Efficient Scalable Hydrothermal Synthesis of MnO2 with Controlled Polymorphs and Morphologies for Enhanced Battery Cathodes

MnO₂ is a versatile, cost-effective transition metal oxide that has attracted interest in multiple domains, including as an active cathode material or catalyst for electrochemical energy storage in batteries. Hydrothermal methods are among the most efficient approaches for MnO₂ synthesis. These approaches enable facile, versatile production of MnO₂ in any of its crystalline phases (α, β, δ, etc.), with the dominant product being determined by reaction conditions such as precursor concentration and temperature. These benefits unfortunately come with impractically low product yields (∼9%) and long reaction times. Here, we report that low-molecular weight, water-soluble polymers function as effective nucleating agents for the hydrothermal synthesis of MnO₂. For fundamental reasons, these polymers are also reported to promote large (10- to 40-fold) increases in reaction rate and yield. We evaluate the physical and crystallographic features of the synthesized MnO₂ and find that depending upon the reaction conditions, the polymer-assisted synthesis yields dominantly δ-MnO₂ or β-MnO₂. The as-prepared δ-MnO₂ materials with extra interlayer water are studied as electrodes for aqueous Zn-ion battery applications and are shown to support long-duration storage at both moderate and high rates.