Role of PO4 tetrahedron in LiFePO4 and FePO4 system

Using high resolution transmission electron microscopy with image simulation and Fourier analysis, the Li_1?xFePO₄ (x < 0.01), Li_1?x FePO₄ (x ～ 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 Li_1?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 Li_1?xFePO₄ (x ～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. © 2015 Wiley Periodicals, Inc.