Thermal synthesis of conversion-type bismuth fluoride cathodes for high-energy-density Li-ion batteries

Towards enhancement of the energy density of Li-ion batteries, BiF₃ has recently attracted considerable attention as a compelling conversion-type cathode material due to its high theoretical capacity of 302 mAh g^-1, average discharge voltage of ca. 3.0 V vs. Li⁺/Li, the low theoretical volume change of ca. 1.7% upon lithiation, and an intrinsically high oxidative stability. Here we report a facile and scalable synthesis of phase-pure and highly crystalline orthorhombic BiF₃ via thermal decomposition of bismuth(III) trifluoroacetate at T = 300 °C under inert atmosphere. The electrochemical measurements of BiF₃ in both carbonate (LiPF₆-EC/DMC)- and ionic liquid-based (LiFSI-Pyr_1,4TFSI) Li-ion electrolytes demonstrated that ionic liquids improve the cyclic stability of BiF₃. In particular, BiF₃ in 4.3 M LiFSI-Pyr_1,4TFSI shows a high initial capacity of 208 mA g^-1 and capacity retention of ca. 50% over at least 80 cycles at a current density of 30 mA g^-1.