Reaction Pathways of Iron Trifluoride Investigated by Operation at 363 K Using an Ionic Liquid Electrolyte

Abstract

FeF 3 possesses a high theoretical capacity of 712 mAh g −1 owing to the three-electron reaction. However, various drawbacks, such as the large voltage hysteresis of the conversion reaction, prevent its practical use in lithium secondary batteries. In this study, the charge-discharge behavior of FeF 3 in an ionic liquid electrolyte at 363 K was investigated to elucidate the mechanisms and cause of the reduced overpotentials of the charge-discharge reactions. An evident plateau with an equilibrium potential of 3.42 V vs. Li + /Li during the initial discharge, indicating the two-phase reaction of FeF 3 to form another phase nominally composed of non-trirutiletype LiFe 2 F 6 , was confirmed. Lithium cation was inserted into LiFe 2 F 6 , resulting in a gradual decrease in the rest potential. The lithium-inserted phase was finally converted to LiF and FeF 2 at the end of the one-electron discharge. The conversion of FeF 2 to LiF and Fe in the ionic liquid electrolyte at 363 K was completed at >2.0 V and 71.2 mA g −1 , even though the reaction did not occur at 298 K unless the electrode was discharged below 2.0 V. This difference in the operating voltage of the conversion reaction was mainly due to the suppression of the Li + diffusion overpotential at 363 K.

Phase Evolution of Trirutile Li0.5FeF3 for Lithium-Ion Batteries

“…The second charge capacity is 159.5 mAh g -1 (Figure 2 26,27 However, there is no sign of rhombohedral FeF3 (R−3c) after the second charge to 4.3 V, contrary to other reports that confirmed the reconversion from LiF/FeF2 to FeF3. 68,75,87 Such a difference indicates that the starting material strongly affects the phase evolution of the Li-Fe-F systems. In the overall conversion chemistry of FeF3 at room temperature, the role of trirutile Li0.5FeF3 is considered to be limited because its electrochemical activity is extremely low.…”

“…The second charge capacity is 159.5 mAh g –1 (Figure c), corresponding to less Li + extraction ( x ∼ 0.35 for Li 0.5– x FeF 3 ) compared to the first charge capacity ( x ∼ 0.39 for Li 0.5– x FeF 3 ), thus providing evidence of the lower delithiated state after the second charge process. Previous publications have described the formation of trirutile Li 0.5 FeF 3 as an intermediate obtained by inserting Li + into a FeF 3 framework with a distorted rhenium trioxide structure ( R 3̅ c ). , However, there is no sign of rhombohedral FeF 3 ( R 3̅ c ) after the second charge to 4.3 V, contrary to other reports that confirmed the reconversion from LiF/FeF 2 to FeF 3 . ,, Such a difference indicates that the starting material strongly affects the phase evolution of the Li–Fe–F systems. In the overall conversion chemistry of FeF 3 at room temperature, the role of trirutile Li 0.5 FeF 3 is considered to be limited because its electrochemical activity is extremely low.…”

Unveiling the Role and Mechanism of Nb Doping and In Situ Carbon Coating on Improving Lithium‐Ion Storage Characteristics of Rod‐Like Morphology FeF3·0.33H2O

“…The pair of peaks at a relatively low voltage can be ascribed to reversible conversion between Fe + LiF and LiFeF 3 , while the other pair at a relatively high voltage can be ascribed to the intercalation and de-intercalation of Li + . [39] As the cycling number increases, the reduction peak gradually shifts toward the high-voltage direction because of the reduced particle size and activation of the surface or structure of the active substance after the initial cycle, thus facilitating better contact between the conductive agent and the active material, and elevating the working voltage. Notably, a weak reduction broad peak at 2.4 V during the first discharge and a long slope at 3.7-4.5 V during the initial charge are observed in both CV diagrams, yet do not appear in the subsequent cycles, which can be attributed to the generation of a cathode-electrolyte interface (CEI) film due to decomposition of the electrolyte.…”

Enabling Conversion‐Type Iron Fluoride Cathode by Halide‐Based Solid Electrolyte

“…Even without sophisticated structural engineering, the FeF 2 -LYC cathode composite consisting of crystalline FeF 2 , glass-ceramic LYC, and carbon additive, already demonstrated a better cycling stability and lower voltage hysteresis (especially for the first cycle) than FeF 2 cathodes tested in liquid electrolytes (Figure S10, Supporting Information). [3,7,8,16,20,23,49,[57][58][59] Since micron-sized regions that are rich in Fe and F are still observed in the cathode composites after 20 cycles (Figure S11, Supporting Information), further enhancing the performance is still possible by achieving a more uniform distribution of the components at nanoscale. We then tried to further improve the electrochemical performance of FeF 2 -LYC, by amorphizating it through high-energy ball-milling, as amorphous transition metal sulfide cathodes have been reported to exhibit enhanced kinetics and cycling stability compared with the crystalline ones.…”

“…extremely low-cost materials (e.g., iron: ≈0.1 US$ kg −1 ), are representative examples that yield very high specific capacities at attractive voltages. [15][16][17][18][19][20][21][22][23][24] It should be noted that while sulfur as a conversion-type cathode has already been used in the batteries for unmanned air vehicles, [25] FeF 2 offers a higher volumetric capacity (2002 mA h cm −3 ) than sulfur (1935 mA h cm −3 ) at a slightly higher theoretical potential and both fluorine and iron are more abundant in the Earth's crust than sulfur. [8] Despite the great promise of storing two to three times more energy per given unit mass than the conventional cathodes, the practical application of iron fluoride cathodes has been hindered by multiple challenges.…”