High-Pressure Synthesis of Trigonal LiFe₂F₆: New Iron Fluoride with Li⁺ Tunnels as a Potential Cathode for Lithium-Ion Batteries

Abstract

To overcome the challenge of synthesizing new and promising iron fluoride cathode materials, high-pressure synthesis was employed to obtain a new form of LiFe2F6 crystallized in the trigonal P321 α-LiMnFeF6 structure. Starting from solid fluorine sources, the trigonal structure was obtained for the first time for LiFe2F6 at 2.7 GPa, 500 °C, and characterized by synchrotron X-ray diffraction. Rietveld refinements revealed that a new form of LiFe2F6 possesses the Na2SiF6-type structure, which would provide enhanced Li+ diffusions. A Mössbauer spectrometry analysis was done with a paramagnetic structure obtained at 300 K and a magnetic sextet at 77 K with a 53 T hyperfine field for Fe3+ similarly to α-LiMnFeF6. The first electrochemistry tests indicate a good first cycle capacity of ∼150 mAh·g–1 with a charge process corresponding to Li+ disinsertion, a value doubled when compared to α-LiMnFeF6. These first experimental results show a new synthetic idea to discover materials designed for next generation batteries.

Fluorinated Materials as Positive Electrodes for Li- and Na-Ion Batteries

“…α-LiMnFeF 6 , synthesized by the sol−gel TFA route, exhibits a modest electrochemical performance, as compared to that of the other LiM II M III F 6 fluorides with a capacity of 73 mAh g −1 after 10 cycles (4.3−2.3 V). 165 Lately, high-pressure condition (2.7 GPa, 500 °C) was employed to obtain a new form of LiFe 2 F 6 crystallizing in the trigonal P321 structure, 295 with a higher capacity of 102 mAh g −1 after 10 cycles (4.5−2.0 V) compared to α-LiMnFeF 6 .…”

Insertion of fluorine into a LiFePO4 electrode material by gas–solid fluorination

“…10,11 With that, inorganic fluorides such as iron fluoride (FeF 3 ) and lithiated iron fluorides (e.g., LiFe 2 F 6 or Li 3 FeF 6) were studied as potential positive electrode candidates. [12][13][14][15][16] Li 2 FePO 4 F and LiFePO 4 F fluorophosphates and LiFeSO 4 F fluorosulfate, among others, were investigated as possible iron-based solutions for the positive side of secondary batteries and are the subject of numerous studies, offering a strong framework that acts as a host for Na or Li insertion. [17][18][19][20][21][22] As a result, a classical picture is that the introduction of a fluorine atom in the fluorophosphate host allows high potentials to be obtained, while keeping the benefit of phosphate groups.…”