First-principles study on promoting the performance of graphene as an anode material for alkali metal ion batteries by covalent cross-linking of rigid molecules

In this work, to study the influence of covalent cross-linking on the properties of a layered carbon structure, bilayered graphenes with a larger layer distance were built, supported by covalently cross-linked molecules, 1,4-phenylene (GPG) or <i>p</i>-phenylenediamine (GPPG). Based on first-principles calculations, it was found that both structures are thermally stable at a temperature of 500 K. The internal covalently cross-linked molecule can effectively expand the layer distance, enhance the adsorption stability, increase the theoretical specific capacity, and reduce the diffusion energy barrier. GPG has a theoretical specific capacity of 770.65/1541.29/1233.04 mA h g^-1 for Li/Na/K ions, while GPPG has larger values of 2307.56/1442.23 mA h g^-1 for Na/K ions. As anode materials of Li/Na/K ion batteries, both GPG and GPPG display a stable voltage output during the charging/discharging process. The diffusion energy barriers of Li, Na and K ions far from the supported molecule in GPG/GPPG are 0.21/0.31, 0.05/0.15 and 0.03/0.16 eV, respectively. Furthermore both GPG and GPPG show excellent cycling stability based on appropriate variation of their interlayer spacing during the adsorption process.