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[Py ]FSI-NaFSI Based Ionic Liquid Electrolyte for Sodium Batteries: Na Solvation and Interfacial Nanostructure on Au(111) +

Timo Carstens, Abhishek Lahiri, Natalia Borisenko, and Frank Endres J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04729 • Publication Date (Web): 22 Jun 2016 Downloaded from http://pubs.acs.org on June 26, 2016

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The Journal of Physical Chemistry

[Py1,4]FSI-NaFSI Based Ionic Liquid Electrolyte for Sodium Batteries: Na+ Solvation and Interfacial Nanostructure on Au(111) Timo Carstens,1 Abhishek Lahiri,1* Natalia Borisenko,1* and Frank Endres1*

1

Institute

of

Electrochemistry,

Clausthal

University

of

Technology,

Arnold-Sommerfeld-Str. 6, 38678 Clausthal-Zellerfeld, Germany

1. ABSTRACT In this paper, the NaFSI-[Py1,4]FSI/Au(111) interface was investigated using cyclic voltammetry (CV) and in situ atomic force microscopy (AFM). Raman spectroscopy was used to evaluate the Na+ solvation in [Py1,4]FSI. It was found that Na coordinates with three FSIforming [Na(FSI)3]2-. In situ AFM revealed that the interaction of [Py1,4]FSI with Au(111) is much stronger compared to other ionic liquids measured using the same technique. On applying a potential, a force of about 50 nN is required to penetrate through the innermost layer. On addition of low concentration of NaFSI (0.05 M), an insignificant change in the innermost solvation layer was observed, whereas on addition of 0.25 M and 0.5 M NaFSI, a significant change in the interfacial structure was noted. The present study clearly shows that Na+ ions vary the ionic liquid/Au(111) interface and could provide insight into the interfacial processes in ionic liquid based sodium batteries.

2. INTRODUCTION For portable energy storage, lithium battery technology has become the most successful commercially available device. However, for large-scale storage technology such as grid 1 ACS Paragon Plus Environment


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storage, these portable devices will not be sustainable, mainly due to the cost and lack of available lithium. Also, the organic based electrolytes which are used in Li batteries are inflammable. Therefore, a lot of research has been done to find an alternative energy storage device with low cost, high capacity and reliability.1-3 Recently, there has been a lot of interest in sodium-ion battery technology, primarily due to the low cost and abundant availability of sodium.4, 5 Na-S and Na-NiCl2 based batteries are already applied in practice for grid storage application.6 However, the working temperature for these batteries is in the range of 523-573 K due to the presence of β-alumina electrolyte. Therefore, there is a need to develop more suitable electrolytes which can be used at or near room temperature for sodium based batteries. Ionic liquids (ILs) are considered as potential electrolytes for sodium-ion batteries7 due to their extraordinary physicochemical properties such as low vapor pressure, wide electrochemical window and nonflammability. They have been studied and used as safe electrolytes for electrochemical applications. Few studies using ionic liquid based electrolytes with sodium salts have been shown.8-11 Ding et al.8 studied the physicochemical properties of sodium

bis(fluorosulfonyl)imide

(NaFSI) – N-methyl-N-propylpyrrolidinium

bis(fluorosulfonyl)imide ([Py1,3]FSI) electrolyte for Na/NaCrO2 cell and a stable discharge capacity of 92 and 106 mAh g-1 at 298 and 353 K, respectively, was shown. Chagas et al.9 reported

a

higher

discharge

capacity

of

200 mAh g-1

for

100

cycles

Na0.45Ni0.22Co0.11Mn0.66O2 in a sodium bis(trifluoromethylsulfonyl)imide (NaTFSI)

for –

N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide ([Py1,4]FSI). The physicochemical properties of NaTFSI-[Py1,3]FSI mixture was characterized by Yoon et al.10 The ionic conductivity was found to decrease with an increase in salt concentration due to the increase in viscosity. From

23

Na NMR, the authors reported that Na+ strongly coordinates with the

TFSI anion. Raman spectroscopy revealed the formation of [Na(TFSI)3]2- complex in the case 2 ACS Paragon Plus Environment

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of NaTFSI – 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm]TFSI) mixture.11 As the solid-electrolyte interface governs the electrochemical phenomena, it is important to understand the interface. The solid-liquid interface in ILs has been explored experimentally using various techniques such as in situ scanning tunneling microscopy (STM), in situ atomic force microscopy (AFM), X-ray reflectivity and surface force apparatus (SFA).12-19 The experimental data shows that the ionic liquid at the solid-liquid interface forms an orderly multilayered structure. Recently, the solid-liquid interface has been explored in the presence of lithium salts.20-25 In situ STM revealed that an underpotential deposition (UPD) of Li occurs at ~ 1 V positive to lithium bulk deposition on Au(111) in 0.5 M LiTFSI-[Py1,4]TFSI.20 In situ AFM showed that the addition of 0.05 wt% LiCl in 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([HMIm]FAP) influences significantly the interfacial structure: an attractive force is obtained in the presence of LiCl, while a repulsive force is measured for the pure ionic liquid.21 Furthermore, in situ STM revealed that by reducing the electrode potential of Au(111), it undergoes (22 × √3) reconstruction in [Py1,4]FAP (1-butyl1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate) containing 0.1 M LiCl and between -0.8 V and -1.4 V vs. Pt quasi ref. the herringbone superstructure is probed. At more negative electrode potentials the dissolution of gold surface occurs.22 Hu et al.23 presented the in situ

STM

of

LiTFSI

in

[Py1,3]FSI

and

in

N-methyl-N-propylpiperidinium

bis(trifluoromethylsulfonyl)imide on highly oriented pyrrolitic graphite (HOPG) and Au(111). It was found that in pure ionic liquids, the cations intercalate the HOPG resulting in exfoliation of HOPG layers. However, in the case of FSI anion and in the presence of Li salt, the decomposed product formed on HOPG surface protected the exfoliation process. We recently showed using Raman spectroscopy and in situ AFM the influence of Li ion concentration in [Py1,4]TFSI on the Au(111)-IL interface.24 It was found that the Li+ solvation 3 ACS Paragon Plus Environment


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changes with concentration thus altering the Au(111)-electrolyte interface. AFM reveals that on addition of Li salt the [Py1,4]+ cation is replaced by Li+ ion at the interface. Furthermore, with increasing the Li ion concentration a significant decrease in the number of interfacial layers is observed.24 Elbourne et al.25 presented the molecular layer structure of [EMIm]TFSI in presence and absence of a Li salt using amplitude modulated atomic force microscopy (AM-AFM). An ordered molecular structure in the form of anion-cation-cation-anion was observed in the pure [EMIm]TFSI. On addition of Li+ ion, the cations and anions of [EMIm]TFSI were shown to be displaced.25 In this paper, the interfacial structure of NaFSI-[Py1,4]FSI on Au(111) was probed using cyclic voltammetry and in situ AFM. The Na ion concentration in the ionic liquid was 0.05 M, 0.25 M and 0.5 M. Raman spectroscopy was used to investigate the solvation structure of Na+ ion in [Py1,4]FSI. It is shown that the [Py1,4]FSI/Au(111) interfacial structure is strong and only the addition of high concentrations (0.25 M, 0.5 M) of NaFSI significantly changes the IL/Au(111) interfacial structure.

3. EXPERIMENTAL SECTION [Py1,4]FSI was purchased in the highest available quality from Solvionic and was used after drying under vacuum at 100 oC to achieve a water content of below 10 ppm. The water content was measured using Karl-Fisher titration. NaFSI (99.95%) was purchased from Solvionic. Raman spectra were recorded with a Raman module (Ram 2) Bruker Vertex 70 V (Nd: YAG 1064 nm) with a Ge detector. For Raman analysis, the electrolyte was sealed in a glass capillary inside the glove box and the spectra were obtained at an average of 250 scans with a resolution of 2 cm-1.

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For electrochemical measurements, the working electrode in the experiment was Au(111) purchased from Agilent Technologies. Pt ring and Pt wire were used as a counter and a quasi-reference electrode, respectively, which gave good stability in the ionic liquid throughout the experiments. The electrochemical cell was made of Teflon and clamped over a Teflon-covered Viton O-ring onto the substrate, yielding a geometric surface area of 0.3 cm2. Prior to the experiments, the Teflon cell and the O-ring were cleaned in a mixture of 50 : 50 vol% of concentrated H2SO4 and H2O2 (35 %) followed by refluxing in distilled water. The electrochemical measurements were performed inside of an argon-filled glove box with water and oxygen contents below 2 ppm (OMNI-LAB from Vacuum Atmospheres) by using a VersaStat II (Princeton Applied Research) potentiostat/galvanostat controlled by powerCV and power-step software. The scan rate during cyclic voltammetry was 5 mV sec-1. Force curves were collected using a Molecular Imaging Pico Plus AFM in contact mode. In the electrochemical AFM setup, Au (111) was used as working electrode and Pt wires were used as reference and counter electrodes. A silicon SPM-sensor from NanoWorld () was employed for all experiments presented in this study. The spring constant was 6.0±0.5 N/m. All force curves were acquired at room temperature in an argon-filled glove box. The temperature of the experiment was 23°C.

4. RESULTS AND DISCUSSION The Raman spectra of NaFSI-[Py1,4]FSI in the range of 200-600 cm-1 and 2700-3200 cm-1 with varying Na concentrations is shown in Figure 1a and 1b, respectively. In the range between 200 and 600 cm-1, a slight decrease in the peak intensities is noted with NaFSI addition which is related to the formation of conformers. A similar observation has been reported for LiFSI-[EMIm]FSI wherein a prominent decrease in intensity was found on increasing the Li ion concentration.26 Like the TFSI anion, the FSI- anion also shows cis and 5 ACS Paragon Plus Environment


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trans conformers.27 Fujii et al.26 showed that for LiFSI-[EMIm]FSI, Li ion preferentially binds to the cis isomer configuration. However, from Figure 1a, in case of Na ion, it is difficult to predict the preferential binding as only small changes occur in the peak intensities.

Figure 1. Comparison of Raman spectra of NaFSI-[Py1,4]FSI with varying Na ion

concentration in the range (a) between 200 and 600 cm-1 and (b) between 2700 and 3200 cm-1. The Raman was normalized at 520 cm-1 indicated by a black circle. The peaks in figure 1a were assigned based on ref.28, 29 The Raman spectrum of [Py1,4]+ in the region of 2700 and 3200 cm-1 is shown in Figure 1b. No change in the peak intensity in the νCH2 ring modes and alkyl chains is observed until 0.25 M NaFSI. However, on addition of 0.5 M NaFSI, a slight decrease in the intensity occurs which might be related to the formation of [Py1,4]+ cation aggregates. Figure 2a shows the most significant region of the Raman spectra of the ionic liquid wherein on addition of metal salts, changes can be detected and also quantified. The pure FSI peak occurs at 726 cm-1 and with the addition of NaFSI, a slight decrease in the peak intensity as well as a peak broadening is observed (Figure 2a). On increasing the concentration to above 0.25 M, a shoulder is seen at 743 cm-1 which is ascribed to the Na+ ion bound to FSI. In


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Figure 2b, the FSI peak in [Py1,4]FSI can be deconvoluted to three peaks. Peaks I and II are the two conformers of the FSI- anion and peak III may be ascribed to the cation.30

Figure 2 (a) Comparison of Raman spectra of [Py1,4]FSI and NaFSI-[Py1,4]FSI with varying concentrations of NaFSI in the region between 700 and 800 cm-1. (b) The experimental spectrum of [Py1,4]FSI and the Voigt fit components. To identify the Na+ ion solvation number in the FSI ionic liquid, the solvation number was evaluated from the Raman spectra in the region of 680-780 cm-1 using the method described by Fujii et al.26 The Raman bands were deconvoluted to four peaks as shown in the Figure 3a and Figure 3b. Peaks I and II (Figure 3a and Figure 3b) represent the cis and trans conformers of the FSI- anion as seen in the case of the pure IL in Figure 2b. Peak III in figures 3a and 3b is related to the presence of the [Py1,4] cation and peak IV represents the FSI- bound to the Na+ ion.28 The plot of If/CNa versus CT/CNa gave a straight line as seen in Figure 4 wherein If is the integral intensity of the free FSI- anion, CNa is the concentration of Na ions and CT is the total concentration of FSI- anion.



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Figure 3. (a) Raman fit for 0.25 M NaFSI+[Py1,4]FSI and (b) for 0.5 M NaFSI+[Py1,4]FSI

Figure 4. Plot of If/CNa versus CT/CNa at 298 K.

The Na+ solvation number was found to be 2.7 from which it can be suggested that primarily three FSI- anions are bound to the Na+ ion leading to the formation of [Na(FSI)3]2-. In the case of the TFSI anion, the Na solvation number was also found to be 3.11 However, density functional theory (DFT) calculations have shown that the formation of both [Na(TFSI)2]- and


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[Na(TFSI)3]2- are energetically favorable. As in our case the solvation number is 2.7, existence of both [Na(FSI)3]2- and [Na(FSI)2]- might be possible. The cyclic voltammogram of [Py1,4]FSI with different concentrations of NaFSI on Au (111) is shown in Figure 5. In the pure IL (Figure 5, black curve), no noticeable change in current is observed in the cathodic regime until -3.0 V after which a broad reduction wave at -3.4 V is observed which might be related to the reduction of FSI- anion as a similar phenomenon was observed in case of pure [Py1,4]TFSI.31 A significant rise in negative current beyond -3.7 V is related to the decomposition of [Py1,4]+ cation.

Figure 5. Cyclic voltammetry of [Py1,4]FSI with different concentrations of NaFSI on Au(111).

In the anodic regime only a small oxidation peak is observed at +0.4 V that can be related to the oxidation of the decomposed products. On addition of 0.05 M NaFSI to the IL (Figure 5, red curve), no change in the current is observed in the cathodic regime until -3.3 V after 9 ACS Paragon Plus Environment


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which two reduction processes at -3.55 V and -4.0 V occur. It has been previously observed that on addition of metal salts into ionic liquids, the decomposition potential of the cation shifts to more negative potentials.32 Therefore the peaks at -3.55 V and -4.0 V could be related to the reduction of FSI- ions, bulk deposition of Na with some partial decomposition of [Py1,4]+ cation, respectively. In the anodic regime only a small oxidation peak is seen at +0.25 V and can be associated with the oxidation of the decomposed products. On increasing the concentration of NaFSI to 0.25 M, in the cathodic regime, the peak intensity at -3.55 V decreases compared to 0.05 M NaFSI, whereas a prominent peak is observed at -4.0 V (Figure 5, green curve). In the anodic scan two oxidation peaks are seen at -2.15 V and -1.65 V related to the oxidation of Na-Au alloy and deposited Na, respectively. On further increasing the concentration of NaFSI to 0.5 M, only one reduction peak at -4.0 V is seen due to the deposition of bulk Na along with the decomposition of the [Py1,4]+ cation (Figure 5, blue curve). In the anodic scan a slight increase in the peaks related to the oxidation of Na-Au alloy and Na at -2.15 V and -1.65 V, respectively, is observed. On continuation in the anodic scan above 0 V, another oxidation peak at +0.4 V appears, which can be related to the oxidation of the decomposed ionic liquids as a similar peak was observed in case of the pure IL.

In situ AFM force-distance measurements were performed to investigate the interfacial structure of pure [Py1,4]FSI in the presence of NaFSI in the potential regime between OCP (+0.2 V vs. Pt-quasi ref.) and -1.0 V prior to the deposition of Na. Figure S1 shows the expanded region between OCP and -1.5 V. It is evident from the curves that, besides a slight increase in negative current from -0.25 V, there are no visible changes between the pure IL and on addition of NaFSI. Figure 6 represents the AFM force-separation curves for [Py1,4]FSI on Au(111) at various electrode potentials.


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Figure 6. Force-separation curves of an AFM tip approaching Au(111) in [Py1,4]FSI at (a) OCP, (b) -0.5 V and (c) -1.0 V.

At OCP (Figure 6a) five prominent peaks are detected at separation distances of 0.67 nm, 1.55 nm, 2.35 nm, 3.2 nm and 4.0 nm. Each peak represents the rupturing of a solvation layer of the [Py1,4]FSI by the AFM cantilever. The force required to rupture each layer increases as the tip approaches the Au(111) surface and about 10 nN is needed to push through the innermost layer at 0.67 nm. Here, an assumption is made regarding the innermost layer wherein it is considered that there either no more adsorbed layers are present on Au (111) or further adsorbed layers cannot be ruptured. Beyond 5 nm no significant force signal is recorded. The ion pair diameter of [Py1,4]FSI is estimated to be 0.7 nm considering a cubic symmetry. Since the measurement error is ±0.05 nm, the width of each separation layer corresponds to the presence of the [Py1,4]FSI ion pair. The composition of the interfacial structure does not change by reducing the electrode potential. At -0.5 V (Figure 6b) and -1.0 V (Figure 6c) the width of the separation steps corresponds to the IL ion pair. However, the force required to rupture each layer increases significantly. In general, an increase in force and decrease in separation distance in the innermost layer is observed for ionic liquids and has been related to better ordering of the interfacial structures.12-15, 24 However in the case of [Py1,4]FSI, a five times increase in force is required to rupture the 11 ACS Paragon Plus Environment


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innermost layer. At -1.0 V about 50 nN force is required to rupture the innermost layer at 0.68 nm. To the best of our knowledge such high push-through forces in ILs have not been reported in literature to date.17 In our case the high forces can be related to stronger interaction of [Py1,4]FSI with the Au(111) surface. The profile of the force-separation curves do not change significantly with addition of 0.05 M NaFSI in [Py1,4]FSI (Figure 7). Four (at OCP) and five (at -0.5 V and at -1.0 V) solvation layers corresponding each to the IL ion pair are observed at the IL/Au(111) interface. Similar to the pure IL, a force of about 10 nN is required to rupture the innermost layer at 0.68 nm at OCP (Figure 7a). However, in contrast to the pure IL the push-through forces do not increase significantly in the presence of 0.05 M NaFSI by reducing the electrode potential (Figure 7b). Thus, at -1.0 V a force of only 15 nN is needed to push the innermost layer (Figure 7c). This implies that there is some interaction of Na+ with the IL that weakens the IL-surface attraction. Raman spectra reveal the formation of [Na(FSI)3]2- species in the bulk phase which in turn might also affect the IL/Au(111) interfacial structure.

Figure 7. Force-separation curves of an AFM tip approaching Au(111) in [Py1,4]FSI with 0.05 M NaFSI at (a) OCP, (b) -0.5 V and (c) -1.0 V.


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The interfacial structure changes significantly on increasing the concentration of NaFSI to 0.25 M. The number of solvation layers decreases: only four prominent steps can be detected at the applied electrode potentials (Figure 8). At OCP (Figure 8a), the innermost layer width is 0.37 nm, that can be related to the presence of sodium ion at the interface which forms [Na(FSI)3]2-. A similar observation was made on addition of LiTFSI in [Py1,4]TFSI.24

Figure 8. Force-separation curves of an AFM tip approaching Au(111) in [Py1,4]FSI containing 0.25 M NaFSI at (a) OCP, (b) -0.5 V and (c) -1.0 V. The separation distance of the second solvation layer is > 0.9 nm which indicates the presence of both [Na(FSI)3]2- and [Py1,4]FSI ion pairs. On changing the potential to -0.5 V (Figure 8b), no significant change in the separation or force is observed in the force-distance profile which indicates that a well ordered structure is formed at the interface. This is further exemplified by the CV measurements wherein a little change in the current is observed from OCP to -0.5 V. On further reducing the electrode potential to -1.0 V (Figure 8c), again the innermost structure shows a separation distance of 0.3 nm which within the error limit of AFM measurements corresponds to the formation of a [Na(FSI)3]2- layer at the interface. The AFM measurements suggest that an interfacial structure consisting of [Na(FSI)3]2- in the innermost layer and a mixture of both [Na(FSI)3]2- and [Py1,4]FSI ion pair in the second layer forms at the OCP and remains stable with decreasing the electrode potential at least until -1.0 V.



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Figure 9 shows the force distance profile of 0.5 M NaFSI in [Py1,4]FSI. At OCP (Figure 9a), instead of formation of four solvation layers as shown in Figure 8, only three prominent layers are observed. However, the innermost layer shows a separation distance of 0.35 nm which is consistent with that observed in case of 0.25 M NaFSI indicating the presence of [Na(FSI)3]2-.

Figure 9. Force-separation profiles of an AFM tip approaching the Au(111) in [Py1,4]FSI with 0.5 M NaFSI at (a) OCP, (b) -0.5 V and (c) -1.0 V.

The second layer shows a distance of > 0.9 nm suggesting that both [Na(FSI)3]2- and [Py1,4]FSI are present. On changing the potential to -0.5 V and -1.0 V (Figure 9b and Figure 9c), no significant change in distance in the solvation layers are seen, but an increase in rupture force is observed indicating that the interfacial structure has become stronger. Thus, the in situ AFM force-distance measurements reveal that on changing the concentration of NaFSI from 0.25 M to 0.5 M, a stable interfacial structure is formed which is not influenced by changing the potential from OCP to -1.0 V.

5 CONCLUSIONS


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In this paper the interfacial nanostructure of NaFSI-[Py1,4]FSI has been investigated using in situ AFM. Raman spectroscopy has shown that Na ions coordinate with three FSI ions forming [Na(FSI)3]2- species. Cyclic Voltammetric studies reveal that the bulk deposition of Na occurs at -3.5 V vs. Pt, however there is no change in the electrochemical behavior of Au(111) with addition of NaFSI in [Py1,4]FSI ionic liquid in the potential region between OCP (+0.2 V) and -1.5 V. In situ AFM measurements showed that a multilayered structure is present at the IL/Au(111) interface. Between ocp and -1.0 V, for [Py1,4]FSI, the innermost layer consist of the IL ion pairs and at -1.0 V a significantly high rupture force (∼50 nN) is needed to break the innermost layer, indicating a strong interaction between the IL and the Au(111) surface. The addition of low concentrations (0.05 M) of NaFSI in the IL does not affect the IL/Au(111) interface significantly. However, by addition of 0.25 M and 0.5 M NaFSI, the structure of the innermost layer changes significantly and indicates the presence of [Na(FSI)3]2-. Thus, both in situ AFM and Raman spectroscopic measurements reveal that the addition of high concentrations (0.25 M-0.5 M) of NaFSI in [Py1,4]FSI leads to the formation of a stable interface in the applied potential regime.

SUPPORTING INFORMATION The expanded region of the CVs between OCP and -1.5 V of [Py1,4]FSI with various concentrations of NaFSI.

AUTHOR INFORMATION Corresponding Authors *Phone: +49-5323-72-2885. E-mail: [email protected]. 15 ACS Paragon Plus Environment


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*Phone: +49-5323-72-3734. E-mail: [email protected]. *Phone: +49-5323-72-3141. E-mail: [email protected].

ACKNOWLEDGEMENTS The authors would like to thank Mrs. Karin Bode, Institute of Inorganic Chemistry (Prof. A. Adam) for help with Raman measurements. This research was supported by DFG EN 370/25-A

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