*4.3. Characterization Methods*

The cell cycling was performed by making a rate capability test (RCT) with 5 cycles, each at 40, 80, 160, 320, 640, and 80 mA·g−<sup>1</sup> (related to active material loading of the manganese dioxide electrode) from 0.8 to 1.7 V with a *VMP3* potentiostat from Biologic Science Instruments, Seyssinet-Pariset, France. The charging process included a constant current (CC) step followed by a constant voltage (CV) step until 10% of the current in the CC-step was reached. The discharge process was performed at CC. EIS measurements were performed between 10 kHz and 20 mHz at 1.7, 1.25 and 0.8 V cell voltage after the five 80 mA·g−<sup>1</sup> charge/discharge cycles, respectively. The test plan procedure is shown in detail in Supplementary Figure S3.

For the mechanical stress test (MST), the electrode coins were put into a transparent PP beaker (20 mL, ø31 × 48 mm) filled with the same amount (~10 mL) of either DI-water or electrolyte solution together with a magnetic stirring bar on top of the electrode sheet. The stirring bar (PTFE, cylindrical, length 20 mm) was operated for 60 s at 200 rpm, respectively. The experimental setup is visualized in Supplementary Figure S4.

For analysing the distribution of the different components in the positive electrode, SEM-EDX images were realized with selected positive electrodes. The positive electrodes were dipped into deionized water and dried overnight before being ion polished. The cutting edges were prepared by ion milling (IM4000 Ion Milling System, Hitachi High-Tech Europe GmbH, Mannheim, Germany) and characterized by SEM (Zeiss Auriga 60, Carl Zeiss Microscopy Deutschland GmbH, Oberkochen, Germany) and EDX (Bruker Quantax XFlash 6160, Bruker Corporation, Billerica, USA).

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/batteries7020040/s1, Figure S1: Overview of the experimental cell setup inside the EL-Cell® ECC aqu, Figure S2: Graphical overview of the test plan procedure for the rate capability test of the herein tested cells, Figure S3: Experimental setup of the mechanical stress test with a magnetic stirring bar in a PP beaker (20 mL, ø31 × 48 mm) filled with ~ 10 mL of liquid, Figure S4: RCT results including the PVP (aq) binder polymer, which is not finishing the RCT due to stability issues of the electrode coating (s. MST), Figure S5: Potential curves including the PVP binder, which was not able to finish the RCT, Figure S6: Comparison of the PAN (aq) and the PAN (DMSO) coating in pristine state. MnO2 particles are coloured green, carbon black is coloured orange, Figure S7: Comparison of the PAN (aq) and the PAN (DMSO) coating in post mortem state. MnO2 particles are coloured green, carbon black is coloured orange, Figure S8: Overview of the EIS spectra for all investigated binder polymers in charged (CH) and discharged (DCH) state after 10 and 30 cycles, respectively, Figure S9: Diagrams of the RCT showing the specific capacity and energy, respectively, together with the efficiency values, Figure S10: SEM+EDX images of the coating with aqueous LA133 (PAN-based) binder with the mixing ration 70/20/10 (AM/CB/BP, wt%) in pristine state. The electrode thickness is ~40 μm, Figure S11: SEM+EDX images of the coating with aqueous LA133 (PAN-based) binder with the mixing ration of 80/15/05 (AM/CB/BP, wt%) in pristine state. The electrode thickness is ~40 μm. (green coloured = MnO2 particles, orange coloured = carbon black (CB)), Figure S12: Comparison of the 70/20/10 and 80/15/05 binders in pristine state. For the 80/15/05 coating, smaller distances of the MnO2 active material particles (coloured green) and a more compact visual impression of the coating could be assumed, which could be referred to the lower CB content and the lower overall passive material (CB+BP) share (20 wt% instead of 30 wt% for the 70/20/10 coating), Figure S13: Overview over the charge/discharge curves of the 10th cycle (80 mA·g−1) of the RCT shown in Figure S4 for the binder variation (a) and mixing variation (b) with insets showing the IR drops directly after the switchover from charge to discharge (marker 1) and the first potential plateau during discharge (marker 2), Table S1: Brief literature research on binder materials, solvents, mixing ratios and current collectors. This literature search was carried out as an example and does not claim to be complete, Table S2: Detailed Overview of the test plan procedure for the RCT.

**Author Contributions:** Conceptualization, O.F.; methodology, O.F., S.I. and C.B.; validation, O.F., S.I. and C.B.; formal analysis, O.F.; investigation, S.I., C.B. and O.F.; data curation, O.F. and S.I.; writing—original draft preparation, O.F.; writing—review and editing, H.G., S.I., C.B., K.P.B. and D.B.; visualization, O.F.; supervision, D.B., K.P.B. and D.S.; project administration, D.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** Oliver Fitz acknowledges the German Federal Environmental Foundation (Deutsche Bundesstiftung Umwelt, DBU) and Christian Bischoff acknowledges the Heinrich Böll Foundation for the support. We thank Volker Kübler for carrying out the SEM+EDX characterizations.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**

