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-Pyr1,4TFSI) Li-ion electrolytes demonstrated that ionic liquids improve the cyclic stability of BiF3. In particular, BiF3 in 4.3 M LiFSI-Pyr1,4TFSI 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.