Electron-Transfer Mechanism in P2-Na0.67MnO2/Graphene Electrodes: Experimental and First-Principles Investigations

The wide application of P2-Na0.67MnO2 as electrode materials is full of challenges, including poor electric conductivity, rate capability, and structural transition. Na0.67MnO2 composited with conductor materials is a universal approach to tackling these issues. However, the electron-transfer mechanism in these composites is not clearly understood. Herein, we prepare Na0.67MnO2 wrapped by graphene oxide rolls (GO/Na0.67MnO2). Electrochemical impedance spectroscopy (EIS) and density functional theory (DFT) calculations suggest a faster diffusion of sodium ions and better electric conductivity in the GO/Na0.67MnO2 system. Additionally, calculated migration barriers indicate that the graphene-Na0.67MnO2 layer can store a large amount of sodium ions with a small ion diffusion barrier of 0.416 eV, confirming the superior rate performance of GO/Na0.67MnO2 than that in pure Na0.67MnO2 at large current densities. These improvements are credited to the massive amount of electrons transferred from graphene to Na0.67MnO2. Importantly, we propose that the electron-transfer mechanism is closely related to the work function of materials. For GO/Na0.67MnO2, graphene with a higher Fermi level and lower work function provides a large number of electrons to Na0.67MnO2, increasing the Fermi level of Na0.67MnO2 and thus enhancing ion diffusion capability and electric conductivity. Our work gives a comprehensive understanding of the electron-transfer mechanism of GO/Na0.67MnO2, and it provides a feasible scheme to study the electrochemical properties of Na0.67MnO2 composites.