YANG Wangchun ¹, WEI Qiankun ², Chen Jun ², GAN Weijiang ², LI Jiacheng ², JIAO Guohao ²

(1. Guodian Power Guangxi Wind Power Development Co., Ltd., Nanning 530000, Guangxi, China;

2. Institute of High-Performance Materials, Guangxi Academy of Sciences,

Nanning 530007, Guangxi, China)

Extended abstract:[Background and purposes] Lithium-ion batteries have become a widely adopted energy storage technology, due to their high energy density, long cycle life and extremely low self-discharge rate. Among various anode materials, transition metal oxides exhibit high theoretical capacity. Fe₃O₄ has garnered significant attention for its high theoretical specific capacity (∼926 mAh·g⁻¹), abundant elemental resources and low cost. However, its practical application faces challenges, such as particle agglomeration, poor rate performance at high current densities and insufficient cycling stability caused by substantial volume expansion, which severely hinder its development. To address these drawbacks, various modification strategies have been developed, including nanostructure control, carbon coating, doping and composite design. Nevertheless, the types of dopant ions for Fe₃O₄ anode materials remain limited, while the synthesis methods for synergistically constructing suitable carbon composite structures still require further exploration. Herein, a novel Fe₃O₄-based composite (Fe₃O₄-Ti@C) was synthesized by using a two-step method involving hydrothermal reaction and high-temperature annealing, achieving in-situ carbon coating and Ti doping. Phase composition, morphology and half-cell electrochemical performance s of the Fe₃O₄-Ti@C composite were systematically characterized.[Methods] The Fe₃O₄-Ti@C was synthesized by using a two-step method involving hydrothermal reaction and high-temperature annealing. Firstly, resorcinol, formaldehyde solution and ammonia water were sequentially added to a mixed solution of ethanol and deionized water (volume ratio of 1:2). The mixture was magnetically stirred to ensure thorough mixing. Subsequently, Fe(NO₃)₃·9H₂O was added to the solution, followed by the dropwise addition of tetrabutyl titanate and stirring until complete dissolution. Next, the final mixture was transferred into a hydrothermal autoclave and reacted at 120 ℃ for 12 h. After natural cooling, the precipitate was collected and washed three times with deionized water and ethanol to remove impurities. The resulting product was separated through centrifugation and dried at 60 ℃ to obtain a yellowish-brown precursor. Finally, the precursor was placed in a tube furnace and subjected to heat treatment at 550 ℃ for 2 h in N₂. After natural cooling, black Fe₃O₄-Ti@C anode material was obtained. Phase and structural analysis of the samples was carried out by using UltimaIV X-ray powder diffractometer (Cu-Kα radiation) and WITec alpha300R Raman spectrometer. The microscopic morphology of the samples was studied by using S-3400N scanning electron microscopy (SEM) and JEOL JEM-2100F transmission electron microscopy (TEM). Elemental composition and valence state of the sample were characterized by using ESCALAB 250Xi X-ray photoelectron spectrometer. Cyclic voltammetry curve (CV, voltage range of 0.01–3.00 V, scan rate of 0.2 mV·s⁻¹) and electrochemical impedance spectroscopy (EIS, 1.0×10⁻²–1.0×10⁶ Hz, amplitude of 5 mV) of the battery were tested using a Chen Hua electrochemical workstation (CHI660E). Performance parameters of the battery, including rate capability, ion diffusion coefficient and cycle stability, were tested using a Xinwei battery testing system CT-4008, with a charge-discharge range of 0.01–3.00 V.[Results] Both Fe₃O₄ and Fe₃O₄-Ti@C exhibit a cubic phase structure (PDF#89-4319). The introduction of Ti expands the lattice of Fe₃O₄, facilitating additional active sites for Li⁺ intercalation and deintercalation. Distinct D-band (disordered sp³ carbon) and G-band (ordered sp² carbon) peaks are revealed, verifying the successful carbon coating on Fe₃O₄. The higher intensity of the D-band than that of the G-band indicates the amorphous nature of the carbon layer, which possesses structural defects that further enhance Li⁺ storage capacity. High-resolution XPS spectra of Fe₃O₄-Ti@C confirm the coexistence of Fe²⁺/Fe³⁺ mixed valence states, along with the presence of O–C, C=O, C–C/C=C bonds and Ti⁴⁺ ions. SEM and TEM images show that pure Fe₃O₄ particles exhibit irregular morphology with sizes ranging from 50 to 150 nm. In contrast, Fe₃O₄-Ti@C maintains a near-spherical morphology but with rougher surfaces and interparticle agglomeration due to high-temperature carbonization. Particle sizes of Fe₃O₄-Ti@C are reduced to 10–50 nm, with a more uniform distribution. CV curves of Fe₃O₄-Ti@C exhibit nearly overlapping profiles after the second cycle, indicating high reversibility and minimal side reactions. The consistent positions and intensities of reduction/oxidation peaks confirm excellent cycling stability. Galvanostatic charge-discharge curves show coincident profiles in the second and third cycles, suggesting stable SEI formation and electrode activation within the initial cycles. The Fe₃O₄-Ti@C anode delivered a reversible capacity of 1065.1 mAh·g⁻¹ at 0.2 A·g⁻¹ and maintained at 668.9 mAh·g⁻¹ even at 5.0 A·g⁻¹, representing 4.1 and 22.3 times the capacity of pure Fe₃O₄, respectively. EIS results reveal a smaller semicircle radius for Fe₃O₄-Ti@C, indicating lower charge transfer resistance. The steeper slope in the low-frequency region of EIS and higher Li⁺ diffusion coefficients (4.28×10⁻¹¹ to 1.39×10⁻⁸ cm²·s⁻¹), as compared with those of  Fe₃O₄ (5.19×10⁻¹² to 1.01×10⁻⁸ cm²·s⁻¹), confirm enhanced ion transport kinetics. Long cycle testing results show that, after 200 cycles at 0.2 A·g⁻¹, Fe₃O₄-Ti@C retains a discharge capacity of 1080.8 mAh·g⁻¹, with a capacity retention rate of 98.7%, significantly outperforming pureFe₃O₄ (28.6%).[Conclusions] The Fe₃O₄-Ti@C anode delivers a remarkable reversible capacity of 1065.1 mAh·g⁻¹ at 0.2 A·g⁻¹ and maintains at 668.9 mAh·g⁻¹ even at a high current density of 5.0 A·g⁻¹, which are 4.1 and 22.3 times higher than those of pure Fe₃O₄, respectively. Moreover, it exhibits excellent cycling stability, retaining a discharge capacity of 1080.8 mAh·g⁻¹ at 0.2 A·g⁻¹ after 200 cycles with a high capacity retention rate of 98.7%, significantly outperforming pure Fe₃O₄ (28.6%). The superior performance is attributed to the synergistic effect of Ti doping and in-situ carbon coating: Ti-ion doping expands the interplanar spacing of Fe₃O₄, facilitating faster lithium-ion diffusion, while the carbon coating provides a stable conductive network that enhances electron transfer and effectively suppresses volume expansion during cycling. Such Fe₃O₄-Ti@C composite has application potential as the anode material for lithium-ion batteries.

Key words:Fe₃O₄; carbon coating; lithium-ion battery; anode; cycle stability