Are transition-metal borides promising for Na ion batteries? A first-principles study on transition-metal boride monolayer

Although two-dimensional (2D) materials have been proposed as the promising candidates for Na ion batteries (NIBs), electrode materials with high specific capacity and moderate diffusion energy barriers are still scare. Here, we firstly demonstrated that transition-metal borides (TMB) are much more promising electrode materials than other transition metal compounds. Density functional theory (DFT) calculations are performed to investigate the electronic properties and Na storage capability of TMB monolayer, which is realized in recent experiments. TMB monolayer forms strong chemical interaction with Na atoms, and the diffusion energy barrier of Na atoms is much lower than LIBs. Importantly, TMB monolayer exhibits a very high Na storage capacity. Such exceptional properties, including high stoichiometry (namely TMBNa₂), excellent electronic conductivity, moderate Na diffusion and high operating voltage, endow TMB monolayers as very promising anode materials for NIBs.