Pomegranate‑Shaped Fe₂O₃/C Core‑Shell Nanocomposites: A One‑Step Hydrothermal Route for High‑Performance Li‑Ion Battery Anodes

Abstract

Transition‑metal‑oxide anodes remain limited by severe volume swelling and poor cycling stability. Here, we report a straightforward one‑step hydrothermal synthesis of core‑shell pomegranate‑shaped Fe₂O₃/C nanocomposites, a first for this architecture. As an anode, the material delivers an outstanding 705 mAh g⁻¹ reversible capacity after 100 cycles at 100 mA g⁻¹, and maintains 480 mAh g⁻¹ at an ultra‑high rate of 2000 mA g⁻¹. The superior performance stems from the pomegranate‑like morphology, which guarantees excellent electrical contact, buffers the enormous volume change, and facilitates rapid Li⁺ transport.

Background

Lithium‑ion batteries (LIBs) dominate portable electronics and electric vehicles owing to their high energy density, long cycle life, and low self‑discharge. However, the graphite anode’s theoretical capacity (372 mAh g⁻¹) cannot meet future demands. Transition‑metal oxides (TMOs) such as Fe₂O₃ offer a theoretical capacity of 1007 mAh g⁻¹ and are abundant, non‑toxic, and environmentally friendly. Yet, Fe₂O₃ suffers from dramatic volume expansion and rapid capacity fade during cycling.

Nanostructuring Fe₂O₃ has proven effective in accommodating strain and shortening Li⁺ diffusion paths. Moreover, integrating Fe₂O₃ into conductive matrices—often via carbon coatings—enhances electronic conductivity and mitigates pulverization. Current fabrication routes typically involve multi‑step hydrothermal reactions followed by high‑temperature calcination, which are costly and time‑consuming. A single‑step, scalable synthesis of a robust Fe₂O₃/C core‑shell architecture is therefore highly desirable.

Methods

Iron nitrate nonahydrate (Fe(NO₃)₃·9H₂O) and anhydrous dextrose (C₆H₁₂O₆) were dissolved in 40 mL deionized water at a 10:1 carbon‑to‑iron molar ratio. The solution was sealed in a 100 mL Teflon‑lined autoclave, heated to 190 °C for 9 h, then cooled to room temperature. The resulting precipitate was washed, centrifuged, and dried at 60 °C for 12 h. The composition was confirmed by XRD (Rigaku D/Max 2500 V/pc), Raman spectroscopy, TGA (45.2 wt% C), SEM, TEM, EDS, and XPS. For electrochemical testing, an 80 wt% Fe₂O₃/C, 10 wt% Super‑P, and 10 wt% PVDF slurry was cast on Cu foil, dried at 100 °C, and assembled into CR2025 coin cells with Li foil counter electrode, 1 M LiPF₆ in EC/DMC (1:1), and Celgard 2300 separator. Cycling and rate tests were conducted at 25 °C with a CT‑4008 cycler; CV was measured on a Zahner Im6e workstation.

Results and Discussion

**Phase Identification**
XRD patterns matched the hematite Fe₂O₃ structure (JCPDS 33‑0664) with no discernible carbon peaks, confirming successful coating at 190 °C. Raman spectra displayed the Fe₂O₃ two‑magnon peak (~1306 cm⁻¹) and sharp D/G bands at 1396 and 1571 cm⁻¹, indicating a graphitic carbon layer with a low ID/IG ratio.

**Morphology**
SEM images revealed uniformly dispersed spherical particles (30–40 nm) forming a 3D network. TEM confirmed a core‑shell arrangement: Fe₂O₃ cores (~34 nm) enveloped by ~1.75 nm carbon shells, producing a pomegranate‑like structure. HRTEM showed lattice fringes corresponding to Fe₂O₃ (104) and (012) planes; SAED confirmed polycrystalline Fe₂O₃.

**Electrochemical Performance**
In the first cycle, the discharge capacity reached 917 mAh g⁻¹, with a reversible capacity of 760 mAh g⁻¹ after charging—typical SEI formation loss. Subsequent cycles stabilized at ~760 mAh g⁻¹. After 100 cycles at 100 mA g⁻¹, the capacity retained 705 mAh g⁻¹ (≈90 % of the second‑cycle value) with near‑100 % coulombic efficiency, underscoring excellent cycling stability. Rate capability tests yielded 710, 620, 580, and 480 mAh g⁻¹ at 100, 200, 500, and 2000 mA g⁻¹, respectively; the capacity recovered to 680 mAh g⁻¹ upon returning to 100 mA g⁻¹.

For comparison, plain Fe₂O₃ nanoparticles (25–50 nm) delivered only 720.9 mAh g⁻¹ initially, dropping to 396.5 mAh g⁻¹ after 100 cycles, and exhibited inferior rate performance. The core‑shell design therefore markedly mitigates volume expansion and improves conductivity.

**Theoretical Capacity**
The composite’s theoretical capacity (C_theo = C_Fe₂O₃,theo × Fe₂O₃ % + C_carbon,theo × C %) equals 720 mAh g⁻¹, consistent with the measured values. The slight shortfall is attributed to the inevitable SEI formation and minor structural defects.

**Impedance Analysis**
EIS showed a smaller high‑frequency semicircle for the Fe₂O₃/C electrode, indicating lower charge‑transfer resistance compared to bare Fe₂O₃. The porous pomegranate architecture enhances Li⁺ diffusion and alleviates stress during cycling.

**Comparison with Literature**
When benchmarked against reported Fe₂O₃/C anodes, the pomegranate‑shaped nanocomposite exhibits the highest post‑cycling capacity, affirming the synergistic benefits of the hierarchical core‑shell structure and high graphitic carbon content.

Conclusions

We have developed a scalable, one‑step hydrothermal route to synthesize pomegranate‑shaped Fe₂O₃/C core‑shell nanocomposites. The resulting anode delivers exceptional capacity retention, rate capability, and cycle life, primarily due to its unique morphology that buffers volume change and promotes rapid Li⁺ transport. This material represents a promising candidate for next‑generation LIB anodes.