Amorphous Iron Phosphate

Abstract

In the previous chapter it was shown that a mixture of amorphous 3Fe2O3•2P2O5·10H2O, prepared by air oxidation of iron(II)phosphate, exhibited good electrochemical properties as cathode of a lithium cell (Prosini et al., J. Electrochem. Soc. 148, A1125–A1129, 2001). However, two different types of iron were formed during the heating treatment of iron(II)phosphate, namely iron(III)phosphate and iron(III)oxide. Since the latter form is electrochemically non-active for lithium intercalation in the investigated voltage range (4.0–2.0 V) (Prosini et al., Int. J. Inorg. Mater. 2, 365–370, 2000), the electrochemical properties of such material was only due to the amorphous iron(III)phosphate phase. For this reason we investigated a new synthetic route to prepare pure amorphous iron(III)phosphate. The new route involved the spontaneous precipitation from equimolar aqueous solutions of Fe(NH4)2(SO4)2·6H2O and NH4H2PO4, using hydrogen peroxide as an oxidizing agent. The material was characterized by chemical analysis, TG/DTA, XRD, and SEM. The material was tested as a cathode in non-aqueous lithium cells. Galvanostatic intermittent titration technique (GITT) was used to follow the lithium intercalation process. To evaluate the effect of firing on the specific capacity the material was heated at various temperatures up to 650°C. The material heated at 400°C showed the best electrochemical performance, delivering about 108 Ah kg?1 when cycled at 170 A kg?1 rate. The capacity fade upon cycling was found as low as 0.075% per cycle.