ИСТИНА

Fluoride-phosphates with a general formula of (Li,Na)2MPO4F (M = Mn, Fe, Co) were recognized as potential cathode materials for rechargeable batteries of various large-scale applications [1,2,3,4]. Depending on the nature of alkali and transition metal there exist three basic types of frameworks: stacked Li2MPO4F (M = Ni, Co), 3D Na2MnPO4F and layered Na2MPO4F (M = Fe, Co) [1,2,3]. Among them isostructural Li2MPO4F (M = Ni, Co) are considered as high-voltage cathodes, operating at potentials up to 5.5V [2,3,4]. Nonetheless, a comprehensive examination of the materials is impeded due to a severe degradation of commercial electrolytes at these voltages [5]. The solid-solution intercalation mechanism ascribed to Li2MPO4F is supposed to allow altering the potential when substituting of Co and Ni by Fe and Mn with lower M3+/M2+ potentials [3,5]. Thus the aim of this work is synthesis and electrochemical investigation of (Li,Na)2Co1-xMxPO4F (M = Mn, Fe) cathode materials. For the preparation of (Li,Na)2Co1-xMxPO4F cathode materials conventional solid-state and freeze-drying methods were applied. The solid-solution ranges of Li2Co1−xFexPO4F and Li2Co1−xMnxPO4F were limited to x = 0.3 and x = 0.1 respectively. A deeper substitution of Co by metals resulted in multi-phase samples. With the increase of x the annealing temperature should be raised up to 750C [5]. Taking into account the “elasticity” of Li2MPO4F framework [6] the further substitution was believed to be possible by a concurrent substitution of Li by Na. As a result Li1.6Na0.4Co0.6Fe0.4PO4F, LiNaCo0.5Fe0.5PO4F and LiNaCo0.67Mn0.33PO4F compounds were prepared. The substitution of all Li by Na brings to Na2Co0.5Fe0.5PO4F with quasi-2D framework [1]. The Rietveld refinement of LiNaCo0.5Fe0.5PO4F structure, based on joint synchrotron and neutron diffraction data, showed the ordering in alkali metals positions. Fe oxidation states in the materials were characterized by Mössbauer spectroscopy, that found only two doublets attributed to Fe2+. The chemical composition of the materials was confirmed by EDS microanalysis. Electrochemical behavior of LiNaCo0.5Fe0.5PO4F and Na2Co0.5Fe0.5PO4F was investigated by means of galvanostatic and cycling voltammetry techniques. The materials demonstrated a reversible electrochemical activity towards lithium in a potential range of 2.5–5.3 V. It should be noted that an average Fe3+/Fe2+ potentials in LiNaCo0.5Fe0.5PO4F and Na2Co0.5Fe0.5PO4F are 50÷100mV higher comparing to pure LiNaFePO4F and Na2FePO4F that confirms the idea of fine potential tuning by means of substituting. The comparison between synthetic routes and electrochemical properties of different types of fluoride-phosphate materials will be presented. Acknowledgement. The work was supported in part by Russian Foundation for Basic Research (RFBR grant 13-03-00495a) References: [1] B. L. Ellis, W. R. Michael Makahnouk, W. N. Rowan-Weetaluktuk, D. H. Ryan, L. F. Nazar, Chem. Mater., 22 (2010) 1059. [2] S. Okada, M. Ueno, Y. Uebou, J. Yamaki, J. Power Sources, 146 (2005) 565. [3] N. R. Khasanova, A. N. Gavrilov, E. V. Antipov, K. G. Bramnik, H. Hibst, J. Power Sources, 196 (2011) 355. [4] M. Nagahama, N. Hasegawa, S. Okada, J. Electrochem. Soc., 157 (2010) A748. [5] N. R. Khasanova, O. A. Drozhzhin, S. S. Fedotov, D. A. Storozhilova, R. V. Panin, E. V. Antipov, Beilstein J. Nanotechnol., 4 (2013) 860. [6] N. R. Khasanova, O. A. Drozhzhin, D. A. Storozhilova, C. Delmas, E. V. Antipov, Chem. Mater., 24 (2012) 4271.