Role of PO4 tetrahedron in LiFePO4 and FePO4 system

Using high resolution transmission electron microscopy with image simulation and Fourier analysis, the Li1-xFePO4 (x<0.01), Li1-x FePO4 (x approximate to 0.5), and FePO4 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 Li1-xFePO4 (x<0.01) contains some missing Li ions leading PO4 group distorted around M1 tunnel of the unit cell; (2) the texture of the particles of Li1-xFePO4 (x approximate to 0.5) has homogeneously distributed compositional domains of LiFePO4 and FePO4 resulting from spinodal decomposition which promote Li ion easily getting into the particle due to uphill diffusion, (3) the particles of FePO4 formed in charging have heavily distorted lattice and contain some isolated LiFePO4, (4) interface between LiFePO4 and FePO4 and between amorphous and crystal region provides the lattice distortion of small polarons. Microsc. Res. Tech. 78:462-471, 2015. (c) 2015 Wiley Periodicals, Inc.