SiC₃N₃ monolayer as a universal anode for alkali metal-ion batteries

Our density functional theory calculations show that silicon doping in g-CN (SiC₃N₃) can improve the electrochemical performance of g-CN as an anode of alkali metal-ion batteries and solve the problems of too high adsorption ability and migration energy barrier commonly found in porous carbon nitride. The stability of SiC₃N₃ was verified by molecular dynamics simulations and phonon spectroscopy. Elastic constant calculations revealed that the Si doping in g-CN can improve its mechanical properties. Specifically, Li/Na/K has a suitable adsorption capability (-0.71/-0.52/-0.98 eV) and a lower migration barrier (0.73/0.43/0.21 eV) on SiC₃N₃, where the barrier of a single Li-ion is the lowest among the doped porous carbon nitride materials studied so far. Moreover, SiC₃N₃ exhibits a high theoretical capacity (253/1512/1512 mA h g^-1) and a low open-circuit voltage (0.48/0.18/0.31 V) for Li/Na/K ion batteries. Compared with B-doped g-CN previously studied, Si doping can more effectively improve the electronic conductivity of g-CN owing to greater charge transfer between Si and g-CN; the migration energy barrier of alkali metal ions on SiC₃N₃ is reduced more significantly due to its puckered structure instead of a planar structure; and the capacity of SiC₃N₃ is nearly doubled for alkali metal ion batteries because it has more feasible adsorption sites for alkali metals. These results suggest that Si-doped g-CN can be a universal anode material for alkali metal ion batteries.