Reversible Mn ²⁺ Dissolution-Deposition in Rechargeable Zn-MnO ₂ Cells Revealed by ¹ H Nuclear Magnetic Resonance Spectroscopy

Aqueous rechargeable zinc-ion batteries with MnO2 cathodes are well-suited for stationary energy storage; however, Mn2+ dissolution and related effects during discharge are thought to contribute to decreasing cell performance. The paramagnetic Mn2+ ion induces large perturbations to the magnetic resonance signature of H2O solvent molecules in the electrolyte solution, namely the chemical shift and R1 relaxation rate. In this work, linear correlations are established between the concentration of Mn2+ and the 1H chemical shift and R1. This relationship is used to follow the Mn inventory and quantify dissolved Mn2+ in the cells over many cycles. We show that Mn2+ dissolution is largely reversible and that Mn2+ dissolution-redeposition is a primary charge-storage mechanism in these cells. This work develops and implements an in situ methodology to monitor dissolution of Mn2+ during battery operation, leading to scientific insight into the reaction mechanisms governing the performance and capacity fade of rechargeable Zn-MnO2 batteries.