ICP-OES for the investigation of reaction and aging mechanisms in aqueous acidic Zn-MnO2 batteries

Summary:

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 mechanisms (RMs) of ARZMBs reported in the literature are still controversially discussed, namely the following:
(1) Zn plating/stripping,
(2) Zn2+ (de-) intercalation (theoretically “Zn2+ neutral” during cycling together with mechanism (1)),
(3) MnO2 conversion with H+,
(4) Mn2+/MnO2 deposition/dissolution (with high influence on the Mn2+ concentration, highlighted by latest literature),
(5) Metal hydroxide (MHO) precipitation (e.g. zinc hydroxide sulfate (ZHS) leading to Zn2+ ion loss),
(6) Hydrogen/Oxygen Evolution Reaction (HER/OER, leading to H2O and electron loss).
Another important topic of research is the corrosion of the current collectors in the aqueous environment, e.g. of highly-available stainless steel material. The knowledge about both the RMs and the corrosion phenomena is highly important for the construction of industry-oriented and long-term stable battery cells.
On this basis, this poster uses Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) for the determination of the elemental composition of a 2 M ZnSO4 + 0.5 M MnSO4 electrolyte in different states of charge (SOC, here: rest phase, initial discharge, 1st charge/1st discharge). For this purpose, an appropriate experimental cell setup was designed with the ability of electrolyte sample collection. The ICP-OES results show differences in the Mn2+ and Zn2+ concentration in relation to the SOC, which can be linked to the cell cycling results and, by using Faraday’s law, transferred to theoretical charge differences. Regarding the above-mentioned RMs, the following preliminary conclusions can be drawn: (1) deviations from the charge conservation principle during the cycling of the ARZMB (especially regarding Zn) could be linked to Zn corrosion/HER and MHO precipitation reactions, (2) the Mn2+/MnO2 deposition/dissolution could be confirmed with regard to the correlation of the cell cycling capacity and the observed Mn2+ charge difference. Furthermore, the current collector corrosion was investigated, however, with no corrosion products detectable during the first cycles in the scope of this work.
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 RMs in ARZMBs. 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.
Further investigations (e.g. comparison of different electrolyte compositions and current collector materials) can deepen the understanding of ion concentration changes during the cycling of a ARZMB.