Switching between solid solution and two-phase regimes in the Li1-xFe1-yMnyPO4 cathode materials during lithium (de)insertion: Combined PITT, in situ XRPD and electron diffraction tomography study

Abstract The electrochemical properties and phase transformations during (de)insertion of Li + in LiFePO 4 , LiFe 0.9 Mn 0.1 PO 4 and LiFe 0.5 Mn 0.5 PO 4 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 Li 1-x Fe 1-y Mn y PO 4 . 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 Li 1-x Fe 0.5 Mn 0.5 PO 4 during charge and discharge is discussed. For the first time, the crystal structures of electrochemically deintercalated Li 1-x Fe 0.5 Mn 0.5 PO 4 with different Li content – LiFe 0.5 Mn 0.5 PO 4 , Li 0.5 Fe 0.5 Mn 0.5 PO 4 and Li 0.1 Fe 0.5 Mn 0.5 PO 4 – are refined, including the occupancy factors of the Li position. This refinement is done using electron diffraction tomography data. The crystallographic analyses of Li 1-x Fe 0.5 Mn 0.5 PO 4 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.