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LaSrMnO4: Reversible Electrochemical Intercalation of Fluoride Ions in the Context of Fluoride Ion Batteries Mohammad Ali Nowroozi,† Kerstin Wissel,† Jochen Rohrer,‡ Anji Reddy Munnangi,§ and Oliver Clemens*,†,∥ †

Technische Universität Darmstadt, Institut für Materialwissenschaft, Fachgebiet Materialdesign durch Synthese, Alarich-Weiss-Straße 2, 64287 Darmstadt, Germany ‡ Technische Universität Darmstadt, Institut für Materialwissenschaft, Fachgebiet Materialmodellierung, Jovanka-Bontschits-Straße 2, 64287 Darmstadt, Germany § Helmholtz Institute Ulm for Electrochemical Energy Storage, Helmholtzstraße 11, 89081 Ulm, Germany ∥ Karlsruher Institut für Technologie, Institut für Nanotechnologie, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein Leopoldshafen, Germany S Supporting Information *

ABSTRACT: This article reports on the investigation of LaSrMnO4 with K2NiF4 type structure for use as an intercalation based high voltage cathode material with high capacity for fluoride ion batteries (FIBs). Charging was performed against PbF2 based anodes and shows that fluoride intercalation proceeds stepwise to form LaSrMnO4F and LaSrMnO4F2−x. Ex-situ X-ray diffraction experiments were recorded for different cutoff voltages for a deeper understanding of the charging process, highlighting additional potential of the method to be used to adjust fluorine contents more easily than using conventional fluorination methods. A discharging capacity of approximately 20−25 mAh/g was found, which is ∼4−5 times higher compared to what was reported previously on the discharging of BaFeO2.5/BaFeO2.5F0.5, approaching discharge capacities for conversion based fluoride ion batteries. Density functional theory based calculations confirm the observed potential steps of approximately 1 and 2 V for the first (LaSrMnO4 → LaSrMnO4F) and second (LaSrMnO4F → LaSrMnO4F2−x) intercalation steps against Pb-PbF2, respectively. Additionally, a detailed structure analysis was performed for chemically prepared LaSrMnO4F2−x (x ∼ 0.2), showing strong similarity to the product which is obtained after charging the batteries to voltages above 2 V against Pb-PbF2. It was observed that charging and discharging kinetics as well as coulomb efficiencies are limited for the batteries in the current state, which can be partly assigned to overpotentials arising from the use of conversion based anode composites and the stability of the charged sample toward carbon black and the current collectors. Therefore, the structural stability of LaSrMnO4F2 on the deintercalation of fluoride ions was demonstrated by a galvanostatic discharging to −3 V against Pb-PbF2, which can be used to compensate such overpotentials, resulting in almost complete recovery of fluorine free LaSrMnO4 with a discharge capacity of ∼100 mAh/g. This is the first report showing that selective extraction of fluoride ions from an oxyfluoride matrix is possible, and it highlights that compounds with K2NiF4 type structure can be considered as interesting host lattices for the reversible intercalation/deintercalation of fluoride ions within intercalation based FIBs.

1. INTRODUCTION

chemically most stable and mobile anion, relating to the high electronegativity of fluorine and the high redox potential of the F−/F2 redox couple. In contrast to the use of chloride ions, the voltage window of fluoride ion batteries (FIBs) is not limited by the electrochemical stability of the ionic charge carrier. Previous investigations2,6,7 of FIBs were entirely based on conversion type reactions for the electrode materials (trans-

Over the past few decades a rise in energy applications caused high demands for energy storage and battery technologies. The overwhelming part of battery research was undertaken in the field of reversible lithium ion battery (LIB) systems, related to the fact that Li+ is an electrochemically stable cation, with a high mobility allowing for good battery kinetics. Nevertheless, there are also other cations (e.g., Na+1) and even anions (e.g., F− (ref 2) and Cl− (refs 3−5)) which have been considered as mobile and stable charge carriers for battery applications. For anion based batteries, the fluoride ion represents the electro© 2017 American Chemical Society

Received: November 29, 2016 Revised: March 1, 2017 Published: March 2, 2017 3441

DOI: 10.1021/acs.chemmater.6b05075 Chem. Mater. 2017, 29, 3441−3453

Article

Chemistry of Materials formation of metals to the respective metal fluorides and vice versa). Since many metals can be obtained as di- or trifluorides, this automatically results in high energy densities from the high theoretical capacities of such systems. However, conversion reactions are known to have a negative impact on the reversibility, cyclability, and transport kinetics, which is related to large volume changes during the conversion process as well as crystallization phenomena.8,9 This is known to result in high overpotentials, which lowers the energy efficiency of such systems.9 Apart from the fact that intercalation based electrode materials (i.e., materials which can intercalate/deintercalate ions into vacancies/interstitials/interlayers within a host lattice) have been shown to be advantageous for use in state-of-the-art LIB systems, using an intercalation based instead of a conversion based cathode could imply similar improvements for FIBs as well: Remarkably, the highest oxidation states for most metals/transition metals (e.g., Mn, Fe, Co, Ni) can be found in binary oxides or ternary oxyfluorides, but not in binary fluorides, related to simple rules of solid state chemistry10,11 (Cations with high oxidation states tend to have low coordination numbers due to their small ionic radius. In contrast, anion rich compounds have higher overall coordination numbers of the cations. This is contradictive and therefore often structurally prohibitive for binary compounds MFy). Evidently, high transition metal oxidation states often imply that the material becomes a strong oxidizer, which translates to a high suitability as a high voltage cathode material (the terms cathode/anode material are being used as it is common in the battery community, i.e. describing the role of the material during the discharging process).8 However, identification and exploration of intercalation based materials which are capable of both high capacities and high energy densities remain an unsolved task for FIBs. Polymorphs of the perovskite structure (such as brownmillerite type compounds, A2B2O5), as well as perovskite intergrowth structures (e.g., K2NiF4 type structures), have been previously studied for their fluorination chemistry with a focus on changes of properties, including superconductivity, magnetism, electronic structures, etc.12−14 However, less attention has been paid to electrochemical fluorination as a synthesis technique due to experimental limitations,15,16 and no attention was paid yet to develop intercalation based electrode materials for fluoride ion batteries (in a single preliminary article, we reported on the electrochemical fluorination of BaFeO2.5 and found bad reversibility of the charging reaction with very low discharge capacities of 0) reactions according to y A + F2 → AFy 2

La/Sr which did not require an increase in the size of the unit cell (see Figure 2).

Therefore, with the formation of Li being the natural limit for LIBs, the formation of F2 will determine a natural limit for any material used for fluoride ion batteries on the cathode side. To take this into account, voltages were calculated against a hypothetical F2 gas electrode (FGE). This electrode was modeled by calculating the energy of an F2 molecule in a box of 34.5 × 34.5 × 34.5 Å3; the density of F2 within this box would correspond to a pressure of 1 bar at 298 K. We emphasize that referring to this electrode reverses the electrochemical scale: A material with a low voltage against a FGE would correspond to being a good cathode material, whereas a material with a high voltage against a FGE would correspond to a good anode material (compare voltages against FGE and Li/LiF given in Table 1) . Table 1. Voltages against a Hypothetical Fluorine Gas Electrode (FGE) Calculated for a Variety of Conversion Based Materials for Fluoride Ion Batteries

System

calculated Voltage against FGE [V]

voltage of respective metallic species against F2 gas [V] (values taken from ref 35)

calculated voltage against Li/LiF for comparison [V]

Li/LiF Mg/MgF2 Ce/CeF3 Pb/PbF2 Cu/CuF2 Ag/AgF Ag/AgF2

5.95 5.36 5.29 3.69 2.42 2.12 1.73

6.09 5.42 5.39 3.39 2.70 2.27

0 0.59 0.66 2.26 3.53 3.83 4.22

Figure 2. (a) Possible ordering scenarios (A, B, C) of La and Sr within LaSrMnO4 without increase of the size of the unit cell. (b) Possible ordering scenarios for fluoride ion for a compound of composition LaSrMnO4F and ordering A of the LaSrMnO4 host, labeled A1 to A4 with c lattice parameters obtained after structure optimization. Fluorinated LaSrMnO4F with B- or C-ordered LaSrMnO4 hosts (not shown) are correspondingly labeled as B1 to B4, and C1 to C4.

For LaSrMnO4F different configurations of fluoride ions (see Figure 2b) were tested for all configurations of La/Sr ions. In this respect, it is found that configurations with an occupation of a single interstitial anion layer by fluoride ions are energetically lower by approximately 0.4−0.8 eV per LaSrMnO4 formula unit. This confirms previous findings on the structure of LaSrMnO4F by Aikens et al.,19 and supports findings on charging of the battery (see section 3.4) which indicate subsequent filling of layers. Furthermore, the theoretical calculations also predict that ordered and disordered scenarios could be easily distinguished for LaSrMnO4F from the lattice parameters by means of XRD (c ∼ 14.3 Å for ordered configurations (A, B, C) (1, 2) vs c ∼ 15.4−15.7 Å for disordered configurations (A, B, C) (3, 4)). Table 2 summarizes calculated voltages against a FGE which were found for the different configurations of La and Sr ions for the first charging step LaSrMnO4/LaSrMnO4F. Those findings can be summarized as follows: the voltages for the first oxidative intercalation step of LaSrMnO4 are found to be relatively homogeneous around 2.8−2.9 V against a FGE. This agrees with what was observed for the charging and discharging reactions performed against PbF2 (voltages against Pb-PbF2 are given as a separate column in Table 2 to allow easier comparison to experimental values reported in subsequent sections). Since all voltages against a FGE are >0, the phases can be considered as accessible metastable states for a topochemical intercalation of fluoride ions. Since all interstitial anion sites are filled for LaSrMnO4F2, only a single structural setting has to be considered for each configuration of La/Sr ions. The voltage obtained for the charging step LaSrMnO4F/LaSrMnO4F2 again depends on the detailed choice of the La/Sr distribution. In this respect,

To test the suitability of the DFT method for FIBs, voltages were calculated for a selection of metals which are well-known as conversion electrodes for FIBs, and for which thermochemical data are available. The voltage of such systems can be calculated according to y M + F2 → MFy 2 y U (against a FGE) = [E(MFy) − [E(M) + E(F2)]] 2 /( −y 96485 C·mol−1)

with y being the number of fluoride ions (respectively number of transferred electrons), and E being the calculated energy which is obtained from the structure and energy optimization in J·mol−1 (0 K approximation neglecting entropy contributions). Table 1 gives an overview of the voltages against a FGE for a selection of conversion based metal electrodes. The differences of those voltages are in excellent agreement with what was found in previous experimental studies on conversion based materials2,6,7 and with what would be approximately expected from the well-known standard potentials (also included in Table 1).35 This procedure was then adopted to calculate voltages against a FGE for the two redox systems LaSrMnO4/ LaSrMnO4F and LaSrMnO4F/LaSrMnO4F2. The use of computationally expensive superstructures was avoided, and energies were calculated for four different ordering scenarios of 3444


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fluorine content LaSrMnO4F2−x phase by heating the oxide together with an excess of AgF2, which is known to be a strong oxidizer,37 even stronger than CuF2/XeF2 (which was found to induce lower fluorination degrees for LaSrMnO4). The fact that CuF2 cannot and AgF2 can be used for the preparation of LaSrMnO4F2−x also agrees with predictions from DFT based calculations (see section 3.1). The best results were obtained for heating the mixture of LaSrMnO4 and AgF2 at 250 °C for 48 h. The formation of silver together with its relative phase fraction (∼20 wt % in the final mixture) gives strong support for the reaction following the scheme shown below:

Table 2. Calculated Voltages for the Redox Couples LaSrMnO4/LaSrMnO4F and LaSrMnO4F/LaSrMnO4F2 for Different Distributions of La, Sr, and F Ions Calculated voltage [V] for LaSrMnO4 / LaSrMnO4F against

Calculated voltage [V] for LaSrMnO4F/ LaSrMnO4F2 against

Configuration of fluoride and La/Sr ions as given in Figure 2

FGE

Pb/ PbF2

FGE

Pb/ PbF2

A1 A2 B1 B2 C1 C2

2.87 2.87 2.83 2.92 2.77 2.77

0.82 0.82 0.86 0.75 0.92 0.92

1.03 1.03 1.74 1.65 1.80 1.80

2.63 2.63 1.99 2.04 1.89 1.89

LaSrMnO4 + (1 − x /2)AgF2 → LaSrMnO4 F2 − x + (1 − x /2)Ag

In order to determine the structural characteristics of LaSrMnO4F2−x, high quality XRD data were collected and a structure analysis using the Rietveld method was performed (see Supporting Information, Figure S2). It is clear that a close structural relationship exists between the as-prepared phase of LaSrMnO4F2−x and LaSrMnO4F/ LaSrMnO4. In this respect, it is found that the pattern can be indexed using a tetragonal unit cell with a ∼ 3.77 Å and c ∼ 15.36 Å. Therefore, the fluorination results in a significant cell expansion of 17.3% along the c-axis compared to LaSrMnO4 (a = 3.7890(6) Å, c = 13.1231(9) Å), and of 8.6% compared to LaSrMnO4F19 (P4/nmm, a = 3.7749(1) Å, c = 14.1049(3) Å). For LaSrMnO4F, Aikens et al.19 found that only every second layer of anion interstitials was filled, resulting in a symmetry lowering to P4/nmm due to the loss of body centering. In this respect, it is justified to assume (and also in agreement with what was obtained out of the DFT based calculations, see section 3) that both interstitial anion layers are filled for the highly fluorinated compound LaSrMnO4F2−x. This is supported by the fact that the superstructure reflections which result from symmetry lowering to P4/nmm for LaSrMnO4F (the most intense superstructure reflections for the known structural model of LaSrMnO4F19 are (0 1 2), (1 1 1), (2 1 4), (0 2 7), and (0 1 10)) disappear on our attempt to prepare LaSrMnO4F2−x, indicating the recovery of body-centered symmetry. Since the intensity of the superstructure reflections is mainly arising from the differences of the fluorine contents of the interstitial anion layers for LaSrMnO4F, one can conclude that the fluorine contents are similar for the interstitial layers in LaSrMnO4F2−x. The structure refinement was performed using a model with I4/mmm symmetry, and a good fit to the recorded pattern was obtained (see Supporting Information, Figure S2). The refined structural data are listed in Table 3.

reasonable convergence and energies could only be obtained for the settings B and C of LaSrMnO4F2 (with setting A resulting in implausible diamagnetic Mn species). The calculated voltages given in Table 2 transformed to the PbF2 reference system agree with what was found for the charging against a Pb-PbF2 composite described in section 3.4, highlighting the potential of LaSrMnO4F to act as a high voltage cathode material for fluoride ion batteries. Remarkably, the calculated voltages for the second charging step are found at a voltage against a FGE which is lower by ∼0.6−0.8 V compared to CuF2 (respectively higher by 0.6−0.8 V in comparison to CuF 2 against PbF 2 ), which was used experimentally as the cathode material with the highest voltage for FIBs so far.2,7 The corresponding calculated voltage profile for the charging against Pb/PbF2 as the reference electrode is shown in Figure 3 for easier comparison to the experimental charging curves reported in section 3.4.

Figure 3. Calculated charging profile for the stepwise charging of LaSrMnO4 to LaSrMnO4F and LaSrMnO4, neglecting possible overpotentials. Dark gray error bars indicate intervals of confidence from the distribution of La and Sr atoms; light gray bars indicate intervals of confidence from the choice of the value of +Ueff (5 ± 2 eV).

Table 3. Structural Data for LaSrMnO4F2−x (x ∼ 0.16, space group I4/mmm) from Rietveld Analysis of XRD Data Atom

3.2. Preparation of Chemically Fluorinated LaSrMnO4F2‑x. As described in the Introduction, no details have been reported previously on the crystallographic structure of LaSrMnO4F1.7 prepared by heating LaSrMnO4 under a flow of fluorine gas.19 One can assume that highly oxidizing agents are required to prepare phases containing pentavalent manganese species. It was attempted to synthesize a high 3445

x

Wyckoff site

La/Sr Mn O1 O2 F

4e 2a 4c 4e 4d

a [Å] Rwp [%]

3.77036(8) 3.48 GOF

0 0 0 0 0

z

Occupancy

0 0 1 /2 0 1 /2

y

0.6429(6) 0 1 /2 0.8800(4) 1 /4

0.5/0.5 1 1 1 0.92(1)

c [Å] 1.72

15.36195(2) RBragg [%]

1.98


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To test new materials, it is important to reduce performance limiting factors of the counter electrode on the cell performance. Therefore, different compounds (among them CeF3, MgF2, PbF2, CuF2, and AgF) have been examined for being used on the anode side. Good results were obtained for the use of PbF2 in the initial investigations, presumably due to the following reasons: PbF2 is known to possess good intrinsic fluoride ion conductivity.39 Moreover, elemental lead has a relatively low melting point of ∼327 °C, which could lower crystallization overpotentials arising after the reduction of PbF2 on charging of the cells. However, the reader should be aware that PbF2 would not be an ideal anode material for a high voltage battery, since its voltage against a FGE is not very high (∼3.6 V). However, its voltage against a FGE can be considered to be high enough for preparing the battery in a discharged state. Additionally, it will be shown within this paragraph that also PbF2 anodes can be altered regarding their performance on changes in their exact composition (e.g., by using Pb-PbF2 composites). In addition, kinetic limitations arise from the solid electrolyte and cannot be totally avoided within our current battery setup. Overall, this makes the unambiguous evaluation of the cathode side very difficult, which we will emphasize within section 3.5 by a detailed discussion of limiting factors. To rule out the possibility of changes in the phase composition by pure heating of the battery, a test cell was heated for 100 h (which is a typical time needed for charging) at the conditions used for the electrochemical measurements, but without applying a charging current. XRD confirms that both anode and cathode composites can be considered to be stable upon long heat treatments at temperatures used for the cell testing (see Supporting Information, Figure S3). Especially, the lattice parameters of LaSrMnO4 remain nearly unchanged, indicating no intercalation of fluoride ions into the interstitial anion layers of the K2NiF4 type structure. To determine the charging kinetics of LaSrMnO4, we investigated the galvanostatic charging with a current of 10 μA (24 μA·cm−2) at 170 °C. Figure 5a shows a typical charging curve of the LaSrMnO4/La0.9Ba0.1F2.9/PbF2 cell. First, it is observed that the charging capacity of the cell is higher than what would be expected from the theoretical capacities of LaSrMnO4 (155 mAh/g for the full transformation to LaSrMnO4F2). This is indicative of possible side reactions occurring during the charging, which will be discussed in a subsequent section (see 3.5). However, two characteristic voltage plateaus can always be identified for the charging process against PbF2: one short plateau around 0.3 V and a further plateau typically occurring between 1 and 1.5 V. Interestingly, no clear high voltage plateau (which would be expected for the charging step LaSrMnO4F/LaSrMnO4F2−x) was observed against PbF2 (but this can be accessed on modifying the anode composite by the addition of lead; see later in this section). Anode and cathode sides were examined before and after the electrochemical charging to 3 V using ex-situ XRD; recorded patterns are presented in Figure 5c and d. It was observed (Figure 5c) that charging to high voltages against PbF2 resulted in the formation of two K2NiF4 type phases, which can be assigned from an investigation of lattice parameters to be LaSrMnO4F and LaSrMnO4F2−x. A summary of the lattice parameters of the cathode precursor LaSrMnO4 and the products after charging are given in Table 4. This, together with the additional formation of elemental Pb on the anode side

Although the unit cell shows a strong volume increase, Mn−O bond distances (4 × 1.885 Å, 2 × 1.844 Å) decrease further compared to what was reported for LaSrMnO4F19 (4 × 1.892 Å, 1 × 1.783 Å, 1 × 2.157 Å) and Jahn−Teller distorted LaSrMnO4 (4× 1.893 Å, 2× 2.267 Å from our own analysis). Again, those bond distances are in agreement with what would be expected for pentavalent manganese species from Shannon’s radii (Mn4+ (CN = 6): 0.53 Å, O2− (CN = 6): 1.40 Å, as well as an interpolated value for Mn5+ (CN = 6): 0.47 Å).38 Due to the use of AgF2 as oxidizing reagent, diffraction peaks of Ag and a phase which can be assigned to be an intermediate product from the reduction of the fluorination agent (presumably AgF2 − Ag2O) are found in the XRD pattern. Distinction between the different silver oxides/fluorides/ oxyfluorides proves to be difficult due to the fact that all structures are related to a cubic close packing of silver ions with the anions filling parts of the octahedral or tetrahedral gaps. The presence of such unreacted/partially reacted residues of the fluorination agent renders the use of titration techniques prohibitive for the determination of the detailed fluorine content of the target phase. Therefore, attempts were made to refine the occupancy of the interstitial anion site (4d), which resulted in a composition LaSrMnO4F1.84(2). Within the experimental limitations of determining the exact anion composition from powder diffraction data (as discussed in previous articles12), the result is in good qualitative agreement with the previous report by Aikens et al.19 on the F2 based fluorination of LaSrMnO 4 , which found formation of LaSrMnO4F1.7. 3.3. Cell Morphology. The average thickness of each cell component was measured by scanning electron microscopy (SEM) of a cross section of the pellet, and found to be 80 μm, 1200 μm, and 80 μm for anode, electrolyte, and cathode, respectively (Figure 4). The obtained SEM image shows that

Figure 4. Scanning electron microscopy (SEM) cross-sectional view of a battery pellet.

both active electrode composites stick uniformly to the electrolyte, ensuring good contact, which is necessary for the electrochemical reactions. 3.4. Investigation of Charging of LaSrMnO4. Currently, FIBs cannot be considered to be a state of the art technology. In this respect, none of the previously reported conversion based electrode materials6 works well enough to serve as a wellestablished standard against which new materials can be tested. 3446


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Figure 5. (a) Galvanostatic charging of LaSrMnO4 with 10 μA at 170 °C against PbF2 (various cutoff voltages are marked with a cross); (b) XRD measurements of the cathode composite containing LaSrMnO4 before and after charging to various cutoff voltages; (c) XRD patterns of the cathode before and after charging; (d) XRD patterns of the anode before, at Vc = 0.45 V, and after charging (* marks an unknown phase).

Table 4. Microstructural Detail of the Active Cathode Material at Various Cut-off Voltages (I = 10 μA and T = 170 °C) LaSrMnO4 Vc [V] Before charging 0.45 1.20 1.43 1.55 2.00 3.00

a [Å]

c [Å]

3.820(1)

12.957(2)

3.810(1) 3.816(2)

13.094(11) 12.988(12)

LaSrMnO4F a [Å]

c [Å]

3.780(1) 3.770(1) 3.768(1) 3.770(1) 3.767(1) 3.769(1)

14.000(1) 14.212(1) 14.218(1) 14.231(2) 14.235(3) 14.237(3)

LaSrMnO4F2−x a [Å]

c [Å]

Ratio of LaSrMnO4F/ LaSrMnO4

Ratio of LaSrMnO4F2−x / LaSrMnO4F

5.73 37.7 3.769(1) 3.773(1) 3.771(1)

16.007(7) 15.628(7) 15.631(7)

0.08 0.78 0.75

for charging of Bi cathodes, where BiOF phases were found to crystallize first from the transformation of surface oxidized Bi particles.6 Further charging up to Vc = 1.2 V led to the complete disappearance of the starting LaSrMnO4 phase, with further increase of the c-axis up to ∼14.2 Å. The formation of elemental lead is indicated on the anode side by broad peaks, which are hardly visible. This means that the process now can be assigned to the potential difference of the redox couples Pb/ PbF2 and LaSrMnO4/LaSrMnO4F1−y. The charging process corresponding to this plateau proceeds up to a voltage of approximately 1.5−1.7 V. Increasing fluorine incorporation in LaSrMnO4 to form LaSrMnO4F within this plateau can also be followed from changes of the lattice parameters (Table 4). On increasing the cutoff voltage to 1.43 and 1.55 V, the c-axis increases further up to 14.21 and 14.23 Å, respectively, which is accompanied by a further shrinking of the a-axis to ∼3.77 Å (Table 4). This is in agreement with an increased filling of one of the anion interstitial layers (increase of c), which is accompanied by an oxidation of Mn3+ to form Mn4+ (decrease

(Figure 5d), shows that LaSrMnO4 is accessible for oxidative charging in a FIB setup. Additional reflections of an unknown phase (marked by * in Figure 5d) are found on the anode side after charging, indicating the formation of an intermediate phase. This phase appears to be reversibly transformed back to PbF2 on discharging of the battery (see section 3.6). To obtain a deeper understanding of the charging reaction, ex-situ XRD measurements were recorded for different cutoff voltages Vc as indicated in Figure 5a. The composition of the cathode side together with lattice parameters was examined by means of XRD (see Figure 5b and Table 4). Already for a cutoff voltage of 0.45 V (see Figure 5b and Table 4), the majority of the starting LaSrMnO4 phase has disappeared, and a phase with increased c-axis of ∼14.0 Å had formed. The formation of elemental lead was not indicated on the anode side (Figure 5d) at the same time. With respect to the observations on heating of the cell, those changes have to be assigned to an electrochemical process, possibly related to a small fraction of inserted fluoride ions or a voltage induced partial substitution process. The latter was previously assumed 3447


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Figure 6. LaSrMnO4/Pb-PbF2 electrochemical cell charged at T = 200 °C and I = 10 μA: (a) the charging curve; (b) XRD measurement of the cathode side before and after charging.

Figure 7. (a) Galvanostatic charging of carbon black against PbF2 at T = 170 °C and I = 10 μA. (b) Charging curves of the LaSrMnO4/PbF2 electrochemical cell charged at various temperatures of 150 °C, 170 °C, and 200 °C (I = 10 μA).

of the a-axis results in shortening of Mn−O bonds). Up to Vc = 1.55 V, only a small amount of the highly fluorinated phase LaSrMnO4F2‑x is obtained in addition (Table 4), which significantly increases only upon further charging to 2.0 V and above. At 3.0 V the lattice parameters and composition of the cathode do not strongly differ compared to the lower cutoff voltage of 2.0 V. In this respect, we found that the detailed design and composition of the anode side can have a tremendous impact on the galvanostatic charging. Since a pronounced third charging plateau corresponding to the couple LaSrMnO4F/ LaSrMnO4F2−x was not expressed in the charging curve, even though the highly fluorinated phase LaSrMnO4F2−x was partly formed (see Figure 5b,c and Table 4), attempts were made to investigate a slightly modified anode composite based on the use of the same redox couple. On using a composite anode made of Pb-PbF2, the profile of the galvanostatic charging curve changed strongly, especially at voltages above the LaSrMnO4/ LaSrMnO4F charging plateau. As shown in Figure 6, the LaSrMnO4F/LaSrMnO4F2−x charging plateau is expressed much sharper on modifying the anode composite (but not the anode redox couple). After charging the battery to 2.2 V (the end point of the third plateau as shown in Figure 6a), almost all LaSrMnO4 was transformed to LaSrMnO4F2−x (Figure 6b), with nearly complete disappearing of the intermediate LaSrMnO4F phase. We emphasize that the charging differences comparing PbF2 and Pb-PbF2 were found to be highly reproducible. This finding is surprising, since the redox potential of the anode side should be independent of the admixing of lead. However, it also highlights that the development of well-

defined, highly reversible reference electrodes would be appreciable for FIBs. In agreement with previous findings,2,6,17,21,40 the influence of morphology and composition of conversion electrodes is still not understood in full detail. The conversion process, which is complicated in its nature due to the need for the presence and growth of crystallization seeds, remains a challenge for the investigation of intercalation based materials for FIBs. Therefore, the authors are currently investigating intercalation based compounds for the use as anode materials. Acknowledging the complexity of the battery setup and its components, limiting factors have been investigated and will be described in section 3.5. We also emphasize that the cell setup reported here, apart from being used for FIBs, has further potential as a synthesis technique for adjusting distinct fluorine contents. Phase pure preparation of LaSrMnO4F could only be achieved by chemical reactions in a two-step topochemical synthesis so far.19 We have shown that this phase can be prepared in a one-step synthesis from an explicit choice of the cutoff potential to ∼1.43 V. Therefore, the current setup might provide access to highly adjustable fluorine contents, which would be difficult to control with oxidative fluorination agents such as F2 gas and AgF2 in addition to an increase in reaction safety. In terms of capacity, LaSrMnO4 has shown to intercalate the highest amounts of fluoride ions within an electrochemical reaction so far, since both charging steps are in principle accessible. Its capacity is therefore ∼4 times higher in terms of fluoride ions per transition metal than what was found for the charging of BaFeO2.517 with an identical cell setup, ∼10 times higher than what was reported for the fluorination of La2CuO4 3448


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Figure 8. Charging curves (a) and XRD measurements (b) of the LaSrMnO4/PbF2 electrochemical cell at T = 170 °C, Vc = 3.0 V, and various currents of 5 μA, 10 μA, 40 μA, and 80 μA.

Table 5. Lattice Parameters and Composition of the Active Cathode Material after Charging to 3.0 V Using Different Currents (T = 170 °C) LaSrMnO4F

LaSrMnO4F2−x

Current [μA]

a [Å]

c [Å]

a [Å]

c [Å]

Ratio of LaSrMnO4F2−x /LaSrMnO4F

5.0 10.0 40.0 80.0

3.770(1) 3.769(1) 3.767(1) 3.766(1)

14.221(3) 14.237(3) 14.300(3) 14.288(2)

3.782(1) 3.771(1) 3.776(2)

15.613(7) 15.631(7) 15.542(21)

0.87 0.75 0.34

in liquid media,15 and ∼480 times higher than what was reported for the fluorination of YBCO.16 3.5. Critical Examination of Performance Limiting Factors. From a quantitative phase analysis, a final overall composition of ∼LaSrMnO4F1.3−1.5 is indicated after charging against PbF2 (with further increase to a single phase composition close to LaSrMnO4F2−x (with presumably small x) for charging against Pb-PbF2). As already discussed in the previous section, experimental charging capacities are as high as 600−800 mAh/g, which is clearly above the theoretical capacity of LaSrMnO4 even if one would assume the formation of fluorine saturated LaSrMnO4F2. Therefore, the coulomb efficiency of the charging process can be regarded to be low, indicating that side reactions are occurring during the charging reaction. Such decrease of coulomb efficiency could result from the oxidation of the conductive additive (black carbon). To examine this, a cell made of C II La0.9Ba0.1F2.9 II PbF2 (using the same amount of active carbon as present in the LaSrMnO4 composite cathode) was charged. The absolute capacity of such a carbon based cell is much lower compared to the composite containing LaSrMnO4 in addition (Figure 7a). However, the short plateau is found at a similar voltage as needed for the charging of LaSrMnO4. Therefore, fluorinated LaSrMnO4 might act as a type of catalyst for the fluorination of carbon (or the steel current collector which should be oxidized at even lower voltages). The theoretical capacity of carbon belonging to the reaction “C + F− → CF + e−“ would be as high as ∼2200 mAh/g (1.54 mAh for the typical amount of carbon used in the cells reported here). Clearly, carbon could therefore contribute to the high capacities observed during the charging reaction. In contrast, for the use of a cathode made of LaSrMnO4 only (or a LaSrMnO4 + La0.9Ba0.1F2.9 composite alternatively), no charging plateau was expressed, indicating the need for a conductive additive on the cathode side. This problem of “composite stability” is well-known for high voltage cathode materials for LIBs41 and is therefore an intrinsic problem for high voltage systems. The decomposition of carbon must also

result in a strong impact on the discharging of the cell (see section 3.6). The fluorination of carbon, which results in the formation of covalent bonds with sp3 carbon, will result in a strong decrease of the conductivity of the system with an increase of the band gap42 of carbon. In section 3.6, we will show that this is most likely the reason for the deterioration of reversibility of the cell. In future studies, the authors will therefore aim to investigate the influence of other conductive additives (e.g., carbon nanotubes, nanocrystalline silver, ITO). Additionally, limitations of the cell performance were examined by investigating the effect of current (i.e., the Crate) and temperature on charging of the battery. The effect of charging current was investigated by preparing identical electrochemical cells and charging them at 170 °C with different currents equal to 5 μA, 10 μA, 40 μA, and 80 μA up to a cutoff voltage of 3.0 V. The charging curves (Figure 8a) show that by increasing the charging rate the plateau of the charging process shifts to higher voltages. This indicates that the current cells still suffer from high transport and/or reaction overpotentials, confirmed from an investigation of the composition of the cathode side after charging (see Figure 8b and Table 5). At high charging currents (e.g., 80 μA) the formation of highly fluorinated LaSrMnO4F2‑x was not indicated, with formation of only LaSrMnO4F. This might indicate that the filling of a second anion interstitial layer could have slower kinetics compared to the filling of the first layer. For the lowest charging current under investigation (5 μA), the highly fluorinated LaSrMnO4F2−x phase can be found to larger extents, and an overall composition close to LaSrMnO4F∼1.5 is indicated from a quantitative phase analysis. When using a current of 10 μA at different temperatures of 150 °C, 170 °C, and 200 °C, a strong decrease in the overpotentials is also indicated for increasing temperature (see Figure 7b). The overpotentials seem to saturate at charging temperatures between 170 and 200 °C; however, low temperatures are preferable to avoid destruction of the PTFE sealing of the cell. It was also found that an increase of the 3449


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Figure 9. (a) Discharging curves of LaSrMnO4/PbF2 and LaSrMnO4/Pb-PbF2. (b) XRD measurements of the anode side of LaSrMnO4/PbF2 electrochemical cells. Experimtents were performed at T = 170 °C with I = −1.0 μA.

Table 6. Structural Detail of the Cathode Side (LaSrMnO4/PbF2 electrochemical cell) after Charging and Discharging LaSrMnO4F After After After After

Charging against PbF2 Discharging against PbF2 Charging against Pb-PbF2 Discharging against Pb-PbF2

LaSrMnO4F2−x

a (Å)

c (Å)

a (Å)

c (Å)

Ratio of LaSrMnO4F2−x / LaSrMnO4F

3.769(1) 3.778(1)

14.237(3) 14.067(4) 14.054(7)

15.631(7) 14.950(12) 15.419(3) 15.193(15)

0.75 0.79

3.778(1)

3.771(1) 3.863(2) 3.777(1) 3.800(1)

0.98

cell after charging against PbF2. The discharging voltage is in reasonable agreement with what would be expected from DFT calculations for the LaSrMnO4/LaSrMnO4F charging step (see section 3.1), therefore indicating that the discharging reactions can be assigned to the deintercalation of fluoride ions from the LaSrMnO4F phase. This is strongly supported by the lattice parameters of the K2NiF4 phase determined after discharging, which show that the phase basically transforms back to a phase similar to what was found when charging to voltages as low as 0.45 V only (see Figure 5b). On discharging, the amount of elemental lead found on the anode side strongly decreases (see Figure 9b), with a nearly complete recovery of the PbF2 phase that was used before charging. On a further note, the unidentified (presumably metastable) phase which was found after charging (as described in section 3.4) disappears. To minimize the influence of the anode composite on the discharging reaction (e.g., from the consumption of lead formed on charging), we also examined charging and discharging of the cell against the composite anode made of Pb-PbF2. Such a cell shows a significant increase (Figure 9a) of the discharge capacity by a factor of 2−3 compared to pure PbF2, reaching approximately 20−25 mAh/g. This is much larger than what was obtained previously for BaFeO2.5.17 Overall, the need to use a conversion based anode and stability problems within the cathode composite (Table 6) represent an intrinsic limitation of the current study. A limited cell voltage together with high overpotentials for the discharging reaction (presumably due to carbon decomposition arising during the charging) must necessarily result in strong limitations. To take this into account, a detailed study of progressive charging and discharging of the cell is not considered at the current state. A fundamental question regarding the use of oxyfluoride materials as electrode materials for FIBs is if the intercalation process can be fully reversible. This reversibility of the charging of LaSrMnO4 can be demonstrated by the following: full recovery of the discharged state was attempted by trying to compensate overpotentials on reducing the cutoff voltage to negative potentials. In this regard, the cell was galvanostatically

temperature can serve to reduce overpotentials for higher charging rates. On the other hand, lower temperatures can be considered to reduce the amount of possible side reactions during the charging process, since the overall charging capacity is reduced. Therefore, it is noticed that the optimization of the charging/discharging conditions is of strong importance for advancing the methodology and will depend on the exact cell composition and reference anode in use. We summarize that the study of the reversibility of the high voltage cathode LaSrMnO4, detached from the influence of limiting factors and overpotentials from the cell setup, is not possible for the cell in its current state-of-the-art. The reader should be aware that an unambiguous assignment of the source of overpotentials to electrolyte transport or to reaction overpotentials cannot be made, as indicated e.g. from the differences observed for changing against a Pb-PbF2 based anode system. 3.6. Study of Cell Discharging: Confirming the Reversibility of LaSrMnO4 for Fluoride Deintercalation. For all attempts to discharge the battery, it is found that the discharging process suffers from higher kinetic limitations in comparison to the charging process; that is, discharging overpotentials are higher than charging overpotentials. As highlighted in the previous section, this could result from the fact that the carbon additives cannot be considered to be inert for high voltage cathode materials. It is also important to keep in mind that Pb-PbF2 is an anode with low potential against FGE (as outlined by the DFT calculation reported in section 3.1), which limits the cell voltage and reduces the driving force for the discharge reaction. Attempts were made to use anodes which would allow for higher cell voltages (e.g., Mg-MgF2 or CeF3); however, we found that this resulted in impacts on the charging process, which makes the interpretation of subsequent discharging data even more difficult. To take such limitations into account, the discharging of the cell has been examined using a relatively low current of −1.0 μA (which is reduced by a factor of 8 from what was previously used for intercalation based FIBs, e.g.6). Figure 9a shows the discharging curve of the 3450


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Figure 10. Galvanostatic forced discharging (a) and XRD pattern of the cathode side before charging, after charging, and after “forced discharging” (b) of LaSrMnO4 against PbF2.

charged to +3 V at a current of +10 μA, followed by subsequent discharging to −3 V reversing the current to as high as −10 μA. A discharging to negative voltages was also reported previously, e.g. to investigate the reversibility of Mg intercalation into ReO3 type host structures43 against a supercapacitor. The voltage profile (see Figure 10a) shows two discharging plateaus, with an overall discharge capacity of ∼100 mAh/g. This is close to the theoretical capacity of LaSrMnO4F2−x, taking into account that the fluorination of carbon is usually not reversible, as discussed in section 3.5, and confirms the approximated composition of LaSrMnO4F1.3−1.5 as determined from phase analysis of the products after charging. This “forced discharging” results in an almost full recovery of the nonfluorinated starting phase of LaSrMnO4 (Figure 10b). The authors think that this structural reversibility is most likely facilitated from anion ordering within the K2NiF4 structure, which allows for different local site potentials for oxide and fluoride ions.12 Furthermore, the selectivity of anion deintercalation could be promoted from different mobilities of oxide and fluoride ions. This could result from the fact that the fluoride ions on the 4d site do not form bonds to the manganese cations, and are only bonded to the softer lanthanide/alkaline earth cations.12−14 The findings reported here highlight that oxyfluorides indeed can serve as stable host lattices with mutual selectivity for the deintercalation of anion species during a discharging reaction. This is the first report highlighting the reversibility of the fluorination of K2NiF4 type host lattices, with the fluorination reaction being considered in much detail by chemical methods in previous studies.12−14

and it is surprising that such intercalation chemistry can be achieved using a larger anion instead of one of the smallest cations. This study further highlights current limitations of FIBs as well as the need to improve solid−solid interfaces of the electrode composites. The identification of a suitable reference electrode (intercalation or conversion based) without significant overpotentials could help for the unambiguous identification of limiting factors for the study of new electrode composites. Further limitations must be considered in finding suitable compatible materials for use as electronically conducting admixtures (and/or current collectors). The problem of composite stabilities is currently also unsolved for LIBs and SIBs in an all-solid-state, and it was acknowledged that the optimization of interfaces is a key challenge for pushing the methodology further.45 For high voltage cathodes for FIBs, the finding of alternative conductive admixtures could prove to be beneficial, and we will examine this in detail in future studies.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b05075. Scheme of the setup of a fluoride ion battery, the refined pattern of chemically fluorinated LaSrMnO4, and the diffraction pattern comparing cathode and anode composites before and after heating (PDF)

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4. CONCLUSIONS The findings reported here show that the development of intercalation based cathodes for FIBs is in principle possible. In agreement with DFT based calculations and results from chemical synthesis, LaSrMnO4 can serve as a high capacity, high voltage cathode. This originates from the fact that two fluoride ions per transition metal species can be incorporated as well as from the high oxidation states of the manganese cation. Intercalation based materials for FIBs are therefore not restricted to low capacities and can have in principle similar capacities as intercalation based cathodes for the well-studied LIB systems. Furthermore, we have given the first evidence that the intercalation of fluoride ions is fully reversible. In comparison, the reversible uptake/removal of more than one lithium ion per active transition metal species has rarely been observed for intercalation/insertion type electrodes for LIB,44

AUTHOR INFORMATION

Corresponding Author

*Fax: +49 6151 16 20965. E-Mail: [email protected]. ORCID

Oliver Clemens: 0000-0002-0860-0911 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the German Research Foundation within the Emmy Noether program (Grant No. CL 551/2-1). Calculations for this research were conducted on the Lichtenberg high performance computer of the TU Darmstadt. 3451


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