# Electrochemical behaviour of low temperature grown iron fluoride thin films

Yoshinari Makimura, Aline Rougier, Lydia Laffont, Manfred Womes,

Jean-Claude Jumas, Jean-Bernard Leriche, J.-M. Tarascon

► To cite this version:

Yoshinari Makimura, Aline Rougier, Lydia Laffont, Manfred Womes, Jean-Claude Jumas, et al. Electrochemical behaviour of low temperature grown iron fluoride thin films. Electrochemistry Communications, 2006, 8 (11), pp.1769-1774. 10.1016/j.elecom.2006.08.004. hal-00353431

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Electrochemical behaviour of low temperature grown
iron fluoride thin films

Y. Makimura<sup>a</sup>, A. Rougier<sup>a,*</sup>, L. Laffont<sup>a</sup>, M. Womes<sup>b</sup>, J.-C. Jumas<sup>b</sup>,
J.-B. Leriche<sup>a</sup>, J.-M. Tarascon<sup>a</sup>

<sup>a</sup> Laboratoire de Réactivité et de Chimie des Solides, CNRS UMR-6007, Université de Picardie Jules Verne, 33 rue Saint Leu, 80039 Amiens Cedex, France

<sup>b</sup> Laboratoire des Agrégats Moléculaires et Matériaux Inorganiques, CNRS UMR-5072, Université Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex 5, France

Abstract

By coupling a homemade substrate holder with a refrigerated ethanol cryogenic system, we succeeded in growing thin films by Pulsed Laser Deposition at substrate temperatures as low as –50 °C. The benefit of enlarging substrate temperatures to negative values is illustrated through the example of iron fluoride thin films, for which the substrate temperature is a key factor governing the FeF<sub>2</sub> or/and FeF<sub>3</sub> phase deposition. Using a FeF<sub>3</sub> target, the X-ray diffraction study shows that the “FeF<sub>x</sub>” thin films grown at 600 °C correspond to a single well-crystallized FeF<sub>2</sub> phase (S.G.: *P*4<sub>2</sub>/m*m*m) as opposed to a mixture of FeF<sub>3</sub> and FeF<sub>2</sub> phases for room temperature substrate, and a single FeF<sub>3</sub> phase (S.G.: *R*3c) having quite an intense (012) Bragg peak at low temperature substrate (–50 °C). Such assignments were confirmed by complementary HRTEM and Mössbauer measurements with the exception of the –50 °C grown film that was shown to contain amorphous FeF<sub>2</sub> together with crystallized FeF<sub>3</sub> phases. The electrochemical behaviour of the FeF<sub>x</sub> thin films, namely their voltage profiles, was found to be dependent on the substrate grown temperature.

Keywords: Iron fluorides; Thin film; Pulsed laser deposition; Lithium-ion battery; Low temperature substrate; <sup>57</sup>Fe Mössbauer data

**1. Introduction**

In recent years, nano-sized transition-metal oxides have
received an extensive interest regarding their use as elec-
trode materials in lithium-ion batteries [1]. Such interest
has recently been amplified with the arrival of Li-driven
conversion reactions leading to M–O based electrodes with
outstanding capacity gains over classical insertion elec-
trodes. These reactions, which are taking advantage of elec-
trochemistry at the nano-scale, have been extended to other
classes of materials such as binary metal sulphides, nitrides,
phosphides and fluorides [2–8]. Indeed transition-metal flu-
orides, which as iron-based compounds are attractive in
terms of their non-toxicity and cost [6,7], present the

advantage of having a high ionic character, leading to high
operating voltages compared to oxides in lithium cells.
However owing to their insulator character, iron fluorides
suffer from poor kinetics. Focusing on bulk material, Ama-
tucci et al. reported improved properties for carbon metal
fluorides nano-composites electrodes (e.g. having a 20 nm
crystallite size) cycled at 75 °C. Another interesting alterna-
tive consists in their thin film preparation preventing any
binder or electronic conductor addition. The number of
papers reporting the growth of Fe-based fluoride thin films
is rather low, and more importantly none of them deals
with their electrochemical characterization in lithium cells
[9–14]. In our group, thin films of several families of mate-
rials, among them oxides, phosphides, metal, hydrides,
have been successfully grown using the Pulsed Laser Depo-
sition technique [15–17]. Indeed, the versatility of the PLD
technique provides unique advantages to tune thin films

<footnote>* Corresponding author. Tel.: +33 3 22 82 76 04; fax: +33 3 22 82 75 90.
E-mail address: aline.rougier@sc.u-picardie.fr (A. Rougier).</footnote>
properties by playing with the deposition conditions (target nature, pressure, laser energy, etc.). However, to our knowledge the growth of thin films at substrate temperatures below zero has never been reported. Herein, we embark on the first study reporting deposition at low temperature, namely -50 °C, of iron fluoride thin films used as positive electrode in lithium batteries.

## 2. Experimental

Iron fluoride thin films were grown on stainless steel substrates from high purity $FeF_3$ target under vacuum ($5.0 \times 10^{-5}$ Pa) by pulsed laser deposition using a KrF excimer laser (Lambda Physik, Compex 102, $\lambda$ = 248 nm). To prevent the iron fluoride thin films from any air exposure upon handling and characterization, the PLD system was equipped with a glove box [17]. The classical configuration means a laser energy of 180 mJ, a frequency of 2 or 10 Hz and 1 h duration, the substrate temperature varies from RT to 600 °C. To achieve low substrate temperature deposits, we designed a specific substrate holder, cooled through metallic tubes by refrigerated ethanol flowing from a cryogenic device equipped with a regulator of temperature (Vasse Julabo FP50). Substrate temperatures as low as -50 °C were successfully achieved by decreasing the laser energy to 120 mJ as a 180 mJ energy produces heat inside the chamber limiting the inside temperature to -20 °C.

Thin films were characterized by X-ray diffraction using an X-ray diffractometer (Philips PW 1729) with Cu Kα radiation. TEM and HRTEM imaging were performed using a FEI TECNAI F20 S-TWIN equipped with an EDX analysis detector. The film deposition was achieved directly on copper grids coated with a lacy carbon film. The diffraction patterns were performed using the Selected Area Electron Diffraction (SAED) mode. The SAED patterns obtained on polycrystalline samples were analyzed using the process diffraction software [18].

Conversion Electron Mössbauer Spectroscopy (CEMS) was carried out at room temperature in the constant acceleration mode, using equipment from Wissel and Ortec. $^{57}$Co(Rh) with a nominal activity of 10 mCi was used as a source. The velocity scale was calibrated by means of a room temperature CEMS spectrum of α-Fe, which was also used as the reference for all isomer shifts. The hyperfine parameters Isomer Shift ($\delta$) and Quadrupole Splitting ($\Delta E_q$) were determined by fitting Lorentzian lines to the experimental data, making use of the ISO fit program [19].

The electrochemical behaviour of iron fluoride thin films deposited on stainless steel substrates, and used as positive electrode, was characterized using standard 2035-size coin cells. The negative electrode, a lithium foil was separated from the positive electrode by GF/D borosilicate glass fibre sheet, soaked with 1 M $LiPF_6$ dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC) solution (1/1 by volume) electrolyte. The cells were assembled in a glove box filled with argon. Electrochemical measurements were carried

out galvanostatically at a current density of $0.56 \mu A \cdot cm^{-2}$ (voltage profiles) and of $2.82 \mu A \cdot cm^{-2}$ (cycling tests) in a voltage window of 3.60-0.05 V using a Macpile controller.

## 3. Results and discussion

For a 600 °C substrate temperature, the XRD profile of iron fluoride thin film deposited from a $FeF_3$ target corresponds to the well-crystallized tetragonal $FeF_2$ phase (S.G.: *P4₂/mnm*) (Fig. 1). The signature of the $FeF_3$ phase, represented by the single (012) peak (S.G.: $R\bar{3}c$ [20]), starts to appear with decreasing the substrate temperature (≤400 °C [21]) and becomes clearly visible for a RT substrate. Despite the single peak existence, the $FeF_3$ phase identification is unambiguous as other $FeF_3$ phases, namely the pyrochlore-type structure [22], the HTB-type structure $FeF_3(H_2O)_{0.33}$ [23], do not show any XRD peaks at a similar two theta position. For a RT substrate, the sharpness of the (012) $FeF_3$ peak is opposed to the broadness of the (110) $FeF_2$ one, indicating higher crystallinity for the $FeF_3$ phase compared to the $FeF_2$ one. Using the Scherrer formula, average crystallite sizes of 39 and 15 nm were calculated for $FeF_3$ and $FeF_2$ phases, respectively. First attempts to isolate the $FeF_3$ phase by decreasing the laser energy, increasing the deposition rate, and increasing the fluorine content inside the target (mixture of $XeF_2/FeF_3$ target) remained unsuccessful. On the contrary, thanks to our own homemade low temperature set up, we observed a progressive and full disappearance of the (110) $FeF_2$ XRD peak with decreasing the temperature. Finally for a substrate temperature as low as -50 °C, the (012) $FeF_3$ peak remained as a single peak suggesting, at first sight, the sole existence of the $FeF_3$ phase.

<figure><img src="image_2.png" /><figcaption>Fig. 1. X-ray diffraction patterns of iron fluoride thin films deposited on stainless steel substrates from $FeF_3$ target under vacuum of $5.0 \times 10^{-5}$ Pa at several substrate temperatures, i.e., 600 (a), RT (b) and -50 °C (c), for 1 h at a repetition rate of 10 Hz. Miller indexes were given by assuming a tetragonal lattice of $FeF_2$ (space group: *P4₂/mnm*) and a rhombohedral lattice of $FeF_3$ (space group: $R\bar{3}c$) in hexagonal setting.</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Fig. 2. HRTEM and SAED micrographs of iron fluoride thin films deposited on stainless steel substrates from FeF₃ target under vacuum of 5.0 × 10⁻⁵ Pa at several substrate temperatures, i.e., 600 (a), RT (b) and -50 °C (c), for 1 h at a repetition rate of 2 Hz. Using the process diffraction software, the SAED pattern is transformed to a pseudo powder diffraction pattern (Intensity = f(2θ), λ CuKα) in order to have a precise determination of the phase.</figcaption></figure>
The bright field image of the film deposited at 600 °C is composed of particles of 50–200 nm size with dispersed droplets (100–400 nm size) (Fig. 2a). The XRD-like diagram corresponding to the SAED pattern clearly shows that the film is FeF₂ and is very well-crystallized. On the contrary, the droplets are amorphous. The film deposited at room temperature is still composed of crystallized particles of about 50–200 nm but the line profile of the corresponding SAED pattern indicates the coexistence of the FeF₂ phase together with FeF₃ (Fig. 2b). For the film grown at –50 °C (Fig. 2c), the bright field image shows the existence of an amorphous layer localized around the 20–50 nm crystallized nanoparticles. As deduced from the SAED pattern of the bright field image, crystallized domains correspond to the FeF₃ phase with a proportion of hydrate FeF₃. The presence of FeF₃·H₂O probably results from the reaction to moisture of the LT FeF₃ film, when stored in the glove box, and should not be attributed to the deposition step, which is besides performed in high vacuum (10⁻⁵ Pa). If the HRTEM observations are well consistent with the XRD data for the films deposited above RT, the nature of the co-existing amorphous phase detected for the film deposited at –50 °C remains undetermined.

To further examine the stable phase of iron fluoride thin film in terms of electronic structures, CEMS was carried out. CEMS spectra indicated that the iron fluoride thin film deposited at 600 °C was a single phase of FeF₂ (Fig. 3,

<figure><img src="image_4.png" /><figcaption>Fig. 3. ⁵⁷Fe CEMS spectra of iron fluoride thin films grown from FeF₃ target at 600 (a) and –50 °C (b).</figcaption></figure>

Table 1), which hyperfine parameters ($\delta$ = 1.35 mm/s, $\Delta E_q$ = 2.78 mm/s) are comparable to previous values ($\delta$ = 1.34 mm/s, $\Delta E_q$ = 2.76 mm/s) [24], in agreement with XRD data shown in Fig. 1. Although the XRD pattern of the thin film deposited at –50 °C corresponds to the FeF₃ phase, CEMS spectra indicate a more complex situation enlisting a mixture of iron (III) fluoride (47%) and iron (II) fluoride (53%). The hyperfine parameters of such iron (II) fluoride ($\delta$ = 1.28 mm/s, $\Delta E_q$ = 2.45 mm/s), close to those of crystallized FeF₂, and the line width broadening ($\Gamma$ = 0.54 mm/s) clearly give evidence of the development of an amorphous FeF₂ phase. Concerning the iron (III) fluoride, the hyperfine parameters ($\delta$ = 0.50 mm/s, $\Delta E_q$ = 0.52 mm/s) by comparison with amorphous FeF₃ ($\delta$ = 0.45 mm/s, $\Delta E_q$ = 0.55 mm/s) [25] and the lack of magnetic hyperfine field are in agreement with the formation of poorly crystallized FeF₃. This occurrence of superparamagnetic effects at room temperature for FeF₃ can be explained by the size of grains (10–20 nm) and the thickness of grain boundaries as previously discussed for nanostructured iron fluoride powders [26,27]. These results suggest that the iron fluoride thin film deposited at –50 °C was a two-phase mixture of poorly-crystallized FeF₃ and amorphous FeF₂. Thin film deposition performed at low substrate temperature and under extreme vacuum conditions (10⁻⁵ Pa) was found to be amorphous FeF₂ rather than FeF₃ as expected owing to fluorine volatility.

At this point it is important to describe the electrochemical performances of such films with respect to Li. A rough estimation of the capacity for the 600 °C film, converted in number of lithium inserted and extracted, was calculated from the film thickness considering 80% of the density of the bulk FeF₂ (4.28 g·cm⁻³), i.e. 3.42 g·cm⁻³. For the –50 °C film, based on the Mössbauer estimation, the calculation was done assuming a 50% FeF₂ + 50% FeF₃ film composition and 80% of the bulk density (3.96 g·cm⁻³), i.e., 3.17 g·cm⁻³. However, caution has to be applied here not to overinterpret the data, since the exact film composition is not well determined. The galvanostatic cycling of lithium cells with iron fluoride thin films deposited at 600 and –50 °C as positive electrodes (Fig. 4) exhibit similar trends when cycled between 3.60 and 0.05 V, namely: (1) a large polarization (nearly 1 V) between charges and discharges, (2) a large irreversible capacity on the first cycle owing to a charge capacity being nearly half of the discharge capacity, and (3) a sustained capacity retention

<caption>Table 1</caption>

⁵⁷Fe hyperfine parameters of iron fluoride thin films grown from FeF₃ target at 600 and –50 °C

<table><thead><tr><th>Films</th><th>δ (mm/s)</th><th>ΔE<sub>q</sub> (mm/s)</th><th>2Γ (mm/s)</th><th>C (%)</th><th>Attribution</th></tr></thead><tbody><tr><td>FeF<sub>3</sub> target (600 °C)</td><td>1.37 (1)</td><td>2.78 (1)</td><td>0.22 (1)</td><td>100</td><td>FeF<sub>2</sub></td></tr><tr><td rowspan="2">FeF<sub>3</sub> target (–50 °C)</td><td>1.28 (2)</td><td>2.45 (3)</td><td>0.54 (4)</td><td>53</td><td>Amorph. FeF<sub>2</sub></td></tr><tr><td>0.50 (1)</td><td>0.52 (2)</td><td>0.41 (3)</td><td>47</td><td>FeF<sub>3</sub></td></tr></tbody></table>

The various parameters are: δ = Isomer Shift relative to α-Fe, $\Delta E_q$ = Quadrupole Splitting, $2\Gamma$ = Line Width at Half Maximum, C = contribution of sub-spectra to the spectrum.
<figure><img src="image_5.png" /><figcaption>Fig. 4. Discharge and charge curves of lithium cells using iron fluoride thin films as positive electrode. Films were deposited on stainless steel substrates from $FeF_3$ target under vacuum of $5.0 \times 10^{-5}$ Pa at several substrate temperatures, i.e., 600 (a) and -50 °C (b). The cells were tested in various voltage windows 3.6 to 1.0, 0.5 and 0.05 V at a current density of $1 \mu A/1.77 \ cm^2$ (0.56 $\mu A \cdot cm^{-2}$) (a,b), and $5 \mu A/1.77 \ cm^2$ (2.8 $\mu A \cdot cm^{-2}$) (c) at 25 °C. The electrolyte used was 1 M $LiPF_6$ dissolved in EC/DMC solution (1/1 by volume). (c) Corresponding capacity versus the number of cycles curves.</figcaption></figure>

upon cycling as shown in Fig. 4c. Nevertheless, the voltage traces are quite different for both films. For the 600 °C film (Fig. 4a), the first discharge profile shows first the appearance of a plateau at 1.90 V having a 2 Li amplitude, followed by a continuous voltage decay down to 0.05 V corresponding to an extra storage capacity of 2.5 Li per unit formula. This plateau remains well pronounced upon cycling as long as the discharge cut-off voltage remains greater than 0.50 V. The -50 °C grown film (Fig. 4b)

behaves quite differently as the first discharge curve shows first a plateau near 2.20 V having an amplitude of solely 0.8 Li as opposed to 2 previously, followed by two different voltage sloping ranges going from 2.20 to 1.00 V and from 1.00 to 0.05 V, and corresponding to Li capacities of 2 and 1.5, respectively. As previously, the first plateau remains upon cycling as long as the discharge cut-off voltage is limited to 1 V, but progressively diminishes to disappear after 10 cycles (Fig. 4b) when lowering the discharge cut-off voltage to 0.05 V.

At this point a legitimate question arises as to whether the Li reactivity mechanism reported for the $FeF_2$ and $FeF_3$ powders prevails as we move to thin films. Amtucci's group [7,8] deduced from bulk powder characterizations that the overall lithium uptake and removal in fluorides occurs through the conversion process, $nLi + MX_m \rightarrow M + LiX_{m/n}$, leading therefore to an expected 2 Li per Fe for the complete reduction of $FeF_2$. In contrast, the reduction mechanism was shown to be different for $FeF_3$ since it first enlists the insertion of Li into $FeF_3$ leading to a $Li_xFeF_3$ phase; the latter was then undergoing a conversion process resulting in the formation of Fe nanoparticles embedded into a LiF matrix, and which were converting back to the $FeF_2$ phases upon subsequent charges. The discharge plateau, which total capacity extends up to 2 lithiums, observed during the reduction of $Li/FeF_{2(600 °C thin film)}$ cells is strongly indicative that our films react towards Li like the powders; that is to say according to the following conversion reaction ($FeF_2 + 2Li^+ + 2e^- \rightleftharpoons 2LiF + Fe$). Such conversion mechanism has been confirmed by HRTEM measurements that have shown the coexistence of both crystallized Fe and LiF phases together with an amorphous layer at the end of the first discharge, and the partial conversion back to $FeF_2$ upon recharge.

A similar comparison between $FeF_3$ powders and thin films is at first not as straightforward, since upon discharge, $Li/FeF_{3( powders)}$ cells have shown a 3 V pseudo plateau corresponding to the insertion of nearly 0.9 Li into $FeF_3$, while no capacity is observed at such voltage for $Li/FeF_{3(-50 °C thin film)}$ cell. This came as a surprise, since the 3 V insertion reaction was shown to have fast kinetics capable of sustaining numerous cycles at C rate. In contrast, upon discharge of such a cell (Fig. 4b) a decrease in the potential down to 1.90 V is followed by a plateau at 2.20 V, characteristic of a nucleation-growth process at the macroscopic scale. Owing to the same amplitude of this plateau, nearly $0.8 Li^+$ per unit formula as the one obtained for $FeF_3$ powders, it is tempting to speculate that it is also indicative of the Li insertion process into $FeF_3$. Therefore, why is the potential so different? A possible answer could be rooted in some kinetic limitations as the $FeF_3$ grown films are carbon-free as opposed to $FeF_3$ powders compositions that contain at least 15% of powders. Interestingly, this improvement with carbon addition is not observed for $FeF_2$ as there is no potential difference between carbon-free $FeF_2$ thin films and carbon-containing $FeF_2$ powder electrodes,