Reversible Electrochemical Intercalation and Deintercalation of Fluoride Ions into Host Lattices with Schafarzikite‐Type Structure

Abstract

Herein, we report the successful electrochemical fluorination and defluorination of schafarzikite‐type compounds with the composition Fe0.5 m 0.5Sb2O4 (M=Mg or Co). We show that electrochemical methods can present a more controllable and less environmentally damaging route for fluorinating compounds in contrast to traditional methods that involve heating samples in F2‐rich atmospheres. The reactivity of the host lattices with fluoride during electrochemical fluorination makes this material an interesting candidate for fluoride‐ion battery applications. However, deleterious side reactions with the conductive carbon matrix during fluorination suggests to the contrary. Regardless of the side reactions, the schafarzikite structure was found to be an alternative reversible host lattice for fluoride incorporation and removal in addition to the previously reported Ruddlesden–Popper‐type compounds.

High cycle life all-solid-state fluoride ion battery with La2NiO4+d high voltage cathode

“…6a), which is the highest discharge capacity that has been obtained so far for an FIB with an intercalation-based cathode material ( Supplementary Fig. 14 Table 7 summarize the discharge capacity of La 2 NiO 4+d as compared to the other previously studied intercalation-based cathode materials including La 2 CoO 4 27,33 , LaSrMnO 4 32 , and Schafarzikite-type structure of Co 0.5 Fe 0.5 Sb 2 O 4 31 showing the highest obtained discharge capacity for the La 2 NiO 4+d /Zn-ZnF 2 electrochemical cell). Note that such a high discharge capacity can only be found by using small discharge current density such as −2.4 µA/cm 2 (−1.0 µA).…”

“…In addition, a partial oxidation of the carbon matrix 15,27,[31][32][33] is overlying the charging of La 2 NiO 4+d in region (II) (U <~2.1 V) and proceeding within region (III) (U >~2.1 V). This oxidation of the CNTs can also be detected by the XPS and Raman measurements, which will be fully discussed later with respect to the different changes happening within the CNT in the different regions.…”

“…In this respect, different oxides including perovskite-type BaFeO 2. 5 30 , schafarzikite-type MSb 2 O 4 31 , and the Ruddlesden-Popper-type (A n+1 B n O 3n+1 or AO(ABO 3 ) n ) compounds LaSrMnO 4 32 and La 2 CoO 4 33 as the cathode material and Sr 2 TiO 3 F 2 34 as the anode material have been previously considered mainly by our research group as potential host candidates for intercalation-based electrode materials for FIBs. The Ruddlesden-Popper-type structure type with n = 1 was so far found to be most suitable for the reversible intercalation of fluoride ions, explained by the presence of interstitial anion sites within the AO rock salt-related layers 35 , allowing for the insertion up to 2F − ions per A 2 BO 4 formula units 36 .…”

Layered electrides as fluoride intercalation anodes

“…However, the gravimetric capacity is low (67 mAh/g theoretically) because the heavy La is "dead weight" from an electrochemical perspective. This is generally true for similar electrodes such as MgFeSb4O8F (39 mAh/g) or CoFeSb4O8F (37 mAh/g), 30 although it can be mitigated by using cations lighter than La or Sb, such as Sr (1.26 Å) in Sr2TiO3F2 (197 mAh/g), which has been proposed as a FiB anode but not yet tested. 31 We conclude that to keep the electrode light, yet retain large interstitial sites, it should ideally have only one type of cation.…”

“…black, reacts irreversibly with F -. 13,30,53 The carbon fluorination may be avoided in the future through the introduction of improved conductors such as SnO2 or carbon nanotubes. 20 In order to compete with Li-ion batteries, the accessible voltage range must be expanded to >3 V, and intercalation cathodes with higher capacities than La2CoO4 or LaSrMnO4 are required.…”