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

A LiFePO₄ material with ordered olivine structure is synthesized from amorphous FePO₄ · 4H₂O through a solid–liquid phase reaction using (NH₄)₂SO₃ as the reducing agent, followed by thermal conversion of the intermediate NH₄FePO₄ in the presence of LiCOOCH₃ · 2H₂O. Simultaneous thermogravimetric–differential thermal analysis indicates that the crystallization temperature of LiFePO₄ is about 437 °C. Ellipsoidal particle morphology of the resulting LiFePO₄ 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 LiFePO₄ 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 LiFePO₄ (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.