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Fe 2 (MoO 4 ) 3 /carbon nanotubes (CNTs) nanocomposite was prepared by a precipitation method and used as a cathode for sodium-ion batteries (SIBs). The field emission scanning electron microscopy (FESEM) test showed that the CNTs and Fe 2 (MoO 4 ) 3 nanosheets formed an interpenetrating network structure. The X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis confirmed that the Fe 2 (MoO 4 ) 3 was a monoclinic crystal structure and mixed with element states in the nanocomposite. The electrochemical properties of the Fe 2 (MoO 4 ) 3 /CNTs nanocomposite were studies by electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge measurement. The results indicated that Fe 2 (MoO 4 ) 3 /CNTs nanocomposite had high specific capacity and good cycling stability as a cathode material for SIBs. As the charge/discharge curent density is at 1 C, the initial specific capacity of Fe 2 (MoO 4 ) 3 /CNTs nanocomposite is 85 mAhg −1 , and it remains 79.81 mAhg −1 after 50 cycles.

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Nowadays, Na-ion batteries have been largely investigated for large scale applications such as grid storage and transportation because of the huge availability of sodium, its low cost and the similarity of both Li and Na insertion chemistries[[i]]. Some efforts have also been devoted to meet requirements in electric and portable devices such as cell phone or laptop. To do this, we must solve problems involving structural collapse due to high volume change during insertion/deinsertion. A good way to design the sodium electrode materials is by starting with analogue ones that function well for Li-ion batteries[i]. Layer structure of NaxMO2 composition seem to be greatly promising[i] due to its high capacity as well as good structural stability. This work presents structure study and electrochemical behaviors of NaxMO2 (M = Mn, Ni and Co) as positive electrode materials synthesized by solid state reaction at 800oC. X-ray diffraction showed that structure of cobalt-based compound was the same as layer P2-Na0.74CoO2 [[ii]]. The single phase of Na0.44MnO2 was obtained by using difference manganese precursors (carbonate, acetate and electrolytic manganese dioxide) and the compound used acetate precursor revealed the best crystallinity. Ni-based compound could not form without O2 oxidation atmosphere. Cyclic voltammetry and charge/discharge testing in Swagelok cell with Na metal as anode between 2 - 3.8 V vs Na+/Na exhibited a series of phase transitions. We also determined the diffusion of sodium ion into tunnel structure by GITT Technique. The results will be further discussed. Acknowledgements This work was supported by National Foundation for Science and Technology Development (Nafosted) through Project Fund granted of 104.03-2012.46. References [i] Simon Engelke, Storage4, 2013, 1, 1. [ii] J.J. Ding, Y.N. Zhou, Q. Sun, X.Q. Yu, X.Q. Yang, Z.W. Fu, Electrochemica Acta, 87, (2013), 488-393.