Manganese oxides transformed from orthorhombic phase to birnessite with enhanced electrochemical performance as supercapacitor electrodes

Abstract

Manganese oxides (MnO₂) are a class of promising electrode materials for high-performance supercapacitors. Among various possible crystal phases of MnO₂, the two-dimensional layered birnessite (δ-MnO₂) is considered to facilitate cation intercalation/deintercalation with little structural rearrangement during the charging and discharging process. Here, we report a facile strategy for synthesis of δ-MnO₂ with significantly enhanced electrochemical capacitive energy storage performance via phase transformation of orthorhombic MnO₂. The resultant δ-MnO₂ delivers a specific capacitance of 251.4 F g⁻¹ at a current density of 1 A g⁻¹, which is almost three times higher than that of the original orthorhombic phase. The solid-state flexible asymmetric supercapacitor based on birnessite MnO₂ as the positive electrode possesses an operating potential window as high as 2.0 V as well as high energy density and power density. This work demonstrates the post crystal engineering of MnO₂ is an effective way to optimize their electrochemical properties.