Investigation into the origin of high stability of δ-MnO₂ pseudo-capacitive electrode using operando Raman spectroscopy

Manganese oxides of different structures, especially α-MnO₂, have been extensively studied as electrodes for pseudocapacitors. However, the poor stability associated with intercalation of proton has been the main obstacle to their commercial applications. To effectively mitigate this problem, it is necessary to fully understand the energy storage mechanism of the MnO₂ phases. In this study, δ phase MnO₂ has been synthesized through controllable electroplating on architectural Ga-doped ZnO (GZO) bones, demonstrating a high specific capacitance of 1068 F g⁻¹ and high stability (slight performance drop focus on the first 2000 cycles and then remained relatively constant in the subsequent 13,000 cycles). The charge storage mechanism of the δ-MnO₂ coated GZO has been carefully investigated under this extreme reaction condition. Results suggest that the amount of charge stored in the electrode material correlates well with the amount of Na⁺ inserted into the electrode material from the electrolyte. It is also noted that no spectral features corresponding to H⁺ insertion were detected during cycling when the sample was probed using in operando Raman spectroscopy. Therefore, for layered δ-MnO₂, a charge storage mechanism of Na⁺ intercalation/deintercalation dominated, accompanied by interlayer spacing expansion/contraction, was proposed. Moreover, theoretical calculations also confirmed that the insertion of Na⁺ is more energetically favorable than H⁺ at all sites of the interlayer in δ-MnO₂, offering a rational explanation of the proposed mechanism and the observed excellent stability.