Thin Film Anodes Demonstrate Sodium Intercalation and De

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Electrochemically Formed NaxMn[Mn(CN)6] Thin Film Anodes Demonstrate Sodium Intercalation and De-intercalation at Extremely Negative Electrode Potentials in Aqueous Media Jeongsik Yun, Florian Schiegg, Yunchang Liang, Daniel Scieszka, Batyr Garlyyev, Anika Kwiatkowski, Thomas Wagner, and Aliaksandr S Bandarenka ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00022 • Publication Date (Web): 18 Dec 2017 Downloaded from http://pubs.acs.org on December 24, 2017

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ACS Applied Energy Materials

Electrochemically Formed NaxMn[Mn(CN)6] Thin Film Anodes Demonstrate Sodium Intercalation and De-intercalation at Extremely Negative Electrode Potentials in Aqueous Media Jeongsik Yun‡,1,2, Florian A. Schiegg‡,1,2, Yunchang Liang1,2, Daniel Scieszka1,2, Batyr Garlyyev1, Anika Kwiatkowski3, Thomas Wagner3, Aliaksandr S. Bandarenka1,2,* ‡ These Authors contributed equally to this paper 1

Physik-Department ECS, Technische Universität München, James-Franck-Straße 1, 85748

Garching, Germany 2

Nanosystems Initiative Munich (NIM), Schellingstraße 4, 80799 Munich, Germany

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Department of Chemistry, Technische Universität München, Lichtenbergstrasse 4, 85747

Garching b. München, Germany *E-mail: [email protected] Keywords: aqueous Na-ion batteries, Na-intercalation, NaxMn[Mn(CN)6], Na-ion battery anodes, electrochemical impedance spectroscopy

ACS Paragon Plus Environment

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Abstract

The development of electrode materials for Na-ion batteries has been substantially accelerated recently with respect to application in grid energy storage systems. Specifically, development of Na-ion batteries operating in aqueous media is considered more promising for this application due to safety issues. Many different types of cathode materials for aqueous Na-ion batteries have been proposed; however, the number and performance of contemporary anode materials are still insufficient for practical deployment. In this work, we demonstrate that electrochemically deposited NaxMn[Mn(CN)6] thin films are very promising anode materials for aqueous Na-ion batteries. NaxMn[Mn(CN)6] films exhibit (i) very low half-charge potential ca -0.73 V vs SHE (ca -0.93 V vs SSC) being one of the lowest among reported in the literature for the electrode materials, which also inhibit hydrogen evolution reaction, (ii) specific capacity of ca 85 mAh g-1 and (iii) only ~3% loss of capacity and high round-trip efficiency (99.6%) after 3,000 cycles. Surprisingly, the choice of the electrolyte composition has a very strong influence not only on the intercalation process, but also on the long-term performance of battery anodes and their electrode surface morphology.


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1. Introduction

Over the past years, the need for decentralized integration of so-called large energy storage systems (ESSs) as buffers to smoothen power fluctuation generated by less flexible renewable energy sources has risen in importance.1-6 Aqueous rechargeable alkali-metal-ion batteries and especially aqueous rechargeable sodium-ion batteries have positioned themselves as promising alternatives to conventional lithium ion batteries (LIBs) to cope with these needs.7,8 From a kinetic point of view, aqueous sodium ion batteries profit from a favorable transport of Na+ over Li+ in both electrodes and electrolyte, enhancing the performance of the system.9-11 Moreover, their electrolyte inherent safety and environmental friendliness coupled to the cost-efficiency due to extremely abundant constituents are major arguments to implement them in large scale ESSs. However, the bigger atomic radius and mass of Na+ ions compared to Li+ ions implying poorer volumetric and gravimetric energy densities, as well as the smaller operating voltage window arising from the electrolysis of the aqueous electrolyte together with the lack of suitable anode materials in aqueous media are among the biggest drawbacks that have to be balanced. Thus, for aqueous Na-ion batteries to become competitive in ESS applications, new materials that effectively overcome these limitations have to be found. Many cathode materials for LIB such as LiCoO2, LiMn2O4 and LiFePO4 are applicable to sodium ion batteries, but commonly used anode materials for LIB such as graphite are not suitable for them due to their instability in the Na+ intercalated state in aqueous electrolytes.12 The anode materials for aqueous Na-ion batteries should fulfill various requirements. They should exhibit (i) as low as possible electrode potentials for sodium intercalation and deintercalation to achieve high full cell voltages and high energy densities, but at the same time (ii)


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they should not trigger any side reactions like hydrogen evolution reaction (HER). Furthermore, the anode materials have to be chemically and mechanically stable at the operating pHs of the electrolyte to prevent dissolution and deformation of the active material. Correspondingly, the anode materials have to operate in the same media (be compatible) with the state-of-the-art cathodes.13 Several types of anode materials have been proposed to meet the above-mentioned requirements. For instance, NASICON-type anodes were firstly proposed by Park et al., and their specific capacity exhibited 100 mAh g-1 at 2 C with the plateau voltage at ca -0.61 V vs SHE and a cycle retention of 95% over 100 cycles.14-16 Qu et al. reported intercalation properties of the nanostructured vanadium oxide V2O5·0.6H2O, presenting a reversible capacity of 43 mAh g-1 in a 0.5 M Na2SO4 aqueous electrolyte.17 Further improvements led to the one-dimensional nanostructured Na2V6O10·nH2O with a lower redox potential and a high initial charge capacity of ca 123 mAh g-1.18 However, it suffers from irreversible structural changes upon Na+ insertion, deteriorating its intercalation properties resulting in a specific capacity of 42 mAh g-1 in subsequent cycles. Recently, a 1,4,5,8-Naphthalenetetracarboxylic dianhydride (NTCDA)derived polyimide was reported by Qin et al., presenting a discharge potential of ca -0.19 V vs SHE when operated in a highly concentrated 5 M NaNO3 aqueous electrolyte.19 It showed an extremely high discharge capacity of 165 mAh g-1 at a discharge rate of 1/3 C. However, its stability is rather questionable, with a capacity decay of 17% after 20 cycles; and the Na+ storage mechanism is still not fully understood. Pasta et al. showed another approach using coprecipitation of the Prussian blue analog (PBA)-based MnII-N≡C-MnIII/II, also operating in aqueous electrolytes.13,20 Its working potential was found to be ca 0.052 V vs SHE, with a specific capacity of 57 mAh g-1 and a high rate capability at C-rates between 1 and 10 C. When


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combining the manganese hexacyanomanganate (MnHCM) anode with the well-known copper hexacyanoferrate (CuHCF) cathode, it showed an almost negligible capacity loss over 1000 cycles, presenting a coulombic efficiency of 99.8% at 10 C. Despite the various approaches, it is evident that further research is necessary to come up with an anode material to achieve significantly higher energy density, specific capacity and a good stability. In this work, we demonstrate that electrochemically deposited MnHCM thin films show extremely low half-charge intercalation potential of ca -0.73 V vs SHE, which is one of the lowest potentials ever reported for the anode material for aqueous Na-ion batteries, to the best of our knowledge. The electrochemically deposited thin film furthermore showed a specific capacity of 85 mAh g-1 with retention of 97% over 3,000 cycles and a high round-trip efficiency of ~99.6%, thus presenting a significant improvement compared to the previously reported systems. The inhibition of HER at the potentials of charging/discharging allows, in principle, achieving the operating potential over 2 V if combined with state-of-the-art cathode materials for aqueous Na-ion batteries. Interestingly, not only ions in the electrolyte involved into the intercalation processes, but also different anions species, which are present in the deposition solutions drastically change the morphology and the performance of the resulting electrode material.21-25

2. Results and Discussion

The electrochemical deposition of NaxMn[Mn(CN)6] thin films was conducted making use of the following tentative redox-schemes, which manifest themselves in Figure 1A and 1D.


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Reduction of the redox active species: 3Na(aq.)+ + Mn(aq.)2+ + [MnII(CN)6](aq.)4- + e- → Na3Mn[MnI(CN)6](s.)

Equation (1)

Oxidation of the redox active species: Na3Mn[MnI(CN)6](s.) →Na2Mn[MnII(CN)6](s.) + Na(aq.)+ + e-

Equation (2)

Figure 1A and 1D show the characteristic cyclic voltammograms (CVs) for electrochemical deposition of NaxMn[Mn(CN)6] thin films from deposition solutions containing 7 mM MnSO4 and 11 mM K3[Mn(CN)6] based on (A) 0.25 M Na2SO4 and (D) 0.25 M NaClO4 electrolytes, respectively. The deposition from the 0.25 M NaClO4-based solution results in a faster growth of the NaxMn[Mn(CN)6] thin films compared to that from the 0.25 M Na2SO4based solution. Figure 1B and 1E demonstrate typical electrochemical quartz crystal microbalance (EQCM) curves describing the mass changes of the Au quartz crystal electrodes during electrochemical deposition of NaxMn[Mn(CN)6] thin films from the 0.25 M Na2SO4based and the 0.25 M Na2ClO4-based deposition solutions, respectively. Resulting mass changes due to the thin film depositions result in ~14 µg cm-2 from the 0.25 M Na2SO4-based solution and ~38 µg cm-2 from the 0.25 M NaClO4-based solution, respectively. The “oscillations” of the mass seeing in Figure 1B and 1E characterize intercalation/de-intercalation of Na-ions during the film deposition. Interestingly, different practically pH-neutral electrolytes (i.e., Na2SO4 and NaClO4) not only change the rate of the deposition, but influence the surface morphology of the deposited films (Figure 1C and 1F). Counterintuitively in this case, the faster deposition results in much smoother and uniform surface than the slower deposition, indicating different modes of deposition. This can be closely related to the nature of the anions species that present in the


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deposition solutions; anions should likely participate in the deposition process through specific adsorption at the surface. Recent studies on intercalation mechanism support this hypothesis.21-25 The studies found that anion species participate in the intercalation processes and have a significant influence on the intercalation. In order to further investigate the influence of electrolyte composition on the intercalation, cyclic voltammetry of NaxMn[Mn(CN)6] thin films was performed in different electrolytes. Figure 2A shows representative CVs of NaxMn[Mn(CN)6] thin films in 0.25 M Na2SO4, 0.25 M NaNO3 and 0.25 M NaClO4 electrolytes. As already found in the previous research, the “irreversibility” of intercalation is different depending on the anions in the electrolytes, exhibiting different shapes of CVs.21-25 Figure 2B shows CVs of NaxMn[Mn(CN)6] in NaClO4 electrolytes of different concentration (0.25 M, 5 M and 10 M). As can be seen in Figure 2B, the voltammetric behavior of the film is more reversible in electrolytes with higher ionic strength. Since Na2SO4 shows only limited solubility in water (lower than 2 M), NaClO4 was chosen due to its extremely high solubility. The higher concentration of NaClO4 electrolytes, the more intercalation/de-intercalation reversibility can be achieved. Especially, at 10 M NaClO4, the CV shows almost symmetric shape. The peak separations were reduced from ∆E0.25M=100 mV at a 0.25 M NaClO4 electrolyte over ∆E5M = 50 mV to ∆E10M = 1 mV, significantly improving the systems’ efficiency, and this cannot be explained just with the changes in the ionic conductivity of the electrolyte. Some PBAs such as nickel hexacyanoferrate (NiHCF) and cobalt hexacyanoferrate (CoHCF) have different electrode potential for intercalation of alkali metal cations, which can be associated with different hydration energy of alkali metal cations.21,24 However, MnHCM thin films show very poor intercalation properties for Li+, Rb+ and Cs+ ions (see Figure 2C). Considering the ionic radius of Li+, this can be found as rather surprising fact,


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which, however, can be explained taking into account the influence of alkali metal cations on the HER catalytic activity of various electrodes.26 The above-mentioned results were also supported by the XPS-analysis, as shown in Figure 3. In the case of Cs-intercalation, only HER takes place at the potential where Na- or Kintercalation occurs, showing that the nature of the cations drastically influence the HER catalytic activity of electrodes.26 A simple assumption that the size of the intercalating species or the related hydration energy would determine the nature of intercalation process is not true in this particular case; the catalytic properties of the whole system likely influence the interfacial process.21 Figure 2D shows CVs of co-intercalation of Na+ and K+ ions, which are simultaneously present in the electrolyte, into the MnHCM thin films. The CVs show that cointercalation of both ions is possible, but the films suffer from severe degradation in a number of cycles. Figure 2E and 2F are the used equivalent electric circuit (EEC) model for fitting and collected EIS spectra for MnHCM thin films, respectively. As shown in Figure 2F, the “loopshaped” EIS spectra indicate at least three stages during the intercalation processes.21-25 The Krammer-Kronig test didn’t reveal any problem with the quality of the fittings. Detailed explanation and interpretation for physical meaning of the EEC model and its corresponding fittings are given in the electronic supporting information (ESI). All key elements for MnHCM thin films were found from the XPS analysis. The comparison of MnHCM thin films in the Na-intercalated state and the Na-deintercalated state confirmed that not all Na-ions are expelled from the films as expected from Equation 2 as evident from Figure 3A. Figure 3B shows XPS peaks regarding different elements. Interestingly, characteristic XPS peaks for Na were seen at all MnHCM thin films, which shows that some amount of Na is present in the MnHCM thin films regardless of the intercalation of different alkali metal cations


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from the corresponding solutions containing Li- or K-ions. These Na-ions are the ones of the main composition of the MnHCM structure. Mn-peaks are clearly shown in all the XPS measurements. Characteristic XPS peaks for lithium were very small, verifying the analysis of the CV of MnHCM thin films in 0.25 M LiNO3 the limited, negligible intercalation of Li ions into the MnHCM thin films. On the other hand, XPS peaks related to potassium prove that intercalation of K-ions had taken place not only in 0.25 M K2SO4, but also in the mixture of 0.125 M Na2SO4 and 0.125 M K2SO4 electrolytes. The comparison of MnHCM thin films in the Na-intercalated state and the Na-de-intercalated state confirmed that not all Na-ions are expelled from the films. The shift of the binding energy of Mn elements between Mn 2p1/2 and Mn 2p3/2 peaks of the Na-intercalated/deintercalated films suggests that the intercalation of sodium is accompanied with the change of the oxidation state of Mn. The binding energies of Mn 2p3/2 of NaxMn[Mn(CN)6] are 641.15 eV for Mn2+ (Na-intercalated film) and 642.19 eV for Mn3+ (Nadeintercalated film). Compared to the binding energies of Mn 2p3/2 of Na2Mn[Mn(CN)6], the difference of the binding energy shows only 0.05 eV for Mn2+ and 0.09 eV for Mn3+, respectively.20 Those values are in a good agreement with the other literature values.30 Based on the above discussed findings, an optimized system was set up and its properties were analyzed. 10 M NaClO4 was chosen as electrolyte to provide the charge carrier of the system due to its considerably higher solubility limit compared to Na2SO4 and the fact that the investigated concentration effect outweighs the anion-effect. The system was cycled 3,000 times with a scan rate of 50 mV s-1 between -1.18 V and -0.6 V. Figure 4A displays CVs of MnHCM thin films in 10 M NaClO4, where every 100th cycles are shown. Other than postulated so far, no HER was observed in this range. The cathodic and anodic half-charged potentials were found to be as low as -0.958 V and -0.930 V, after 100 cycles, respectively. After the full 3,000 cycles,


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they had slightly shifted towards less negative values (ca -0.937 V and -0.910 V respectively), thus showing a superior cycling stability. The system presented a specific discharge capacity of ~85 mAh g-1 at a high C rate (60 C, i.e., ca 5A g-1) as shown in Figure 4B. The capacity retention over the 3,000 cycles was as high as 97.0% and the coulombic efficiency improved from 95.1% in the first cycle to 99.6% in the last cycle (shown in Figure 4C). Figure 5 shows the different classes of the electrode materials for aqueous sodium ion batteries. Electrochemically deposited NaxMn[Mn(CN)6] thin films exhibit one of the lowest electrode potentials among different types of electrode materials for aqueous Na-ion batteries. Notably, no HER was observed (in presence of Na+ and K+) even at such a low potential.

3. Conclusion

An extremely low electrode potential for Na-intercalation/de-intercalation in aqueous media was achieved with electrochemically deposited sodium manganese-hexacyanomanganate thin films, which at the same time do not catalyze the hydrogen evolution reaction. The halfcharged potential of NaxMn[Mn(CN)6] thin films in a 10 M NaClO4 aqueous electrolyte exhibited -0.93 V vs SSC with a high coulombic efficiency and a good specific capacity of 85 mAh g-1 showing only marginal losses over 3,000 cycles. Interestingly, electrolyte composition and its concentration have an influence not only on the battery performance, but also the morphology of the NaxMn[Mn(CN)6] thin film during the deposition. Intercalation of Li+ and Rb+ into NaxMn[Mn(CN)6] thin film triggered severe degradation of the films upon cycling, and Cs+ does not intercalate into the films at all. Notably, hydrogen evolution reaction doesn’t take


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place at the NaxMn[Mn(CN)6] thin films at the potentials even lower than -1.15 V vs SSC in 0.25 M NaNO3 and 0.25 M KNO3 electrolytes, but, surprisingly, hydrogen evolution reaction takes places at the NaxMn[Mn(CN)6] thin films in 0.25 M CsNO3, which suggests the electrolyte composition has strong influence not only on the intercalation properties, but also on side reactions at the electrode. For the optimization of the aqueous Na-ion batteries for future applications, the role of the electrolytes composition should be comprehensively studied.

4. Experimental Section

Deposition, in-situ characterization and battery performance tests of NaxMn[Mn(CN)6] thin films were performed in an argon-flooded three-electrode setup consisting of an AT-cut Au quartz crystal wafer (5 MHz, 1’’ diameter) or an Au-sputtered glass arrandeeTM as working electrode, a Pt gold wire as counter electrode and an Ag/AgCl (SSC) reference electrode, against which all the potentials are referred to (unless other are indicated explicitly), with the exception of Figure 5, summarizing the data from the literature and obtained in this work. Potentials and currents were controlled with a VSP-300 potentiostat (Bio-Logic, France). A QCM 200 (Stanford Research Systems, USA) electrochemical quartz crystal microbalance (EQCM) was used to monitor the mass changes related to the deposited film. The schematics of experimental setup is provided in Figure S4. For the deposition of MnHCM thin films, an aqueous solution containing 0.25 M Na2SO4, 7 mM MnSO4 and 11 mM K3[Mn(CN)6] (see ESI) was prepared, and the potential of the working electrode was cycled at a scan rate of 50 mV s-1 in the range between -1.18 V


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and -0.6 V. The same deposition procedure was modified by replacing 0.25 M Na2SO4 with 0.25 M NaClO4 in the preparation of the deposition solution. Potassium hexacyanomanganate(III) was synthesized following a slight alternation of the methods of Grube, Brause, and Meyer, respectively.27 All other chemicals used are referred to in more detail in ESI. The reaction mechanism was analyzed using electrochemical impedance spectroscopy (EIS) using a 10 mV ac-perturbation with probing frequencies between 10 kHz and 0.5 Hz in the potential range described above. The fitting quality was controlled by the normalized root-meansquare deviations, and the fitting was performed with the home-made “EIS Data Analysis 1.1” software.28 The morphology of the synthesized film was investigated using a multimode atomic force microscopy (AFM) instrument (Veeco Instruments Inc.) with a Nanoscope IIID controller using the Nanoscope 5.31r1 software. All measurements were performed in tapping mode making use of Bruker REESP-300 AFM tips. All microscopy data were analyzed by the WSxM 5.0 Develop 8.0 software.29 The MnHCM thin film chemical composition was investigated by means of X-ray photoelectron spectroscopy (XPS) using a SPECS XPS spectrometer (SPECS, Germany) employing an Al anode (12kV, 200W) and setting the pass energy to 20 eV to ensure a high resolution of the spectra. Due to the high vulnerability of the film in the presence of oxygen, water and possibly light, special precautions, like the use of an Ar purged preconditioning cell and the shielding from light had to be taken.20 Further details on experimental procedures, instrumentation and chemicals are given in the ESI.


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Figure 1. Electrochemical and AFM characterization of NaxMn[Mn(CN)6] thin films. The films were deposited from aqueous solutions containing 7 mM MnSO4 and 11 mM K3[Mn(CN)6] together with (A-C) 0.25 M Na2SO4 and (D-E) 0.25 M NaClO4. The figures show characteristic cyclic voltammograms of NaxMn[Mn(CN)6] thin films during deposition from the aqueous solution containing (A) 0.25 M Na2SO4 and (D) 0.25 M NaClO4, respectively. Corresponding EQCM curves (B, E) show the mass changes of the deposited NaxMn[Mn(CN)6] thin films as (B) ca 14 µg cm-2 and (E) ca 38.5 µg cm-2, respectively. AFM images of NaxMn[Mn(CN)6] thin films from the deposition solution containing (C) 0.25 M Na2SO4 and (F) 0.25 M NaClO4 reveal drastically different surface roughness. Note the different scale of the z-axis of profile-line across the bottom of each AFM image in (C) and (F).


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Figure 2. Characterization of the MnHCM thin films. (A) CVs of MnHCM thin films in 0.25 M Na2SO4 (black), 0.25 M NaNO3 (blue) and 0.25 M NaClO4 (red), respectively. (B) CVs of MnHCM thin films in NaClO4 electrolytes of different concentrations; 0.25 M (black), 5 M (blue), and 10 M (red). (C) Intercalation/deintercalation of different alkali metal cations (Li+, Na+, K+, Rb+ and Cs+) into MnHCM thin films. (D) CVs of MnHCM thin films in the mixture of 0.125 M Na2SO4 and 0.125 M K2SO4 electrolytes (scan rate in all the cases is 50 mVs-1). (E) The equivalent electric circuit proposed to model the intercalation mechanism (see ESI) and (F) the collected EIS data (open symbols) of MnHCM thin films in 0.25 M Na2SO4 electrolyte and their fitting data (solid lines) are shown respectively.


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Figure 3. (A) XPS data for MnHCM thin films in “Na-intercalated” and “Na-deintercalated” states. (B) XPS data for MnHCM thin films at intercalated states with different alkali metal cations (Li, Na and K). All XPS peaks confirmed the presence of intercalated species in the MnHCM thin films. The intensity of the Na-peaks is different according to the state of the intercalation, but their positions are same, while Mn-peaks were shifted according to the intercalation/deintercalation of Na ions.


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Figure 4. Battery tests of MnHCM thin films. (A) CVs of MnHCM thin films in 10 M NaClO4 for over 3,000 cycles (only every 100th cycles are shown; deconvolution of the peaks is also presented). (B) Battery capacity determination of MnHCM thin films at 60 C (ca. 5 A g-1). The resulting specific capacity was ca 85 mAh g-1 with almost overlapped hysteresis of charge/discharge. (C) Anodic and cathodic capacity retention and coulombic efficiency of the deposited thin film.


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Figure 5. The working electrode potentials of NaxMn[Mn(CN)6] in comparison with different classes of electrode materials for aqueous Na-ion batteries. Adapted with the permission from reference.12 Copyright © 2014 American Chemical Society.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Synthesis of potassium hexacyanomanganate(III) trihydrate, K3[Mn(CN)6] x 3H2O, Used chemicals, Fitting of EIS spectra and their interpretation (Word file), Experimental Setup, Scanning Electron Microscope Measurement


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AUTHOR INFORMATION Corresponding Author Aliaksandr S. Bandarenka Physik-Department ECS, Technische Universität München, James-Franck-Straße 1, 85748 Garching, Germany Nanosystems Initiative Munich (NIM), Schellingstraße 4, 80799 Munich, Germany E-mail: [email protected] (A.S. Bandarenka)

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡ Jeongsik Yun, Florian A. Schiegg contributed equally.

Funding Sources

ACKNOWLEDGMENT Financial support from the cluster of excellence Nanosystems Initiative Munich (NIM) is gratefully acknowledged. Jeongsik Yun is thankful for the financial support from Nagelschneider Stiftung. T.W. and A.K. thank the Department of Chemistry TUM for courtesy granting access to lab space and equipment.


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TOC Picture 361x175mm (96 x 96 DPI)


Thin Film Anodes Demonstrate Sodium Intercalation and De

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