Reversible Insertion in AFeF₃ (A = K⁺, NH₄⁺) Cubic Iron Fluoride Perovskites

Abstract

The search for new cathode materials is primordial for alkali-ion battery systems, which are facing a constantly growing demand for high energy density storage devices. In quest of more performing active compounds on the positive side, anhydrous iron(III) fluoride demonstrated to be a good compromise in terms of high capacity, operating voltage, and low cost. However, its reaction toward lithium leads to complicated insertion/conversion reactions, which hinder its performances in Li-ion cells. Cycling this material against larger ions such as sodium and potassium is hard or simply impossible due to the size of the channels of the FeF 3 framework impeding ions diffusion. Herein, we propose a strategy based on the use of cubic perovskite AFeF 3 (A = K + , NH 4 + ) as starting materials, allowing the straightforward insertion (after a first disinsertion of the alkali and/or NH 4 + ion) of lithium within the structure and enabling the cycling toward larger alkali ions such as sodium and potassium. For example, a cubic KFeF 3 perovskite, produced by a facile synthesis method, shows superior rate capability toward lithium retaining a capacity of up to 132 mA•h•g −1 at 5 C or of 120 mA•h•g −1 at 5 C toward sodium and enabling cycling toward potassium. Moreover, cubic NH 4 FeF 3 perovskite is discussed for the first time as the suitable cathode material for alkali-ion batteries.

Electrochemical and Structural Behavior of Trirutile-Derived FeF3 During Sodiation and Desodiation

“…At the charged state of the 10th cycle (Pattern 5), the 100 diffraction peak belonging to the cubic FeF 3 ( Pm 3̅ m ) is noted at 13.3°, which confirms that the new reaction previously visualized by the new charge–discharge curves is related to a reversible phase transformation from the orthorhombic NaFeF 3 ( Pnma ) phase to the cubic FeF 3 ( Pm 3̅ m ) phase. This is consistent with the previous study on the charge–discharge behavior of nanosized orthorhombic NaFeF 3 . , Pattern 5 also highlights the presence of a tetragonal phase (hereafter, tetragonal-3 FeF 3 ) that exhibits a disordered trirutile structure akin to tetragonal-2 FeF 3 despite their different lattice parameters due to different Fe occupancies (see Figure S9e and Table ).…”

“…For instance, density functional theory calculations on the orthorhombic NaFeF 3 ( Pnma ) revealed that the oxidation of Fe 2+ to Fe 3+ was engendered by Na + extraction from the orthorhombic NaFeF 3 to form fully desodiated FeF 3 ( Pnma ): a slightly more stable phase than the trigonal ( R 3̅ c ) and cubic ( Pm 3̅ m ) phases . The theoretical works further envisaged an energetically stable, intermediate phase of orthorhombic Na 0.5 FeF 3 , as a line compound in the extraction process. , A recent experimental work on nanosized materials reported a phase transformation from orthorhombic NaFeF 3 ( Pnma ) to cubic FeF 3 ( Pm 3̅ m ), albeit without forming trirutile Na 0.5 FeF 3 . , Although the phase transformation was not observed, this study was the first to mention trirutile Na 0.5 FeF 3 in the context of the Na–Fe–F system.…”

Ultra-stable potassium storage and hybrid mechanism of perovskite fluoride KFeF3/rGO

“…S5a. † Compared to the sodium storage behavior of KFeF 3 , [15][16][17] the CV curves in PIBs are rather smooth without sharp peaks because of the larger ion radius of K + , which is consistent with the results reported in the literature. 21,27,28 The CV curves of KFeF 3 -PVA-500 and KFeF 3 -500 show the same characteristics (Fig.…”

“…are potential cathode materials for LIBs and SIBs since they have a robust structure with three-dimensional diffusion channels for alkali metal ions. [15][16][17] However, they also have low intrinsic electronic conductivity, which greatly limits their electrochemical performances. Hence, applying a carbon coating or forming an AMF 3 -carbon composite is a natural strategy to facilitate electron conduction.…”

Electro-Magnetic Behavior of Highly Correlated Fluorides KFeF3, KCoF3and KNiF3: A ComparativeAb-initioStudy of Cation Effect

“…A. S. Verma in different works as well many other researchers have given wide attention to this type of Fluorides[66][67][68][69][70][71][72][73][74] because they show specific physical properties due to their crystal structures and unique ferroelectric dielectric, (anti)magnetic and optoelectronic properties. Revised structural phase transition are made in KMnF 3 Fluoroperovskite crystals as well as Joanna Kapusta et al[75] works, and many other scientist researchers have also mentioned that exist structural phase transition in the KMnF 3 Fluorides (Mn are transition metals)[76][77][78][79][80][81][82] which has been considered cubic for a very long time, critical behavior magnetic transitions are given specially for two KCoF 3 and KNiF 3 Fluorides by A. Oleaga et al[83], also Atsushi Okazaki et al[57] studied the crystal structures of the anti-ferromagnetic KMnF 3 , KFeF 3 KCoF 3 , KCNiF 3 and KCuF 3 have been determined above and below their Néel temperatures (T N ) by X-ray diffraction using single crystals. They reported that the structures of these compounds are the ideal perovskite type (cubic) except for that of KCuF 3 Which crystallizes as a tetragonal modification (a > c) of the perovskite type.…”