A Facile Approach to Prepare Rattle/Hollow-Type α-Fe2O3 Submicron Spheres with a Thin Carbon Layer for High-Performance Lithium Ion Battery Anodes

Recently, the hollow structures of various transition metal oxide materials (cobalt oxides [1], iron oxides [2], nickel oxides [3]) have received particular attention for use in lithium-ion batteries (LIBs) because they not only have a higher theoretical capacity than commercial graphite (372 mAh g^-1), but also possess a large void space that can accommodate huge changes in volume during electrochemical reactions. In our previous report, hollow Fe₃O₄ microspheres exhibited the improvement of cyclability. Although the hollow Fe₃O₄ microspheres show remarkable improvement in cycling stability, most still exhibited capacity fading at high current rates due to their poor electrical conductivity. In order to improve cycling performance at high current rates, we have prepared rattle/hollow-type α-Fe₂O₃ submicron spheres with thin carbon layer through the diffusion of metal ions into the hydrogel template. In this synthetic process, an additional carbon coating process is not necessary such as two-step hydrothermal process and chemical vapour deposition (CVD). In addition, we report a strategic synthesis method to control the internal structures of α-Fe₂O₃ submicron spheres. Of the two types of samples, the rattle-type α-Fe₂O₃ submicron spheres have a high surface area (1415 m² g^-1) because α-Fe₂O₃ nano-grains are formed in polymer networks. During removal process of template particles by heat treatment, because the cross-linked networks surrounding tiny Fe metal precursors act as a barrier to prevent particle growth, the rattle structured α-Fe₂O₃ submicron spheres consisting of nano-sized grains with a high surface area can be formed. In this process, the thin carbon layer is formed on the inside surface as well as outer surface of the α-Fe₂O₃ nano-grains, which was generated from a decomposition of the polymer templates. These rattle/hollow α-Fe₂O₃ submicron spheres with thin carbon layer exhibit excellent cyclability at high current rates (~10C). We demonstrated that the void space and thin carbon layer covering most of the α-Fe₂O₃ surface acts as buffer layer to accommodate variation in the volume of α-Fe₂O₃ and improves charge transfer at high current rates.

References

[1] J. Wnag, N. Yang, H. Tang, A. Dong, A. Jin, M. Yang, D. Kisailus, H. Zhao, Z. Tang and D. Wang, Angew. Chem. Int. Ed., 2013, 52, 6417.

[2] H. –S. Lim, B. –Y. Jung, Y. –K. Sun, K. –D. Suh, Electrochimica Acta, 2012, 75, 123.

[3] N. H. Idris, J. Mater. Res., 2011, 26, 860.