Crystal chemistry of the olivine-type Li_xFePO_4 system (0 < = x <= 1) between 25 and 370 deg C

摘要

The crystal chemistry of two initial mixtures of LiFePO_4 and heterosite FePO_4 (0.5LiFePO_4 + 0.5FePO_4; 0.75LiFePO_4 + 0.25PePO_4) was investigated through Neutron diffraction, at 350 and 370 deg C, respectively, and at room temperature after cooling. At 350 and 370 deg C, Li_(0.5)FePO_4 and Lio,75FeP04 are refined as olivine-type single phases, in which Li+ ions are disordered. Significant anisotropic microstrains, within the (001)pmnt, planes, occur, whichmay be accounted for by heterogeneous distance distributions within and between the [100]pmnb Li+ channels. On cooling back to room temperature, LirjjFePO_4 and Li_(0.75)FePO_4 single phases separate into mixtures of LiFePO_4 + Li_(.0.64)FePO_4; respectively. The Li content of this metastable intermediate phase may correspond to the occupancy of 2 out of 3 Li sites within [100]min-b Li+ channels. In Li_(0.64)PePo_4, average Li-0 bonds are longer than in LiFePO_4, whereas Fe-0 bond lengths are shortened, due to a fraction of Fe(III). This may be at the origin of the metastability of such intermediate phase, and thus of the two-phase mechanism between LiFePO_4 and FePO_4.