Enabling Zinc-Ion Batteries with Acidic Aqueous Electrolytes for Stationary Energy Storage: Challenges of the Reaction Mechanism

Zinc-ion batteries (ZIB) with mildly acidic aqueous electrolytes represent a promising alternative to the currently dominant lithium-ion batteries, particularly due to their advantages in safety, material availability, environmental sustainability, and cost reduction potential. As alternative battery chemistries are essential for a successful energy transition, research in this area must advance rapidly. At the Fraunhofer Institute for Solar Energy Systems (ISE), ZIB have been intensively investigated for several years, focusing on manganese dioxide as the cathode active material. The reaction mechanisms at both the cathode and anode have been extensively studied, yet a clear understanding of their proportion and interaction at different operating conditions remains elusive in the scientific community. At ISE, in-depth investigations into the pH of the electrolyte and its relationship to the mechanisms at the anode and cathode have been conducted, identifying pH as a central influencing factor in the functionality of ZIB (s. Figure 1). To facilitate these studies, several in-operando technologies have been developed and novel cell designs have been introduced that allow for these in-operando measurements, complemented by optical and electrochemical measurement methods for detecting the side reactions such as e.g. the precipitation of zinc hydroxide species. These investigations have led to numerous insights and fostered active exchange with the scientific community. Furthermore, ISE has produced and put into operation first ZIB laboratory prototypes as part of research projects in collaboration with industry (s. Figure 2). Our current research focuses on three main topics: Electrolyte formulations including additives and pH buffers to balance the pH shifts, varying separator strategies to optimize the electrode-electrolyte interface, and adapting electrode compositions by selecting appropriate materials to render ZIB marketable. This presentation will provide an overview of the current challenges in the field of the aqueous ZIB technology, the state of research and key findings at ISE, and a forward-looking perspective on this promising cell technology.