High‐Rate LiFePO₄ Electrode Material Synthesized by a Novel Route from FePO₄ · 4H₂O

Abstract

Abstract A LiFePO 4 material with ordered olivine structure is synthesized from amorphous FePO 4 · 4H 2 O through a solid–liquid phase reaction using (NH 4 ) 2 SO 3 as the reducing agent, followed by thermal conversion of the intermediate NH 4 FePO 4 in the presence of LiCOOCH 3 · 2H 2 O. Simultaneous thermogravimetric–differential thermal analysis indicates that the crystallization temperature of LiFePO 4 is about 437 °C. Ellipsoidal particle morphology of the resulting LiFePO 4 powder with a particle size mainly in the range 100–300 nm is observed by using scanning electron microscopy and transmission electron microscopy. As an electrode material for rechargeable lithium batteries, the LiFePO 4 sample delivers a discharge capacity of 167 mA h g –1 at a constant current of 17 mA g –1 (0.1 C rate; throughout this study n C rate means that rated capacity of LiFePO 4 (170 mA h g –1 ) is charged or discharged completely in 1/ n hours), approaching the theoretical value of 170 mA h g –1 . Moreover, the electrode shows excellent high‐rate charge and discharge capability and high electrochemical reversibility. No capacity loss can be observed up to 50 cycles under 5 C and 10 C rate conditions. With a conventional charge mode, that is, 5 C rate charging to 4.2 V and then keeping this voltage until the charge current is decreased to 0.1 C rate, a discharge capacity of ca. 134 mA h g –1 and cycling efficiency of 99.2–99.6 % can be obtained at 5 C rate.