Simultaneous Elucidation of Solid and Solution Manganese Environments via Multiphase Operando Extended X-ray Absorption Fine Structure Spectroscopy in Aqueous Zn/MnO2 Batteries

Abstract

Aqueous Zn/MnO2 batteries (AZMOB) with mildly acidic electrolytes hold promise as potential green grid-level energy storage solutions for clean power generation. Mechanistic understanding is critical to advance capacity retention needed by the application but is complex due to the evolution of the cathode solid phases and the presence of dissolved manganese in the electrolyte due to a dissolution-deposition redox process. This work introduces operando multiphase extended X-ray absorption fine structure (EXAFS) analysis enabling simultaneous characterization of both aqueous and solid phases involved in the Mn redox reactions. The methodology was successfully conducted in multiple electrolytes (ZnSO4, Zn(CF3SO3)2, and Zn(CH3COO)2) revealing similar manganese coordination environments but quantitative differences in distribution of Mnn+ species in the solid and solution phases. Complementary Raman spectroscopy was utilized to identify the less crystalline Mn-containing products formed under charge at the cathodes. This was further augmented by transmission electron microscopy (TEM) to reveal the morphology and surface condition of the deposited solids. The results demonstrate an effective approach for bulk-level characterization of poorly crystalline multiphase solids while simultaneously gaining insight into the dissolved transition-metal species in solution. This work provides demonstration of a useful approach toward gaining insight into complex electrochemical mechanisms where both solid state and dissolved active materials are important contributors to redox activity.