Tailoring Manganese Oxide Cathodes via Basic-Medium Synthesis for Enhanced Zn2+ Storage: Unraveling Interfacial Kinetics and Transport Limitations

Abstract

Manganese-based aqueous rechargeable zinc-ion batteries (AZIBs) are emerging as highly viable alternatives to lithium-ion systems for grid-scale energy storage, driven by their cost-effectiveness, intrinsic safety, and high theoretical capacity. However, their commercial deployment is currently hindered by performance degradation stemming from structural instability during prolonged cycling. This study systematically investigates the influence of the synthesis environment on the structural evolution and electrochemical performance of manganese oxide cathodes. By manipulating the reaction medium, we demonstrate that alkaline environments yield Hollandite-type NaxMn8O16 (h-MO) with hierarchical flower-like morphology, whereas near-neutral conditions produce aggregated Mn3O4 (MO) spherical particles. Comprehensive characterization via XRD, ICP-OES, Raman spectroscopy, XPS, SEM, TEM, and BET corroborates this distinct phase and morphological divergence. Electrochemical evaluations reveal that the h-MO cathode delivers a superior reversible capacity (252 mAh·g–1 at 50 mA·g–1) compared to MO counterpart (52 mAh·g–1). Furthermore, h-MO exhibits good cycling stability, displaying an 8% capacity increase over 160 cycles at 200 mA g–1. Ex situ XRD and SEM analyses reveal a reversible H+/Zn2+ co-storage mechanism accompanied by reversible zinc hydroxide sulfate formation. Crucially, operando electrochemical impedance spectroscopy (EIS) coupled with distribution of relaxation times (DRT) analysis was employed to deconvolute the dynamic electrochemical processes of the h-MO cathode during cycling. This advanced diagnostic approach provides profound insights into the evolution of interfacial charge-transfer and ion-transport kinetics. These findings highlight the importance of synthesis-driven control in tailoring manganese oxide cathodes and offer guidance for the rational design of advanced AZIB systems.