In this context, ZIBs show great potential for development. The processes underlying the charging and discharging dynamics in ZIBs are characterised by a high degree of complexity, which has only been partially understood until now. In order to unravel these complex electrochemical interactions, the Institute of Theoretical Chemistry at the University of Ulm is carrying out state-of-the-art quantum chemical modelling, which is then incorporated into continuum modelling at the German Aerospace Center (DLR) in Ulm. The aim of this research is to gain a detailed understanding of the characteristic biphasic cycles in ZIBs, which could have a decisive influence on the performance parameters and long-term stability of these batteries. This detailed analysis will provide a fundamental understanding of the underlying electrochemical mechanisms, which is essential for optimising and developing this promising battery technology.

Background: Why zinc-manganese oxide batteries?

ZIBs have zinc anodes and manganese oxide cathodes that work in a zinc sulphate solution. They are unique in that they use a water-based electrolyte. This makes them safer and more environmentally friendly than many other battery types. Compared to their alkaline counterparts, these batteries have already shown better rechargeability in earlier experiments.

Charge storage mechanisms

Charge storage in ZIBs is a multi-facetted process, made possible by the interaction of several chemical reactions. A schematic overview illustrates the underlying charge storage mechanisms in ZIBs: The redox reaction of the zinc metal anode (I) is shown on the left-hand side. The electrochemical reactions take place at the cathode: the insertion of zinc ions (II.a), the dissolution of manganese ions (Mn²⁺) (II.b) and the insertion of hydrogen ions (H⁺) (II.c). The lower right corner of the figure shows the deposition of ZHS (III), a phenomenon observed experimentally at the cathode.

The study of these mechanisms shows that ZIBs go through two phases during both charging and discharging:

• First phase: Characterised by a rapid insertion of zinc and hydrogen ions into the cathode material, resulting in a relatively stable and predictable voltage.

• Second phase: Begins when the dissolution of manganese ions becomes increasingly relevant. This phase is characterised by slower kinetics and a changed voltage curve, which is often accompanied by a voltage drop. This transition reflects the changed electrochemical conditions within the battery.

Integration of density functional theory and continuum cell model in battery research

Density functional theory (DFT) and the continuum cell model are two modern approaches used in battery research to understand and optimise processes at the molecular level. Scientists at the University of Ulm are using DFT to analyse the electronic structure and the associated reaction processes. This analysis makes it possible to calculate the changes in total energy during the insertion of hydrogen and zinc and to derive the voltage of the battery.

In combination with this, the continuum cell model simplifies the complex electrochemical processes in ZIBs by introducing "quasi-particles". These abstract entities represent groups of ions. Instead of looking at each ion individually, researchers can now analyse the total concentrations of these "quasi-particles" in the battery electrolyte. Such continuum modelling is carried out at DLR, with the necessary interaction parameters being determined using DFT calculations at the University of Ulm.

Simulation results

Research supported by the Dr Barbara Mez-Starck Foundation has shown that the insertion of hydrogen ions takes place at higher voltages, while the dissolution of manganese oxide-zinc structures and the insertion of zinc ions take place at lower voltages. This has important implications for understanding the charging and discharging processes in ZIBs. In particular, it is shown that the dissolution reactions of the cathode structure play an essential role in the discharge and could explain the phenomenon of the two-phase voltage curve observed in experiments.

The findings on electrolyte pH changes and the formation of solids such as zinc sulphate hydroxide (ZHS) during battery operation are equally important. It can be shown that the pH value increases during discharge and leads to the precipitation of ZHS when a saturation limit is reached, which in turn stabilises the pH value. This stabilisation is critical, as an uncontrolled change in the pH value can lead to a degradation of the battery performance. Therefore, the management of zinc and hydrogen ion concentrations, as well as the dissolution of manganese oxide, is essential to prolong the life of ZIBs.

The new model could pave the way for more powerful zinc-manganese oxide batteries

The researchers led by Dr Birger Horstmann and Dr Axel Groß have developed a theoretical model that explains the charging and discharging of ZIBs better than before. This model agrees well with the experimental results. 

The two-phase nature of the discharge in ZIBs can be simulated with the developed simulation. Particularly important is the formation of zinc sulphate hydroxide (ZHS), which causes the voltage to drop during the second discharge phase and stabilises the pH. Experiments support this theory and show that the models accurately reflect the real charge and discharge cycles.

To reduce the power loss and shape changes that occur over time, the scientists recommend an optimised charging process that minimises cathode dissolution. In addition, adjusting the electrolyte composition could help to improve the stability of the batteries. These findings provide valuable starting points for the further development of ZIBs, which are playing an increasingly important role in modern energy storage.