Morphological investigation of sub-micron FePO4 and LiFePO4 particles for rechargeable lithium batteries

Microstructural variations of amorphous FePO4 and LiFePO4 (the latter obtained by chemical lithiation of the former) as a result of the annealing temperature have been studied by Thermogravimetric Analysis (TGA)/Differential Thermal Analysis (DTA), chemical analysis, Brunauer–Emmet–Taylor (BET) and Scanning Electron Microscopy (SEM) techniques. Round-shaped amorphous FePO4 particles 40–80 nm in size are obtained after heating (at 400 °C) amorphous FePO4·2H2O in air (previously prepared by a precipitation route). On further heating at 650 °C, in air, crystalline trigonal FePO4 of crystallite size <200 nm is formed. Round-shaped amorphous LiFePO4 particles 40–80 nm in size crystallize by heating at 550 °C in Ar+5%H2 for 3 h. After thermal treatment, LiFePO4 particles are interconnected by necks, which resembled a sintering process. The particle size of LiFePO4 increases with an increase of temperature up to 750 °C, but an abnormal growth is evident at annealing temperatures above 650 °C. DTA analysis showed two exothermic peaks at 547 and 768 °C for FePO4 due to phase transitions, whereas for LiFePO4 two exothermic effects at 496 and 567 °C are shown.