Olivine Type NaFePO4 Prepared By Electrochemical Delithiation for Sodium Ion Batteries

Abstract

Nowadays, sodium batteries attracted great interest due to the low cost and the environmental abundance of the sodium. Layered sodium-transition metal oxides, i.e. NaMO2, have been widely investigated as cathode materials for these batteries [1]. However, although Dahn’s group showed the high thermal stability for NaCrO2recently, the results so far obtained are not encouraging since this cathode material suffers of poor cycle life and low thermal stability [2].Metal-phosphate based electrodes, such as Nasicon type Na3V2(PO4)3 and NaFePO4, are believed to be very promising alternative cathodes for sodium batteries [3]. Considerable attention has been addressed to maricite NaFePO4, a phase characterized by the presence of (PO4 3-) tetrahedra that totally surround the Na+ cations, namely a structure with no free channels for Na+ diffusion and accordingly with a poor electrochemical behavior in sodium cells [4]. The NaFePO4 phase with an olivine structure is expected to have better electrochemical properties, as suggested by recent studies conducted on chemically delithiated olivine LiFePO4 [5,6]. The results reported in the quoted references showed in fact that FePO4 heterosite can be obtained by a chemical Li-Na exchange process; the formed NaFePO4, however, could not be cycled due to severe decays in capacity presumably associated with the presence of residual Li in its crystal structure, as speculated by Zaghib’s group.In this work we show that, by adopting a refined, electrochemical process, the LiFePO4 can be successfully and efficiently totally converted in NaFePO4and demonstrate that this material can be used as cathode in a sodium battery having excellent performance in terms of cycle life and rate capability.The process of transformation of olivine LiFePO4 to FePO4 was conducted electrochemically with conditions such as to assure a full delithiation of LiFePO4 and its total exchange in FePO4 (see experimental section for details). Figure 1A shows the voltage profile of this first process, evolving on the expect 3.5V plateau and delivering a capacity almost matching the theoretical value [7], both evidences demonstrating that the transformation of LiFePO4 into FePO4 was indeed almost completed. The achievement of the reaction process is confirmed by the XRD analysis, Figure 1B black and red patterns (LiFePO4 peaks were neglibable), demonstrating that the olivine LiFePO4 phase (triphylite) is fully delithiated to form the olivine FePO4(heterosite, JCPDS card No. 42-0579, Pnma space group) phase.