In Situ X-Ray Diffraction Studies of Na4Fe3(PO4)2(P2O7)/C Nanocomposite for Sodium-Ion Batteries

The sodium-ion batteries (SIB) have attracted wide interest as a competitive alternative to Li-ion batteries for electric energy storage applications due to low cost and wide availability of sodium resources ^[1]. The new mixed-polyanion phosphate phases Na₄M₃(PO₄)₂(P₂O₇) (M = transition metal) are considered as promising candidates ^[2]. Na₄Fe₃(PO₄)₂(P₂O₇) shows a high average discharge voltage around 3.2 V, which is higher than 3.0 V for Na₂FePO₄F, 3.0 V for Na₂FeP₂O₇, 2.7 V for olivine NaFePO₄, and 2.4 V for amorphous FePO₄ ^[3].

In this work, we prepared the Na₄Fe₃(PO₄)₂(P₂O₇)/C nanocomposite by a facile sol-gel route. Typically, 0.01 mol Fe powder and 0.015 mol citric acid were mixed firstly in 30 mL deionized water under vigorous magnetic stirring at 80 ^oC. Then stoichiometric amounts of NaH₂PO₄·2H₂O (0.0133 mol) was added into the solution and stirred at 80 ^oC for 2 h. 0.01 mol ethylene glycol was added into the solution and the oil bath temperature was increased to 120 ^oC and kept for 2 h for gel formation. After the drying process, the obtained xerogel was calcined at 500, 600 ^oC for 10 h in a flowing argon atmosphere to obtain the products, respectively.

The structural evolution and electrochemical Na extraction/insertion mechanism of Na₄Fe₃(PO₄)₂(P₂O₇) electrode for sodium ion batteries during the charge and discharge processes were investigated by using the in situX-ray diffraction. As shown in Fig. 1, the results confirm that Na extraction/insertion reaction proceeds via a one-phase process.

Fig. 1 The in situ X-ray diffraction patterns of the Na₄Fe₃(PO₄)₂(P₂O₇)/C electrode during the 1st charge/discharge process.

Acknowledgements

Financial support from National Basic Research Program of China (973 program, Grant No. 2011CB935903) and National Natural Science Foundation of China (Grant No. 21233004 and 21021002) are gratefully acknowledged.

References

[1] B. Dunn, H. Kamath, J.M. Tarascon, Science 334 (2011) 928.

[2] F. Sanz, C. Parada, J.M. Rojo, C. Ruíz-Valero, Chemistry of Materials 13 (2001) 1334.

[3] H. Kim, I. Park, D.H. Seo, S. Lee, S.W. Kim, W.J. Kwon, Y.U. Park, C.S. Kim, S.W. Jeon, K. Kang, Journal of the American Chemical Society 134 (2012) 10369.