First principles investigation on Na-ion storage in two-dimensional boron-rich B₂N, B₃N, and B₅N

Na-ion batteries (SIBs) are emerging as a promising alternative to Li-ion batteries for large-scale energy storage in light of abundant Na resources and their low cost. Development of appropriate electrode materials that can conquer some critical issues such as low theoretical storage capacity and sluggish redox kinetics resulting from the larger radius of Na is urgently needed for their practical applications. In this work, boron-rich 2D BN (x = 2, 3, and 5) has been explored as promising anode materials for high-performance SIBs based on density functional theory calculations. BN electrodes exhibit moderate affinity toward Na-ions with adsorption energies of -0.41 to -1.21 eV, which allows stable Na-ion intercalation without the formation of metal dendrites. Moreover, both B₃N and B₅N deliver low diffusion barriers (0.28 and 0.08 eV) for Na-ion migration, guaranteeing a high charging/discharging rate. More importantly, these BN anodes exhibit not only a remarkably high theoretical capacity of 1129-1313 mA h g^-1 but also a low open-circuit voltage (0.45-0.87 V), which is important to achieve high energy density. AIMD simulations have confirmed the excellent cyclability of BN electrodes during reversible lithiation/delithiation. These results suggested that the BN electrode could be used as a new lightweight SIB anode with high capacity, cyclability, and desired rate performance.