Nanocrystalline FeF3 and MF2 (M = Fe, Co, and Mn) from metal trifluoroacetates and their Li(Na)-ion storage properties

Abstract

With demands placed on batteries constantly increasing, new positive electrode materials with higher energy density, satisfactory power density, and long-term cycling capabilities, composed of highly abundant elements with low-cost, are desired. One such low-cost cathodic material is iron(III) fluoride – FeF₃. Its theoretical capacity for single-electron reduction, accompanied by the insertion of Li⁺ ions, is 237 mA h g⁻¹. Herein we present a new synthesis for nanocrystalline FeF₃ using inexpensive iron trifluoroacetate as a molecular single-source precursor. We also report an adaptation of this simple chemistry to several transition metal difluorides (M = Fe, Co, and Mn). With FeF₃, a high capacity of 220 mA h g⁻¹ was attained at a moderate current density of 100 mA g⁻¹ (∼0.5 C). In addition to high capacity, we see the evidence for high rate-capability. Capacities of up to 155 mA h g⁻¹ were observed with 1-minute (10 A g⁻¹) charge–discharge ramps, and at least 88% of this capacity was retained after 100 cycles. When tested as a sodium cathode, FeF₃ exhibits capacities of up to 160 mA h g⁻¹ at a current rate of 200 mA h g⁻¹.