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

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Abstract
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.

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