Ionic Liquid Based Synthesis of α-Fe2O3 nanocrystallite and Its Application in Lithium Ion Battery

Hematite (α-Fe₂O₃), which is the most stable iron oxide with n-type semiconducting properties under ambient conditions, is of great scientific and technological importance. Especially in the energy storage field, it has attracted considerable interest because of their low cost and excellent electrochemical properties in rechargeable lithium ion batteries (LIBs).[1,2] Stimulated by both the promising applications of iron oxides and the novel properties of nanoscale materials, considered efforts have been made in the synthesis of α-Fe₂O₃ nanoparticles, nanorods, nanowires and nanotubes. However, most of the synthesis approaches are complicated and beyond the large scale production. Herein, we report the preparation of α-Fe₂O₃ nanocrystalline using ionic liquid (BMIBF₄) as morphology tuning agent through a facile, one step solvethermal method. 

The role of ionic liquid (IL) played in the preparation process was investigated and a possible formation mechanism was proposed. Fig. 1 showed the morphologies of the products with and without ionic liquid in the reaction system. From which, we can see that uniform nanocrystallites were obtained with the addition of ionic liquid, demonstrating it a good morphology controllable agent for the products. Fig.2 showed that ionic liquid did not affect the phase of the products.

A series of experiments about the formation process ofα-Fe₂O₃ nanocrystallites have been carried out by changing the reaction temperature and time. The results showed that the product obtained at 180 °C for 36 hours exhibited the most homogenous morphology and the best dispersibility. The electrochemical performances of them were examined in rechargeable lithium batteries. Due to the poor conductivity of them, graphene nanosheets (GNSs) were used to composite with them to improve the electrochemical performance. As Fig.3 demonstrated, α-Fe₂O₃ nanocrystallites were successfully dispersed on the surface of GNS (Fig. 3a,b). It exhibited an initial discharge capacity as high as 1900 mAh g^-1, which is much higher than that of primitiveα-Fe₂O₃ electrode of 1600 mAh g^-1in Fig. 3c.

Figure 1. SEM image (a) and TEM image (b) of the product obtained at 180°C for 36 h with IL; SEM image (c) and TEM image (d) of the product obtained at 180°C for 36 h without IL.

Figure 2. XRD patterns of the product obtained at 180°C for 36 h with and without IL.

Figure 3. SEM images (a,b) of GNS supportedα-Fe₂O₃ nanocrystallites and the first charge-discharge curves of the electrodes at a current density of 50 mA g^-1 in LIBs (c).

References

[1] J. Chen, L. Xu, W.Y. Li, X.L. Gou, Advanced Materials, 17 (2005) 582-586.

[2] J.Y. Zhong, C.B. Cao, Y.Y. Liu, W.S. Khan, Chemical Communications, 46 (2010) 3869-3871.