Ternary Metal Fluorides As High-Energy Cathodes with Low Cycling Hysteresis

Transition metal fluorides (MF_x) are promising for use as high-capacity cathodes in rechargeable lithium batteries for large-scale applications, such as electric vehicles and grid-scale storage, but energy-efficiency and kinetics related issues remain a major hurdle to their commercial use. Cu based fluorides are particularly attractive due to the 3.55 V redox potential and extraordinarily high specific energy (1874 Wh/kg). Here we report on the synthesis, structural and electrochemical properties of new ternary metal fluorides (M1yM21-yFx: M1, M2 = Fe, Cu), which may overcome these issues. By substituting Cu into the Fe lattice, forming the solid solution Cu_yFe_1-yF₂, reversible Cu and Fe redox reactions were achieved with surprisingly small hysteresis (<150 mV; Figure 1). This finding indicates that cation substitution may provide a new pathway for tailoring electrochemical properties of conversion electrodes.

The Li storage/release mechanisms and limits to cycling stability of Cu_yFe_1-yF₂ were investigated by combining electrochemical measurement with comprehensive structural and chemical analysis using in-situ X-ray absorption spectroscopy, X-ray diffraction, and transmission electron microscopy-electron energy loss spectroscopy (TEM-EELS). The lithium reaction process is much more complicated in Cu_yFe_1-yF₂ than the binary metal counterparts (i.e. FeF₂, CuF₂) [2-4]. Some of the recent results on synthesis, structural and electrochemical characterization of the ternary metal fluorides will be presented. Detailed lithium reaction mechanisms, and Cu-loss related issues along with possible remedy solutions in the Cu_yFe_1-yF₂system, will be discussed.

 [1] Wang et al., “Ternary Metal Fluorides as High-Energy Cathodes with Low Cycling Hysteresis”, Nat. Commun.6:6668 (2015).

[2] Wang et al., “Conversion Reaction Mechanisms in Lithium Ion Batteries: Study of the Binary Metal Fluoride Electrodes”, J. Am. Chem. Soc., 133, 18828 (2011).

[3] Wang et al., “Tracking Li Transport and Electrochemical Reaction in Nanoparticles”, Nat. Commun., 3, 1201(2012).

[4] Hua et al., “Comprehensive Study of the CuF₂ Conversion Reaction Mechanism in a Lithium Ion Battery”, J. Phys. Chem. C 118, 15169 (2014).