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

Abstract

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-y)Ba(y)F(3-y) (0 ≤ y ≤ 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.

Modified coin cells to evaluate the electrochemical properties of solid-state fluoride-ion batteries at 150 °C

“…300 kHz) for the entire semi-circle to be visible. A value of approximately Re = 750 Ω for both cells is found, which is concomitant with the resistance given by a layer of LBF in our experimental conditions (σi(150 °C) ≈ 9.4×10 -5 S.cm -1 for La0.95Ba0.05F2.95) [9].…”

“…The electrolyte selected for our electrochemical investigation is Ba-doped LaF3. Ball-milling LaF3 and BaF2 is an easy method to produce nanocrystalline La1-xBaxF3-x (0 ≤ x ≤ 0.15) solid solutions of tysonite-type structure (trigonal, P-3c1) which offer a fluoride-ion conductivity of about 10 -5 S·cm -1 at 150 °C [9,22]. For our study, La0.95Ba0.05F2.95 (denoted LBF hereafter) was used.…”

“…However, considering the relatively poor conductivity of solid fluoride electrolytes [8][9][10][11][12][13], it is required to evaluate solid-state FIBs at higher temperatures, typically 150 °C. Indeed, performing electrochemical testing at 150 °C decreases the cell internal resistance and enables the use of higher current densities (10 µA/cm² [7,14]).…”

Materials and Electrochemistry: Present and Future Battery

“…It can therefore be regarded as a suitable and stable ion for charge transfer in a battery. There are only a few works on fluoride ion battery [59][60][61].The transfer of the fluoride ion between two electrodes enables reversible storage of electrons by charge compensation when the electrochemical couple is either charged or discharged by the outer electric circuit of the battery.The theoretical working principle of a FIB is illustrated with an example of BiF 3 /Mg as electrochemical couple. The discharge is accompanied by the oxidation of the anode, e.g., Mg → MgF 2 , which releases two electrons to the electric circuit.…”

F anion dynamics in cation-mixed nanocrystalline LaF3: SrF2

“…They constitute an important part in many devices such as sensors [3] and those constructed for electrochemical energy storage [4,5]. Apart from Li and Na ion conductors, also materials with high F anion conductivity have increasingly attracted attention [6], because there is renewed interest in the realization of batteries with fluorine anions as main charge carriers [7][8][9]. Apart from that, binary earth alkaline fluorides, especially CaF 2 and BaF 2 , easily accept rareearth-elements and, thus, allow the preparation of luminescent solid solutions with improved optical properties.…”