Electrospinning Equipment for Research| Effect of annealing atmosphere on the phase composition andelectrochemical properties of iron-oxide-based electrospun nanofibers

Prof. Wang Yiqian (Qingdao University): Effect of Annealing Atmosphere on Phase Composition and Electrochemical Properties of Iron Oxide-Based Electrospun Fiber Membranes

Iron oxides have attracted wide attention due to their unique advantages such as environmental sustainability, low cost, and excellent electrochemical performance. Iron oxide fiber membranes prepared by electrospinning combined with high-temperature annealing show great potential as binder-free anode materials for lithium-ion batteries. While previous studies have made progress in understanding the effects of precursor solution composition and annealing temperature on the properties of iron oxide fiber membranes, the influence of annealing atmosphere - a critical factor - on their phase composition and electrochemical performance has not been reported.To address this, the current study successfully prepared iron oxide fiber membranes using electrospinning and systematically investigated the effects of different annealing atmospheres on the phase composition and electrochemical properties of iron oxide-based electrospun fiber membranes. 

Recently, Prof. Wang Yiqian's team at Qingdao University published their latest research findings titled "Effect of annealing atmosphere on the phase composition and electrochemical properties of iron-oxide-based electrospun nanofibers" in the Journal of Energy Storage.The research team successfully prepared free-standing, lightweight, and flexible iron oxide-based nanofiber membranes through electrospinning combined with annealing (specific process shown in Figure 1), and studied the influence of annealing atmosphere on product morphology, structure, and electrochemical performance. 

Figure 1: Schematic diagram of the preparation process for FO-P 600 and F-P 600.

Figure 2: SEM images of FO-P 600 and F-P 600.

Annealing at 600°C in argon produced carbon fibers loaded with Fe3O4 nanoparticles (F-P 600), while annealing in air at the same temperature yielded fibers composed of Fe2O3 nanoparticles (FO-P 600).As shown in Figure 2, FO-P 600 consists of nanoparticle-assembled fibers with an average diameter of about 150 nm; F-P 600 shows carbon fibers loaded with nanoparticles, featuring uniform size, relatively smooth surface, no pores, and an average diameter of about 190 nm. F-P 600 exhibits better cycling stability because the carbon in the F-P 600 electrode can alleviate the volume expansion of Fe3O4 nanoparticles during cycling, thereby improving the cycling stability and electrochemical performance of the electrode.

Figure 3: (a) CV curves of F-P 600 and (b) FO-P 600; (c) Cycling performance of F-P 600 and FO-P 600 at 0.2 A g-1; (d) Rate performance of F-P 600 and FO-P 600 at different current densities.

Figure 4: (a) EIS spectra of F-P 600 and FO-P 600 electrodes, with inset showing equivalent circuit diagram; (b) Fitting plot of Zre versus ω-0.5 for F-P 600 and FO-P 600 electrodes, with inset table showing slopes after linear fitting.

From the CV curves of F-P 600 and FO-P 600, it can be seen that the lithium storage mechanisms of products obtained under different annealing atmospheres are different: the F-P 600 electrode mainly stores lithium through conversion reactions; while the FO-P 600 electrode can store lithium not only through conversion reactions but also through pseudocapacitive processes. The numerous pores in FO-P 600 can provide abundant channels for lithium insertion.

Furthermore, cycling and rate performance tests were conducted on F-P 600 and FO-P 600. The F-P 600 electrode maintained a specific capacity of 709.0 mAh g-1 after 100 cycles at a current density of 0.2 A g-1, significantly higher than the FO-P 600 electrode (187.2 mAh g-1). Notably, F-P 600 shows superior rate performance at high currents. 

The Rct values of F-P 600 and FO-P 600 electrodes are 312.0 Ω and 395.0 Ω respectively, while their Rs values are 6.6 Ω and 3.8 Ω respectively (Rct and Rs represent electrolyte impedance and charge transfer impedance, respectively). The lithium ion diffusion coefficients of F-P 600 and FO-P 600 electrodes are 5.8×10-18 and 9.5×10-18 cm2 s-1, respectively.These results indicate that the FO-P 600 electrode has larger charge transfer impedance, while the F-P 600 electrode has higher conductivity than the FO-P 600 electrode. 

As anode materials for lithium-ion batteries, compared with FO-P 600, the F-P 600 electrode demonstrates superior cycling stability and rate performance. This excellent performance is mainly attributed to two aspects: In the FO-P 600 electrode, Fe2O3 nanoparticles undergo severe volume expansion during charge/discharge processes, leading to pulverization and agglomeration of electrode materials, which adversely affects electrode performance. In contrast, in the F-P 600 electrode, the introduction of carbon material effectively mitigates the volume expansion of Fe3O4 nanoparticles during charge/discharge processes, thereby significantly improving the electrochemical performance of the electrode.

Paper link: https://doi.org/10.1016/j.est.2025.116851