Jahn-Teller Effect Directed Bandgap Tuning of Birnessite for Pseudocapacitive Application

Birnessite MxMnO2 (M = Na+, K+, etc.) has emerged as a promising alternative to the classical MnO2 material owing to its improved pseudocapacitive performance for energy storage. Understanding their structure-property correlation is essential for the development and application of advanced supercapacitors. Herein, we adopt the crystal field theory and density functional simulation to reveal the structural dependence of the pseudocapacitive property of KxMnO2. Attributing to the Jahn-Teller effect of Mn3+, the bandgap of KxMnO2 can be tuned by changing the x value (i.e., the Mn(III)/Mn(IV) ratio). Then, we design a narrow-bandgap K0.25MnO2 (0.84 eV), which affords a high capacitance of 415 F g(-1) at 1 A g(-1) and a desirable rate capability of 293 F g(-1) at 20 A g(-1). Operando Raman spectroscopy confirms that the Jahn-Teller induced structure evolution of [MnO6] octahedron accounts for the superior pseudocapacitive behavior of K0.25MnO2. This finding offers theoretical guidance to the design and application of birnessite materials for pseudocapacitors.