In the search for novel battery systems with high energy density and low cost, fluoride ion batteries have recently emerged as a further option to store electricity with very high volumetric energy densities. Among metal fluorides, CuF 2 is an intriguing candidate for cathode materials due to its high specific capacity and high theoretical conversion potential. Here, the reversibility of CuF 2 as a cathode material in the fluoride ion battery system employing a high F − conducting tysonite-type La 0.9 Ba 0.1 F 2.9 as an electrolyte and a metallic La as an anode is investigated. For the first time, the reversible conversion mechanism of CuF 2 with the corresponding variation in fluorine content is reported on the basis of X-ray photoelectron spectroscopy measurements and cathode/electrolyte interfacial studies by transmission electron microscopy. Investigation of the anode/electrolyte interface reveals structural variation upon cycling with the formation of intermediate layers consisting of i) hexagonal LaF 3 and monoclinic La 2 O 3 phases in the pristine interface;ii) two main phases of distorted orthorhombic LaF 3 and monoclinic La 2 O 3 after discharging; and iii) a tetragonal lanthanum oxyfluoride (LaOF) phase after charging. The fading mechanism of the cell capacity upon cycling can be explained by Cu diffusion into the electrolyte and side reactions due to the formation of the LaOF compound.

Use of lithium ion batteries is currently the method of choice when it comes to local stationary storage of electrical energy. In the search for an alternative system, fluoride ion batteries (FIBs) emerge as a candidate due to their high theoretical capacity, and no lithium is needed for its operation. To improve the cycling performance and lower the working temperature of a solid-state battery, one of the critical components is the electrolyte, which needs advanced performance. This paper aims at developing an electrolyte with enhanced ionic conductivity for fluoride ions, to be used in a FIB. Tysonite LaBaF (0 ≤ x ≤ 0.15) solid solutions were synthesized by a facile wet chemical method, and its ionic conductivity was analyzed using electrochemical impedance spectroscopy. A composition study shows that the conductivity reaches a maximum of 1.26 × 10 S·cm at 60 °C for the LaBaF pellet sintered at 800 °C for 20 h, which is 1 order of magnitude higher than that for the as-prepared pellet and 2 times higher than the conductivity of sintered ball-milled batches. The reason for this dramatic increment is the more efficient decrement of grain boundary resistance upon sintering. Morphological, chemical, and structural characterizations of solid electrolytes were studied by X-ray diffraction, scanning electron microscopy , energy dispersive X-ray spectroscopy, physisorption by the Brunauer-Emmett-Teller method, and transmission electron microscopy. Electrochemical testing was carried out for the FIB cell using LaBaF as electrolyte due to its highest conductivity among the compositions, Ce as anode, and BiF as a cathode. The cycling performance was found to be considerably improved when compared to our earlier work, which used the ball-milled electrolyte.