Synthesis and Thermal Behavior of Crystalline Hydrated Iron(III) Phosphates of Interest as Positive Electrodes in Li Batteries

Four different hydrated iron phosphates were prepared as pure crystalline powders by precipitation in aqueous solutions under controlled pH: strengite, FePO 4 ·2H 2 O ( Pbca; a = 9.8923(6) Å, b = 10.125(1) Å, c = 8.729(1) Å); metastrengite I, FePO 4 ·2H 2 O ( Pbnm; a = 5.226(2) Å, b = 10.026(3) Å, c = 8.917(3) Å); metastrengite II, FePO 4 ·2H 2 O ( P 2 1 / n; a = 5.3276(3) Å, b = 9.800(1) Å, c = 8.7129(6) Å, β = 90.532(6)°); and spheniscidite, NH 4 (Fe 2 (PO 4 ) 2 OH·H 2 O)·H 2 O ( P 2 1 / n; a = 9.808(3) Å, b = 9.727(3) Å, c = 9.866(3) Å, β = 102.85(2)°). Thermogravimetric analysis revealed slightly more than 2 H 2 O molecules per FePO 4 for the first three compounds, the excess being considered as adsorbed on the powders' surface. Temperature-controlled X-ray diffraction allowed isolation and determination of the stability domains of a large number of FePO 4 polymorphs which may, or may not, reabsorb H 2 O upon exposure to ambient air. All these phases, after careful optimization of the electrode preparation, may be reversibly reduced to ∼90% of their theoretical capacities within the 2−4 V vs Li + /Li region.