Solid Electrolytes for Fluoride Ion Batteries: Ionic Conductivity in Polycrystalline Tysonite-Type Fluorides

Batteries based on a fluoride shuttle (fluoride ion battery, FIB) can theoretically provide high energy densities and can thus be considered as an interesting alternative to Li-ion batteries. Large improvements are still needed regarding their actual performance, in particular for the ionic conductivity of the solid electrolyte. At the current state of the art, two types of fluoride families can be considered for electrolyte applications: alkaline-earth fluorides having a fluorite-type structure and rare-earth fluorides having a tysonite-type structure. As regard to the latter, high ionic conductivities have been reported for doped LaF₃ single crystals. However, polycrystalline materials would be easier to implement in a FIB due to practical reasons in the cell manufacturing. Hence, we have analyzed in detail the ionic conductivity of La_1–<i>y</i>Ba_<i>y</i>F_3–<i>y</i> (0 ≤ <i>y</i> ≤ 0.15) solid solutions prepared by ball milling. The combination of DC and AC conductivity analyses provides a better understanding of the conduction mechanism in tysonite-type fluorides with a blocking effect of the grain boundaries. Heat treatment of the electrolyte material was performed and leads to an improvement of the ionic conductivity. This confirms the detrimental effect of grain boundaries and opens new route for the development of solid electrolytes for FIB with high ionic conductivities

Nanostructured Fluorite-Type Fluorides As Electrolytes for Fluoride Ion Batteries

Fluoride ion batteries (FIB) provide an interesting alternative to lithium ion batteries, in particular because of their larger theoretical energy densities. These batteries are based on a F anion shuttle between a metal fluoride cathode and a metal anode. One critical component is the electrolyte that should provide fast anion conduction. So far, this is only possible in solid so-called superionic conductors, at elevated temperatures. Herein, we analyze in detail the ionic conductivity in barium fluoride salts doped with lanthanum (Ba_1–<i>x</i>La_<i>x</i>F_2+<i>x</i>). Doping by trivalent cations leads to an increase of the quantity of point defects in the BaF₂ crystal. These defects participate in the ionic motion and therefore improve the ionic conductivity. We demonstrate that further improvement of the conductivity is possible by using a nanostructured material providing additional conduction paths through the grain boundaries. Using electrochemical impedance spectroscopy and AC conductivity analysis, we show that the ionic conduction in this material is controlled by the motion of vacancies through the grain boundaries. The mobility of the vacancies is influenced by the quantity of dopant but decrease for too large dopant concentrations. The optimum compositions having the highest conductivities are Ba_0.6La_0.4F_2.4 and Ba_0.7La_0.3F_2.3. The compound Ba_0.6La_0.4F_2.4 was successfully used as an electrolyte in a FIB