Tuning of superdiamagnetism in La₂CuO₄ by solid-state electrochemical fluorination and defluorination

Voltage control for the switching and tuning of functional properties such as magnetism¹ and optical properties^2,3 as well as superconductivity⁴ is a current hot-topic in materials research. Such voltage control can be achieved via liquid- or solid-state systems and can be based on charge doping via double layers or the intercalation of ionic species, thus introducing holes or electrons into the structure, which are of keen relevance for the properties detailed above. For superconductors, liquid-gating was used predominantly in previous studies,^5,6 which can be complicated due to the complexity of the detailed reaction happening at the liquid/solid interface. In contrast, solid ion conductors have an advantage in having a transport number of t ∼ 1 for a single ionic species,⁷ thus facilitating a clear reaction chemistry at the solid to solid interface. For the typical oxide-based cuprate superconductors, an oxide ion transport to control the superconducting properties is hard to achieve due to the fact that oxide ion conduction requires high temperatures, at which the cuprate systems have a strong sensitivity on the applied oxygen partial pressure. Thus, we have previously investigated the use of fluoride ions, a charge carrier with high electrochemical stability and high ionic mobility in solid-state devices, in order to switch functional properties, a concept not considered for superconductors in large detail.

The La₂CuO₄ system is among the most extensively studied high-T_c cuprate superconductors. The stoichiometric compound is an antiferromagnetic insulator, but hole doping according to La₂CuO_4+d or La_2−xSr_xCuO₄⁸ or electron doping according to La_2−xCe_xCuO₄⁹ turns the material into a superconductor and thus a superdiamagnet, i.e., a compound with ideal intrinsic diamagnetism as found for superconductors due to the unique Meißner–Ochsenfeld effect. While the critical temperatures (T_c) of the La₂CuO₄-based superconductors are relatively low (≤45 K) compared to other cuprates, their relatively simple crystal structures and easily tunable superconducting properties have made them invaluable “model materials” for detailed studies of high-T_c superconductivity from the very beginning up to the present day.^10–19 

La₂CuO₄ has a K₂NiF₄-type structure (also known as the n = 1 member of the A_n+1B_nO_3n+1 Ruddlesden–Popper series), which contains perovskite-like LaCuO₃ blocks separated by rock-salt-like LaO layers (Fig. 1). However, compared to the ideal tetragonal K₂NiF₄ structure, La₂CuO₄ is orthorhombic with the space-group Bmab.¹¹ Hole doping can be done either by heterovalent cation substitution on the La site or by intercalating additional anions into interstitial vacancies found in the rock-salt layer, as shown in Fig. 1 (see, e.g., Refs. 20 and 21 and the references therein). This anion intercalation route is especially interesting because the anions can be topochemically inserted post-synthesis and they can be mobile at low temperatures, which allows for the tuning of the hole doping in La₂CuO₄ in benign conditions.²² Such tuning can be very useful in examining the effects of doping on superconductivity in detail, if it can be precisely controlled.

As can be expected, the most common choice of an anion to be intercalated in La₂CuO₄ is oxygen, and the effects of oxygen doping have been extensively studied (see, e.g., Ref. 23 and the references therein). However, La₂CuO₄ can also incorporate other anions, such as fluorine, and understanding the differences between the different dopants can be important. While fluorine intercalation has been much less studied than that of oxygen, topochemical fluorination of La₂CuO₄ has been reported before using F₂ gas,^24–31 XeF₂,³² NH₄F,³³ or ZnF₂³⁴ as the fluorinating agents. The chemical fluorination typically takes place at temperatures between 150 and 400 °C and at or slightly above ambient pressure. While the exact results from these methods differ somewhat, a general finding is that at low temperatures of 150–250 °C, fluorination leads to the formation of a superconducting La₂CuO₄F_x phase with x up to ∼0.20, increased orthorhombic distortion as well as increase in the c-axis as compared to the non-fluorinated La₂CuO₄, and T_c of 35–42 K. In the case of the fluorination by XeF₂, Abakumov et al.³² further showed that at 250 °C, a non-superconducting, body-centered tetragonal K₂NiF₄-type phase was formed with an ordered anion sublattice. Increasing the temperature to 300 °C leads to anion exchange and a formation of a monoclinic superstructure with a composition of La₂CuO_3.6F_0.8. Despite these efforts, understanding of the phase formation and superconducting properties of La₂CuO₄F_x with different x is still lacking compared to the oxygenated La₂CuO_4+d.

While the chemical fluorination methods are effective, it can be hard to control the exact composition of the resulting phase, which is necessary for detailed studies on doping. In contrast, electrochemical intercalation of ions offers a controllable way to adjust doping in La₂CuO₄; in such methods, the target material is placed as an electrode in an electrochemical cell and can be charged (ions inserted into the structure) or discharged (ions removed from the structure) with precision simply by adjusting the current and time of charging/discharging. Electrochemical intercalation of oxygen into La₂CuO₄ has been shown previously with liquid electrolytes, which has allowed for the determination of a detailed phase diagram for La₂CuO_4+d (see, e.g., Ref. 23 and the references therein), and Paulus et al.³⁵ have also demonstrated in situ neutron powder diffraction measurements of La₂CuO_4+d during cell charging and discharging. To our knowledge, comparative electrochemical intercalation of fluorine into La₂CuO₄ has been reported only once by Delville et al.³⁶ using an organic fluorinated liquid electrolyte. While they were successful in making a superconducting La₂CuO₄F_x phase with x ≈ 0.18, they did not demonstrate tuning of the doping level nor the effects of different levels of doping to the material’s properties.

In this work, we use an all-solid-state electrochemical cell to intercalate fluorine into La₂CuO₄. The cell used is a common setup conventionally used within so-called fluoride ion batteries (see Ref. 37 for a recent review) and is based on the solid fluoride ion conductor La_0.9Ba_0.1F_2.9, which has a wide electrochemical operation window especially toward oxidation, which occurs at around ∼6 V vs lithium under release of F₂; a counter-electrode composed of Pb and PbF₂ can facilitate both oxidation and reduction dependent on the choice of the current direction on a working electrode. We have demonstrated this method previously by tuning the magnetic properties of La_2−2xSr_1+2xMn₂O₇ from ferromagnetic to non-ferromagnetic by fluorine intercalation/deintercalation,³⁸ and here, we apply it to the tuning of superdiamagnetism in the case of La₂CuO₄ as the working electrode. We show that the fluorination can be controlled precisely, resulting in the superdiamagnetic La₂CuO₄F_x phase with controllable properties, and that the fluorination can be reversed on re-establishing the antiferromagnetic defluorinated state.

To synthesize La₂CuO₄, stoichiometric amounts of La₂O₃ and CuO were thoroughly ground with an agate mortar and pestle. La₂O₃ was heated to 1000 °C before use. The powder mixture was calcined at 900 °C for 12 h in air. After calcination, the powder was reground, pelletized, and sintered twice at 1000 °C in air for 24 h with an intermediate regrinding and pelletizing. A sample of La_1.85Sr_0.15CuO₄ was prepared in the same manner, using additional SrCO₃.

The electrochemical cell preparations were done in an argon-filled glove box to prevent any contact with air. The electrolyte material La_0.9Ba_0.1F_2.9 was prepared by ball milling stoichiometric amounts of LaF₃ and BaF₂ for 12 h at 600 rpm (based on Ref. 39). The cathode composite was prepared with a weight ratio of 30:60:10 of La₂CuO₄, the electrolyte, and carbon black, respectively. The mixture was ball milled for 1 h at 250 rpm. For the preparation of the anode composite, elemental Pb, PbF₂, and carbon black were ball milled in a weight ratio of 45:45:10 for 12 h at 600 rpm. Cells were prepared by pressing together 200 mg of the electrolyte, 7 mg of the cathode composite, and 7 mg of the anode composite at a pressure of 2 tons. The dimensions of the resulting pellets were measured to be ∼1.3 mm in thickness and 7.3 mm in diameter. The pellets were spring-loaded in a modified Swagelok-type cell.³⁸ 

X-ray powder diffraction measurements were done on a Bruker D8 Advance diffractometer in Bragg–Brentano geometry with CuKα radiation (VANTEC detector). Measurement ranges of 10°–130° and 20°–80° (2θ) were used for the powder samples and for the electrochemical cells, respectively, with a step size of 0.0066°. To prevent possible side reactions with air, the electrochemical cell samples were loaded into an air-tight low-background specimen holder (Bruker A100B36/B37) and sealed inside an argon-filled glove box before the measurements. Rietveld refinement was done using TOPAS V6. A reference scan of LaB₆ (NIST 660a) was used as a basis for empirical determination of the instrumental intensity distribution. This method allows for a precise determination of lattice parameters as well to extract phase quantities (more information can be found in Refs. 40 and 41), which were previously shown to be a precise measure to follow oxidative electrochemical fluorination in Ruddlesden–Popper-type materials,³⁷ especially if the relation of fluorine content on lattice parameters is known, as is the case for La₂CuO₄F_x.^24–31 

For cyclic voltammetry and galvanostatic charge/discharge measurements, the cells were heated to 170 °C in order to improve ionic conductivity through the electrolyte. Cyclic voltammetry was done using a Biologic SP-150 potentiostat with a scan rate of 0.1 mV/s. A potentiostat (either Biologic SP-150 or Solartron Analytical 1400 CellTest System) was also used for the galvanostatic charging/discharging. The current was chosen such that the respective cathode material was charged/discharged with a C/20 rate; in the case of 0.0021 g of the active material of La₂CuO₄, this leads to a charging current of 6.9 μA.

Magnetic measurements were done using a Quantum Design MPMS superconducting quantum interference device (SQUID) magnetometer. The cathode composite was scratched off the cells and filled into a gelatin capsule and fixed into a plastic sample holding straw. Magnetic susceptibility was measured in zero-field-cooled (ZFC) and field-cooled (FC) modes between 10 and 70 K at a 20 Oe field.

The as-synthesized La₂CuO₄ was found to be single phase with no impurities. Rietveld refinement with the space group Bmab resulted in unit-cell parameters of a = 5.356 31(4), b = 5.404 00(4), and c = 13.148 88(9) Å, in good agreement with earlier studies.^22,24,25,32 Magnetic measurements showed a weak diamagnetic response below 28 K (χ = −3 × 10⁻⁴ emu/mol Oe at 10 K with a 20 Oe field), evidencing a slight oxygen excess, and La₂CuO_4+d with d ≈ 0.01.²³ It should be noted that this susceptibility was two to three orders of magnitude weaker than any of the electrochemically fluorinated samples (see later in this section) and was thus not a concern for the current work.

The La₂CuO₄ cathode composite was a mixture of La₂CuO₄, electrolyte, and carbon black, made to optimize ionic and electronic conductivities. To ensure that La₂CuO₄ was not modified within the cathode composite due to sample preparations or heating during charging, the structural parameters of the parent oxide, the as-prepared cathode composite, and a heated cathode composite were compared. The La₂CuO₄ unit-cell parameters of these samples did not differ significantly from each other (0.07% or less), showing that La₂CuO₄ is stable in the cell environment. However, after the cathode preparation by ball milling, the crystallite size of La₂CuO₄ did reduce to about one third of the original, as-synthesized sample (100–150 nm, based on Rietveld refinement). Figure 2(a) shows x-ray diffraction (XRD) and Rietveld refinement results for a cathode composite, which was held at 170 °C for 5 days, but not charged: only the electrolyte and La₂CuO₄ reflections are visible with no indications of other phases or changes in the La₂CuO₄ structure.

To study the electrochemical fluorination process of La₂CuO₄, cycling voltammetry measurements were first done on several La₂CuO₄|La_0.9Ba_0.1F_2.9|Pb–PbF₂ cells. Figure 3 shows cyclic voltammograms of two measurements done using different switching potentials, both for three cycles. The open-circuit voltage was 0.30–0.33 V vs Pb/PbF₂. A broad peak can be seen at 0.78 V in the forward direction of the first cycle, which we assign to the intercalation of fluoride ions into La₂CuO₄. This peak is partially overlapped by a larger peak with a maximum at 1.62 V, which is caused by the fluorination of the carbon additive in the cathode composite; this has previously been observed by Nowroozi et al.^42–44 for similar cells using LaSrMnO₄, La₂CoO₄, or La₂NiO₄ cathodes. The carbon fluorination is not reversible, as shown by the lack of a corresponding peak in the backward direction. When the cell potential is swept above the carbon fluorination peak before switching direction (∼1.8 V vs Pb/PbF₂), a broad peak is seen at 0.36 V in the backward direction, belonging to the deintercalation of fluoride ions from La₂CuO₄. The peak current is much higher than that of the fluorination peak at 0.78 V (6.3 vs 4.3 μA), which may indicate partial defluorination of the carbon additive or other side reactions taking place during discharging. Using a lower switching potential of ∼1.0 V vs Pb/PbF2, above the La₂CuO₄ fluorination peak, but below the maximum of the carbon fluorination peak (inset, Fig. 3), a clearer peak corresponding to the La₂CuO₄ defluorination can be seen at 0.32 V in the backward direction. Here, the peak currents are equal in both directions, showing improved reversibility of the La₂CuO₄ fluorination/defluorination process. Thus, both the electrochemical intercalation and deintercalation of fluoride ions to/from La₂CuO₄ are possible. However, regardless of the switching potential, both the fluorination and defluorination peaks disappeared with additional cycling, showing poor repeatability after the first charge–discharge cycle. Limiting the charging/discharging to a narrower potential range might improve the cell cyclability, as has been demonstrated by Nowroozi et al.⁴⁴ in the case of La₂NiO₄ cathodes, but was not considered in the present study.

The electrochemical fluorination of La₂CuO₄ was done by galvanostatic charging. Figure 4 shows a representative charging curve of a La₂CuO₄|La_0.9Ba_0.1F_2.9|Pb–PbF₂ cell. Here, we plot the charging curve as a function of the transferred F⁻ ions per La₂CuO₄ unit available in the cathode composite (mole per mole), as calculated from the charged capacity. It should be noted that due to a possible overlap with the carbon fluorination or other side reactions, the amount of transferred F⁻ does not necessarily correspond to the amount of F⁻ intercalated into La₂CuO₄, at least in the case of cells charged to high potentials. However, it does correspond to the maximum possible amount of F⁻ in the La₂CuO₄F_x structure, i.e., the upper limit of x. As is seen in Fig. 4, in the beginning of the charging process, the cells showed a rapid increase in potential and then a plateau at 0.60–0.72 V. After the transfer of ∼0.04 F⁻, the cell potential began to gradually rise, leading to a long plateau at 0.95–1.20 V. This plateau extended from ∼0.10 to ∼1.80 transferred F⁻, after which the potential rose again in two steps and then began to rise linearly until at least 3 V (corresponding to 4 F⁻ transferred per La₂CuO₄, not shown). The plateaus above 1 V may have overlap with the carbon fluorination, as indicated by the cyclic voltammetry and also found in the case of other similar cathode materials.^42–44 

We acknowledge already at this state that the direct observation of fluoride in the host lattice is difficult due to the indistinguishability of O/F by diffraction methods;⁴⁵ to our experience, determination of fluoride contents by, e.g., TEM and energy dispersive X-ray (EDX) can only be performed reliably for larger degrees of fluorination (1–2 fluoride ions per La₂MO₄ unit) due to the fact that residues of nanocrystalline La_0.9Ba_0.1F_2.9 always reside on the particle surface of the active material.⁴⁴ Since the structural changes are small and not fully specific for fluoride incorporation, it is acknowledged that the detailed oxidative change of La₂CuO₄ cannot be fully verified. However, in contrast to liquid electrolytes, solid electrolytes have transference numbers of 1 for a specific charge carrier,⁷ thus allowing for very specific electrochemical reactions, e.g., oxidative fluorination or reductive defluorination, in the case of fluoride ion batteries. Thus, fluoride incorporation represents the most likely and plausible scenario, and for this reason, we will refer to the electrochemically oxidatively modified samples of La₂CuO₄ as “La₂CuO₄F_x” or as “fluorinated La₂CuO₄,” respectively, in the following.

We would like to point out that since the cell potential is a function of the state of charge x in La₂CuO₄F_x, galvanostatic charging was chosen here to adjust the state of charge most precisely; potentiostatic charging would require the choice of potentials slightly too high at early state of charges, thus potentially facilitating the occurrence of side reactions. Separate cells were galvanostatically charged for different times (corresponding to different amounts of transferred F⁻) and the structural and magnetic properties of the resulting La₂CuO₄F_x were determined ex situ. The electrochemically fluorinated La₂CuO₄F_x phase retained the orthorhombic distortion of La₂CuO₄, and Rietveld refinements were successfully done using the Bmab space group. The fluoride ions appeared to enter the interstitial rock-salt layer vacancies (as presented in Fig. 1), partially filling both of the layers simultaneously, as has been found also in previous studies on La₂CuO₄ fluorination.^25,32 Figure 2(b) shows XRD and Rietveld refinement results for a cathode charged to a nominal composition of 0.18 F⁻ per La₂CuO₄ as an example: For all the cells charged up to a nominal 0.35 F⁻, only reflections related to the electrolyte and La₂CuO₄F_x were visible. On the other hand, cells charged to a nominal 0.37 F⁻ or higher showed the appearance of a new phase with a broad reflection at 30°–31° [see Fig. 2(c) for a cell charged to a nominal 0.56 F⁻ as an example]. This phase could not be reliably identified, but the reflection could correspond to LaOF, which would evidence partial decomposition of La₂CuO₄. The intensity of this broad reflection did not notably change between cells charged from 0.37 to 0.56 nominal F⁻, but the refined phase fraction of La₂CuO₄F_x in the cathode composite decreased from ∼30% down to ∼20%, indicating a partial phase decomposition and/or a formation of other fluorinated and possibly amorphous phases. Finally, Fig. 2(d) shows XRD and Rietveld refinement results for a cell charged to a nominal 1.88 F⁻ (i.e., the first potential step after the long plateau, Fig. 4). While the electrolyte peaks appear intact, no La₂CuO₄F_x reflections can be seen. Instead, new peaks appeared between 29° and 32° and at 41°, which could not be identified. They did not match with the tetragonal or monoclinic structures of chemically fluorinated samples reported by Abakumov et al.³² These results show that the La₂CuO₄F_x phase is stable up to the nominal 0.35 F⁻ without degradation. With higher nominal fluorine contents, other phases begin to form and between 0.56 and 1.88 nominal F⁻ per La₂CuO₄, the phase is completely transformed.

Changes in the unit-cell parameters between the samples charged to different nominal fluorine contents were small but could be precisely determined with Rietveld refinement. Figure 5 shows the refined La₂CuO₄F_x unit-cell parameters for all the charged cells as a function of transferred F⁻ per La₂CuO₄. The a parameter decreased with the charging, while the b and c parameters increased, in good accordance with previous studies on La₂CuO₄ fluorination.^{24,25,28,29,33} As is evident in Fig. 5, the c lattice parameter was especially sensitive to the fluorination due to the intercalated F⁻ ion pushing the perovskite-like layers apart, whereas the a and b parameters changed less, as they are mainly affected by the small changes in copper ionic radius and anion positions. A linear change can be seen in the a and b parameters from 0 to ∼0.25 transferred F⁻ and in the c parameter from 0 to ∼0.15 transferred F⁻, after which the rate of change decreased. However, the unit-cell volume increased up to 0.56 transferred F⁻ (inset, Fig. 5), indicating that fluorine does enter the La₂CuO₄ structure up to the highest fluorine content tested here. There was an increased amount of scatter in the unit-cell parameters above ∼0.20 transferred F⁻, possibly due to the overlap with the carbon fluorination taking place at higher potentials, which could lead to less reliable fluoride intercalation into La₂CuO₄. This overlap with the carbon fluorination could also be the reason why the unit-cell parameters did not change linearly above the ∼0.20 transferred F⁻, if part of the fluoride reacted with the carbon. However, the c lattice parameter of a sample charged to a nominal 0.18 F⁻ per La₂CuO₄ [c = 13.201(1) Å] was found to closely match that of the La₂CuO₄F_x sample with x ≈ 0.18 fluorinated electrochemically in a liquid-electrolyte cell by Delville et al.³⁶ [c = 13.205(1) Å]. Furthermore, the c parameter was similar to that of a chemically fluorinated La₂CuO₄F_x sample with estimated x ≈ 0.18, reported by Tuilier et al.²⁵ (c = 13.194 Å), indicating that at least up to this amount of transferred F⁻, our results are consistent with previous ones. The x ≈ 0.18 composition appears to be close to the maximum fluorine content that previous studies have reached,^25,26,29,36 so comparison between samples with higher fluorine content was not possible.

All of the electrochemically fluorinated La₂CuO₄F_x samples tested here (with 0.05–0.56 nominal F⁻ per La₂CuO₄) showed a diamagnetic behavior below ∼40 K, indicative of superconductivity. This was in good accordance with previous results on fluorinated La₂CuO₄F_x, where T_c values of 35–42 K have been verified by both transport and magnetic measurements^{24–30,33,36} and related to the fluorine content x as well as the amount of hole doping it implies. Figure 6 shows representative magnetic susceptibility curves of a La₂CuO₄F_x sample charged to a nominal 0.19 F⁻. As can be seen from Fig. 6, the sample showed a strong diamagnetic signal below T_c = 38(1) K, a notable change from the as-synthesized La₂CuO₄. The inset of Fig. 6 shows a comparison between this La₂CuO₄F_x sample and a bulk sample of La_1.85Sr_0.15CuO₄: The T_c values of the two samples are nearly identical, but the fluorinated sample shows a weaker diamagnetic response. In fact, the maximum calculated superconducting volume fraction among the electrochemically fluorinated samples was only about 8% at 20 Oe. While this low volume fraction could indicate inhomogeneous fluorination of the samples, it is most probably caused by the small particle size of La₂CuO₄ after the cathode preparation by ball-milling and the fact that the La₂CuO₄ particles are separated by other non-superconducting compounds in the cathode composite: The London penetration depths of high-T_c superconductors tend to be large, and if the particles size is small in comparison and the particles are isolated, the superconducting volume fraction may appear erroneously small.⁴⁶ Thus, the true volume fraction of the La₂CuO₄F_x samples is likely to be much higher than was apparent.

Figure 7 shows T_c and the magnetic susceptibility (at 10 K) values for the La₂CuO₄F_x samples as a function of transferred F⁻ ions per La₂CuO₄. The diamagnetic response was already present at 0.05 transferred F⁻, the smallest amount tested here. T_c increased rapidly with increasing fluorination and reached the maximum value of 39(1) K at ∼0.14 transferred F⁻, resulting in a similar behavior in the low doping region as has been found for high-T_c cuprates in general.⁴⁷ As can be seen in Fig. 7, this degree of fluorination dependent increase in critical temperature is very similar to what is observed for hole-doping in La_2−xSr_xCuO₄ with a comparable hole content, giving further proof that the electrochemical fluorination goes without significant side reactions for low fluoride contents. However, the La₂CuO₄F_x samples do show higher T_c values than La_2−xSr_xCuO₄ at the low doping region. The diamagnetic susceptibility of La₂CuO₄F_x reached its maximum at 0.20–0.25 transferred F⁻, but the highly fluorinated samples showed increased amounts of scatter in the susceptibility values, possibly due to the same problem of overlapping carbon fluorination as was discussed in the case of the unit-cell parameters. The fairly linear dependence of diamagnetic susceptibility for 0 < x < 0.25 gives good support of nearly stoichiometric transfer of fluoride ions into the La₂CuO₄ host structure. Above ∼0.25 transferred F⁻, the diamagnetic response began to decrease, which appeared to be mainly due to the decrease in the La₂CuO₄F_x phase fraction in the cathode composite rather than overdoping of the La₂CuO₄F_x phase itself. This was evidenced by the fact that T_c remained relatively high (36–38 K) even for the highly fluorinated samples, whereas overdoping would be expected to quickly lower T_c.⁴⁷ 

Besides the electrochemical intercalation of fluorine into La₂CuO₄, electrochemical deintercalation of fluorine from La₂CuO₄F_x was also tested. Figure 8 shows as an example galvanostatic charging/discharging curves of a cell charged to a nominal 0.25 F⁻ per La₂CuO₄ and then discharged down to 0 F⁻. The discharging curve showed two clear, but slightly sloping plateaus corresponding to the two plateaus in the charging curve; the sloping plateaus could evidence irreversible changes in the La₂CuO₄ structure caused by the fluoride ion intercalation. A third, small “pseudo-plateau” can also be seen in the beginning of the discharging curve, which could have been caused by side reactions, such as carbon defluorination.

To study the defluorination of La₂CuO₄F_x, five cells were charged up to 0.25 nominal F⁻ per La₂CuO₄ (where the diamagnetic response was at the maximum) and then discharged for different times. Figures 9(c) and 9(d) show the changes in unit-cell parameters for the discharged cells as a function of the nominal amount of F⁻ per La₂CuO₄, compared to the charged cells [Figs. 9(a) and 9(b), same data as in Fig. 5]. The unit-cell parameters appeared to change fast at the beginning of discharging and then slow down. Cells discharged completely to a nominal F⁻ content of x = 0.00 had unit-cell parameters very close to those of the original, non-charged La₂CuO₄, indicating a relatively ideal behavior for the first charging/discharging cycle, as was also demonstrated by the cyclic voltammetry measurements. The La₂CuO₄ weight fraction of the discharged cathodes, as determined from Rietveld analysis (Fig. 10), decreased from ∼30% to ∼27% compared to the charged or uncharged state, indicating that very little amorphization or decomposition had taken place. The two most discharged cells with nominal contents of 0.00 and 0.05 F⁻ per La₂CuO₄ did not show signs of diamagnetism, well confirmed by the fact that the overall magnetic moment in the composite was so low that the sample could not even be centered appropriately in the SQUID magnetometer. Thus, this study clearly demonstrates that the superdiamagnetic properties of La₂CuO₄F_x can be adjusted by intercalating and fully switched off by deintercalating fluoride ions to/from the structure. By this, we demonstrate that fluoride ion batteries offer the unique capability to access novel electrochemically tunable properties, such as superdiamagnetism, which have not been achieved using any other battery technology so far. We acknowledge that further charging and/or discharging steps were found to be less ideal, as the plateaus quickly disappeared, consistent with the cyclic voltammetry results. Therefore, further work is needed to optimize mechanical cell stability and material degradation beyond the first charge/discharge cycle and to minimize the extent of side reactions caused by carbon fluorination.⁴⁴ 

La₂CuO₄ was electrochemically modified in an all-solid-state cell; the degree of electrochemically oxidative change, most plausibly an electrochemical oxidative fluorination, was found to be controllable, and the results were reproducible between separate cells. This method offers a precise adjustment of hole doping in La₂CuO₄ and thus tuning of superdiamagnetic properties with x up to at least ∼0.20, above which side reactions due to the fluorination of the carbon additive used in the cathode composite began to interfere with the process. Furthermore, the fluorination process could be reversed, as fluorine could be deintercalated from La₂CuO₄F_x, returning the samples to a non-superdiamagnetic state. However, the fluorination/defluorination worked only for one charge–discharge cycle, after which the performance degraded.

Interestingly, by using an all-solid-state setup, we were able to reach higher apparent fluorine contents than previous studies on La₂CuO₄ fluorination, evidenced by the significantly higher change of lattice parameters achieved, although the exact fluorine content of the highly fluorinated samples could not be verified due to the overlap with the carbon fluorination. Eliminating or replacing the carbon additive could allow for a better examination of the La₂CuO₄F_x structure and associated fluoride content and any possible new phases forming at high fluorine contents, if electrical conductivity of the cathode can be maintained at a sufficiently high level. Further work is also needed in order to understand why the fluorination/defluorination process is not reversible beyond the first charge–discharge cycle, although similar materials have provided excellent cycling stability for similar fluoride contents.⁴⁴ Limiting the potential range of charging/discharging may alleviate this problem, and a better understanding of the degradation may be gained by studying the La₂CuO₄F_x crystal structure in detail, and in situ, using high-resolution x-ray diffraction, neutron diffraction, or local structural analysis methods such as extended x-ray absorption fine structure measurements. In addition, we would like to emphasize that this route might serve as a mild synthesis route for anion-doped T′-type La₂CuO₄, which we aim to study in the future.

This paper is dedicated to Professor Dr. Horst P. Beck on the occasion of his 80th birthday.

This work was funded within the Emmy Noether Programme (Grant No. CL551/2-1) by the German Research Foundation (DFG).

The authors declare no conflict of interest.

S.V. performed the experimental studies and wrote the manuscript. L.A. contributed to the magnetic measurements and contributed to writing the manuscript. O.C. designed the study and contributed to writing the manuscript.

The data that support the findings of this study are available from the corresponding author upon reasonable request.