ИСТИНА

Complex phosphates LiMPO4 (M = Fe, Mn, Co) and fluorophosphates Na2FePO4F became an object of a great interest as cathode materials for Li-ion and Na-ion batteries. These materials exhibit a rich diversity of its electrochemical properties, including phase transformation during charge/discharge, depending on the composition, particles’ size and orientation, temperature, electrolyte etc. So, two-phase mechanism of Li+ deintercalation-intercalation is typical for LiFePO4, but for Mn-substituted olivines extraction and insertion of Li+ occurs via single phase in broad ranges of composition [1], [2]. Phase transformations in LixFe1-yMnyPO4 were studied in a number of works with in situ and ex situ diffraction methods, but X-ray diffraction data provides only one-sided notion about the processes which occur during Li+ (de)intercalation in these compounds. As regards to Na2FePO4F cathode materials, its phase transition behavior in Li- or Na-ion electrolyte was not systematically studied. The aim of the present work is detailed study of phase transformations in LixFe1-yMnyPO4 (0≤x≤1, 0≤y≤0.5) and (Na,Li)xFePO4F (1≤x≤2) by means of in situ X-ray powder diffraction (XRPD) and potentiostatic intermittent titration technique (PITT). The electrochemical properties and phase transformations during (de)insertion of Li+/Na+ in LiFePO4, LiFe0.9Mn0.1PO4, LiFe0.5Mn0.5PO4 and Na2FePO4F are studied by means of galvanostatic cycling, potential intermittent titration technique (PITT) and in situ X-ray powder diffraction in Li-ion and Na-ion electrolytes, respectively. Different modes of switching between the solid solution and two-phase regimes are revealed which are influenced by the Mn content in Li1-xFe1-yMnyPO4 and by the type of intercalated ion (Li or Na) – in (Na,Li)2FePO4F. Additionally, an increase in electrochemical capacity with the Mn content is observed at high rates of galvanostatic cycling (10C, 20C), which is in good agreement with the numerically estimated contribution of the solid solution mechanism determined from PITT data [3]. This work was supported by the Russian Foundation of Basic Research (grant No. 14-29-04064), Skolkovo Institute of Science and Technology, and the Lomonosov Moscow State University Program of Development