Reversible Mn2+ Dissolution-Deposition in Rechargeable Zn-MnO2 Cells Revealed by 1H 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.