Microwave–Hydrothermal Crystallization of Polymorphic MnO2 for Electrochemical Energy Storage

Abstract

We report a coupled microwave–hydrothermal process to crystallize polymorphs of MnO2 such as α-, β-, and γ-phase samples with plate-, rod-, and wirelike shapes, by a controllable redox reaction in MnCl2–KMnO4 aqueous solution system. MnCl2–KMnO4 redox reaction system was for the first time applied to MnO2 samples under the coupled microwave–hydrothermal conditions, which shows clear advantages such as shorter reaction time, well-crystallized polymorphic MnO2, and good electrochemical performances as electrode materials for lithium ion batteries. For comparison, we also did separate reactions with hydrothermal only and microwave only in our designed MnCl2–KMnO4 aqueous system. The present results indicate that MnCl2–KMnO4 reaction system can selectively lead to α-, β-, and γ-phase MnO2, and the as-crystallized MnO2 samples can show interesting electrochemical performances for both lithium-ion batteries and supercapacitors. Electrochemical measurements show that the as-crystallized MnO2 supercapacitors have...