Amorphous FePO4 as 3 V cathode material for lithium secondary batteries

The structural and electrochemical properties of amorphous FePO4.xH(2)O (x = 2, 1, 0) and hexagonal FePO4 powders have been investigated using differential thermal analysis-thermogravimetry (DTA-TGA), X-ray diffractometry (XRD), X-ray absorption spectroscopy (XAS), scanning electron microscopy (SEM), cyclic voltammetry (CV), and charge/discharge cycling. On heating, amorphous FePO4.2H(2)O was transformed into hexagonal FePO4 at 380 degreesC, through amorphous FePO4.H2O and FePO4 phases. Reversible lithium insertion and extraction into and from the amorphous FePO4 occurred at 2.8 and 3.2 V vs. Li/ Li+, respectively, which potentials are lower than those of the olivine-structured LiFePO4. All samples show a large capacity loss at the 1st cycle, but their discharge capacities are gradually increased from 65 mA h g(-1) (2nd cycle) to 75 mA h g(-1) (15th cycle) at a current density of 17 mA g(-1) and kept up to the 50th cycle. The discharge/charge dependence on the current densities (17, 34, and 85 mA g(-1)) and operating temperatures (20, 50, and 80 degreesC) were also investigated for amorphous FePO4. Fe K-edge X-ray absorption spectroscopy has been performed on amorphous LiyFePO4, to determine the changes in the local electronic and geometric structures during the discharge. For the Fe K-edge, the pre-edge and edge are shifted in accordance with the oxidation state of the Fe3+/Fe2+ redox couple in LiyFePO4.