Monolayer BC₂: an ultrahigh capacity anode material for Li ion batteries

There is great interest in developing promising candidate materials for high-capacity, low cost, environmentally friendly, longer cycle life anodes for lithium ion batteries. Due to better Li adsorption properties than graphene, boron doped graphene has been considered to be an attractive anode material for Li-ion batteries. Using first principles density functional theory calculations, we investigate the effect of increasing boron concentration on the gravimetric capacity of monolayered boron doped carbon sheets. The calculations are performed for uniformly boron doped carbon sheets, BC_x (x = 7, 5, 3, 2 and 1) as well as their non-uniformly doped counterparts, which are found to be energetically preferable for x = 5, 2 and 1. Our results indicate pronounced enhancement in gravimetric capacity with increasing concentration of B, up to x = 2. The storage capacity of the uniformly doped BC₂ turns out to be the highest ever reported for B doped graphene sheets, which is 1.9 times (1667 mA h g^-1) that of the previously reported value for BC₃ (J. Phys. Chem. Lett., 2013, 4, 1737-1742). This dramatic increase in the capacity of uniformly doped BC₂ occurs because of the availability of significantly more empty states above the Fermi level compared to the other BC_x sheets. Moreover, the diffusion energy barriers and open circuit voltage are found to be lower in uniformly doped BC₂, leading to better Li kinetics. For x = 1, Li binds very strongly to the uniformly doped BC and higher diffusion energy barriers are found for non-uniformly doped BC, rendering them ineffective as anode materials. Our study reveals that BC₂ is the most promising candidate as an anode material for Li ion batteries owing to its high Li storage capacity combined with low diffusion barrier and low open circuit voltage.