Development of New Anode Composite Materials for Fluoride Ion Batteries

The increasing demand for high energy storage systems has prompted the interest to alternative chemistries for new battery systems, with better performances than currently used Li-ion batteries. Recently, the reversible charge and discharge of a battery based on a fluoride shuttle has been demonstrated [1]. Such a fluoride ion battery (FIB) can theoretically achieve a high energy density, i.e. above 5000 Wh.L-1 for some electrode combinations.  This type of batteries is at an early stage of development and large improvements need to be done concerning the performance of the electrodes or of the electrolyte. In the first study, different types of metal fluorides were already tested as cathode (BiF3, CuF2, SnF2 or KBiF4) and Ce metal was used as anode. However, this material, as well as other potential anode materials like La, Ca, Li or Mg, is very sensitive to oxidation, even when stored under argon. We therefore decided to prepare electrode materials in the discharged state to investigate new anode materials, using Bi metal as cathode. We found that it was possible to obtain several cycles of charge/discharge using CeF3 or doped-CaF2 as anode but not when using MgF2. Moreover, the discharge capacities obtained after the initial charge were low. To ensure both electronic and ionic conductivity in the electrodes it is usually necessary to prepare a composite material. Here, only activated carbon was added to CeF3 and doped-CaF2 as these compounds are also good ionic conductors. In the case of MgF2, it was necessary to add also a certain amount of the electrolyte as MgF2 itself shows very poor ionic conductivity. The low performances obtained for this anode composite are thus most likely linked to interfaces issues between the different compounds of the electrode composite. In order to improve the reactivity of the interfaces, a new composite was prepared by adding Mg and MgF2 (the charge and discharge products) to the electrolyte and carbon in the anode composite. Using this new anode composite, it was possible to obtain much better cycling performances with higher charge/discharge capacities and lower polarization. Different characterization techniques were used to understand the reactions occurring when using the different anode material and understand the improvement observed when mixing Mg and MgF2.

[1] M. Anji Reddy & M. Fichtner – J. Mater. Chem. 43 (2011) 17059