Microwave-hydrothermal crystallization of polymorphic MnO₂ for electrochemical energy storage

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. MnCl ₂-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. MnCl ₂-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.