La₂CoO₄: a new intercalation based cathode material for fluoride ion batteries with improved cycling stability

Abstract

La2CoO4, a new promising intercalation-based cathode material for fluoride ion batteries with improved cycling stability.

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

“…Furthermore, we note that the potential for the fluorination of the schafarzikite compounds, which only involves the Fe 2+ /Fe 3+ redox couple, is higher than Mn 3+ /Mn 4+ and Co 2+ /Co 3+ in LaSrMnO 4 and La 2 CoO 4 , respectively (ca. 1.2 V for LaSrMnO 4 and 0.9 V for La 2 CoO 4 against a composite of Pb+PbF 2 at the same condition), and this would not be expected intuitively from the electrochemical series . This could either result from an unusually high electrochemical potential of Fe 2+ within this structure type, or from higher overpotentials for the schafarzikite‐type structure as compared to the Ruddlesden–Popper‐type compounds.…”

“…However, a rearrangement of polyhedra within the schafarzikite structure to result in a three‐fold rotational axis (required for cubic symmetry) does not appear possible. Therefore, no simple group–subgroup relationships could be identified, which would explain a change to cubic symmetry, and this is in agreement with previous symmetry analyses …”

“…The volume changes of the active cathode material structures also calculated to be approximately 1.8 and 1.7 % for Co 0.5 Fe 0.5 Sb 2 O 4 and Mg 0.5 Fe 0.5 Sb 2 O 4 , respectively (from 433.8(2) to 426.2(4) Å 3 on fluorination for Co 0.5 Fe 0.5 Sb 2 O 4 , and from 433.0(2) to 425.7(2) Å 3 on fluorination for Mg 0.5 Fe 0.5 Sb 2 O 4 . We would like to point out that those changes are very low as compared to Ruddlesden–Popper‐type compounds, which are on the order of 10–20 % …”

“…These findings show that the schafarzikite‐type structure allows for reversible intercalation/deintercalation of fluoride ions through electrochemical fluorination, making it the second structure type known for the structurally reversible incorporation of fluoride ions so far. As with the fluorinated Ruddlesden–Popper‐type compounds, the oxide and fluoride in the fluorinated schafarzikite structure have been shown to occupy two different crystallographic sites, see Figure . These sites were determined from neutron powder diffraction studies, where the bond distances from the structural solutions were used to calculate bond valences sums to support the validity of the proposed models.…”

“…Chemical fluorination of the oxides has predominantly been performed via chemical reactions, for example, by heating samples under flowing F 2 gas or with milder fluorination agents such as PVDF . The use of oxidative agents (F 2 , CuF 2 , AgF 2 ) is challenging, and can often lead to the decomposition of the target compounds . The reason for this originates from the fact that such reagents always work at a certain chemical fluorination potential, which can only be altered by the choice of the metal fluoride.…”