Journal of Power Sources 160 (2006) 698–703 The preparation of NaV 1-x Cr x PO 4 F cathode materials for sodium-ion battery Haitao Zhuo a , Xianyou Wang a,∗ , Anping Tang a , Zhiming Liu a , Sergio Gamboa b , P.J. Sebastian b a College of Chemistry, Xiangtan University, Hunan 411105, China b Solar-Hydrogen-Fuel Cell Group, CIE-UNAM, Temixco 62580, Morelos, Mexico Received 1 December 2005; received in revised form 18 December 2005; accepted 22 December 2005 Available online 14 February 2006 Abstract The key to development of sodium-ion battery is the preparation of cathode/anode materials. Cr doped NaV 1-x Cr x PO 4 F(x = 0, 0.04, 0.08) were prepared by the high temperature solid-state reaction for the application of cathode material of sodium-ion batteries. The structures and morphologies of the cathode materials were characterized by Flourier-infrared spectra (FT-IR), X-ray diffraction (XRD) and scanning electron microscope (SEM). The effects of Cr doping on performances of the cathode materials were analyzed in terms of the crystal structure, charge–discharge curves and cycle performances. The results showed that the as-prepared Cr-doped materials have a better cycle stability than the un-doped one, an initial reversible capacity of 83.3 mAh g -1 can be obtained, and the first charge–discharge efficiency is about 90.3%. In addition, it was also observed that the reversible capacity retention of the material is still 91.4% in the 20th cycles. © 2006 Elsevier B.V. All rights reserved. Keywords: Sodium ion battery; Cathode materials; Capacity; Capacity retention; Cr doping; NaV 1-x Cr x PO 4 F 1. Introduction Lithium rechargeable batteries are now well established as power sources for portable electronic equipments such as cellular phone, camcorder and laptop computer. If recharge- able sodium-ion batteries with good performance characteristics could be developed, it would have some significant advantages over lithium-ion batteries, notably a reduction in raw materials cost and the ability to utilize electrolyte systems of lower decom- position potential (due to the higher half-reaction potential for sodium relative to lithium) [1]. So, sodium-ion batteries will be a kind of promising novel batteries. With the development of anode materials in sodium-ion bat- teries [1–7], the studies of cathode materials with high perfor- mances will become more and more important. Recently, a num- ber of studies about cathode materials have been reported [8,9], for example, Na x CoO 2 , Na x MnO 2 [8], Na x TiS 2 , Na x NbS 2 Cl 2 , ∗ Corresponding author. Tel.: +86 732 8293371; fax: +86 732 8292061. E-mail address: wxianyou@yahoo.com (X. Wang). Na x WO 3-x , Na x V 0.5 Cr 0.5 S, Na x MoS 3 , Na x TaS 2 [9–13] and so on. However, the bad reversibility of intercalation and deinterca- lation of sodium-ion in above materials and the lower reversible capacity or the greater transferring resistance of the materials have limited their commercial use. Recently, Barker et al. reported that the NaVPO 4 F was used in conjunction with commercially available hard carbon mate- rial to form viable sodium-ion batteries [14–17]. The average discharge voltage for the sodium-ion battery was determined to be about 3.7 V, a figure comparable with commercially available lithium-ion cells. But the discharge capacity of NaVPO 4 F was declined to less than by 50% of its initial discharge capacity after 30 charge/discharge cycles. Although NaVPO 4 F is cur- rently considered one of the most hopeful cathode materials for sodium-ion battery, its performance should be highly improved by modification or doping technology. It is well known that dop- ing technology has been widely used in lithium-ion battery to improve the capacity retention [18,19]. In this paper, NaVPO 4 F were synthesized with a high temperature solid-state reaction, and Cr was doped into this material to prepare NaV 1-x Cr x PO 4 F (x = 0–0.1) powders. The effects of Cr doping on structure and 0378-7753/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2005.12.079