Electrochemical Approach for the Synthesis of Carbon-Metal Fluoride Nanocomposites for Lithium Batteries

Metal fluorides are assuming importance as electrode materials for rechargeable lithium batteries as they possess high gravimetric and volumetric energy densities and are thermally stable. However, metal fluorides are insulators, exhibits sluggish kinetics and undergo large volume changes during electrochemical discharge and charge. Mechanochemical synthesis of carbon metal fluoride nanocomposites (CMFNCs) was suggested to address these problems [1]. Recently we reported a facile chemical approach for the synthesis of C-FeF₂ nanocomposites by reductive intercalation method using CF_x as a precursor [2]. The C-FeF₂ nanocomposites obtained by this method showed high reversible lithium storage due to the incorporation of nanocrystalline FeF₂ in electronically well-connected graphitic domains.

Here we propose and demonstrate a rather general approach for the synthesis of CMFNCs at room temperature using CF_x as precursor. CF_x is known as a cathode material for lithium primary batteries. During discharge process lithium reacts with CF_x and form C+LiF. In other words this is an electrochemical method for the synthesis of C-LiF nanocomposites. Fundamentally, it is possible to extend this approach for the synthesis of other C-MF_x nanocomposites if the reaction of metal with CF_x is thermodynamically feasible provided with suitable metal ion transporting electrolyte. Figure 1 show the discharge curve obtained with CF_x cathode and Fe metal foil as anode and Fe²⁺ transporting liquid electrolyte. During the discharge process the Fe²⁺ is released into the electrolyte at anode and the CF_x is reduced at cathode to form C-FeF₂. The discharged electrode was analyzed by XRD and formation of FeF₂ is confirmed. The as synthesized C-FeF₂ nanocomposites showed excellent reversible lithium storage properties. Further, we have succeeded in the synthesis of various C-MF_x nanocomposites by the proposed electrochemical approach. 

References:

1. F. Badway, N. Pereira, F. Cosandey, G. G. Amatucci, J. Electrochem. Soc. 2003, 150, A1209.
2. M. Anji Reddy, B. Breitung, V.S.K. Chakravadhanula, C. Wall, M. Engel, C. Kübel, A. K. Powell, H. Hahn and M. Fichtner, Adv. Energy Mater. 2013, 3(3), 308.