Rhombohedral iron trifluoride with a hierarchized macroporous/mesoporous texture from gaseous fluorination of iron disilicide

Abstract

Stable low temperature rhombohedral iron trifluoride has been obtained by the fluorination under the pure fluorine gas of iron disilicide. The combination of both unusual fluorination process and precursor avoids to get unhydrated crystalline FeF 3 particles and allows the formation of hierarchized channels of mesoporous/macroporous texture favorable for lithium diffusion. The fluorination mechanism proceeds by temperature steps from the formation, for a fluorination temperature below 200 °C, of an amorphous phase and an intermediate iron difluoride identified mainly by 57 Fe Mössbauer spectroscopy before getting, as soon as a fluorination temperature of 260 °C is reached, the rhombohedral FeF 3 . Both amorphous and crystallized samples display good ability for electrochemical process when used as cathode in lithium-ion battery. The low diameter of rhombohedral structure channels is balanced by an appropriate mesoporous texture and a capacity of 225 mAh.g -1 after 5 cycles for a discharge cut-off of 2.5 V vs. Li + /Li at a current density of C/20 has been obtained and stabilized at 95 mAh.g -1 after 116 cycles.

Room Temperature Pseudo-Solid State Iron Fluoride Conversion Battery with High Ionic Conductivity and Low Interfacial Resistance

“…The value of σ for gel-1 (σGel-1) at room temperature (0.33 mS cm -1 ) is ~5-6× times lower than that expected for the unencapsulated ILE (~1.5 mS cm -1 ) 26 and neat PYR14TFSI (~2.0 mS cm -1 ). 26,27 By contrast, σ of gel-2 (σGel-2) at room temperature (1.92 mS cm -1 ) is close to that previously reported for the unencapsulated LHCE (1.6-2.4 mS cm -1 ). 28,29 There are several important takeaways from Table 1.…”

“…As discussed in Figure S13, ionic conductivity differences alone are an inadequate explanation for this trend. Instead, we attribute these results to three factors: viscosity (~20-26× lower for the LHCE; [27][28][29] vide supra), RCT (~4x lower for gel-2 than gel-1; Figure 2b), and the unique solvent structure of LHCEs. Lower viscosity for the LHCE in gel-2 allows for greater Li + ion mobility, which is increasingly important at faster rates.…”

“…We hypothesize that higher σ and lower Ea for gel-2, relative to gel-1, is due to the substantially lower viscosity of the LHCE (3.7-4.8 cP) 28,29 relative to the IL (PYR14TFSI; 95.1 cP). 27 Raman spectroscopy. Figure S2a shows the complete Raman spectra of the PVDF-HFP gels before (gel-1) and after (gel-2) electrolyte exchange, with accompanying comparison to their pure components (ILE, LHCE, and PVDF-HFP).…”

Unravelling lithiation mechanisms of iron trifluoride by operando X-ray absorption spectroscopy and MCR-ALS chemometric tools

“…Especially, the rhombohedral structure is less studied because of the high temperature required to obtain this structure 4 , however it is also the most stable as it does not exist in an hydrated form in opposite to the other structures. Yet, to overcome hydration or high temperature phase formation, solid-gas fluorination using pure molecular fluorine is the most efficient fluorination way 13 .…”

Operando Mössbauer Spectroscopy Investigation of the Electrochemical Reaction with Lithium in Bronze-Type FeF3·0.33H2O

“…While the mechanism of the electrochemical reaction with Li + has been thoroughly studied and is widely accepted nowadays for anhydrous FeF 3 , − there is still uncertainty concerning the mechanisms of the main electrochemically active hydrated phases, i.e., FeF 3 ·0.33H 2 O, FeF 3 ·0.5H 2 O, and FeF 3 ·3H 2 O. Since the synthesis conditions for the anhydrous trifluoride can be particularly constraining, , the hydrated phases, and, in particular, hexagonal tungsten bronze FeF 3 ·0.33H 2 O, represent the bulk of the recent efforts in optimization of these ionic compounds capable of high-voltage electrochemical reactions. Most of the efforts, however, were devoted to the optimization of the electrochemical performance and not to the understanding of the underlying mechanisms .…”