Microwave–Hydrothermal Crystallization of Polymorphic MnO2 for Electrochemical Energy Storage
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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 Faradaic reactivity sequence α- > γ- > β-MnO2 upon their tunnel structures, the intercalation–deintercalation reactivity of these MnO2 cathodes follows the order γ- > α- > β-phase, and the conversion reactivity of these MnO2 anodes follows the order γ- > α- > β-phase. MnCl2–KMnO4 reaction system can also lead to the mixed-phase MnO2 (β- and γ-MnO2), which can provide better anode performances for lithium-ion batteries. The current work deepens the fundamental understanding of several aspects of physical chemistry, for example, the chemical reaction controllable synthesis, crystal structure selection, electrochemical property improvement, and electrochemical reactivity, as well as their correlations.
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