Entwicklung einer Zink-Ionen-Batteriezelltechnologie mit wässrigen Elektrolyten für stationäre Anwendungen

The doctoral thesis deals with the development of a battery storage system based on a zinc-manganese dioxide battery technology with aqueous electrolytes (ARZMB). The work is divided into three parts: (a) evaluation of the electrode fabrication process, (b) investigation of the reaction mechanism of the battery cell chemistry and (c) production of a battery module prototype for techno-economic evaluation of the battery technology. The evaluation of the electrode fabrication process (a) comprises two different methods for the cathode: The doctor blade coating process (i) enables reproducible electrode fabrication by coating a stainless-steel current conductor foil with a paste. The use of water-based binder compositions shows increased coating stability and higher specific capacities in the aqueous electrolyte compared to non-water-based compositions. Investigations of electrode cross-sections before and after cycling show pore blockages after cycling, which are attributed to precipitation reactions and identified as a major ageing mechanism of battery cell technology. Electrodeposition (ii), as a second electrode fabrication process, offers a high degree of flexibility in the choice of current-conducting substrate material, allowing for porous structures. Deposition at voltages in the range of 2-2.1 V enables efficient deposition of the active material MnO2. For the fabrication of the anode, electrodeposition of zinc from a Zn2+-based electrolyte onto a copper foil as support material and current conductor shows long-term stable cycling. The investigation of the reaction mechanism of the ARZMB technology (b) by means of the pH investigation in the electrolyte during cycling or cyclic voltammetry shows periodic pH fluctuations in ZnSO4-based electrolytes. These pH fluctuations are triggered by the Mn2+/MnO2 dissolution/deposition mechanism as an essential part of the reaction mechanism of the cathode. Due to the pH fluctuations, precipitation reactions of metal hydroxides occur on the surface or within the pores of the cathode. To reduce the pH fluctuations, pH buffer substances are evaluated: Acetate- and propionate-based electrolyte compositions (EC) show pH buffering properties and can be used in different electrolyte concepts in combination with different zinc salts and buffer systems. For the specific formulation of EC for ARZMB technology, requirements regarding the concentration of Mn2+/Zn2+ ions (~1 mol.l-1), their concentration ratio (1:1), the buffer acid/base ratio related to the initial pH and pH operating range (pH 4.5 ±1), the ionic conductivity (> 10 mS.cm-1), electrochemical stability in the potential range considered (~0.5-2.2 V vs. Zn/Zn2+), and safety and toxicity. The evaluation of different EC shows a high buffer capacity in the range of ~2.5 mol.l-1 while addressing the formulated requirements. The nominal discharge potential increases by approx. 15-20 % when using buffered EC compared to the unbuffered ZnSO4 reference electrolyte (RE). The long-term stability (by 80 % residual capacity) compared to the RE increases by a factor of 12 to approx. 240 cycles. The construction of a battery module prototype (c) is carried out considering different approaches for the components of the battery module housing, the electrical interconnection, the electrodes, the separator, the current collector, and the electrolyte. The previous findings are used as a basis. The successful electrochemical characterisation of a battery module with approx. 0.8 Wh (0.2 Ah) validates the concept. The techno-economic evaluation is carried out qualitatively based on the materials and manufacturing processes used and shows a significant cost reduction potential compared to the state of the art in lithium-ion battery technology.