Energy Storage Materials Based on Iron Phosphate
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V1.9.1
Energy Storage Materials Based on Iron Phosphate
Pier Paolo Prosini, Maria Carewska, Marida Lisi, Stefano Passerini, Silvera Scaccia, and Mauro Pasquali1 ENEA, C.R. Casaccia. Via Anguillarese 301, 00060 Roma, Italy. 1 Dipartimento ICMMPM, Facoltà di Ingegneria, Università di Roma “La Sapienza”, Roma, Italy.
ABSTRACT
Several iron phosphates were synthesized by solution-based techniques and tested as cathodes in non aqueous lithium cells. The addition of phosphate ions to a solution of iron (II) produced crystalline Fe3(PO4)2. This material is easily oxidized by air to form an amorphous phase that is able to reversibly intercalate lithium. The amorphous compound was identified to be a mixture of FePO4 and Fe2O3. A new synthetic route was developed to prepare pure amorphous FePO4. Amorphous LiFePO4 was obtained by chemical lithiation of FePO4. The material was heated at 500°C under reducing atmosphere to obtain nano-crystalline LiFePO4. This latter material showed excellent electrochemical performance when used as cathode of lithium cells.
INTRODUCTION
Iron is a very attractive material to be used in the field of lithium battery, especially to build large size batteries for powering electric vehicles or for realizing dispersed electrical power sources. However, for a series of reasons, iron and its derivatives have not met with success as cathode materials. In fact, in the iron-based oxides containing O2- as anion, the Fe4+/Fe3+ redox energy tends to lie too far below the Fermi energy with respect to a lithium anode, while the Fe3+/Fe2+ couple is too close to it. The use of polyanions such as XOnm- (X= Mo, W, S, P, As) has been shown to lower the Fe3+/Fe2+ redox energy to useful levels. The effect of the structure on the Fe3+/Fe2+ redox energy of several iron phosphates was investigated by Padhi et al. [1]. Among these, lithium iron phosphate (ordered olivine-type structure) is emerging as the most promising material to replace lithiated transition metal oxides as cathode material for secondary lithium-ion batteries [2-4]. In this paper we report the synthesis of several iron phosphates and their behavior as cathodes in lithium batteries.
EXPERIMENTAL DETAILS
Crystalline Fe3(PO4)2 was prepared by spontaneous precipitation from iron (II) and phosphate aqueous solutions. A 0.06 M Fe(NH4)2(SO4)2⋅6H2O (Carlo Erba, RPE) solution was
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V1.9.2
added at ambient temperature to a constantly stirred 0.04 solution of K2HPO4 (Carlo Erba, RPE), in a 1:1 volume proportion. A pale-blue gel started to form after the addition was completed. The gel was collected on a membrane filter, washed and dried. Amorphous iron phosphate was obtained by oxidation of crystalline Fe3(PO4)2 in air at 100°C. After the heating treatment, the color of the powder changed from white to dark-yellow. Amo
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