Facile molten salt synthesis of Cs-MnO₂ hollow microflowers for supercapacitor applications

A facile molten salt technique is an interesting preparation method as it enables mass production of materials. With the use of CsNO₃ salt, Cs-intercalated MnO₂ hollow microflowers are obtained in this work. δ-MnO₂ with a layered structure, instead of other allotropes with smaller structural cavities, is formed and stabilized by large Cs⁺ ions. Formation of the hollow microflowers is explained based on the Ostwald ripening process. The salt to starting agent ratio has little effect on the crystal structure and morphologies of the products but does influence the crystallinity, the interlayer distance, and the intercalating Cs⁺ content. The capacity of Cs⁺ in the structure and the interlayer distance are maximized when the weight ratio of CsNO₃ : MnSO₄ is 7 : 1. Cs-MnO₂ obtained from this optimum ratio has most suitable crystallinity and interlayer distance, and consequently shows a highest specific capacitance of 155 F g^-1 with excellent cycling performance. The obtained specific capacitance is comparable to that of other alkaline-intercalated MnO₂, suggesting that Cs-MnO₂ could be another interesting candidate for supercapacitor electrodes.