Boron-doped g-CN monolayer as a promising anode for Na/K-ion batteries

Graphitic carbon nitride (CxNy) has a two-dimensional structure similar to N-doped graphene, which is nitrogen-rich and contains porous defect sites that can serve as atomic storage sites for alkali metals. Compared with C3N4, the most widely reported carbon nitride family, g-CN is structurally stable without graphitic nitrogen. However, the adsorption energy of Li/Na/K in the pore size is too negative (-2.60/-2.99/-3.11 eV) to be desorbed, and the high diffusion barrier of Li/Na/K ion (2.94/3.11/2.43 eV) implies the low charge/discharge rate for metal-ion batteries. Here, we showed that doping B atoms into the pore sites of g-CN (BC3N3) possess some advantages for application in batteries. Specifically, the BC3N3 monolayer has excellent stability, with moderate adsorption ability for Na/K (-0.54/-0.98 eV) and a much lower diffusion barrier of ion (0.73/0.35 eV). BC3N3 shows excellent theoretical specific capacity (603.30/904.95 mAh/g) and low average open-circuit voltage as an anode material for Na/K-ion batteries. Moreover, under a small amount of alkali metal adsorption, BC3N3 exhibits metallic properties and the electrical conductivity of BC3N3 is improved. These favorable properties indicate that the B-doped g-CN monolayer is a promising anode material for Na/K ion batteries and this approach can be extended to other porous carbon-nitrogen structures.