Performance degradation of Li_xFePO₄(x = 0, 1) induced by postannealing

Olivine LiFePO₄has been studied for more than a decade as a promising cathode material for rechargeable lithium batteries. However, the low electric conductivity and tap density still hinder its large-scale commercialization. Micro-sized LiFePO₄is prepared by an optimized hydrothermal method in this paper. The influence of postannealing on the physicochemical properties of LiFePO₄and FePO₄is investigated to understand the plausible mechanism for performance degradation. It is found that postannealing even chemical delithiation greatly affects the particle size, morphology, pore distribution, surface area, and probably the lattice strain of Li_xFePO₄(x = 0, 1). Consequently, the electrochemical performances of annealed materials are severely deteriorated because of the sluggish lithium diffusion, difficult electrolyte accessibility, and incomplete phase transition during charge/discharge. In addition, the "self-healing" process along with cycling is analyzed by in-situ synchrotron X-ray diffraction.