Alkaline Earth Metals As Anode for High Energy Rechargeable Fluoride Ion Batteries

With increasing demand on energy supply together with reducing fossil sources, many efforts have been put into the research and development of environmentally benign, high energy density and low-cost rechargeable battery systems as an alternative. Fluoride Ion Battery (FIB), which is based on fluoride ion electrochemistry rather than lithium ion (in case of lithium ion battery), has emerged as a potential alternative. The element fluorine possesses an outstanding potential compared to Li owing to its high electronegativity and, thus, electrochemical stability, relatively low weight, more natural abundance and comparably low cost. By choosing an appropriate pair of electrodes comprising metal fluoride and metal together with a suitable fluoride-containing electrolyte, a high voltage electrochemical FIB cells can be built [1, 2]. Recently the first proof of concept for rechargeable fluoride ion batteries based on all-solid-state setup has been demonstrated [2].

Alkaline earth metals have recently attracted much attention for their employment in energy storage technologies due to potentially high energy density, low cost and environmental friendliness. Hence, in this study Ca and Ba pure metals were chosen and tested as anode in rechargeable FIB system. Ca_xBa_1-xF₂ (x ~ 0.5), a highly ion conducting metastable fluoride compound, was selected and synthesized via high energy mechanical milling route as fluoride ion conducting electrolyte. Powder X-ray diffraction confirms the formation of solid solution and electrochemical impedance measurement of Ca_xBa_1-xF₂ shows ionic conductivity in the order of 10^-4 S.cm^-1 at elevated temperature around 200 ⁰C, which is in agreement with earlier report [3]. The assembly of all-solid-state electrochemical FIB cells was carried out with Ca or Ba as anode, selected metal fluorides (BiF₃, CoF₃, and CuF₂) as high voltage cathode, and Ca_xBa_1-xF₂ (x ~ 0.5) as electrolyte. The cells were assembled in the charged state and electrochemically tested at ~ 200 ⁰C. Initial results showed that the specific capacities of Ca/BiF₃ and Ca/CuF₂ cells for the first discharge are about 260 mAh/g and 350 mAh/g, respectively. These capacities are about 86% (in case of BiF₃) and 66% (in case of CuF₂) of their theoretical specific capacities and higher than those of Ce/BiF₃ and Ce/CuF₂ systems reported previously [2]. Electrode analysis together with cycling behaviour reveal the fluoride ion transfer between anode and cathode, as well as the cyclability of alkaline earth metal based FIB system. Further structural and electrochemical characterization will be presented and discussed.

References

1.   N. I. Sorokin and B. P. Sobolev, Crystallography Reports, 52, 2007, 842-863.

2.   M. A. Reddy and M. Fichtner, J. Mater. Chem., 21, 2011, 17059-17062.

3.   A. Duvel, B. Ruprecht, P. Heitjans and M. Wilkening, J. Phys. Chem. C, 115, 2011, 23784-23789.