(Invited) New Insight of NaLiFePO4f As a High Voltage Cathode Material for Lithium-Ion Battery

The lithium-ion battery is one of the most promising electrochemical energy storage system having great potential for a wide range of applications. However, there are certain challenges that need to be addressed such as low practical energy density, short lifetime and high price. Three-dimensional polyanion compounds (XO 4 ) m- (X=P, S) with impressing electrochemical performance and stability are considered as the cathode material for next generation lithium-ion battery[1, 2]. Meanwhile, combining high ionic M-F bond with (XO 4 ) m- (X=P, S) is able to enhance the operating voltage, such as LiVPO 4 F which has already been reported with good performance[3]. Comparing with other fluorophosphates, NaLiFePO 4 F has received particular interest due to its high operating voltage and the potential of more than one alkali ion can be activated[4, 5]. Meanwhile, several problems are associated with this material such as low electronic conductivity and limited knowledge about the mechanism of the second lithium-ion activation during the charging and discharging process[6]. In order to solve these problems, first principle simulation based on density functional theory, is employed to investigate the mechanism of lithium-ion extraction of NaLiFePO 4 F (Li 2 FePO 4 F). It is found that not only the transition metal Fe 2+ but also O 2- will lose electron during the delithiation process by electronic structure analysis. Operating voltages and structural evolution are obtained after the calculation of lithiated phases Li x FePO 4 F (x=2, 1.5, 1, 0.5, and 0). The oxidation potentials for different lithium ion concentration in Li x FePO 4 F compounds are located at ~3.41, 3.51, 4.80, and 5.60V respectively. The lithium-ion activation exhibits in the form of a single phase. The NaLiFePO 4 F is synthesized by annealing the mixture of LiFePO 4 and NaF with molar ratio 1:1 at 650ºC for 1.5h. The Li 2 FePO 4 F can be obtained by ion-exchange during electrochemical testing. The electrochemical performance is characterized by galvanostatic charge-discharge test between 2~4.5V at rate 0.1C with an average potential~3.4V vs Li + /Li. A reversible specific capacity ~127mAh g -1 can be obtained. References Padhi, A.K., K.S. Nanjundaswamy, and J.B. Goodenough, Phospho ‐ olivines as Positive ‐ Electrode Materials for Rechargeable Lithium Batteries. Journal of The Electrochemical Society, 1997. 144 (4): p. 1188-1194. Nanjundaswamy, K.S., A.K. Padhi, J.B. Goodenough, S. Okada, H.Ohtsuka, H. Arai, and J.Yamaki., Synthesis, redox potential evaluation and electrochemical characteristics of NASICON-related-3D framework compounds. Solid State Ionics, 1996. 92 (1): p. 1-10. Barker, J., M.Y. Saidi, and J.L. Swoyer, Electrochemical Insertion Properties of the Novel Lithium Vanadium Fluorophosphate, LiVPO 4 F Journal of The Electrochemical Society, 2003. 150 (10): p. A1394-A1398. Khasanova, N.R., O. A. Drozhzhin, D. A. Storozhilova, C. Delmas, and E. V. Antipov, New Form of Li 2 FePO 4 F as Cathode Material for Li-Ion Batteries. Chemistry of Materials, 2012. 24 (22): p. 4271-4273. Ben, Y. H., M. Shikano, H. Sakaebe, S. Koike, M. Tabuchi, H. Kobayashi, H. Kawaji, M. Avdeev, W. Miiller, and C.D..Ling, Synthesis and characterization of the crystal structure, the magnetic and the electrochemical properties of the new fluorophosphate LiNaFe[PO 4 ]F. Dalton Trans, 2012. 41 (38): p. 11692-9. Antipov, E.V., N.R. Khasanova, and S.S. Fedotov, Perspectives on Li and transition metal fluoride phosphates as cathode materials for a new generation of Li-ion batteries. IUCrJ, 2015. 2 (Pt 1): p. 85-94. Figure 1