Novel two-dimensional C6B4 monolayer as an anode for Li-/Na-ion batteries with high theoretical capacity

•k-C6B4 on the adsorbed Li/Na exhibited metallic characteristics.•The maximum values of Young's modulus and Poisson's ratio of k-C6B4 are 272.39 N/m and 0.4, respectively, with good ductility.•k-C6B4 has a low open circuit voltage and a relatively low diffusion energy barrier of 0.195 eV and 0.123 eV for Li/Na ions.•k-C6B4 has a high specific capacity of 1395 mAhg−1 for both Li/Na ions.