# Insertion of fluorine into a LiFePO 4 electrode material by gas-solid fluorination

Lemoine Kevin, Kenmogne-Debah Roméo, Doubtsof Léa, Frezet Lawrence, Petit Elodie, Guerin Katia, Bertrand Devouard, Sougrati Moulay-Tahar, Delbègue Diane, Bonnet Pierre

► To cite this version:

Lemoine Kevin, Kenmogne-Debah Roméo, Doubtsof Léa, Frezet Lawrence, Petit Elodie, et al. In-sertion of fluorine into a LiFePO 4 electrode material by gas-solid fluorination. Dalton Transactions, 2024, 53 (17), pp.7546-7554. 10.1039/D4DT00603H. hal-04586473

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Submitted on 12 Nov 2024

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# Insertion of fluorine in LiFePO₄ electrode material by gas-solid fluorination

Lemoine Kevin*ª, Kenmogne-Debah Roméoª, Doubtsof Léaª, Frezet Lawrenceª, Petit Elodie ª,
Guerin Katiaª, Devouard Bertrandᵇ.c, Sougrati Moulay-Taharᵈ, Delbègue Dianeᵉ and Bonnet Pierre*ª

ªUniversité Clermont Auvergne, CNRS, ICCF, 24, Avenue Blaise Pascal, 63178 Aubière, France.

ᵇLaboratoire Magmas et Volcans, CNRS UMR 6524, Université Clermont Auvergne, IRD, OPGC, 63170 Aubière, France.

ᶜCEREGE, Aix Marseille Univ, CNRS, IRD, INRAE, 13545 Aix-en-Provence, France.

ᵈICGM, University of Montpellier, CNRS, ENSCM, Montpellier, France.

ᵉCentre National des Etudes Spatiales, 18 avenue Edouard Belin 31 401 Toulouse Cedex 9, France.

*corresponding authors :

Lemoine Kevin : kevin.lemoine@uca.fr

Bonnet Pierre : pierre.m.bonnet@uca.fr

**Keywords:** Fluorination, solid/gas reaction, iron-based materials, secondary batteries.

**Abstract:** In this study, the insertion of fluorine in LiFePO₄ was carried under molecular fluorine F₂ at different temperatures. The reactivity strongly depends on the applied fluorination temperature leading to very different products: a core delithiation of the material is observed at low temperatures with the formation of a LiF shell around the particles, while the material decomposes to gradually form a mixture of α-FeF₃ and α-Li₃FeF₆ iron fluorides at higher temperatures. A second thermal treatment under N₂ leads to the formation of LiFePO₄F according to a new way not yet reported. Supported by X-Ray Diffraction, Raman, infrared and Mössbauer spectroscopies, ⁷Li Nuclear Magnetic Resonance, and electrochemical characterizations of the different materials, this overall view demonstrates the various fluorination mechanisms for LiFePO₄, from the chemical delithiation to then stabilize the pure fluorinated form LiFePO₄F or illustrates an innovative way that could be extendable to obtain the triphylite form of NaFePO₄.

## 1 Introduction

With the average energy needs of each individual growing over the past two decades, the improvement of the energy storage such as the lithium-ion batteries (LIBs) is a major challenge.¹⁻³ Also, the lithium cobalt oxide employed as the positive electrode in Li-ion batteries (LiCoO₂ with a capacity of 150 mAh·g⁻¹ at 3.7V) is formed with mined cobalt obtained under deplorable conditions. A non-toxic and environment friendly alternative is to look at the iron-based compounds with an abundant element in the earth's crust. But iron oxides demonstrate low potentials (~1V) and are therefore more often studied on the side of the negative electrode.⁴,⁵ Tailoring electrodes materials by following the solid-state chemistry principles allows to modify and modulate their electrochemical
properties. Goodenough and later Tarascon et al. have well demonstrate the effect of different chemical groups nature on the material potential values.⁶ For instance, the potential values are following the inductive effect of oxides lower than the corresponding phosphates, also lower than sulfates: O²⁻ < PO₄³⁻ < SO₄²⁻. For that reason, LiFePO₄ (LFP) with the olivine crystal structure was one of the most widely used cathode materials for LIBs and is still studied nowadays.⁷⁻⁹ Similarly, fluorine atom led to higher potentials than oxygen due to the larger ionicity of M–F bonds as compared to M–O bonds.¹⁰,¹¹ With that, inorganic fluorides such as iron fluoride FeF₃, lithiated iron fluoride like LiFe₂F₆ or Li₃FeF₆ were studied as potential positive electrode candidates.¹²⁻¹⁶ Li₂FePO₄F and LiFePO₄F fluorophosphates, LiFeSO₄F fluorosulfate, among others, were investigated to be possible iron-based target solutions for the positive side of secondary batteries and are the subject of numerous studies offering a strong framework acting as host for Na or Li insertion.¹⁷⁻²² As a result, a classical picture is that the introduction of a fluorine atom in the fluorophosphate host allows to achieve high potentials, while keeping the benefit of the phosphate groups. Indeed, LiFePO₄F potential value is close to the one obtained for the classic iron fluoride FeF₃ (2.8 and 3.0V, respectively), in addition to the tavorite-like structure offering a 3D network made of 1D chains of metal octahedra interconnected by phosphate tetrahedra, allowing a Li⁺ diffusion compared to α-FeF₃ dense perovskite structure.²³,²⁴ In the following, the paper is focused on the fluorination of LiFePO₄ iron phosphate to directly incorporate F atoms within the 3D network. Different synthesis methods for the obtention of fluorophosphate or fluorosulfate, for example, have been reported in literature such as the solvothermal or ionothermal synthesis allowing to obtain the expected phase at lower temperature unlike the classic ceramic process.¹⁷,²⁵,²⁶ In this later case, the compound is directly elaborated by a stoichiometric mixture of lithium fluoride and iron phosphate (when LiFePO₄F is aimed), the solid-state reaction occurs at high temperature (575°C for LiFePO₄F).²³,²⁷ Generally, the ceramic method gives micrometric particles (~1 µm), while reactions in solution can lead to nanostructured materials, maximizing the specific surface for better contacts with the liquid electrolyte.²⁸ But at the best of our knowledge, it should be noticed that no direct gas-solid fluorination to synthesize LiFePO₄F is described in the literature. The purpose of this exploratory work is the understanding of LiFePO₄ reactivity with gaseous fluorine (F₂), and then to illustrate the possible direct modification of this compound with the insertion of fluorine atoms in the matrix. Carefully following the characterizations of all the fluorinated phases under different conditions with Powder X-Ray Diffraction (PXRD), Raman and Mössbauer spectrometry but also gases emitted by *in situ* Fourier-transform infrared spectroscopy (FTIR), Transmission Electronic Microcopies (TEM), and ⁷Li Nuclear Magnetic Resonance (NMR), the phases obtained were determined and the different chemical fluorination mechanisms were proposed. All the electrochemical behaviors were also investigated when used as positive electrodes for LIB with a special attention is focused on the LiFePO₄F obtained compound, but also the possible cationic exchange to show an interesting way to form NaFePO₄ with the triphylite structure, an allotropic form only obtained with a Li/Na exchange in LiFePO₄.
# 2 Results and discussion

## 2.1 Structural modifications

The LiFePO₄/C composite material is initially black due to the carbon coating around the grains, and its fluorination is triggering a color change, even at room temperature (Figure 1, insert). The composite was treated under molecular fluorine (F₂) in a nickel furnace under different temperatures (room temperature or RT, 125°C, 250°C, 300°C and 500°C) during 5h. For the fluorinations carried out between room temperature and 350°C, the results are quite similar, in contrast to the product obtained after fluorination at 500°C that turns out to be very different. For all PXRD patterns of materials fluorinated at temperature T < 500°C (Figure 1), the main phase can be indexed with the monoclinic anhydrous non-lithiated iron phosphate (FePO₄, *Pnma*²⁹), a result confirmed for LiFePO₄-125 by Raman Spectrometry where the delithiated phase is identified in good agreement with the literature values (Figure S1).³⁰ The delithiation process seems to occur with the formation of LiF, with intensities of peaks at 39 and 45° for LiFePO₄-RT and LiFePO₄-125, also for LiFePO₄-250 and LiFePO₄-300 but with a decrease in intensity. Furthermore, close examination of the diffractograms shown in Figure 1 demonstrates that the Bragg peaks relative to the α-Li₃FeF₆ cryolite phase³¹ (monoclinic, C2/c) start to be formed from the fluorination at 250°C, whereas they are not observed at lower temperature, thus indicating a decomposition of the initial FePO₄ matrix. This is induced by the departure of phosphorus atoms as highlighted on the fluorinated product at 500°C, but already beginning at 250°C even if not fully completed. For the material fluorinated at 500°C, the fluorination of the phosphate matrix is complete with Bragg peaks indexed by a mixture of iron fluorides : α-Li₃FeF₆ cryolite is formed with the lithium from LiFePO₄ and α-FeF₃ formation is the result of the excess of iron fluorinated after all Li⁺ ions reacted.
<figure><img src="image_2.png" /><figcaption>Figure 1: Powder X-ray diffraction patterns of products obtained after fluorination at different temperatures: room temperature (RT, blue), 125 (light blue), 250 (green), 350 (red) and purple (500). Images for different materials in insert. Theoretical calculated XRD for FePO₄ and LiFePO₄ are shown in black.</figcaption></figure>

For LiFePO₄-125, the Li⁺ non-presence in the material could be elucidated with samples characterized by TEM. The starting material is coated by a carbon layer, a layer not observable anymore after fluorination at 125°C, but the formation of a new layer around the particles is observed. A chemical analysis by Energy Dispersive Spectrometry (EDS) was performed on the particles core and no fluorine atoms are significantly detected, resulting in an atomic composition close to FePO₄ (Figure S2). The TEM images show particles surrounded by a layer made of dendrites (Figure 2, left). Their chemical analysis mostly shows a fluorine rich area leading to the conclusion that fluorine is here combined with a very light element not measurable by EDS, in our case Li (Figure S3). This hypothesis is well consistent with the diffuse electron diffraction patterns obtained by selected area electronic diffraction on this layer (insert in Figure 2, left) showing distinct diffraction fringes which are in good concordance with the LiF structure, also confirmed by ⁷Li NMR with a spectrum with spinning sidebands, and its isotropic position at -1.4 ppm, all similar to LiF. The ⁷Li NMR peaks are broader compared to the classic signature for LiF, indicating an important coupling between Li and Fe atoms in FePO₄, known to be paramagnetic, and confirms that FePO₄ particles are intimately coated by
LiF layers (Figure 2, right), a phenomenon observed already in metal oxides coated by LiF.³² All these indications lead to conclude that after fluorination at low temperature, T < 250°C, the material is organized in a core shell type structure: a delithiated iron phosphate in the centre surrounded by a LiF coating layer.

<figure><img src="image_3.png" /><figcaption>Figure 2: TEM images (left) and ⁷Li NMR spectra of LiFePO₄ + F₂ 125°C (right).</figcaption></figure>

During the fluorination, the gases emitted are collected and analyzed by *in situ* FTIR. As can be seen on the spectra presented in Figure 3, between 250°C and 500°C, the presence of PO₃F molecules is observed with broad peaks around 990 cm⁻¹ and 1420 cm⁻¹, and around 1030 cm⁻¹ for PF₅. During the fluorination, the initial carbon layer, reducing phosphate grains, is also consumed with the formation of volatile species such as CF₄ or C₂F₆ (Figure 3).

<figure><img src="image_4.png" /><figcaption>Figure 3: Infrared spectra of emitted gas during the fluorination of LiFePO₄ up to 500°C.</figcaption></figure>

To fully confirm the thermal behavior with the fluorination, measurements by Mössbauer spectrometry were carried out. The spectra are shown in Figure 4 and were fitted with the hyperfine
parameters reported in Table 1. The spectrum and parameters of the initial material are in perfect
agreement with those given in the literature for the LiFePO₄ phase.³³ For fluorinated materials, it is
noticeable that at room temperature the hyperfine parameters obtained can be assigned to a typical
FePO₄ signature, in good agreement with previous XRD and Raman results, and up to 350°C.
However, a second paramagnetic contribution (δ = 0.38 and ΔE_Q = 0.36 mm.s⁻¹) increases gradually
with higher fluorination temperature. This contribution is in good agreement with α-Li₃FeF₆ hyperfine
values, even compared with the spectra obtained from α-Li₃FeF₆ synthesized at high temperature by a
stoichiometric mixture of LiF and α-FeF₃ (Figure S4). Finally, for the fluorinated material at 500°C,
the Mössbauer spectrum can be fitted with two contributions: a paramagnetic doublet (δ = 0.43 and
ΔE_Q = 0.36 mm.s⁻¹) corresponding to α-Li₃FeF₆ as well as a magnetic sextet (δ = 0.43 and 2ε = 0.36
mm.s⁻¹, H = 40 T) for α-FeF₃.³⁴ Those two contributions are in perfect agreement with the XRD
assignments made on this compound.

<figure><img src="image_5.png" /><figcaption>Figure 4 : $^{57}$Fe Mössbauer spectra at 300 K of LiFePO$_4$ and fluorinated materials (see Table 1).</figcaption></figure>
Thus, the XRD-Mössbauer cross-analysis clearly shows that the fluorination mechanism of LiFePO₄ seems to take place in two stages: the chemical delithiation followed by the collapsing of the phosphate structure. The complete chemical disinsertion of the Li atoms by fluorine is a favored process even at room temperature (ΔrG°=-612,1 kJ/mol), possible because of the robustness of the phosphate network not changing during this process and also not causing any irreversible change within the material structure. Over the past several years, different models have been proposed to describe the Li⁺ movement and phase transformations in LiFePO₄, resulting in a the two-phase insertion/extraction mechanism in LiFePO₄ happening simultaneously with the oxidation of Fe²⁺ to Fe³⁺.¹⁹ In the second step, with temperatures higher than 250°C, the fluorination of FePO₄ cores leads to the formation of POF₃ and PF₅ gases and the fluorination of iron atoms into α-FeF₃. From 250°C, under the action of temperature, α-FeF₃ reacts with LiF to give α-Li₃FeF₆. Beyond 350°C, the destruction of the phosphate network is complete giving a mixture of α-FeF₃ and α-Li₃FeF₆. It should be noted that in this scenario, the final FeF₃ is therefore the excess unreacted with LiF.

<table><caption>Table 1: Refined values at 300 K for LiFePO₄ and the different fluorinated materials.</caption><thead><tr><th></th><th>Fe<sup>n+</sup></th><th>δ</th><th>ΔE<sub>0</sub>/2ε</th><th>τ</th></tr></thead><tbody><tr><td colspan="5"><strong>LiFePO<sub>4</sub></strong></td></tr><tr><td rowspan="2">2 quadrupolar<br>components</td><td>Fe<sup>2+</sup></td><td>1.22</td><td>2.97</td><td>0.93</td></tr><tr><td>Fe<sup>3+</sup></td><td>0.37</td><td>0.99</td><td>0.07</td></tr><tr><td colspan="5"><strong>LiFePO<sub>4</sub> + F<sub>2</sub> RT</strong></td></tr><tr><td rowspan="3">3 quadrupolar<br>components</td><td>Fe<sup>3+</sup> (a)</td><td>0.44</td><td>1.52</td><td>0.55</td></tr><tr><td>Fe<sup>3+</sup> (b)</td><td>0.44</td><td>0.59</td><td>0.03</td></tr><tr><td>Fe<sup>3+</sup> (c)</td><td>0.37</td><td>0.00</td><td>0.42*</td></tr><tr><td colspan="5"><strong>LiFePO<sub>4</sub> + F<sub>2</sub> 250</strong></td></tr><tr><td rowspan="2">2 quadrupolar<br>components</td><td>Fe<sup>3+</sup> (a)</td><td>0.38</td><td>1.49</td><td>0.52</td></tr><tr><td>Fe<sup>3+</sup> (b)</td><td>0.38</td><td>0.63</td><td>0.48</td></tr><tr><td colspan="5"><strong>LiFePO<sub>4</sub> + F<sub>2</sub> 350</strong></td></tr><tr><td rowspan="2">2 quadrupolar<br>components</td><td>Fe<sup>3+</sup> (a)</td><td>0.43</td><td>1.52</td><td>0.45</td></tr><tr><td>Fe<sup>3+</sup> (b)</td><td>0.43</td><td>0.45</td><td>0.55</td></tr><tr><td colspan="5"><strong>LiFePO<sub>4</sub> + F<sub>2</sub> 500</strong></td></tr><tr><td rowspan="2">2 quadrupolar<br>components</td><td>Fe<sup>3+</sup></td><td>0.47</td><td>0.39</td><td>0.38</td></tr><tr><td>Fe<sup>3+</sup></td><td>0.53</td><td>0.03</td><td>0.62**</td></tr></tbody></table>

Two hypotheses are possible about the obtained composition of the material: 1/ a mixture of neighboring α-Li₃FeF₆ and α-FeF₃ particles or 2/ a core-shell like structure with an α-Li₃FeF₆ shell

<footnote>*Fe³⁺ in small particles (nano effect) resulting in a large continuous background; **α-FeF₃ with a hyperfine field H = 40 T.</footnote>
around α-FeF₃, after reaction of the external α-FeF₃ with a LiF coating. Because of the sensitivity of α-FeF₃ under the high energy electron beam, the TEM analysis does not allow to give the information by diffraction, but the chemical analysis of the material obtained with the fluorination at 350°C showed no trace of fluorine in the grain center, only at the periphery. In parallel, previous characterizations clearly indicated that a significant amount of Li₃FeF₆ is formed at this temperature. If scenario 1/ is correct, many grains with fluorine in the center of the particle should be found, which is not the case. This concludes that fluorine converts first outside of FePO₄ particles into FeF₃ which immediately reacts with LiF to give Li₃FeF₆. Then when all the LiF around the grain has been consumed, the core of the grain, FePO₄, is then converted to FeF₃. These results suggest a final structure with a α-FeF₃ core and a α-Li₃FeF₆ shell, in good agreement with the hypothesis 2/.

## 2.2 Synthesis of LiFePO₄F

After the fluorination of LiFePO₄ at 125°C, it was shown that an intimate mixture of LiF and FePO₄ was obtained, a mixture generally used to synthesize LiFePO₄F by the classic ceramic method. To follow this idea, an annealing treatment of the LiFePO₄ + F₂ 125°C mixture under unreactive gas (N₂) is done at 550°C during 24h. The product obtained by this treatment is further noted LiFePO₄ -125 + N₂. The XRD pattern of the annealed product is corresponding to LiFePO₄F Brag peaks confirming the total reaction of FePO₄ (Figure 5, left) with TEM photograph in insert showing that the prepared material is formed by individual sub-micronic particles (~500-1000 nm), without carbon layer, in good concordance with the classical solid-state method. This result is also confirmed by Mössbauer Spectrometry with a sharp quadripolar doublet observed at 300 K that can be described by a unique component corresponding to Fe³⁺ in LiFePO₄F (δ = 0.44 and ΔE_Q = 1.13 mm.s⁻¹). Infrared spectroscopy proves the absence of hydroxide groups, totally ruling out the possibility of having formed the LiFePO₄(OH) form of similar structure (Figure S5). Thus, the gas-solid fluorination of LiFePO₄ appears as a new and original method of synthesis of LiFePO₄F.
<figure><img src="image_6.png" /><figcaption>Figure 5: XRD pattern, MET image in insert (left), and Mössbauer spectrum at 300 K (right) of LiFePO₄ + F₂ 125°C – N₂.</figcaption></figure>

To extend the study, similar treatment has been done with LiFePO₄ fluorinated at room temperature (LiFePO₄ – RT) which could result in already carbon coated LiFePO₄F, a favourable aspect when the electrochemical application is aimed. In XRD pattern of the obtained product (noted LiFePO₄ – RT + N₂), all peaks can be attributed to a mixture of LiFePO₄ (triphenylite) and LiFePO₄F. Unlike the fluorinations at higher temperatures, the particles of LiFePO₄–RT are still coated by carbon which reduces a part of iron and can induce a partial defluorination of LiFePO₄F during the thermal treatment under N₂. As a consequence, this indicates that no carbon coated LiFePO₄F particles can be obtained by this way, moreover it shows that no LiFe²⁺(1-x)Fe³⁺xPO₄Fₓ compounds are observed.

## 2.3 Electrochemical study

The galvanostatic curves after 5 cycles for the samples LiFePO₄ + F₂ – 250°C and 500°C are shown in Figure 6. Those results are compared these to LiFePO₄F synthesized after the annealing under N₂. The galvanostatic profile of LiFePO₄F (Figure 6, left) is similar to the one of LiFePO₄ + F₂ – 250°C and classical of a lithium insertion process with a flat plateau both in oxidation and reduction. The case of LiFePO₄ + F₂ – 500°C is totally different with a huge polarization in between oxidation and reduction a classical process of a structural conversion during electrochemistry. The highest potential of use (3.3V) but also the lowest capacity (45 mAh.g⁻¹) is obtained for LiFePO₄ + F₂ – 250°C. 0.26 Li⁺ are inserted for a cut off voltage in discharge of 2.8V. The discharge cut off voltage may be a bit high but, as no more capacity is reached below 2.8V, it was chosen to avoid any conversion mechanism. The potential is in good agreement with previous studies, whereas the capacity reached is quite lower than expected (Cₜh = 178 mAh.g⁻¹).²⁰ Furthermore, the LiF layer around the FePO₄ particles could also
explained the lack of lithium diffusion. A small plateau during the reduction process is also observable, around 3.2V, and can be explained by the potential of insertion for α-Li₃FeF₆ particles, as their formation was shown with the XRD and the Mössbauer spectrometry analysis. With the post-treatment under nitrogen (LiFePO₄ - 125 + N₂ or LiFePO₄F), the potential obtained is 2.7V with a higher capacity up to 55 mAh.g⁻¹ corresponding to 0.34 Li⁺ inserted. As a comparison, a maximum insertion of 0.4 Li⁺ has been obtained for LiFePO₄F from an ionothermal synthesis.³⁵ For higher potential than 2.7V and 0.2 Li⁺, Chen et al. have suggested that the electrochemical process was typical of a single-phase process whereas for lower potential, a two-phase lithium intercalation occurs.²⁰ Our synthesis conditions could prevent from the two phases lithium intercalation process with the increase of the particle size. Similar effect of high polarization coupled to a low capacity was noticed by Ellis et al. for as-prepared LiFePO₄F having large particle size which are not carbon coated.²⁵ This polarization still increases with the increase of fluorination temperature (500°C) not mainly because of the increase of the particle size but because of a new electrochemical process: the conversion reaction of the mixture FeF₃/Li₃FeF₆. Both phases are electrochemical active and leads to iron metal at the lower reduction state with a 3 electrons process but with a high voltage hysteresis. An intermediate insertion process occurs for both FeF₃ and Li₃FeF₆ in between 3.5 and 1.2V leading to Li₀.₅FeF₃ and Li₄FeF₆.¹⁵,³⁶ A capacity of about 40 mAh.g⁻¹ is associated to this phenomenon. Fe²⁺ are then converted into Fe⁰ with a capacity of 199 mAh.g⁻¹. It is possible to compare this value to the theoretical one of 522 mAh.g⁻¹ which results from 2/3 of the theoretical value of FeF₃ (714 mAh.g⁻¹) and 1/3 of the one of Li₃FeF₆ (140 mAh.g⁻¹). The possible core-shell structure could explain the low capacity obtained for this mixture of fluorinated material at 500°C, with the core FeF₃ only reacting after the Li₃FeF₆ shell.

<figure><img src="image_7.png" /><figcaption>Figure 6: Charge/discharge curves for the fifth cycle of LiFePO₄ + F₂ 250°C between 3.8 - 2V and LiFePO₄F between 4.2 - 2.1V (left), and LiFePO₄ + F₂ 500°C between 3.8 - 0.7V (right).</figcaption></figure>

A second test was inspired by the different chemical Li/Na ions exchanges in LiFePO₄ with the triphylite structure, keeping this thermodynamically unstable structure for NaFePO₄, whereas
NaFePO₄ is obtained with the maricite structure when synthesized classically.<sup>37-39</sup> It should be noted that the lithium deinsertion was chemically possible with different techniques such as the use of Br₂ or Cl₂.<sup>40,41</sup> With the obtention of the FePO₄ phase for LiFePO₄ - RT, this material was rinsed with electrolyte solvent to remove the LiF shell. Then, the material was tested with sodium metal as an anode. The galvanostatic curves are shown in Figure. The material is electroactive with a plateau 0.3V below LiFePO₄ as expected versus Na⁺/Na. The capacity obtained is ~130 mAh.g⁻¹ up to 4 cycles (Figure 7), a bit lower than the theoretical one (Cₜh=154 mAh.g⁻¹), a result that could be explained by some residual lithium within the phosphate structure. Further fluorination in time and at a slightly higher temperature should reduce this lithium residue.

<figure><img src="image_8.png" /><figcaption>Figure 7: Charge/discharge curves for the first 4 cycles of LiFePO₄ + F₂ - RT between 4.2 - 2V with a sodium anode.</figcaption></figure>

### 3. Conclusion

In this study of fluorination of LiFePO₄ by molecular fluorine F₂, and the possibility to deeply modify the material depending on the fluorination conditions applied was presented. The first effect of fluorination is the compound delithiation to form a LiF shell around FePO₄ grains. If the temperature is increased, the iron phosphate matrix is converted little by little into α-FeF₃ which immediately reacts with the LiF layer to form α-Li₃FeF₆. When the temperature reaches 500°C the entire phosphate framework is consumed, and the material is completely converted into a mixture of FeF₃ and Li₃FeF₆. If the fluorinated phase is brought up to 550°C under an inert atmosphere as a second treatment, it is possible to obtain the LiFePO₄F phase with an original methodology. The fluorinated phases demonstrated different electrochemical properties after 5 cycles depending on the fluorination
temperature. At 250°C, FePO₄ is surrounded by LiF possibly blocking the Li⁺ pathway giving 45 mAh.g⁻¹ at 3.3V, whereas a mixture of FeF₃/Li₃FeF₆ is obtained at 500°C giving an important voltage hysteresis for the conversion reactions with 199 mAh.g⁻¹. After the second treatment under N₂ of LiFePO₄-125, the material shows a capacity of 55 mAh.g⁻¹ at 2.7V in agreement with the obtention of LiFePO₄F. This study is quite original because it is the first time that direct and total fluorination of LiFePO₄ under molecular F₂ have been reported, resulting in a solid/gas reaction extensible to other polyanionic compounds such as Li₂FeP₂O₇ or Li₃Fe₂(PO₄)₃,⁴²,⁴³ but also LiFePO₄(OH) which was already fluorinated but not by fluorine gas fluorination.⁴⁴ Nevertheless, the first electrochemical characterizations present limited performance and proves the necessity to carry out engineering of formulation on these materials to obtain better performance. Finally, in terms of perspective to this study, a cationic exchange with first the possibility to remove Li⁺ by fluorination, and then the use of FePO₄ with a sodium anode was performed, resulting in NaFePO₄ with the triphylite structure which is electrochemically interesting as its lithium counterpart, and demonstrated a possible chemical delithiation to test any lithiated compound in another alkali ions batteries configuration.

# Materials and methods

## Synthesis

As starting material, we use a commercial nanostructurated LiFePO4, with triphylite structure. The nanosized grains (~150-200nm) are coated by a carbon layer (LiFePO₄/C : 2.5%wt). The material is purchased from “Phostech’ lithium”. Fluorinations are done by gas-solid treatments under pure fluorine gas in a nickel furnace. The furnace is connected to both fluorine and nitrogen bottles, and gases quantities coming from each bottle can be monitored. Two sets of experiments have been performed. Firstly, at “low temperatures” between room temperature (RT) and 350°C, and secondly at “high temperature” at 500°C. Fluorinations are done for 5h. A second treatment was done on some materials at 550°C under N₂ for 24h. All the fluorination treatments are summarized in Table S1. As a reference for Mössbauer spectrometry, α-Li₃FeF₆ with Fe₂O₃ as an impurity at 15 %Fe is obtained mixing Li₂CO₃ + FePO₄·4H₂O in Hfₐq 48% with a second treatment under F₂ at 250°C.

# Material characterization

## Characterization methods

**Powder X-Ray diffraction (PXRD):** X-ray diffraction patterns were collected in the range 10° ≤ 2θ ≤ 95° on a Panalytical MPD-PRO diffractometer equipped with a linear X'celerator detector with a CuKα radiation (1.54 Å). A homemade measurement cell allowed the data to be collected under a static inert argon atmosphere. Data were collected in the [10-95°] 2θ scattering angle range with a 0.0836° step.
**Mössbauer spectroscopy:** Mössbauer spectroscopy has been measured on freshly prepared samples stored under argon. $ ^{57}Fe $ Mössbauer spectra were recorded in the constant acceleration mode and in transmission 20 geometry on a standard Mössbauer spectrometer composed of electronic devices from Ortec and Wissel. A $ ^{57}Co(Rh) $ source with a nominal activity of 370 MBq was used. The source and the absorber were always kept at room temperature. The thickness of the absorbers is about 10 mg/cm$^2$. The isomer shift is given relative to $ \alpha $-Fe standard at room temperature.

**Transmission electron microscopy (TEM):** Powder samples for TEM analysis are deposed on a copper grid with a carbon membrane by drop casting after dispersion in ethanol. Compounds were examined in CM20 Phillips transmission electron microscope (images were recorded at 200kV) and chemical analysis were done with an energy dispersive X-Ray spectroscope.

**Fourier transformation infra-red spectroscopy (FTIR):** Infrared spectra were collected, also during the defluorination step with a Nicolet Summit FTIR spectrometer (Thermo scientific) equipped with a heated MercuryTM gas cell (10 cm cell path, $CaF_2$ windows) coupled with the passivated nickel reactor. Spectra were recorded in the 1000 – 4000 cm$^{-1}$ range with a 4 cm$^{-1}$ resolution and 16 scans.

**Electrochemical measurements:** The electrochemical performances were investigated using galvanostatic cycling. The electrodes were composed of fluorinated sample (45% w/w), acetylene black (45% w/w) as conductive material and polyvinylidene difluoride (PVDF 10% w/w) as binder. After stirring in propylene carbonate (PC), the mixture was spread uniformly onto a stainless-steel current collector disk of 10 mm diameter. After the PC evaporation, the electrodes were dried in a vacuum oven at 120 °C overnight to remove traces of water and solvent before their transfer into an argon-filled glovebox. The anode was a lithium or sodium metal disk, and the separator was Celgard 2034. A two electrodes cell was used (Swagelok cell type), where lithium/sodium was both reference and counter electrodes. The electrolyte was 1.0 M $LiClO_4$ or $NaClO_4$ in propylene carbonate. The cells were assembled in an argon filled dried glove box. Relaxation was performed for at least 5 hours until the open circuit voltage (OCV) stabilization. Galvanostatic cycling, carried out on a VMP2-Z instrument from Biologic, were performed at room temperature by applying a constant current density of 5 mA/g in oxidation and 10 mA/g in reduction.

**Raman Spectroscopy:** A JOBIN YVON T64000 spectrometer with a charge-coupled device multichannel detector was used for Raman investigations at room temperature. The radiation source was a 514.5 nm argon laser line. The laser power was tuned to low power (100 mW) in order to avoid the sample decomposition under the beam. 3 scans of 640 s were made for a single spectrum. The resolution was equal to 0.08 cm$^{-1}$.

**NMR:** $ ^{19}F$ NMR measurements were carried out with a Bruker Advance Spectrometer with working frequency of 282.2, 300.0 and 78.8 MHz, respectively. A Magic Angle Spinning (MAS) probe
operating with 2.5 mm rotors was used allowing a 30 kHz spinning rate. A sequence with a single $\pi/2$ pulse duration of 4.0 µs was used. The chemical shifts were externally referenced to $CFCl_3$.

## Conflicts of interest

There are no conflicts to declare.

## Acknowledgements

All the authors want to thank Willmann Patrick<sup>e</sup> and the Centre National d'Etudes Spatiales (CNES), Hamwi André<sup>a</sup>, Dubois Marc<sup>a</sup>, Louvain Nicolas<sup>a,d</sup>, Fonquernie Claire<sup>a,b</sup>, El-Ghozzi Malika<sup>a</sup> for the fruitful discussions and technical supports during this work.

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