**1. Introduction**

The research on aqueous battery technologies for stationary applications such as the aqueous rechargeable zinc-ion battery (ARZIB) is getting more and more attention. The ARZIB technology combines inherent safety, environmental friendliness, material abundance, low active material costs, and promising cycling stabilities. This publication focuses on the Zn//MnO2 chemistry with zinc on the negative electrode side and manganese dioxide on the positive electrode side, together with an acidic ZnSO4 + MnSO4 based electrolyte.

Recent publications show different ways of fabrication procedures for the positive electrode with doctor blade coating on a current collector foil, electrodeposition of the active material or pasting of the electrode material (summary in Supplementary Figure S1) [1–24].

**Citation:** Fitz, O.; Ingenhoven, S.; Bischoff, C.; Gentischer, H.; Birke, K.P.; Saracsan, D.; Biro, D. Comparison of Aqueous- and Non-Aqueous-Based Binder Polymers and the Mixing Ratios for Zn//MnO2 Batteries with Mildly Acidic Aqueous Electrolytes. *Batteries* **2021**, *7*, 40. https://doi.org/10.3390/ batteries7020040

Academic Editor: Claudio Gerbaldi

Received: 20 May 2021 Accepted: 15 June 2021 Published: 18 June 2021

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As the literature shows, the doctor blade coating is the most prominent fabrication procedure. For this, an electrode slurry with active material (MnO2, AM), a conductive agent such as carbon black (CB), and a binder polymer (BP) in different fractions, most commonly 70/20/10 wt% (AM/CB/BP), is coated on the current collector sheet using a doctor blade [4,5,8–10,13,14,25–28].

Different polymer binders such as *LA133* (based on polyacrylonitrile (PAN)), carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), polytetrafluoroethylene (PTFE), and polyvinylidene fluoride (PVDF) based on different solvents such as DI-water or non-aqueous N-methyl-2-pyrrolidone (NMP) were used in the literature so far ( summary in Supplementary Figure S1) [3,23]. Generally, an aqueous slurry processing enables inexpensive, safe, and environmentally friendly fabrication of the positive electrode. Still, a better understanding of the relation between the binder solvent (aqueous/non-aqueous) and the aqueous electrolyte, together with the reaction mechanism of ARZIBs, can enable a targeted electrode fabrication and an improved cell performance.

For a reasonable and application-oriented electrode fabrication, the consideration of the underlying reaction mechanism of the ARZIB cell chemistry is of high importance: So far, the reaction mechanism for the positive electrode seems to be a combination of multi-step chemical reactions such as a Zn2+ intercalation [12,26–36], a H+/Zn2+- Co-intercalation [17,20,37–39], a dissolution of the MnO2 active material loading and their re-deposition on the positive electrode surface together with the deposition of predissolved Mn2+ ions from the electrolyte (as far as MnSO4 is pre-dissolved in the electrolyte) [19,33,37,40,41]. The reactions are often accompanied by a pH-dependent zinc hydroxide sulfate (ZHS) formation [14,19,28,33,37,39–44]. Furthermore, the formation of inert ZnxMnyOz-species (i.e., ZnMn2O4) on the positive electrode were introduced in the recent literature [11,39,40]. For the negative electrode, there is a reversible Zn plating/stripping at the zinc electrode [9,14,20,42,45,46] as a consequence of the acidic pH value in accordance to the potential-pH diagrams for zinc [36,47]. This is in contrast to the alkaline Zn//MnO2 batteries with the formation of irreversible Zn phases [36,48,49].

In consideration of the different characteristics (intercalation and conversion reactions) of the reaction mechanism, there are various requirements for the positive electrode. For example:


Herein, selected BPs (polyacrylonitrile (PAN), carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), cellulose acetate (CA), nitrile butadiene rubber (NBR)) with aqueous and non-aqueous solvents (DI-water, methyl ethyl ketone (MEK), and dimethyl sulfoxide (DMSO), for details see Section 4.1. Materials) are compared by applying a mechanical stress test (MST) on the positive electrode sheet and rate capability tests (RCT), together with electrochemical impedance spectroscopy (EIS) measurements, on experimental battery cells. The experimental results are compared with SEM+EDX images of cross-sections of pristine and cycled electrode sheets, giving further insights into the homogeneity, material distribution and morphology of the coatings. Furthermore, the mixing ratio of the electrode components is systematically investigated by using a ternary plot visualization to evaluate the influence of the different shares of the positive electrode ingredients on the mechanical stability of the coating and the cycling performance. Finally, recommendations for the selection of the binder polymer and the mixing ratios for the utilization in ARZIBs are made.

This study, besides well-known binder polymers such as CMC, SBR and *LA133*, also considers new BP/solvent combinations such as PAN+DMSO, NBR+MEK and CA+MEK for the utilization in ARZIBs.

As there are only few publications dealing with a comparison of different binder polymers for ARZIBs in literature (to our knowledge, only [4]), this publication is intended to provide a basis for comparing different BP/solvent combinations as well as different mixing ratios for the positive electrode fabrication. Hereby, the focus is set on NMP-free solvents, as NMP-free processing enables lower safety precautions (note: reproductive toxicity of NMP) and a clean-room-free processing, lowering the production costs and being a decisive advantage over the lithium-ion battery technology.
