Amorphous FePO4 as 3 V cathode material for lithium secondary batteries

Abstract

The structural and electrochemical properties of amorphous FePO₄·xH₂O (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 FePO₄·2H₂O was transformed into hexagonal FePO₄ at 380 °C, 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 °C) were also investigated for amorphous FePO₄. Fe K-edge X-ray absorption spectroscopy has been performed on amorphous Li_yFePO₄, 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 Li_yFePO₄.