Role of PO 4 tetrahedron in L i F e PO 4 and F e PO 4 system

Using high resolution transmission electron microscopy with image simulation and Fourier analysis, the Li 1− x FePO 4 ( x  < 0.01), Li 1− x FePO 4 ( x ∼ 0.5), and FePO 4 particles, prepared by charging or discharging the 053048 electrochemical cells (thickness: 5 mm, width: 30 mm, height: 48 mm) and dismantled inside an Ar‐filled dry box, were investigated. The high resolution images reveal: (1) the solid solution of Li 1− x FePO 4 ( x  < 0.01) contains some missing Li ions leading PO 4 group distorted around M1 tunnel of the unit cell; (2) the texture of the particles of Li 1− x FePO 4 ( x ∼0.5) has homogeneously distributed compositional domains of LiFePO 4 and FePO 4 resulting from spinodal decomposition which promote Li ion easily getting into the particle due to uphill diffusion, (3) the particles of FePO 4 formed in charging have heavily distorted lattice and contain some isolated LiFePO 4 , (4) interface between LiFePO 4 and FePO 4 and between amorphous and crystal region provides the lattice distortion of small polarons. Microsc. Res. Tech. 78:462–471, 2015 . © 2015 Wiley Periodicals, Inc.