Abstract: Two-dimensional carbon nitride is considered a very good battery electrode material owing to its uniform-size pores and the presence of nitrogen atoms. First-principles calculations were used to investigate the adsorption and storage of lithium and sodium on monolayer g-C₂N. The capacities of lithium and sodium ion batteries for monolayer g-C₂N are 596 (LiC₂N) and 276 (NaC₄N₂) mAh/g, respectively. The average Li binding energy reaches 2.39 eV relative to isolated Li atoms, which suggests that the lithium capacity achieved on g-C₂N might not be sustained during cycling. By varying the ratio of C to N atoms, it is found that the average Li binding energy is reduced to only 1.69 eV for C:N~5:1, indicating a significant improvement in cycling performance while maintaining the reversible capacity. The mobility barrier energies to Li ion diffusion between two layers in bulk structures with AA and AB stacking sequences are 0.25 and 1.23 eV, respectively, indicating that high Li ion conductivity could be achieved in bulk g-C₂N with AA stacking. These calculations demonstrate that graphitic carbon nitride with uniform-size pores can be used as an electrode material with high capacity and high lithium mobility.