# A Fe-thiolate layered metal organic framework as a high-performance electrode material for potassium-ion batteries

Long Nguyen, Pablo Salcedo-Abraira, Dat Le Thanh, Nusik Gedikoglu, Nathalie Guillou, Robin Coatleven, Philippe Poizot, Fabrice Salles, Nicolas Louvain, Lorenzo Stievano, et al.

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## To cite this version:

Long Nguyen, Pablo Salcedo-Abraira, Dat Le Thanh, Nusik Gedikoglu, Nathalie Guillou, et al. A Fe-thiolate layered metal organic framework as a high-performance electrode material for potassium-ion batteries. Chemical Communications, 2025, 61 (53), pp.9614-9617. 10.1039/D5CC02470F. hal-05136080

HAL Id: hal-05136080

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Submitted on 23 Jul 2025

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# A Fe-thiolate layered metal organic framework as a high-performance electrode material for potassium-ion batteries

Long H.B. Nguyen,<sup>ad</sup> Pablo Salcedo-Abraira,<sup>b†</sup> Dat Le Thanh,<sup>ae</sup> Nusik Gedikoglu,<sup>b</sup> Nathalie Guillou,<sup>c</sup> Robin Coatleven,<sup>b</sup> Philippe Poizot,<sup>b</sup> Fabrice Salles,<sup>a</sup> Nicolas Louvain,<sup>ad</sup> Lorenzo Stievano<sup>ad*</sup> and Thomas Devic<sup>b*</sup>

## Abstract

Here, we report a rare example of an Fe-based MOF capable of accommodating K⁺ ions electrochemically within its layered framework, achieving low polarization, high capacity utilization (~0.8 electrons exchanged per Fe), and stable cycling performance. In situ Mössbauer spectroscopy and operando XRD analyses revealed that the observed electrochemical storage arises from the Fe³⁺/Fe²⁺ redox couple with polyphasic transformations.

## Introduction

Metal Organic Frameworks (MOFs) have been considered as potential electrode materials for metal-ion batteries despite their generally poor electronic conductivity and low densities, thanks to their compositional variety and large diffusion channels, favouring both redox activity and ion transport. Most studies have focused on MOFs application in lithium- and sodium-ion batteries (LIBs and SIBs), but a rising interest exists also for potassium ion-batteries (KIBs).<sup>1,2</sup> Similar to Na, K-containing resources are highly abundant and evenly distributed on Earth's crust. Furthermore, the small Stokes radius of K⁺ and its high ionic conductivity in organic electrolytes represent a beneficial asset for application in electrochemical energy storage systems. Nevertheless, K⁺ diffusion in solids is generally limited due to its large ionic radius, which requires the host structure to possess sufficiently large diffusion paths. Several carboxylate<sup>3-5</sup> and π-d conjugated<sup>6-9</sup> MOFs were proposed as negative (anode) materials. On the cathode (positive) side, excluding Prussian blue analogues, only one example of carboxylate MOF<sup>10</sup> and very few π-d conjugated MOFs<sup>11-14</sup> have been reported, to the best of our knowledge. A large potential hysteresis between charge and discharge is usually observed in these materials, indicating a sluggish K⁺ diffusion. Consequently, the search for new active MOF materials remains an important area of research for electrode material development.<sup>15</sup> A series of layered MOFs consisting of the mixed thiolate-carboxylate ligand DSBDC<sup>4-</sup> (H<sub>4</sub>DSBDC = disulfydrylbenzene-1,4-dicarboxylic acid) and various transition metals cations has recently been identified by multiple groups.<sup>16-19</sup> The cation-sulphur bonds produces a strong ligand-to-metal

<footnote><sup>a</sup> ICGM, Univ. Montpellier, CNRS, ENSCM, Montpellier, France. E-mail: lorenzo.stievano@umontpellier.fr</footnote>

<footnote><sup>b</sup> Nantes Université, CNRS, Institut des Matériaux de Nantes Jean Rouxel, IMN, F-44000 Nantes, France. E-mail: thomas.devic@cnrs-imn.fr</footnote>

<footnote><sup>c</sup> Institut Lavoisier de Versailles, UMR CNRS 8180, Université de Versailles St-Quentin en Yvelines, Université Paris Saclay, 78035 Versailles, France.</footnote>

<footnote><sup>d</sup> RS2E, CNRS, 81000 Amiens, France.</footnote>

<footnote><sup>e</sup> Institut Laue-Langevin (ILL), 38042, Grenoble, France</footnote>

<footnote><sup>†</sup> These authors contributed equally.</footnote>
charge transfer (LMCT) and improved electronic transport, making these solids of interest as photocatalysts for the hydrogen evolution reaction¹⁷ as well as precursors of active materials for supercapacitors.¹⁹ Moreover, Na{Fe(DSBDC)}, consisting of anionic {Fe(DSBDC)} layers separated by Na⁺ ions, is electroactive toward various alkali ions (Li⁺, Na⁺, K⁺).¹⁸ Here, a potassium-containing thiolate-carboxylate MOF, K{Fe(DSBDC)}, acting as host structure for Li⁺, Na⁺, and K⁺ in electrochemical reactions is reported. Regardless of the cycle rate, the structure is highly selective towards K⁺ with quite low polarization on the electrochemical profile during the K⁺ (de)insertion process. *In situ* Mössbauer analyses show that the electrochemical activity of K{Fe(DSBDC)} is due to the Fe³⁺/Fe²⁺ redox couple, and depending on the size of the inserted alkali ion, the structure undergoes a series of biphasic or solid solution reactions as evidenced by operando powder X-ray diffraction (PXRD).

## Results and Discussion

K{Fe(DSBDC)}·3H₂O is obtained under solvothermal conditions from FeCl₃·6H₂O, H₄DSBDC and KOH in a mixture of water and N,N-dimethylformamide (DMF).¹⁸ The Rietveld refinement of the PXRD pattern (Figure S1) shows that it is structurally similar to the Na phase. It consists of anionic layers made of chains of edge-sharing FeS₄O₂ octahedra connected by the organic ligands (Figure 1a). These layers are separated by K⁺ ions that interact both with dangling oxygen atoms from the carboxylate groups and water molecules (Figure 1b). This defines interlayer channels containing additional water molecules. The dehydration occurs in two steps at 90° and 130°C, as evidenced by temperature dependent PXRD, and is fully reversible upon air exposure (Figure S2). In the structure of fully dehydrated K{Fe(DSBDC)}, the anionic {Fe(DSBDC)} layers remain almost unchanged, but are slipped to accommodate the departure of water molecules bound to K⁺ ions (Figure 1c and Figure S4). In the hydrated state, the K⁺ ions are 6-fold connected to oxygen atoms in a distorted octahedral environment, while they are only surrounded by 4 oxygen atoms in a square configuration in the dehydrated state. Such an unusual environment is stabilized by secondary, weaker interactions with both O and S atoms (K···O = 3.10(4) and 3.16(5) Å, K···S = 3.36(2) Å, cf. Figure S4 and Table S2). Nevertheless, channels potentially favouring ion intercalation remain also in the anhydrous phase (Figure 1b). The room temperature Mössbauer spectrum of the hydrated form (Figure S5) shows the presence of two different high spin (HS) Fe³⁺ environments. The major component (90 %) is attributed to HS Fe³⁺ in a FeS₄O₂ environment¹⁸,²⁰,²¹, in line with the structure of K{Fe(DSBDC)}. The minor component also corresponds to HS Fe³⁺, possibly only surrounded by oxygen neighbours (FeO₆). Such an environment might correspond to Fe³⁺ defects residing in interlayer channels, as found in the Na analogue¹⁸ and in line with related materials based on divalent cations (Ni, Co).¹⁶,¹⁷ The spectrum evolves very slightly upon dehydration (Figure 3), but the hyperfine parameters remain consistent with the exclusive presence of HS Fe³⁺ (Table S3).
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Crystal structures of K{Fe(DSBDC)}·nH₂O. a) Anionic layer; b) hydrated and c) dehydrated forms. One channel is drawn taking into account the van der Waals radii of the atoms to highlight the residual porosity.</figcaption></figure>

The electrochemical activity of dehydrated K{Fe(DSBDC)} was evaluated by galvanostatic cycling in K half-cell in the potential range of 3.5–2.0 V vs. K⁺/K. Since K{Fe(DSBDC)} contains only Fe³⁺, thus offering negligible capacity via direct oxidation, a discharge step was first applied, accompanied by the insertion ~0.8 K⁺. Two main electrochemical steps located in the potential ranges 3.1–2.8 V and 2.2–2.4 V vs. K⁺/K, respectively, are observed during the first discharge (Figure 2a). These processes are fully reversible with a moderate polarization as shown during the subsequent recharge process. The different electrochemical steps are more clearly evidenced by plotting the differential capacity vs. potential curves (Figure 2b). Basically, the overall process occurs through two series of two constitutive electrochemical steps. Since a pair of broad peaks are observed in the 3.1–2.8 V potential region, a solid solution-type electrochemical insertion process is expected. On the contrary, a first-order phase transition should occur in the 2.2–2.4 V vs. K⁺/K range, based on the observed narrow peaks. A capacity retention of 85% after 150 cycles, without no noticeable change in the electrochemical signature (Figure 2c), indicates that the electrochemical mechanism does not change during long-term cycling. About 0.9 K⁺ per formula unit can be reversibly stored, corresponding to a specific capacity value of 79 mAh g⁻¹, close to the theoretical one (83 mAh g⁻¹ for the exchange of one electron per formula unit). Upon increasing the cycle rate, K{Fe(DSBDC)} exhibits remarkable stability with both negligible capacity loss and low polarisation (Figure 2c and Figure S6), indicating that K⁺ diffusion within the framework remains highly efficient even at fast cycling rates. It is worth noting the initial capacity is fully recovered on returning to C/20, confirming that the structural integrity of the material is preserved at high cycle rates.
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Electrochemical behaviour measured in K half-cell within 2.0–3.5 V vs. K+/K potential range using K{Fe(DSBDC)} as the active electrode material and 5 M KTFSI in DME as the electrolyte. a) Potential-composition plot of the first discharge (black), the subsequent charge-discharge (red) compared to the 150th cycle (blue) measured at C/10; Note that the green circle indicates the starting open circuit potential. Corresponding differential capacity vs. potential curves in b), and capacity retention with coulombic efficiency in c). d) Specific capacity vs. cycle number for current rate changes ranging from C/20 to C/2 then C/20 again.</figcaption></figure>

Such a behaviour is unusual among MOFs used in KIBs. Notably, the polarisation of K{Fe(DSBDC)} during electrochemical cycling is remarkably lower than that reported for its Na analogue, Na{Fe(DSBDC)}, which is of 500, 200–300 and 100–250 mV when cycled vs. Li, Na and K, respectively,¹⁸ as well as those of other MOFs.¹⁰⁻¹⁴ Furthermore, the electrochemical profile remains unchanged over prolonged cycling, indicating both structural robustness and stability. Lastly, the presence of four reversible electrochemical steps indicates multiple redox processes or phase transitions during cycling, further underlining the rich and complex electrochemical behaviour of this material. The electrochemical activity of K{Fe(DSBDC)} was also evaluated in half cell vs. Li and Na (Figure S7). Compared to K⁺ insertion, only ~0.6 Li⁺ and ~0.4 Na⁺ can be inserted in K{Fe(DSBDC)} during galvanostatic cycling, with lower reversible capacity and a sensibly larger polarization. This could reflect a less efficient diffusion of smaller alkali metal ions in K{Fe(DSBDC)}.

<figure><img src="image_5.png" /></figure>
**Figure 3.** *In situ* Mössbauer experiment carried out on a $K\{Fe(DSBDC)\}$ half-cell cycled at C/50 current rate from OCV down to 1.8 V vs. $K^+/K$ (electrolyte 5 M KTFSI in DME). The potential was held constant during each acquisition. a) electrochemical profile of the *in situ* cell; $ ^{57}Fe$ Mössbauer spectrum and the corresponding fit recorded on b) pristine $K\{Fe(DSBDC)\}$, c) mid of discharge (2.4 V), and d) end of discharge (1.8 V).

*In situ* Mössbauer spectroscopy experiments were conducted to follow the redox activity of the Fe centres during the electrochemical reaction. As stated above, the spectrum of dehydrated $K\{Fe(DSBDC)\}$ can be deconvoluted into two characteristics HS $Fe^{3+}$ environments, i.e. $FeS_4O_2$ and $FeO_6$ (Figure 3b). At the middle state of charge, ($x \sim 1.4$), two additional Fe doublets with hyperfine parameters in the typical range of HS $Fe^{2+}$ (Figure 3c and Table S3) are observed, corresponding to HS $Fe^{2+}S_4O_2$ and HS $Fe^{2+}O_6$ formed via the reduction of pristine HS $Fe^{3+}S_4O_2$ and HS $Fe^{3+}O_6$. At the end of the discharge, all HS $Fe^{3+}$ is converted to HS $Fe^{2+}$, and only the contributions of HS-$Fe^{2+}S_4O_2$ and HS-$Fe^{2+}O_6$ are visible (Figure 3d), indicating that all Fe centres contribute to the redox activity of $K\{Fe(DSBDC)\}$. Note that the presence of two $Fe^{2+}$ and two $Fe^{3+}$ sites aligns well with the splitting of the two main redox events observed on the differential capacity vs. potential curve (Figure 2b). Similar results are reported for pure carboxylate $Fe^{3+}$-based MOFs evaluated vs $Li^{22-26}$, even though with remarkably low reversibility and/or large hysteresis upon cycling. The outstanding performance of $K\{Fe(DSBDC)\}$, could depend also on the S atoms bridging the Fe centres, favouring electron transport,²⁷ as previously observed in π-d conjugated MOFs. *Operando* PXRD experiments were carried out to shed light on the structural evolution of $K\{Fe(DSBDC)\}$ during electrochemical reaction vs. K and Li. This approach, commonly used for inorganic electrode materials, is scarcely used in the case of MOFs,⁶,⁷,²²,²⁸⁻³⁰ due to their complex crystal structure and low diffracting ability, making quantitative analysis challenging. The low symmetry (triclinic) of dehydrated $K\{Fe(DSBDC)\}$, combined with the relatively low resolution and signal-to-noise ratio of the *operando* experiments are additional difficulties. Nevertheless, at OCV (3.08 V vs. $K^+/K$) the recorded PXRD pattern is similar to that of dehydrated $K\{Fe(DSBDC)\}$ (Figure S8), indicating that no solvent molecule (1,2-dimethoxyethane/DME) is chemically intercalated in the interlayer channels of $K\{Fe(DSBDC)\}$. Additional *ex situ* PXRD experiments confirm this observation, showing that $K\{Fe(DSBDC)\}$ remains unchanged in the presence of DME, while it reforms the $K\{Fe(DSBDC)\} \cdot 3H_2O$ hydrate in the presence of water (Figure S9). At open circuit potential (OCP), four main regions are identified on the XRD pattern (Figure 4): (1) the two most intense peaks at 4.52° and 4.73°, attributed to the 01\bar{1} and 001 reflections, respectively, (2) a broad peak centred at 11.92° associated to 1\bar{1}0, 1\bar{1}1 and 11\bar{1} Bragg peaks, (3) another one at 14.40° encompassing at least 6 reflections and (4) multiple peaks between 16 and 18.5°. On discharging to ~2.4 V vs $K^+/K$ (corresponding to the insertion of ~0.35 $K^+$), a continuous peak shift towards lower angles is observed in all regions, even though to different extents. Upon further reduction from $x \sim 1.35$ to ~1.8 (2.4 to 2.0 V vs. $K^+/K$), in regions (2), (3) and (4), the initial peaks are replaced by new peaks, mainly at lower angles. These data indicate that $K^+$ insertion in $K\{Fe(DSBDC)\}$ occurs through two main steps as previously anticipated based on the electrochemical features, including a first solid solution process from $K\{Fe(DSBDC)\}$ to $K_{1.35}Fe(DSBDC)$ followed by a biphasic one from $K_{1.35}\{Fe(DSBDC)\}$ to $K_{1.8}\{Fe(DSBDC)\}$, corresponding to the low-potential plateau. The shift of the Bragg peaks towards lower angles in the first process indicates an increase in cell volume during $K^+$ insertion in the interlayer channels. On the other hand, the structure of $K_{1.8}\{Fe(DSBDC)\}$ formed at the end of the biphasic step cannot be determined due to the relatively low resolution and signal-to-noise ratio of the *operando* XRD data. Nonetheless, a further increase is unit cell is expected for accommodating more $K^+$ ions in the structure. The main two-step reaction is fully reversed during charge (Figure 4), in line with the electrochemical data. The same experiment carried out at a higher current rate (C/2) shows a similar mechanism (Figure S10), indicating that such a structural transformation is not kinetically limited, at least in the range of investigated current rates. In contrast, a similar experiment vs. Li (C/5 rate, electrolyte LP30) shows a
continuous shift of all Bragg peak positions (Figure S11), in line with a solid solution mechanism and consistent with the shape of the electrochemical profile (Figure S7).

<figure><img src="image_6.png" /><figcaption>**Figure 4.** Operando XRD data recorded during the 1st cycle of a K5M KTFSI/DME [{Fe(DSBDC)} cell galvanostically cycled at C/20 in the potential range of 2–3.5 V; the corresponding electrochemical profile of the operando cell is also shown.</figcaption></figure>

**Conclusion**

Summarising, a novel Fe-based thiolate-carboxylate layered material, K{Fe(DSBDC)}, synthesized by a solvothermal route, shows reversible K⁺ de insertion) with low hysteresis, structural stability and very good cyclability in half cell vs. K metal. Such performance is extremely rare among MOF-based electrodes, not only for KIBs, but also in LIB and SIB systems. These remarkable properties likely stem from a synergy between efficient electron transport, facilitated by sulphur bridges, and fast K⁺ diffusion through interlayer channels of appropriate size. In contrast, Li⁺ and Na⁺ insertion is less favourable, likely due to their smaller ionic radii. While further optimisation is needed to enhance the redox potential and capacity in order to reach state-of-the-art existing organic or inorganic electrode materials,³¹ this work highlights the strong potential of thiolate-carboxylate MOFs as a versatile platform for designing next-generation electrode materials for rechargeable metal-ion batteries.

**Author contributions**

LHBN, PSA, DLT, NGe, RC, NGu, FS: investigation, formal analysis; PP: formal analysis, NL, LS, TD: conceptualization, funding acquisition, supervision, writing.

**Data Availability Statement**

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

**Conflicts of interest**

There are no conflicts to declare.
## Acknowledgments

This work was supported by the Agence Nationale de la Recherche (ThioMOFs, ANR-19-CE08-0029-01, -03 and -04; Labex STORE-EX, ANR-10-LABX-76-0). Authors acknowledge the synchrotrons Soleil and Elettra for providing access to the CRISTAL and MCX beamlines, respectively. We thank Erik Elkaïm (CRISTAL) for the PXRD data collection, Laura Gigli (MCX) for her help in the XRD *operando* experiments, Stéphane Grolleau (IMN) for the thermogravimetric analyses, and Moulay T. Sougrati (ICGM) for the Mössbauer spectroscopy measurements.

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