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 MnO 2 cathodes are well-suited for stationary energy storage; however, Mn 2+ dissolution and related effects during discharge are thought to contribute to decreasing cell performance. The paramagnetic Mn 2+ ion induces large perturbations to the magnetic resonance signature of H 2 O solvent molecules in the electrolyte solution, namely the chemical shift and R 1 relaxation rate. In this work, linear correlations are established between the concentration of Mn 2+ and the 1 H chemical shift and R 1 . This relationship is used to follow the Mn inventory and quantify dissolved Mn 2+ in the cells over many cycles. We show that Mn 2+ dissolution is largely reversible and that Mn 2+ dissolution-redeposition is a primary charge-storage mechanism in these cells. This work develops and implements an in situ methodology to monitor dissolution of Mn 2+ during battery operation, leading to scientific insight into the reaction mechanisms governing the performance and capacity fade of rechargeable Zn-MnO 2 batteries.