LaSrMnO₄: Reversible Electrochemical Intercalation of Fluoride Ions in the Context of Fluoride Ion Batteries

Abstract

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.

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

“…The lengths of the observed charging plateaus in region 2 exceed this capacity significantly, which can be explained from an overlap of the charging plateau with the electrochemical fluorination of the conductive additive of carbon to C−F species (the amount of carbon added can contribute to a charging capacity which is at least 10 times higher than the absolute capacity originating from the amount of the schafarzikite compounds). As found previously, this can impede the discharging (defluorination) of the target compounds, owing to the destruction of the electronic conductive matrix under formation of C−F species .…”

“…As found previously, this can impede the discharging (defluorination) of the target compounds, owing to the destruction of the electronic conductive matrix under formation of C−F species . This would prohibit their use as cathode materials for reversible fluoride‐ion batteries when carbon is used as an additive for achieving electronic conductivity within the active composite. For Co 0.5 Fe 0.5 Sb 2 O 4 , the plateau region is significantly longer, which might be explained by a higher catalytic activity for the fluorination of carbon.…”

“…0.04 F − ) for Co 0.5 Fe 0.5 Sb 2 O 4 and Mg 0.5 Fe 0.5 Sb 2 O 4 , respectively (see Figure a). This observation is similar to our previous findings for LaSrMnO 4, for which the charging plateau also was found to overlap with the decomposition of the carbon matrix. However, on discharging to negative potentials against Pb/PbF 2 (“forced discharging” due to accessing potentials that would correspond to an endergonic or hindered process, as would be the case for the destruction of carbon on charging), lattice parameters were found to change back to close to the values observed for the starting materials (see Table and Figure b, c).…”

“…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.…”

Anion Doping of Ferromagnetic Thin Films of La0.74Sr0.26MnO3−δ via Topochemical Fluorination

“…Since fluorine ions are more mobile than oxygen ions [43], their reversible insertion and extraction may be realized by means of electrochemical methods [34,44]. Such methods, e.g., by using electrochemical oxidative fluorine insertion (AMnO 3−y → AMnO 3−y F y ), or reductive fluorine deintercalation (AMnO 3−y F y → AMnO 3−y ), shall open new opportunities for tuning material properties.…”