ИСТИНА

Complex phosphates LiFe1-yMnyPO4 became an object of a great interest as cathode materials for Li-ion batteries. Partial substitution of Fe by Mn in LiFePO4 allows to increase specific energy density of the cathode material because of higher electrochemical potential Mn2+/Mn3+, keeping proper stability and low degradation rate during galvanostatic cycling. 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. The aim of the present work is detailed study of phase transformations in LixFe1-yMnyPO4 (0≤x≤1, 0≤y≤0.5) 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+ in LiFePO4, LiFe0.9Mn0.1PO4 and LiFe0.5Mn0.5PO4 are studied by means of galvanostatic cycling, potential intermittent titration technique (PITT) and in situ X-ray powder diffraction. 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. 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. The observed asymmetric behavior of the phase transformations in Li1-xFe0.5Mn0.5PO4 during charge and discharge is discussed. For the first time, the crystal structures of electrochemically deintercalated Li1-xFe0.5Mn0.5PO4 with different Li content - LiFe0.5Mn0.5PO4, Li0.5Fe0.5Mn0.5PO4 and Li0.1Fe0.5Mn0.5PO4 - are refined, including the occupancy factors of the Li position. This refinement is done using electron diffraction tomography data. The crystallographic analyses of Li1-xFe0.5Mn0.5PO4 reveal that at x = 0.5 and 0.9 the structure retains the Pnma symmetry and the main motif of the pristine x = 0 structure without noticeable short range order effects.