# Naked F – Provided by Imidazolium Fluoride-Based Electrolytes Promotes Low Overpotential in Fluoride-Ion Batteries

Jules Valentin, Fabien Boeda, Vincent Maisonneuve, Edouard Boivin

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Jules Valentin, Fabien Boeda, Vincent Maisonneuve, Edouard Boivin. Naked F – Provided by Imida-zolium Fluoride-Based Electrolytes Promotes Low Overpotential in Fluoride-Ion Batteries. European Journal of Inorganic Chemistry, 2026, 10.1002/ejic.202500566. hal-05509753

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Submitted on 16 Feb 2026

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# Naked F⁻ Provided by Imidazolium Fluoride-Based Electrolytes Promotes Low Overpotential in Fluoride-Ion Batteries.

Jules Valentin¹, Fabien Boeda¹, Vincent Maisonneuve¹*, Edouard Boivin¹,*

1. Institut des Molécules et Matériaux du Mans (IMMM, UMR CNRS 6283)

* Corresponding authors: edouard.boivin@univ-lemans.fr; vincent.maisonneuve@univ-lemans.fr

## Abstract

As an alternative to lithium-based batteries, fluoride-ion batteries (FIBs) offer a promising pathway owing to their high theoretical energy density. However, their progress has been hindered by the lack of room-temperature electrolytes, either solid or liquid, that exhibit sufficiently wide electrochemical stability windows and that allow F-ion shuttling between electrodes with low overpotential. In this work, we identify 1-butyl-3-methylimidazolium fluoride (BMimF) dissolved in acetonitrile (MeCN) as a particularly promising electrolyte candidate. This system exhibits a high room-temperature ionic conductivity of 15.3 mS·cm⁻¹ and an electrochemical stability window ranging from -0.37 to +1.59 V vs. Pb/PbF₂. ¹⁹F NMR reveal that BMimF provides a “naked” F⁻ that enable the stable cycling of a symmetric Pb + PbF₂ cell with a very low overpotential of 75 mV (at 6 mA·g⁻¹) and 160 mV while paired with BiF₃. XRD, AES and SEM-EDX reveal that, compared with the widely used TBAF·3H₂O in THF electrolyte, BMimF in MeCN provides a more stable chemical environment for CsMnFeF₆, a previously reported insertion-type electrode material, by effectively limiting cation dissolution.

## Introduction

The development of next-generation batteries must consider the projected shortage of lithium reserves. Alternative cationic charge carriers such as Na⁺, K⁺, Ca²⁺, Mg²⁺, Zn²⁺, and Al³⁺ are being widely investigated, but their performance is still far from lithium-based technologies. Anionic charge carriers, particularly F⁻ ions, offer attractive opportunities that could enable significant breakthroughs in energy density.¹,² Among the different approaches, conversion-type positive electrode materials are the most appealing due to their high theoretical capacities. For instance, CuF₂ can theoretically deliver 527 mAh·g⁻¹ at a relatively high voltage of 3.66 V vs. Li/LiF. Consequently, early studies on Fluoride-Ion Batteries (FIBs) primarily focused on such conversion-type electrodes. More recently, insertion-type materials have also demonstrated remarkable electrochemical performance. For instance, the perovskite-type Ca₀.₈Sr₀.₂FeO₂□ (□ refers to O vacancy) delivers 580 mAh·g⁻¹ at 1.0 V vs. Pb/PbF₂, corresponding to 3.85 V vs. Li/LiF.³ Tysonite-type La₁⁻ₓBaₓF₃⁻ₓ solid electrolytes are the most commonly employed for both conversion and insertion-type reactions in FIBs due to their wide electrochemical stability windows (ESWs). However, their relatively low fluoride-ion conductivity requires elevated operating temperatures (typically ranging from 140 to 170°C).⁴ By contrast, electrolytes such as BaSnF₄ operate at room temperature but exhibit a limited ESW.⁴ Several liquid electrolytes combine high ionic conductivity at RT with wide ESW.⁵⁻¹² However, to date, only 1 M TBAF·3H₂O (TetraButylAmmonium Fluoride trihydrate) in THF (TetraHydroFurane) has been tested against the potential insertion-type cathode pyrochlore CsMnFeF₆,¹³ fluorides known to be fairly sensitive to water. Since such fluorides are sensitive to water, this raises questions regarding their compatibility with hydrated electrolytes and highlights the need for dry fluoride-containing solutions.

Dry TBAF provides a “naked” F⁻ anion due to the large and hydrophobic cation, which does not engage in hydrogen bonding. However, such free F⁻ is strongly basic and reacts with the β-hydrogens of TBA⁺, leading to Hofmann elimination and the formation of HF₂⁻ ions, among other byproducts.¹⁴ In contrast, hydrated TBAF (TBAF·3H₂O) mitigates this issue: coordinated water molecules form hydrogen bonds with F⁻, lowering its basicity and preventing Hofmann elimination, thereby
improving electrolyte stability. TBAHF₂ (TetraButylAmmonium bifluoride) is another chemically stable anhydrous salt but it provides HF₂⁻ ions rather than free F⁻ ions. Recently, TBAF·2DPU (DPU = DiPhenyl Urea) has been identified as a promising alternative with DPU shielding the basicity of F⁻ ion in a dry salt, through hydrogen bonds.⁹ However, the rather high dissociation energy required to release “naked” F⁻ from F·2DPU lead a large overpotential (≈600 mV at 50 µA·cm⁻¹) in symmetric Pb/PbF₂ cell. Replacing the butyl groups in TBA⁺ by methyl groups to form TMA⁺ (TetraMethylAmmonium) prevents Hofmann elimination due to the absence of β-hydrogens, but this modification drastically reduces the salt's solubility in aprotic solvents which can be improved by using anions acceptors¹⁵ or ionic liquid solvents¹⁶, or even fluorinated solvents.¹⁷ Moreover, grafting trimethyl groups on TMA⁺ increases the solubility while preventing the presence of β-hydrogens but still provides a strongly basic F⁻ ion, that can deprotonate the solvents, forming HF₂⁻.¹⁸,¹⁹ Imidazolium fluorides, in which the F⁻ basicity is lowered due to hydrogen bonding with the C2-H of the imidazolium ring, are expected to be less basic. Moreover, the delocalization of the positive charge in imidazolium ring makes the N–C bonds more stable against Hoffman elimination, unlike the localized positive charge in tetraalkylammonium cations. Indeed, contrary to TBAF, 1-butyl-3-methylimidazolium fluoride (BMimF), although hygroscopic, is rather stable at RT in its anhydrous form.²⁰ To the best of our knowledge no imidazolium fluorides have been employed as electrolyte salt for FIB, only imidazolium bifluoride²¹ and imidazolium-based ionic liquids as solvents.¹⁰,²²

In this work, we report the use of a 0.5 M BMimF solution in acetonitrile (MeCN) as a liquid electrolyte for FIBs. The nature of the fluorinated charge carrier was characterized by ¹⁹F NMR spectroscopy, while electrochemical measurements were carried out to determine the ionic conductivity and the ESW of this new electrolyte. The performance of BMimF in MeCN was first evaluated in a symmetric cell composed of Pb + PbF₂ + VGCF (Vapor Grown Carbon Fibers) composite electrodes. Subsequently, its chemical compatibility with the pyrochlore CsMnFeF₆ cathode, a previously reported intercalation-type electrode material for FIBs, was examined using XRD, SEM-EDS, and MP-AES analyses. Finally, galvanostatic measurements were conducted to assess the ability of CsMnFeF₆ to support reversible fluoride-ion intercalation.

## Electrolytes characterization

Solutions of TBAF·3H₂O in THF and TBAHF₂ in MeCN were obtained commercially and used without further purification. In contrast, anhydrous BMimF was synthesized following a previously reported procedure.²⁰ Its purity and dryness were confirmed by ¹H NMR spectroscopy in CDCl₃ (Figure S1). BMimF is insoluble in THF, appears instable in carbonate-based (PC and DMC) solvents but can be dissolved at concentrations of at most 0.5 M in MeCN. Expect where otherwise specified this concentration has been used in this study. To identify the fluorinated species present in these different electrolyte solutions, ¹⁹F{¹H} NMR spectra were recorded using No-D experiments (Figure 1).
<figure><img src="image_2.png" /><figcaption>Figure 1: ¹⁹F{¹H} NMR spectrum of TBAF·3H₂O in THF, TBAHF₂ in MeCN, fresh and two weeks aged BMimF in MeCN.</figcaption></figure>

For TBAHF₂ in MeCN, a main ¹⁹F resonance is observed at δ ≈ -147.5 ppm, corresponding to the HF₂⁻ ion. A similar but much weaker signal is also detected in TBAF·3H₂O in THF, accompanied by an intense peak at δ ≈ -115.4 ppm, which is attributed to F⁻ ions solvated by three H₂O molecules. In the case of BMimF in MeCN, both HF₂⁻ (δ ≈ -147.5 ppm) and F⁻ (δ ≈ -108.6 ppm) signals are present, with a HF₂⁻/F⁻ intensity ratio close to 0.3:1. This HF₂⁻ species would most probably arise from the deprotonation of slightly acidic proton of MeCN by a strongly basic F⁻, as ¹⁹F{¹H} NMR spectra of the BMimF in MeCN solution aged during two weeks doesn't show any significant evolution of the HF₂⁻/F⁻ ratio (Figure 1) after the acid/base equilibrium is reached, ruling out the instability of BMimF towards Hoffman elimination while dissolved in MeCN. Crucially, the amount of HF₂⁻ is significantly smaller than previously reported for anydrous tetraalkylammonium fluorides dissolved in MeCN, suggesting the lower basicity of the F⁻ provided by BMimF (23 % against 37% of F involved in HF₂⁻ moeity).¹⁵ As an explanation, in dissolved BMimF, the F⁻ ion would likely engage in hydrogen bonding with the C2–H proton of the imidazolium ring, thus reducing the F⁻ basicity.¹⁶ However, the less negative F⁻ chemical shift of BMimF in MeCN compared with TBAF·3H₂O in THF would indicate that F⁻ ions are more weakly hydrogen-bonded in the former solution. The conductivity of the electrolytes was measured at RT using electrochemical impedance spectroscopy (Figure S2). BMimF in MeCN exhibits an ionic conductivity of 15.3 mS·cm⁻¹ - a value decreasing to 6.2 mS·cm⁻¹ at a concentration of 0.1M - whereas the TBAF·3H₂O in THF electrolyte displays a lower value of 5.5 mS·cm⁻¹. This reduced conductivity can be ascribed to the presence of three coordinated H₂O molecules, which form strong hydrogen bonds with F⁻ ions, thereby hindering their mobility compared to the more “naked” F⁻ ions supplied by BMimF in MeCN. However, the high ionic conductivity measured for the BMimF-based electrolyte should be interpreted with caution, as HF₂⁻ species may account for a substantial fraction of the total conductivity. Indeed, some imidazolium bifluorides have been
reported to exhibit exceptionally high ionic conductivities, approaching 100 mS·cm⁻¹.²³ Although a reliable determination of the fluoride-ion transference number (e.g., via the Bruce–Vincent method) would help quantify the specific contribution of F⁻ transport, our attempts did not yield stable measurements in a symmetric Pb + PbF₂ + VGCF setup. The inherently high conductivity of bifluoride species may also explain why TBAHF₂ in MeCN exhibits a nearly fivefold higher ionic conductivity compared with TBAF·3H₂O (25.7 vs. 5.5 mS·cm⁻¹, see Figure S2), although difference in solvent, salt concentration and H-bonding with water molecules likely influence these values.

<figure><img src="image_3.png" /><figcaption>Figure 2: LSV measurements on TBAF·3H₂O in THF and BMimF in MeCN.</figcaption></figure>

The Figure 2 shows a comparison of the Linear Sweep Voltammetry (LSV) measurements of the electrolytes in a three electrode cell made of a glassy carbon working electrode, a Pt counter electrode and a Pb + PbF₂ + VGCF reference electrode. BMimF in MeCN and TBAF·3H₂O in THF exhibit similar EWSs of –0.37 to 1.59 V and –0.44 to 1.57 V vs. Pb/PbF₂, respectively (Figure S3). Although these EWS are smaller than those reported previously in EC/DEC solvent,⁹ TBAF·3H₂O in THF and BMimF in MeCN appear to be suitable in terms of ESW for the activation of both Mn²⁺/Mn³⁺ and Fe²⁺/Fe³⁺ redox couples in the pyrochlore CsMnFeF₆.¹³

### Electrochemical measurements on conversion-type materials

Pb/PbF₂ is the most widely employed counter and reference electrodes in FIBs. While the orthorhombic α-PbF₂ polymorph is the most stable at RT, the cubic β-PbF₂ allotropic form is preferred due to its superior ionic conductivity.²⁴ Since, commercial PbF₂ is a mixture of relatively well cristallized α and β polymorphs, ball milling was first employed to reduce particle size. However, it tends to favor the formation of α-PbF₂ at the expense of β-PbF₂. Complete transformation into β-PbF₂ was achieved by annealing at 380 °C followed by quenching (Figure S4, Table S1), as reported previously.²⁵

In order to evaluate the ability of studied electrolyte to trigger conversion reaction in FIBs, symmetric Pb + PbF₂ + VGCF//electrolyte//Pb + PbF₂ + VGCF cells were cycled in galvanostatic mode. The optimization of PbF₂ is efficient, since, when using TBAF·3H₂O in THF electrolyte, a decrease of the overpotential from ca. 1000 mV to ca. 360 mV (Figure S5) is observed. Moreover it delivers the highest reversible capacity, reaching nearly 210 mAh·g⁻¹ during the first full cycle (i.e. first charge, 2nd discharge because cells are prepared as half charged) as displayed on Figure 3a. Despite its superior ionic conductivity, BMimF in MeCN enables only partial conversion of Pb to PbF₂ (150 mAh·g⁻¹) but provides a much lower overpotential (75 mV) at the same current density of 6 mAh·g⁻¹. The high overpotential observed with TBAF·3H₂O in THF, similar to that previously reported for TBAF·2DPU in EC/DEC,⁹ can be attributed to the energy required to dissociate solvated species and release free F⁻
ions at the electrode/electrolyte interface. BMimF in MeCN supplies more “naked” F⁻ ions, hence the overpotential is significantly reduced, since no dissociation is required. The smaller capacity obtained in the voltage range (-0.25 - +0.25 V vs. Pb/PbF₂), could originate from the limited fraction of available F⁻ ions induced by the lower concentration and the presence of 23 mol% of F involved in HF₂⁻ moieties that do allow fluorination of Pb but at a significantly higher potential, as observed for TBAHF₂ in MeCN (see Figure S6).

<figure><img src="image_4.png" /><figcaption>Figure 3: (a) 1<sup>st</sup> full cycle of Pb+PbF<sub>2</sub>+VGCF symmetrical cells with TBAF·3H<sub>2</sub>O in THF and BMimF in MeCN as the electrolyte (b) Galvanostatic cycling of Pb/PbF<sub>2</sub>/VGCF symmetrical cells at 6mA.g<sup>-1</sup> with a cut-off capacity of 20% of theoritical capacity (c) : 1<sup>st</sup> cycle of BiF<sub>3</sub> + C// BMimF in MeCN // Pb + PbF<sub>2</sub> + VGCF cell</figcaption></figure>

In terms of cyclability, TBAF·3H₂O in THF exhibits the most stable performance, retaining 60% of its
initial capacity after 10 cycles, whereas BMimF in MeCN retains only 38% (Figure S7). It should be
emphasized, however, that Pb/PbF₂ serves primarily as a counter (CE) and reference electrode (RE)
rather than a practical negative electrode, due to its relatively large working voltage. For such a role,
it is usually used in a large excess with respect to the capacity of working electrode. Under such
conditions (i.e. cut of capacity limited to 20% of the theoretical capacity), the lead-based electrodes
can reversibly cycle with an overpotential remaining below 160 mV for at least 200 h (Figure 3b). From this perspective, BMimF in MeCN appears to be the most suitable option among the electrolytes studied, since a flat and stable voltage profile as well as a low overpotential are more desirable than high capacity for a RE/CE. To assess the suitability of the BMimF-based electrolyte for enabling reversible conversion reactions in FIBs, BiF₃ was employed as the positive electrode material (Figure S8). A reversible capacity of 215 mAh·g⁻¹ was achieved with a low overpotential of 160 mV at 15 mA·g⁻¹, confirming that the “naked” F⁻ ions supplied by BMimF allow (de)fluorination reactions with improved kinetics.

## Stability of CsMnFeF₆ against the electrolytes

To evaluate the suitability of electrolytes for insertion-type reactions, CsMnFeF₆ + VGCF // electrolyte // Pb + PbF₂ + VGCF cells were prepared, since the pyrochlore CsMnFeF₆ has previously been reported as an insertion-type material for FIBs.¹³ Phase purity and structure were confirmed by Rietveld refinements (Figure 4, Table S2), revealing a single-phase pyrochlore structure cristallizing in the cubic *Fd*−*3m* space group with a single 16c site accomodating both Mn²⁺ and Fe³⁺ cations. The Cs−F, Mn−F and Fe−F coordination octahedra yield average bond lengths 3.23(1) Å for Cs−F of and 1.991(3) Å for Mn−F/Fe−F similar to its previously reported structure,²⁶,²⁷ with a lattice parameter of 10.5183(6) Å. Scanning Electron Microscopy (SEM) images reveal agglomerates of submicronic particles while EDS measurements shows a homogeneous distribution of elements with cationic ratio close to that expected for CsMnFeF₆ (Figure S8).

<figure><img src="image_5.png" /><figcaption>Figure 4: Rietveld refinement of the structure of the pyrochlore CsMnFeF₆ against XRD data.</figcaption></figure>

*Ex situ* XRD (Figure 5) analyses were carried out to assess the stability of the active material in different electrolytes after 7 days of rest at open circuit voltage (OCV). The XRD pattern of CsMnFeF₆ + VGCF composite electrode exposed to BMimF in MeCN shows no significant changes, in contrast to electrodes exposed to TBAF·3H₂O in THF, where CsMnFeF₆ partially transforms into a new phase with slight changes in the lattice parameters of the remaining pyrochlore (Figure S9, Table S3). Although this phase could not be unambiguously identified, its diffraction pattern closely resembles that previously observed after several charge−discharge cycles of CsMnFeF₆ in the same electrolyte.¹³ While this earlier study suggested that this secondary phase might arise from reversible F⁻ insertion/extraction into/from CsMnFeF₆, our results demonstrate that it forms in the absence of applied current, indicating that it is not an electrochemically driven reaction.
<figure><img src="image_6.png" /><figcaption>Figure 5: XRD patterns of CsMnFeF₆ composite electrode at pristine state and after 7 days at OCV in TBAF·3H₂O in THF and BMimF in MeCN.</figcaption></figure>

To evaluate the potential partial solubilization of cations from CsMnFeF₆ in various electrolytes, Microwave Plasma-Atomic Emission Spectroscopy (MP-AES) measurements were performed on electrolyte solutions obtained after immersing CsMnFeF₆ powder for 7 days, complemented by ex-situ SEM-EDS analyses of composite electrodes after 7 days of rest at OCV (Figure 6). In pure THF and MeCN solvents, no significant dissolution (< 1 mmol·L⁻¹) of any cations was detected by MP-AES, and no change in the cationic ratios of the CsMnFeF₆ + VGCF composite cathodes was observed, consistent with the very low solubility of inorganic fluorides in organic media (Figures S10 and S13). In contrast, a TBAF·3H₂O solution in THF dissolved approximately at 10 mmol·L⁻¹ of Cs and Mn, along with a higher concentration of Fe (≈15 mmol·L⁻¹), indicating partial dissolution of CsMnFeF₆ most probably induced by the presence of water molecules in TBAF·3H₂O. Correspondingly, SEM-EDS analysis on the electrode composite exposed to the TBAF·3H₂O solution over the same period of time revealed a decrease in the Fe content from 34(1)% to 27(1)%, leading to the formation of a Mn-enriched secondary phase. Meanwhile, the remaining Fe in solution may diffuse across the electrolyte and precipitate at the Pb/PbF₂ counter electrode consistent with the detection of Fe (with Fe/(Fe+Pb) ≈ 5 mol%) and no Mn by SEM-EDS (Figures S11 and S13). In the case of BMimF in MeCN, only Fe exhibits a significant solubilization (≈10 mmol·L⁻¹ against <1 mmol·L⁻¹ for Mn and Cs), although to a lesser extent than in TBAF·3H₂O. Hence, the evolution of the cationic ratios obtained by SEM-EDS on the composite electrode remains within standard error (Figure S12 and S13). This behavior suggests that the absence of water in the BMimF-based solution prevents adverse reactions with the active material, making it a more compatible electrolyte for CsMnFeF₆, despite the presence of HF₂⁻.
<figure><img src="image_7.png" /><figcaption>Figure 6: Mn (a), Fe (b) and Cs (c) cationic ratio on positive composite electrodes after 7 days of OCV in anhydrous MeCN, anhydrous THF, TBAF·3H₂O in THF, BMimF in MeCN. Results of MP-AES measurements of the electrolyte solutions obtained after immersing CsMnFeF₆ powder for 7 days (d).</figcaption></figure>

Electrochemical measurements

The electrochemical properties of CsMnFeF₆ were evaluated by galvanostatic cycling in CsMnFeF₆ + VGCF // electrolyte // Pb + PbF₂ + VGCF cells. As shown in Figure 7, the initial discharge with TBAF·3H₂O in THF displays a smooth voltage profile at an average potential of ca. –0.25 V vs. Pb/PbF₂, consistent with the Fe³⁺ reduction. However, the delivered capacity is limited to 25 mAh·g⁻¹ (≈ 33% of the theoretical capacity assuming full reduction of Fe³⁺ and the formation of CsMn²⁺Fe²⁺F₅). The subsequent charge exhibits a sloping voltage increase up to 1.0 V vs. Pb/PbF₂, likely corresponding to Fe²⁺ re-oxidation, followed by a relatively flat plateau at ≈ 1.2 V vs. Pb/PbF₂, consistent with activation of the Mn²⁺/Mn³⁺ redox couple. During the next discharge, a two-step staircase profile suggests successive reduction of Mn³⁺ and Fe³⁺, though with a modest reversible capacity of ≈ 25 mAh·g⁻¹. After this activation cycle, the voltage profile remains relatively stable, and the capacity gradually decreases upon prolonged cycling. These results are mostly in line with previous studies of CsMnFeF₆ cycled in TBAF·3H₂O in THF vs. Bi/BiF₃.¹³

Using TBAHF₂ in MeCN (Figure S14) and BMimF in MeCN (Figure 7b) electrolytes, the first discharge shows a similar voltage profile than with TBAF·3H₂O in THF, albeit with slightly different capacities (23 and 18 mAh·g⁻¹, respectively). It is worth noting that since TBAHF₂ in MeCN enables the Pb to PbF₂ conversion reaction in a symmetric cell, F⁻ extraction from CsMnFeF₆ during discharge becomes feasible, since it no longer relies on HF₂⁻ dissociation. This interpretation aligns with the similar first-discharge voltage profiles observed across the three electrolytes. In contrast, using BMimF in MeCN
as the electrolyte yields a recharge profile characterized by a plateau near 0.7 V vs. Pb/PbF₂ accounting for ca. 55 mAh·g⁻¹. By analogy with TBAF·3H₂O, this feature likely reflects the activation of the Fe²⁺/Fe³⁺ but in an irreversible manner, as the plateau is absent in subsequent discharge cycles. Since CsMnFeF₆ appears to be more stable versus BMimF in MeCN than TBAF·3H₂O in THF, the ability of CsMnFeF₆ to undergo a reversible insertion-type reaction is questionable. Reversible electrochemical signatures are only observed in TBAF·3H₂O in THF; however, in this electrolyte Mn and Fe partially dissolves, leading to the formation of an unknown Mn-enriched secondary phase. It is therefore unclear whether the observed electrochemical response using TBAF·3H₂O in THF electrolyte originates from CsMnFeF₆, the secondary phase and/or from dissolved transition-metal species. In any case, using BMimF in MeCN, electrolytes that provide a more stable environment, CsMnFeF₆ delivers poor electrochemical performance.

<figure><img src="image_8.png" /><figcaption>Figure 7: Charge-discharge curves of CsMnFeF₆ in (a) TBAF·3H₂O in THF (b) BMimF in MeCN</figcaption></figure>

## Conclusion

In this work, we report the use of a 0.5 M BMimF solution in MeCN as a liquid electrolyte for FIBs, exhibiting an ionic conductivity of 15.3 mS·cm⁻¹ at room temperature and an ESW of -0.37 V to +1.59 V vs. Pb/PbF₂. Despite these advantages, ¹⁹F NMR reveals the coexistence of F⁻ and HF₂⁻ species in solution, likely arising from MeCN deprotonation. This electrolyte enables the reversible cycling of a composite Pb + PbF₂ + VGCF in symmetric cell with an initial overpotential of 75 mV at 6 mA·g⁻¹ remaining below 160 mV over at least 200 h. This overpotential is among the smallest reported so far in Pb/PbF₂ symmetric cell most probably due to the “naked” F⁻ provided by BMimF in MeCN that doesn't require any dissociation at the electrode/electrolyte interface to induce fluorination of lead. When tested against a pyrochlore CsMnFeF₆ cathode, previously reported as a potential insertion-
type cathode material, BMimF-based electrolyte provides a rather stable environment but delivers
poor performance, in contrast to that observed with TBAF·3H₂O. In the latter case, CsMnFeF₆
behaves similarly to that reported previously but prolonged exposure to TBAF·3H₂O in THF solution
leads to a marked change in the Mn/Fe ratio and the formation of a new phase, thereby raising
concerns regarding the ability of CsMnFeF₆ to sustain reversible F⁻ intercalation reactions.

**Experimental method**

***Materials synthesis***

BMIMF was synthesized following the procedure described by Magnier and co-workers.¹⁶ CsMnFeF₆ was synthesized using CsF (≥ 99%, Sigma-Aldrich), MnF₂ (≥ 99%, Sigma-Aldrich), FeF₃ (≥ 97%, Alfa Aesar). Powders were carefully weighed (total mass 1 g) and inserted into a 45 mL zirconia (ZrO₂) mill (Fritsch) filled with 10 zirconia balls (approximate mass of a ball 3 g); all samples were prepared under a dry argon atmosphere. The mixtures were ball-milled in a Fritsch Pulverisette 7 planetary miller at 400 rpm for 16 h (32 cycles consisting of 15 min milling and 15 min of rest). Commercial 1.0 M TBAF in THF (Acros Organics) and 50% TBAHF₂ in MeCN (Sigma-Aldrich) solutions were used without further purification. To prepare the 0.5 M BMimF in MeCN solution, 79.1 mg of BMimF were dissolved in 1 mL MeCN inside an Ar filled glovebox. To ensure anhydrous conditions, all solvent used in study, MeCN (Sigma Aldrich, 99.5%) and THF (99.9%, Sigma Aldrich, dried through activated alumina columns (Glass Technology GTS 100)), were dried over freshly regenerated 4Å molecular sieves.

**Electrochemical measurements**

Electrochemical measurements were performed using a Biologic VMP-3e potentiostat controlled by EC-lab software. The cell assembly was entirely performed in an argon glovebox. Stainless steel Swagelok cells were used as the electrochemical test cells, and borosilicate glassfiber pads (Whatman GF/D) were utilized as separators exeped while using TBAHF₂-based electrolyte for which PTFE seperators (Whatman GF/D) were employed. Loose-powder working electrodes were prepared by grinding 20 wt % of VGCF conductive carbon and 80 wt % of the active material until a homogeneous color was obtained (ca. 5-10 minutes). The electrodes had a typical mass loading of 10 mg/cm². To produce the lead-based composite electrodes, 1 g of the commercial PbF₂ (Strem, 99.99%) was ball-milled for 16h at 400rpm in 45 mL ZrO₂ jars with 1-cm-diameter-ZrO₂ balls (10) and then sealed in a Pt tube under Ar for thermal treatment at 350°C for 1h 30 before quenching. The as-prepared PbF₂ was grinded with Pb metal powder (Sigma-Aldrich, 99%, 200 mesh) and VGCF in mass ratio of 40:40:20 wt %. Electrochemical stability windows of electrolytes were evaluated by LSV measurements between OCV and ±2.5 V vs. Pb/PbF₂ with a scan rate of 1 mV/s, using a three-electrode set-up with a Pb + PbF₂ + VGCF reference electrode, a platine wire as the counter electrode and glassy carbon as the working electrode. For each electrolytes, two cells were mounted to determine the electrochemical stability towards oxidation and reduction. The ionic conductivity was measured using electrochemical impedance spectroscopy (EIS). Measurements were acquired between 1 MHz and 1 Hz using a stainless steel Swagelock cell. The cell were measured as prepared at room temperature. The ionic conductivity was deduced from the linear fit of the Nyquist plot.

***Materials characterization***

Proton (¹H) and Fluorine (¹⁹F) Nuclear Magnetic Resonance spectra were recorded using either a Bruker NEO 500 or a Bruker Avance 400 spectrometer. X-Ray Diffraction (XRD) patterns were collected using a PANalytical Θ/Θ Bragg-Brentano Empyrean diffractometer) equipped with a CuKα
source ($\lambda_1$ = 1.54056 Å and $\lambda_2$ = 1.54443 Å) and a PIXcel1D detector. The instrumental resolution function of the instrument was empirically determined from a fundamental parameter set determined using a reference scan of LaB₆ (NIST 660a). Le Bail type and Rietveld refinement were performed by using FullProf Suite program.²⁸ Scanning Electron Microscopy (SEM) images were obtained using a JEOL microscope (JSM 6510 LV) at an acceleration voltage of 20 kV. Elementary quantitative microanalyses were performed by Energy-Dispersive X-ray Spectroscopy (EDS) with an Oxford detector and analyzed using Aztec software. Each sample was covered with a thin gold layer to improve the electronic conductivity of the sample. The acquired chemical maps exhibit a local uncertainty of approximately 5% per pixel, and the reported EDS ratios correspond to averages over roughly 800,000 pixels. Accordingly, we adopt an error bar of 1% on the averaged values as a reasonable estimate. From standard solutions containing iron and manganese (100 ppm, VWR Chemicals) as well as Cesium (100 ppm, VWR Chemicals), standard solutions have been prepared in the concentration range of 0-10 ppm for calibration. To explore the solubility of CsMnFeF₆, 10 mg of CsMnFeF₆ were left for 7 days in 1 mL of anhydrous MeCN, anhydrous THF, TBAF·3H₂O in THF and 0.5M BMimF in MeCN solutions. The supernatant was collected and diluted 200-fold with a 0.5% aqueous HNO₃ solution for Microwave Plasma-Atomic Emission Spectroscopy (MP-AES, Agilent technologies MP-4100, spectrometer and ) or Atomic Absorption Spectrometry (AAS, Analytik Jean technologies (ContraA 800) analyses for transition metal and cesium respectevely.

## Acknowledgments

This work was supported by a French government grant managed by the Agence Nationale de la Recherche under the France 2030 program, reference 23-PEBA-0002. The authors gratefully acknowledge the responsible for the technical platforms "X-ray Diffusion and Diffraction", "Electron Microscopy" and "NMR spectroscopy" of IMMM (Le Mans Université).
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<figure><img src="image_9.png" /></figure>

BMimF dissolved in acetonitrile provides “naked” F⁻ ions, enabling efficient fluoride-ion shuttling in fluoride-ion batteries with low overpotential. This electrolyte exhibits an electrochemical stability window comparable to that of TBAF·3H₂O, while offering higher ionic conductivity. As a result, it delivers improved electrochemical performance when paired with Pb/PbF₂ and BiF₃ electrodes.