Performance Studies of CuxFe1-XF2 Conversion Cathode Materials
Authors: Ko Jonathan K. , Fuentevilla Daphne A. , Waller Gordon H. , Mansour Azzam N. 
Published: February 2020

Lithium metal/metal fluoride batteries are potentially more energy dense compared to traditional lithium intercalation batteries; the redox chemistry in conversion materials involves multi electron transfer processes per transition metal atom (e.g., Cu2+ to Cu0, Fe2+/Fe3+ to Fe0) while intercalation of traditional electrodes involves a single electron transfer per transition metal atom. CuF2 is of interest as an active cathode material due to its high theoretical potential of 3.55 V [1] [2] [3]. Along with its theoretical specific capacity of 528 mAh/g, CuF2 provides a high theoretical energy density of 1874 Wh/kg. However, due to Cu dissolution, CuF2 is not fully reversible and has poor cycle life [2]. On the other hand, FeF2 has good cyclability but suffers from large hysteresis due to different reaction pathways during charge and discharge [4]. By substituting Cu into the Fe lattice, CuxFe1-xF2, a ternary metal fluoride, shows promise overcoming the issues with CuF2 and FeF2. Upon charge of CuxFe1-xF2 electrodes, initial reconversion of Fe to Fe2+ and Fe3+ is followed by reconversion of some of the Cu to Cu2+ while the remaining Cu provides conductive pathways during the charge process [5]. Studies have been performed on the Cu0.5Fe0.5F2 which showed improvement over the binary CuF2 and FeF2. The Cu-substituted material showed low hysteresis (<150mV) which allows higher cycling efficiency [5]. In this work, we present further electrochemical testing on Cu0.5Fe0.5F2 as well as investigations of Fe and Cu rich CuxFe1-xF2. The ternary metal fluorides are manufactured by mixing FeF2 and CuF2 in a high-energy ball mill until a solid solution is formed. The structure and phase purity of the materials are characterized by XRD while XPS is used to evaluate the chemistry of the solid electrolyte interfaces.

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