Improved Transport Properties and Novel Li Diffusion Dynamics in van der Waals C ₂ N/Graphene Heterostructure as Anode Materials for Lithium-Ion Batteries: A First-Principles Investigation

In this paper, we report a theoretical investigation of the electronic structures, electron/phonon transport properties, and electrochemical parameters of the C ₂ N/graphene bilayer. The p-type C ₂ N/graphene bilayer, with a direct band gap of 0.2 eV at Î"-point, exhibits promising electric conductivity similar to that of the graphene monolayer. In addition, it also shows excellent lattice thermal conductivity of 1791.1 W/m·K, compared to 82.22 W/m·K of the C ₂ N monolayer. The theoretical capacity of C ₂ N/graphene in Li-ion batteries is found to be 490.0 mA h/g. For Li diffusion, the energy barriers for the energetically favorable diffusion pathways are found to be in the range of 0.2-0.5 eV for both C ₂ N monolayer and C ₂ N/graphene bilayer. The planar diffusion coefficients of the Li atom on C ₂ N and C ₂ N/graphene materials are predicted to be 2.97 × 10 ^-11 and 4.74 × 10 ^-11 m ² /s at 300 K, respectively, comparable with that of the graphene monolayer. With the help of first-principles molecular dynamics (FPMD) simulations at low temperature, it has been revealed that the Li atoms either absorbed or intercalated in the C ₂ N/graphene heterostructure, which could migrate easily in the vertical direction through the large hole of the C ₂ N atomic layer, and these ascended Li atoms together with absorbed Li atoms on the upper surface of the C ₂ N monolayer are able to hop further away from the substrate, giving the strongly absorbed inner Li layer and weakly attached outer Li layer on the top of the C ₂ N atomic layer. The outer Li atoms are mainly responsible for the ionic diffusion at room temperature. The hopping process between the nearest adsorption sites, which is obtained from routine nudge elastic band calculations, is only seen in FPMD simulations at high temperatures (>800 K).