Electron-Transfer Mechanism in P2–Na_0.67MnO₂/Graphene Electrodes: Experimental and First-Principles Investigations

The wide application of P2–Na_0.67MnO₂ as electrode materials is full of challenges, including poor electric conductivity, rate capability, and structural transition. Na_0.67MnO₂ 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 Na_0.67MnO₂ wrapped by graphene oxide rolls (GO/Na_0.67MnO₂). Electrochemical impedance spectroscopy (EIS) and density functional theory (DFT) calculations suggest a faster diffusion of sodium ions and better electric conductivity in the GO/Na_0.67MnO₂ system. Additionally, calculated migration barriers indicate that the graphene–Na_0.67MnO₂ 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/Na_0.67MnO₂ than that in pure Na_0.67MnO₂ at large current densities. These improvements are credited to the massive amount of electrons transferred from graphene to Na_0.67MnO₂. Importantly, we propose that the electron-transfer mechanism is closely related to the work function of materials. For GO/Na_0.67MnO₂, graphene with a higher Fermi level and lower work function provides a large number of electrons to Na_0.67MnO₂, increasing the Fermi level of Na_0.67MnO₂ and thus enhancing ion diffusion capability and electric conductivity. Our work gives a comprehensive understanding of the electron-transfer mechanism of GO/Na_0.67MnO₂, and it provides a feasible scheme to study the electrochemical properties of Na_0.67MnO₂ composites.