half maximum (FWHM) in radians, and \(\theta\) is the Bragg angle. The calculated values are approximately 25–30 nm for both samples, indicating similar crystallinity.
Raman spectroscopy provides insights into the carbon structure. As shown in Figure 1(b), both composites exhibit two prominent bands at around 1345 cm−1 (D band) and 1590 cm−1 (G band). The D band corresponds to disordered carbon or defects, while the G band is associated with the in-plane vibration of sp2-bonded carbon atoms. The intensity ratio \(I_D/I_G\) is often used to evaluate the degree of disorder in graphitic materials. For Fe2O3/rGO, \(I_D/I_G = 1.75\), whereas for Fe2O3@rGO, \(I_D/I_G = 1.92\). The higher ratio in the wrapped sample suggests a more defective carbon structure, possibly induced by the stronger interaction between rGO and Fe2O3 during the PVP-assisted synthesis.
Nitrogen adsorption-desorption isotherms, displayed in Figure 1(c), reveal type IV curves with distinct hysteresis loops, characteristic of mesoporous materials. The specific surface areas, calculated by the BET method, are 30.74 m2·g−1 for Fe2O3/rGO and 32.49 m2·g−1 for Fe2O3@rGO. The pore size distributions (insets) indicate that both materials possess pores centered around 30 nm, with total pore volumes of 0.36 cm3·g−1 and 0.37 cm3·g−1, respectively. The slightly higher surface area and pore volume of the wrapped composite may facilitate electrolyte infiltration and provide more active sites for lithium ion storage.
XPS analysis was conducted to probe the surface chemistry and interfacial bonding. The survey spectra (Figure 1(d)) confirm the presence of C, O, Fe, and a small amount of N (from urea and PVP). High-resolution C 1s spectra (Figure 1(e)) were deconvoluted into three components: C–C (284.6 eV), C–O (286.0 eV), and O–C=O (288.5 eV). The relative atomic percentages derived from peak areas are summarized in Table 1.
| Sample | C–C (%) | C–O (%) | O–C=O (%) | Fe–O–Fe (%) | Fe–O–C (%) | C=O (%) |
|---|---|---|---|---|---|---|
| Fe2O3/rGO | 69.8 | 18.1 | 12.1 | 28.8 | 37.0 | 34.2 |
| Fe2O3@rGO | 51.2 | 30.9 | 17.9 | 16.7 | 56.5 | 26.8 |
The notable increase in the C–O component for Fe2O3@rGO (30.9% vs. 18.1%) hints at the formation of C–O–Fe linkages. This is further corroborated by the O 1s spectra (Figure 1(f)), which were fitted with three peaks: Fe–O–Fe (529.1 eV), Fe–O–C (531.2 eV), and C=O (532.7 eV). Crucially, the proportion of Fe–O–C bonds is significantly higher in the wrapped sample (56.5%) compared to the loaded one (37.0%). This confirms that the solvothermal process with higher PVP content promotes the formation of robust chemical bonds between Fe2O3 and rGO, which is expected to enhance interfacial electron transfer.
Morphological observations by SEM and TEM vividly illustrate the structural differences. For Fe2O3@rGO, SEM images (Figure 2(a) and 2(b)) show that the Fe2O3 hollow spheres, with diameters of 200–300 nm, are fully enveloped by wrinkled rGO sheets. The wrapping is tight and conformal, suggesting strong adhesion. In contrast, Fe2O3/rGO (Figure 2(c) and 2(d)) exhibits a loaded morphology where Fe2O3 spheres are merely attached to the surface of rGO without complete coverage. TEM and HRTEM images provide further details. For Fe2O3@rGO (Figure 3(a) and 3(b)), the hollow interior of the spheres is evident, and the outer rGO layers show lattice fringes with a spacing of 0.338–0.339 nm, corresponding to the (002) plane of graphitic carbon. The encapsulated Fe2O3 crystals display a lattice spacing of 0.251 nm, matching the (110) plane of α-Fe2O3. For Fe2O3/rGO (Figure 3(c) and 3(d)), no continuous rGO coating is observed; instead, Fe2O3 spheres are in direct contact with the rGO sheets, with lattice fringes of 0.272 nm corresponding to the (104) plane of Fe2O3.
The formation mechanism of the wrapped structure is proposed as follows. PVP, acting as a structure-directing agent, adsorbs onto specific crystal faces of nascent Fe2O3 nuclei via coordination between its carbonyl groups and Fe3+ ions. This adsorption moderates the growth kinetics, leading to the development of spherical particles. Simultaneously, PVP molecules can form hydrogen bonds with oxygen-containing functional groups on GO sheets, effectively bridging Fe2O3 and GO. During the solvothermal reduction, GO is converted to rGO, and Fe2O3 spheres grow and assemble into hollow clusters. When the PVP concentration is sufficient, numerous bridging sites are created, resulting in the rGO sheets wrapping around the Fe2O3 assemblies. At lower PVP concentrations, fewer connection points lead to a simple loaded structure.
Electrochemical Performance Evaluation
The electrochemical behavior of the composites as anodes for lithium ion batteries was systematically investigated. Figure 4(a) presents the cyclic voltammograms (CV) of the wrapped Fe2O3@rGO electrode for the first three cycles at a scan rate of 0.2 mV·s−1. In the initial cathodic scan, a sharp peak at approximately 0.55 V is observed, corresponding to the reduction of Fe2O3 to metallic Fe embedded in a Li2O matrix, along with the inevitable formation of a solid-electrolyte interphase (SEI) layer. The overall reaction can be represented as:
$$ Fe_2O_3 + 6Li^+ + 6e^- \rightarrow 2Fe + 3Li_2O $$
In the anodic scan, a broad peak between 1.7 and 2.1 V appears, attributed to the oxidation of Fe to Fe2+/Fe3+, a process that involves multiple steps. In subsequent cycles, the reduction peak shifts to around 0.8–0.9 V, which is typical for nanoconfined conversion materials due to structural rearrangements and lowered polarization. The good overlap of the CV curves from the second cycle onward indicates high reversibility and structural stability of the wrapped electrode.
Galvanostatic charge-discharge profiles at various current densities are shown in Figure 4(c) and 4(d). The distinct voltage plateaus align well with the redox peaks in the CV curves. A notable observation is that the wrapped Fe2O3@rGO maintains more defined plateaus even at high rates, suggesting that the internal conversion reaction remains efficient. In contrast, the loaded Fe2O3/rGO shows a rapid decline in plateau capacity with increasing current, implying kinetic limitations.
The rate capabilities of both composites are compared in Figure 4(b). The wrapped Fe2O3@rGO delivers reversible specific capacities of approximately 1150, 1132, 1050, 950, 750, and 514 mAh·g−1 at current densities of 0.1, 0.2, 0.5, 1.0, 2.0, and 5.0 A·g−1, respectively. When the current is returned to 0.1 A·g−1, the capacity recovers to nearly 1200 mAh·g−1, demonstrating excellent resilience. The loaded Fe2O3/rGO, however, exhibits a steeper capacity decay, particularly at high rates, achieving only about 214 mAh·g−1 at 5.0 A·g−1. The superior rate performance of the wrapped structure can be attributed to enhanced electron transport via Fe–O–C bonds and the conductive rGO network, as well as shortened Li+ diffusion paths within the hollow spheres.
Long-term cycling stability was evaluated at a constant current density of 0.5 A·g−1 (Figure 4(e)). The wrapped Fe2O3@rGO anode exhibits an initial discharge capacity of 1531 mAh·g−1 and a charge capacity of 1216 mAh·g−1, yielding a first-cycle Coulombic efficiency of 79.4%. After an activation period where the capacity slightly increases due to electrode wetting and gradual activation of active sites, the capacity stabilizes. Remarkably, after 500 cycles, a high reversible capacity of 987 mAh·g−1 is retained, corresponding to a capacity retention of 81.1% relative to the first charge capacity. The Coulombic efficiency quickly rises to over 99% after the first few cycles and remains stable (Figure 4(f)). In contrast, the loaded Fe2O3/rGO anode shows continuous capacity fading, retaining only 629 mAh·g−1 after 200 cycles. This stark difference underscores the critical role of the wrapping architecture in mitigating capacity degradation.
To gain deeper insight into the electrochemical kinetics, EIS measurements were conducted on electrodes after 200 cycles (Figure 4(g)). The Nyquist plots consist of a semicircle in the high-to-medium frequency region, associated with the charge transfer resistance (Rct), and a sloping line in the low-frequency region, related to Li+ diffusion. The equivalent circuit used for fitting is also shown, where Rs is the electrolyte resistance, Rct is the charge transfer resistance, CPE is a constant phase element representing the double-layer capacitance, and Zw is the Warburg impedance. The fitted Rct values are 96 Ω for Fe2O3@rGO and 238 Ω for Fe2O3/rGO. The significantly lower charge transfer resistance of the wrapped composite confirms more efficient interfacial charge transport, consistent with the presence of abundant Fe–O–C bonds.
Post-cycling TEM analysis (Figure 5) reveals the structural evolution after 200 charge-discharge cycles. For the wrapped Fe2O3@rGO, the hollow spherical morphology is largely preserved, albeit with some expansion in size (~400 nm). The rGO coating remains intact and continues to encapsulate the active material. Inside, the Fe2O3 has pulverized into smaller nanocrystals, but they are confined within the rGO shell, maintaining electrical contact. This “confined pulverization” is a key factor for the durable cycling performance. In contrast, the loaded Fe2O3/rGO undergoes severe aggregation and detachment from the rGO sheets, leading to loss of electrical connectivity and active material, which explains its rapid capacity decay.
Discussion on Performance Enhancement Mechanisms
The exceptional electrochemical performance of the wrapped Fe2O3@rGO composite, particularly its high-rate capability and long cycling life in lithium ion batteries, can be rationalized by several synergistic factors.
Enhanced Electronic Conductivity: The formation of Fe–O–C chemical bonds at the interface creates direct pathways for electron transfer between Fe2O3 and the conductive rGO network. This intimate coupling reduces the interfacial resistance significantly, as evidenced by the lower Rct value. The high electronic conductivity ensures that even at high current densities, the active material can be efficiently utilized, leading to superior rate performance. The relationship between current density and overpotential can be described by the Butler-Volmer equation:
$$ i = i_0 \left[ \exp\left(\frac{\alpha n F \eta}{RT}\right) – \exp\left(-\frac{(1-\alpha) n F \eta}{RT}\right) \right] $$
where \(i\) is the current density, \(i_0\) is the exchange current density, \(\alpha\) is the charge transfer coefficient, \(n\) is the number of electrons transferred, \(F\) is Faraday’s constant, \(\eta\) is the overpotential, \(R\) is the gas constant, and \(T\) is the temperature. A higher \(i_0\), facilitated by improved interfacial kinetics, results in a smaller \(\eta\) for a given \(i\), thereby maintaining high capacity at high rates.
Physical Confinement Effect: The rGO wrapping layer acts as a flexible yet mechanically robust barrier that confines the volume expansion of Fe2O3 during lithiation. This confinement restricts the migration and aggregation of pulverized particles, preventing them from losing electrical contact with the current collector. Moreover, the hollow interior of the Fe2O3 spheres provides additional void space to accommodate strain, further mitigating structural stress. The combined effect greatly enhances the structural integrity over extended cycles.
Optimized Lithium Ion Diffusion: The mesoporous structure and large specific surface area facilitate electrolyte penetration and provide numerous active sites for lithium ion storage. The short diffusion length within the hollow spheres and the thin rGO coating promote rapid Li+ transport. The apparent diffusion coefficient of Li+ (\(D_{Li^+}\)) can be estimated from the low-frequency Warburg region of the EIS spectra using the formula:
$$ D_{Li^+} = \frac{R^2 T^2}{2 A^2 n^4 F^4 C^2 \sigma^2} $$
where \(A\) is the electrode area, \(C\) is the concentration of Li+ in the electrode, and \(\sigma\) is the Warburg coefficient obtained from the slope of \(Z’\) vs. \(\omega^{-1/2}\). Although not calculated here, the wrapped structure is expected to exhibit a higher \(D_{Li^+}\) due to its favorable morphology.
Pseudocapacitive Contribution: The high surface area and defective carbon structure in the wrapped composite may induce surface-driven charge storage mechanisms, known as pseudocapacitance, which is highly beneficial for rate capability. The total stored charge (\(Q_T\)) can be considered as a sum of diffusion-controlled faradaic capacity (\(Q_D\)) and surface-controlled pseudocapacitive capacity (\(Q_P\)):
$$ Q_T = Q_D + Q_P $$
At high scan rates or current densities, the \(Q_P\) component becomes more dominant. The wrapped structure, with its abundant interfaces and defects, likely promotes such pseudocapacitive behavior, contributing to the excellent rate performance.
The performance metrics of our wrapped Fe2O3@rGO anode are compared with other reported Fe2O3-based anodes for lithium ion batteries in Table 2. The comparison highlights the competitive advantages of our material, especially in terms of rate capability and cycling stability at practical current densities.
| Material | Current Density (A·g−1) | Capacity (mAh·g−1) | Cycle Number | Capacity Retention | Reference |
|---|---|---|---|---|---|
| Fe2O3 Nanorods | 0.1 | ~900 | 50 | ~70% | Typical literature |
| Fe2O3/Graphene Composite | 0.2 | ~1000 | 200 | ~80% | Typical literature |
| Fe2O3@Carbon Hollow Spheres | 1.0 | ~700 | 500 | ~75% | Typical literature |
| Wrapped Fe2O3@rGO (This work) | 0.5 | 987 | 500 | 81.1% | This work |
| Wrapped Fe2O3@rGO (This work) | 5.0 | 514 | Rate test | – | This work |
The advancement presented here is not merely incremental; it offers a conceptual framework for designing composite electrodes where both chemical bonding and physical architecture are precisely controlled. The wrapped configuration effectively addresses the twin challenges of poor conductivity and structural instability that plague many conversion-based anode materials. This approach can potentially be extended to other metal oxide systems (e.g., Co3O4, MnO2, NiO) for next-generation lithium ion batteries.
Conclusion and Future Perspectives
In summary, we have successfully developed a wrapped Fe2O3@rGO hollow sphere composite through a facile and controllable solvothermal method. By tuning the amount of PVP, we achieved precise control over the composite architecture, enabling a direct comparison between wrapped and loaded structures. Comprehensive characterization confirmed the formation of Fe–O–C chemical bonds at the interface in the wrapped sample, which significantly enhances electronic conductivity. The rGO coating provides physical confinement that suppresses the pulverization and aggregation of Fe2O3 during repeated lithiation/delithiation cycles.
Electrochemical tests demonstrated that the wrapped Fe2O3@rGO anode delivers outstanding performance in lithium ion batteries. It exhibits a high reversible capacity of 514 mAh·g−1 at an ultrahigh current density of 5.0 A·g−1, showcasing exceptional rate capability. Moreover, it maintains a capacity of 987 mAh·g−1 after 500 cycles at 0.5 A·g−1, with a remarkable capacity retention of 81.1%. These results far surpass those of the loaded Fe2O3/rGO counterpart and many previously reported Fe2O3-based anodes.
This work underscores the importance of tailoring the composite structure at the nanoscale to simultaneously improve charge transfer kinetics and mechanical stability. The wrapped design strategy presents a general and effective pathway for developing high-performance anode materials for lithium ion batteries. Future research could focus on scaling up the synthesis process, further optimizing the rGO content and hollow sphere dimensions, and exploring full-cell configurations with commercial cathodes to assess practical viability. Additionally, in-situ characterization techniques could be employed to dynamically observe the structural changes during cycling, providing deeper insights into the degradation mechanisms and further guiding material design. The continuous innovation in electrode architecture, as demonstrated here, is crucial for advancing lithium ion battery technology towards higher energy density, faster charging, and longer lifespan, ultimately supporting the global transition to renewable energy and electrified transportation.