ИСТИНА

Sodium and transition metals fluorophosphates with formula Na2MPO4F (M = Fe, Co, Mn) represent a perspective type of compounds for use in cathode materials of lithium- and sodium-ion batteries because of two reasons: the theoretical possibility to extract more than one alkaline ion per formula unit (with corresponding oxidation of d-cation up to +4 oxidation state) and rather high operating voltages in the case of M=Co and Mn [1]. As a result, rather high theoretical energy density may be achieved, in the case of average voltage 4.5 V it amounts 550 W•h/kg per one alkaline metal ion. Na2MPO4F (M=Fe, Co) consists of alternating layers of sodium ions and layers of interconnected FeO4F2 and PO4 polyhedra, while Na2MnPO4F has a framework type of structure which has not good intrinsic sodium diffusion characteristics of layered structure but may be more stable. In the present work the main attention is given to the hydrothermal synthesis of the Na2MPO4F (M=Co, Fe) and investigation of its morphology and electrochemical properties depending on synthetic conditions. Crystal structure study of Na2-xCoPO4F with different amount of deintercalated sodium ions is also performed using in-situ and ex-situ x-ray powder diffraction and EDX analysis. Cathode materials based on obtained Na2CoPO4F samples demonstrate more than 50% of theoretical discharge capacity for one alkaline metal ion in spite of strong amorphization during cycling. Na2FePO4F shows more than 80% of theoretical capacity for one alkaline metal ion. Also the possibility of obtaining Na2MnPO4F and solid solutions with compositions Na2Co1-xMnxPO4F (х=0.2, 0.4, 0.6, 0.8) and Na2Mn1-xFexPO4F (х=0.2, 0.4, 0.6) using hydrothermal route is established. It is shown that compositions Na2Co1-xMnxPO4F (х= 0.2, 0.4) are isostructural to Na2CoPO4F. At the moment cathode materials based on hydrothermally obtained Na2MnPO4F and Na2Mn0.6Fe0.4PO4F demonstrate discharge capacity of 5% and 10% per one alkaline ion respectively. Weak electrochemical activity of the manganese-containing compounds can be attributed to the strong Yahn-Teller distortion of Mn+3 and slow diffusion of alkaline ions. The latter problem could be mitigated due to the small size of hydrothermally obtained Na2MnPO4F particles that was about 100 nm. References [1] Khasanova, N.R., et al. New Form of Li2FePO4F as Cathode Material for Li-Ion Batteries // Chemistry of materials, 2012. 24(22): p. 4271-4273.