ИСТИНА

The interrelation between structure and property is a crucial principle taken into account when developing new functional materials including electrode materials for Li-ion and Na-ion batteries. Analysis and further tuning of crystal structure and chemical composition pave the way towards the enhancement of various electrochemical properties of such materials, mainly specific capacity, working potential and kinetic characteristics of the ion diffusion in the material. Considering these ideas, fluoride-phosphates with a general formula of A2MPO4F (A – alkali metal, M – 3d transition metal) were proposed and then recognized as a perspective class of cathode materials for Li-ion and Na-ion batteries of various large-scale and grid applications [1,2,3,4]. Indeed, phosphate-groups along with fluorine in the structure give rise to a significant potential increase in comparison to oxide materials; a theoretical possibility to extract more than one alkali ion from the lattice makes us expect a higher specific capacity; moreover, a weaker affinity of lithium towards fluorine than oxygen along with solid-solution intercalation mechanism forecast a faster ion transport in the cell. Depending on the nature of alkali and transition metal there exist three basic types of frameworks within the class found up to now: stacked Li2MPO4F (M = Ni, Co), 3D Na2MnPO4F and layered Na2MPO4F (M = Fe, Co) [1,2,3]. Among them isostructural Li2MPO4F (M = Ni, Co) reveal a remarkable prospective as high-voltage cathodes, operating at potentials up to 5.5V [2,3,4]. However, the absence of a commercial electrolyte, whose range of stability covers these operating voltages, hinders a comprehensive examination of the materials [5]. It is supposed that the solid-solution intercalation mechanism attributed to Li2MPO4F enables to linearly shift the potential on substitution of Co and Ni by Fe and Mn, possessing a lower M3+/M2+ potential [3,5]. Thus the aim of this work is synthesis and electrochemical investigation of (Li,Na)2Co1-xFexPO4F cathode materials. [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. Fedoto