Supporting Information

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1 Supporting Information What do Laser Induced Transient Techniques Reveal for Batteries? Na- and K-Intercalation from Aqueous Electrolytes as an Example Daniel Scieszka a,b,(1), Jeongsik Yun a,b,(1), Aliaksandr S. Bandarenka *,a,b a - Physik-Department ECS, Technische Universität München, James-Franck-Straße 1, Garching, Germany b - Nanosystems Initiative Munich (NIM), Schellingstraße 4, Munich, Germany *Corresponding Author: Tel. +49 (0) , bandarenka@ph.tum.de (A. S. Bandarenka) (1)- these Authors contributed equally to this paper S-1

2 1. Fitting of electrochemical impedance spectra The impedance spectra (see Figures 2E and 2F in the manuscript) of the Na 2 Ni[Fe(CN) 6 ] thin films in contact with the 0.25 M Na 2 SO 4 electrolyte cannot be fitted with equivalent electric circuits (EECs) that are frequently used in the literature to model the response of electrochemical systems with single-stage reaction mechanisms. Thus, a more complex EEC is needed to fit the unique loop-shaped spectra, which, in fact, disclose at least three quasi-reversible stages during intercalation/de-intercalation of the alkali metal cations. 1,2 The first step is likely fast reduction or oxidation of Fe-atoms in the electrode material. Due to a good electronic conductivity of the Na 2 Ni[Fe(CN) 6 ] films, the change of the oxidation states of Fe occurs much faster than the intercalation/de-intercalation of alkali metal cations, e.g. Na-ions (see Figure S1). Na 2 Ni[Fe II (CN) 6 ] - e - Na 2 Ni[Fe III (CN) 6 ] + (1) Figure S1. A scheme, which explains the first stage of intercalation/de-intercalation of Na-ions. A commonly used equivalent electric circuit to model the response of a single-stage surface (electrode) limited redox reaction (R u -uncompensated resistance, Z dl the impedance of the double layer, R ct the charge transfer resistance, C a,1 pseudo-capacitance due to a surface (electrode) limited redox reaction). A typical impedance spectrum (open circles) taken for the Na 2 Ni[Fe(CN) 6 ] thin films in contact with S-2

3 the 0.25 M Na 2 SO 4 electrolyte in the potential range of Na-intercalation and the corresponding fitting (solid line) which can adequately describe only the high frequency region. It is possible to fit only the high frequency region of the impedance spectra with a simple model (Figure S1A) which describes a fast oxidation/reduction of Fe in the electrode as shown in Figure S1B. Only the fast redox reaction of Fe II/III can follow the probing AC signals at high frequencies. For the fitting of the lower frequency regions, a more complicated EEC is necessary. Since Na-ion diffusion in the electrode is much slower than the fast redox reaction of Fe (the electron mobility in a conductor is much faster than the ionic mobility in the solid), the electrode surface becomes positively charged and anions get temporarily adsorbed compensating the excess charge of the electrode (see Figure S2). 2Na 2 Ni[Fe III (CN) 6 ] + + SO 4 2- (Na 2 Ni[Fe III (CN) 6 ]) 2 SO 4 (2) Figure S2. Schematic description of the first and the second interconnected reversible stages of Naintercalation/de-intercalation. Equivalent electric circuit describing the surface limited redox reaction at the electrode and subsequent anion adsorption/desorption of anions. Electric circuit elements, C a,1 and R a,1, account for the adsorption/desorption and can formally have either positive or negative values. Typical impedance spectra for the Na 2 Ni[Fe(CN) 6 ] thin films in the 0.25 M Na 2 SO 4 electrolyte in the potential range of Na-intercalation and the corresponding fitting in the high and middle frequency regions. S-3

4 To explain the above mentioned physicochemical processes, an additional set of the passive elements is needed 1,2 in the EEC model as shown in Figure S2A. C a,1 and R a,1 are the representative electric circuit elements that should be considered as a combination and their values can even be negative. With the more complex model, it was possible to fit the collected impedance spectra also within the middle frequency range as shown in Figure S2B. At the last stage, Na + together with anions leaves the surface (see Figure S3). (Na 2 Ni[Fe III (CN) 6 ]) 2 SO 4 2Na + + SO NaNi[Fe III (CN) 6 ] (3) Figure S3. Schematic representation of the three reversible stages of Na-intercalation/de-intercalation. An equivalent electric circuit describing the three-stage intercalation/de-intercalation mechanism. The four electric circuit elements in the dashed square represent the adsorption/desorption of anions and intercalation/de-intercalation. Those elements can formally have either positive or negative values. Typical EIS spectra for the Na 2 Ni[Fe(CN) 6 ] thin films in the 0.25 M Na 2 SO 4 electrolyte in the potential range of Na-intercalation and the corresponding fitting in the whole frequency region. Figure S3A displays the EEC model describing the three-stage intercalation process. As in the previous case, the electric circuit elements in the dashed square should be considered in combination and their values can be even negative. With the above EEC model (Figure S3A), the S-4

5 impedance spectra for the Na 2 Ni[Fe(CN) 6 ] thin films within the full range of frequency and the full range of the potential can be well fitted with a good precision. The Krammers-Krönig check does not reveal any problems with the quality of the spectra. The resulting values of the EEC parameters after the fitting of all the spectra shown in the manuscript are given in Table S1. System R u [Ω] C dl * [µf] n dl R ct [Ω] R a,1 [Ω] R a,2 [Ω] C a,1 [µf] C a,2 V in 0.25 M Na 2 SO V in 0.25 M Na 2 SO V in 0.25 M K 2 SO V in 0.25 M K 2 SO Table S1. Resulting fitting parameters of the equivalent electric circuit (data are shown in Figures 2E, F of the manuscript). C dl * is the double layer capacitance that is estimated according to the ref. 3 (two constant phase element parameters are used to roughly estimate the double layer capacitance). Impedance of the double layer was calculated from CPE (Constant Phase Element) parameters with the program available here ( accessed: ). The time constants (τ) are calculated as ~ 6 ms for both potential in 0.25 M Na 2 SO 4 and ~1.2 ms and 4.5 ms for 0.66 V and 0.76 V in 0.25 M K 2 SO 4, respectively. 2. Influence of cations on the reversibility of charging/discharging In order to assess the reversibility of intercalation/de-intercalation of both alkali metal cations into the thin films, the half-charged potentials ( E 1/2 ) were used. Figure S4. Cyclic voltammograms and charge and discharge curves for the Na 2 Ni[Fe(CN) 6 ] thin films in 0.25 M Na 2 SO 4 and 0.25 M K 2 SO 4 electrolytes, respectively. The black solid lines correspond to the cyclic voltammograms, while the red dashed lines correspond to the charge and discharge curves. S-5

6 Figure S4 shows two different cyclic voltammograms and charge and discharge curves for the Na 2 Ni[Fe(CN) 6 ] thin film in the 0.25 M Na 2 SO 4 and 0.25 M K 2 SO 4 electrolytes. The observed potential shifts ( E 1/2 ) between charging and discharging are ~88 mv and ~74 mv for the 0.25 M Na 2 SO 4 and 0.25 M K 2 SO 4 electrolytes, respectively. 3. Laser induced current transient (LICT) measurements The laser induced current transient (LICT) technique was used to examine the electrode surface charge during the intercalation of Na- or K-ions. (C) (D) Figure S5. LICT characterization of the Na 2 Ni[Fe(CN) 6 ] thin films (full sets of transients) obtained in (A,B) 0.25 M Na 2 SO 4 and (C,D) 0.25 M K 2 SO 4 electrolytes. LICT plot acquired within the potential range of sodium intercalation/de-intercalation. Collected extrema of the current transients from Figure S5A. The extrema are expressed as a function of the electrode potential. (C) LICT plot obtained within the potential range of potassium intercalation/de-intercalation. (D) Collected extrema of the current transients from Figure S5C. The extrema are expressed as a function of the electrode potential. The arrows in Figures S5B and S5D denote the scan direction. S-6

7 Figure S6. Current transients at different electrode potentials for the Na 2 Ni[Fe(CN) 6 ] thin films in 0.25 M K 2 SO 4 electrolyte. The dashed lines show the transients of the potentials at which the hydrogen evolution reaction occurred. The full experimental procedure is described in the manuscript (see section EXPERIMENTAL METHODS). Figure S5 shows the LICT plots (complete sets of transients) obtained for the 0.25 M Na 2 SO 4 and 0.25 M K 2 SO 4 electrolytes within the potential ranges of intercalation/deintercalation of Na + and K +, respectively. S-7

8 4. AFM measurements The AFM images were obtained utilizing a Nanoscope 5.31r1 software, while WSxM 5.0 Develop 8.0 software 4 was used to analyze the recorded images. Figure S6A and B display the resulting surface morphology of the Na 2 Ni[Fe(CN) 6 ] thin films. Electrochemically deposited Na 2 Ni[Fe(CN) 6 ] thin films exhibit a smooth and quasi-uniform surface structure nm 0.00 nm Figure S7. Recorded AFM images of Na 2 Ni[Fe(CN) 6 ] thin film. Representative AFM image of the thin films and its 3D profile. The surface looks uniform and smooth. 5. Chemicals K 3 [Fe(CN) 6 ] (99%, Sigma Aldrich, Germany), NiCl 2 6H 2 O (99.3%, Alfa Aesar, Germany), Na 2 SO 4 (99.0%, Sigma Aldrich, Germany), K 2 SO 4 (99.0%, Sigma Aldrich, Germany) References (1) Yun, J.; Pfisterer, J.; Bandarenka, A. S. How Simple Are the Models of Na Intercalation in Aqueous Media? Energy Environ. Sci. 2016, 9, (2) Ventosa, E.; Paulitsch, B.; Marzak, P.; Yun, J.; Schiegg, F.; Quast, T.; Bandarenka, A. S. The Mechanism of the Interfacial Charge and Mass Transfer During Intercalation of Alkali Metal Cations. Adv. Sci. 2016, 3, (3) (Accessed Mar. 2017) (4) Horcas, I.; Fernández, R.; Gómez-Rodríguez, J. M.; Colchero, J.; Gómez-Herrero, J.; Baro, A. M. WSXM: A software for scanning probe microscopy and a tool for nanotechnology. Rev. Sci. Inst. 2007, 78, S-8