At these days, lithium-ion batteries are the technology of choice for a variety of applications. However, due to the availability of raw materials and safety issues, the research on alternative battery technologies is getting more and more attention in the recent years. Among others, the aqueous rechargeable Zn-MnO2 battery (ARZMB) chemistry is a promising candidate, especially in the field of stationary applications with high requirements in terms of safety, environmental friendliness, material availability, long-term stability and specific energy costs. However, the reaction mechanism of ARZMBs is still controversially discussed. Recent publications question the existence of the Zn2+ (de-)intercalation and, in turn, highlight the Mn2+/MnO2 deposition/dissolution as the major charge storage mechanism. Another important topic of research is the corrosion of the current collectors in the aqueous environment, e.g. of high-available stainless steel material. The knowledge about both the reaction mechanism and the corrosion phenomena is highly important for the construction of industry-oriented and long-term stable battery cells.
On this basis, the present study uses Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) for determining the elemental composition of ZnSO4/MnSO4 based electrolytes in (a) different states of charge (initial state, after resting, initial discharge, 1st charge/discharge) and (b) different aging states (1st cycle, 100th cycle). For this, an appropriate innovative experimental cell setup was designed with the ability of electrolyte sample collection. The results show differences in the Mn2+ and Zn2+ concentration in relation to the state of charge, which can be related to the reaction mechanism with the predominant Mn2+/MnO2 deposition/dissolution. Furthermore, with proceeding cycles, changes in the electrolyte composition in relation to the pristine Mn2+/Zn2+ loading, as well as in consequence of current collector corrosion phenomena can be related to capacity fading and aging mechanisms.
Altogether, this study introduces the ICP-OES as a powerful characterization method, enabling a direct determination of the elemental electrolyte composition for a deeper understanding of the reaction and aging mechanisms in ARZMBs. The results make an important contribution to the question of the ARZMB reaction mechanism, which has not yet been finally clarified. Moreover, the (long-term) stability of the passive materials in the battery cell can rapidly be investigated, which facilitates the material selection for industry-oriented battery cells.