The research on aqueous zinc ion batteries (AZIB) is getting more attention as the energy transition continues to develop and the need for inexpensive and safe stationary storage batteries is growing. As the detailed reaction mechanisms are not conclusively revealed, we want to take an alternative approach to investigate the importance of pH value changes during cycling. By adding a pH-indicator to the electrolyte (2 M ZnSO 4 + 0.1 M MnSO 4 ), the local pH-value change during operation is visualized in operando. The overall pH value was found to increase during cycling whereas a major temporary pH drop in close proximity of the manganese dioxide electrode surface occurs. Additionally, this pH value change was quantified locally by in operando measurements with a pH micro electrode. Different electrolyte compositions with additives (sodium dodecyl sulfate (SDS), sulfuric acid (H 2 SO 4 )) and operation voltages were tested. The pH-potential-diagrams of manganese and zinc reveal pH value and potential limits, leading to active material dissolution at lower pH values and oxygen gas evolution at higher potentials >1.7 V. The procedure of combining a pH indicator, pH microelectrode measurements and pH-potential diagrams can be seen as an appropriate method to determine the recommendable working window of aqueous batteries.

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The reaction mechanisms (RM) during cycling of aqueous rechargeable Zn//MnO2 batteries (ARZIBs) are still controversially discussed. The present study of different acidic electrolyte compositions (0.9 mM H2SO4, 0.5 M MnSO4, 2 M ZnSO4, 2 M ZnSO4+0.5 M MnSO4) and their pH behaviour is therefore designed as an alternative approach to investigate the RM. In operando pH tracking during cycling shows periodic pH changes for each electrolyte, highlighting the role of the pH‐relevant ions OH− and H+ in the chemical processes, the major influence of MnO2 deposition/dissolution mechanisms and the buffering behaviour of the zinc hydroxide sulphate (ZHS) precipitation. Innovative coupled cyclic voltammetry (CV) and pH measurements can link CV redox peaks to a pH change and a corresponding chemical reaction. It was found that a Zn2+ (de‐)intercalation has little or no influence on the capacity. The cycling of the SO42−‐free electrolyte 2 M Zn(CF3SO3)2 underlines the pH‐dependant behaviour of the chemical processes. The results can contribute to the debate of RMs in ARZIBs and other aqueous battery chemistries by introducing a novel measurement technique.

Thermal runaway of single cells within a large scale lithium-ion battery is a well-known risk that can lead to critical situations if no counter measures are taken in today's lithium-ion traction batteries for battery electric vehicles (BEVs), plug-in hybrid electric vehicles (PHEV) and hybrid electric vehicles (HEVs). The United Nations have published a draft global technical regulation on electric vehicle safety (GTR EVS) describing a safety feature to warn passengers in case of a thermal runaway. Fast and reliable detection of faulty cells undergoing thermal runaway within the lithium-ion battery is therefore a key factor in battery designs for comprehensive passenger safety. A set of various possible sensors has been chosen based on the determined cell thermal runaway impact. These sensors have been tested in different sized battery setups and compared with respect to their ability of fast and reliable thermal runaway detection and their feasibility for traction batteries.

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