In situ insertion of shutter ions in MnO2 to boost the supercapacitive performance of flexible supercapacitors

Abstract

Supercapacitors have been acquiring widespread attention as novel energy storage devices, benefiting from their ultrahigh power density, rapid charge/discharge rate, and excellent cycling durability. However, their energy density still faces significant challenges. To meet the industrial level of energy density, herein, the strategy of combining modification of electrode active materials with construction of asymmetric supercapacitors is applied to extend the voltage and thus increase the energy density of energy storage devices. From the aspect of modifying active materials, we exploit a fascinating method of in situ insertion of a series of shutter ions (M⁺ = Li⁺, Na⁺, K⁺) into an akhtenskite-type MnO₂ (ε-MnO₂) matrix, in situ growing on acid-etched carbon cloth (ACC), to enhance the specific capacitance of supercapacitors. The comparison of the supercapacitive performance of the electrodes based on the obtained active materials δ-(M⁺_yMn³⁺_yMn⁴⁺_1−y)O²⁻₂@ACC shows that the δ-(Na⁺_0.48Mn³⁺_0.48Mn⁴⁺_0.52)O²⁻₂@ACC electrode exhibits the best electrochemical performance with a wide voltage window, the largest areal capacitance, and the best cycling durability among them due to the best matching of the Na⁺ hydrated diameter (7.1 Å) with the interlayer distance of δ-MnO₂ (7.0 Å). For constructing asymmetric supercapacitors, an aqueous asymmetric supercapacitor (A-ASC) using 1.5 M Na₂SO₄ aqueous solution as electrolyte was assembled from the δ-(Na⁺_0.48Mn³⁺_0.48Mn⁴⁺_0.52)O²⁻₂@ACC cathode and the Fe₂O₃ anode, in situ growing on ACC. As anticipated, this device exhibits a large energy density (587 μW h cm⁻²) with a wide voltage window (2.5 V) as well as ideal cycling stability with a capacitance retention of 85% after charge/discharge for 10 000 cycles. This work has systematically investigated the size effect of inserted shutter alkali metal ions with step-wise increased ionic diameters on the morphology, structural transformations, charge storage mechanisms and the electrochemical performances of ε-MnO₂ based electrodes for the first time, which provides a new promising synthesis strategy to develop asymmetric supercapacitors with a wide-voltage window and large energy density.