This article reports on the investigation of LaSrMnO 4 with K 2 NiF 4 type structure for use as an intercalation based high voltage cathode material with high capacity for fluoride ion batteries (FIBs). Charging was performed against PbF 2 based anodes and shows that fluoride intercalation proceeds stepwise to form LaSrMnO 4 F and LaSrMnO 4 F 2−x . Ex-situ X-ray diffraction experiments were recorded for different cutoff voltages for a deeper understanding of the charging process, highlighting additional potential of the method to be used to adjust fluorine contents more easily than using conventional fluorination methods. A discharging capacity of approximately 20−25 mAh/g was found, which is ∼4−5 times higher compared to what was reported previously on the discharging of BaFeO 2.5 /BaFeO 2.5 F 0.5 , approaching discharge capacities for conversion based fluoride ion batteries. Density functional theory based calculations confirm the observed potential steps of approximately 1 and 2 V for the first (LaSrMnO 4 → LaSrMnO 4 F) and second (LaSrMnO 4 F → LaSrMnO 4 F 2−x ) intercalation steps against Pb-PbF 2 , respectively. Additionally, a detailed structure analysis was performed for chemically prepared LaSrMnO 4 F 2−x (x ∼ 0.2), showing strong similarity to the product which is obtained after charging the batteries to voltages above 2 V against Pb-PbF 2 . It was observed that charging and discharging kinetics as well as coulomb efficiencies are limited for the batteries in the current state, which can be partly assigned to overpotentials arising from the use of conversion based anode composites and the stability of the charged sample toward carbon black and the current collectors. Therefore, the structural stability of LaSrMnO 4 F 2 on the deintercalation of fluoride ions was demonstrated by a galvanostatic discharging to −3 V against Pb-PbF 2 , which can be used to compensate such overpotentials, resulting in almost complete recovery of fluorine free LaSrMnO 4 with a discharge capacity of ∼100 mAh/g. This is the first report showing that selective extraction of fluoride ions from an oxyfluoride matrix is possible, and it highlights that compounds with K 2 NiF 4 type structure can be considered as interesting host lattices for the reversible intercalation/deintercalation of fluoride ions within intercalation based FIBs.

Fluoride-ion batteries: a comprehensive review.

The Ruddlesden-Popper (KNiF) type phase LaNiOF was prepared via a polymer-based fluorination of LaNiO. The compound was found to crystallize in the orthorhombic space group Cccm ( a = 12.8350(4) Å, b = 5.7935(2) Å, c = 5.4864(2) Å). This structural distortion results from an ordered half occupation of the interstitial anion layers and has not been observed previously for KNiF-type oxyfluoride compounds. From a combination of neutron and X-ray powder diffraction and F magic-angle spinning NMR spectroscopy, it was found that the fluoride ions are only located on the apical anion sites, whereas the oxide ions are located on the interstitial sites. This ordering results in a weakening of the magnetic Ni-F-F-Ni superexchange interactions between the perovskite layers and a reduction of the antiferromagnetic ordering temperature to 49 K. Below 30 K, a small ferromagnetic component was found, which may be the result of a magnetic canting within the antiferromagnetic arrangement and will be the subject of a future low-temperature neutron diffraction study. Additionally, density functional theory-based calculations were performed to further investigate different anion ordering scenarios.