Prediction of boridenes as high-performance anodes for alkaline metal and alkaline Earth metal ion batteries

We conducted a comprehensive density functional theory investigation using the r2SCAN-rVV10 functional on the structural stability and electrochemical properties of boridenes for their use as anode materials in rechargeable alkaline (earth) metal-ion batteries (Li+, Na+, K+, Mg2+ and Ca2+). According to first-principles molecular dynamics simulations and reaction thermodynamic calculations, Mo4/3B2(OH)2 and Mo4/3B2F2 are unstable in the presence of alkaline (earth) metal ions due to the surface-conversion reactions between the surface terminations and adsorbates. Meanwhile, the bare Mo4/3B2 and Mo4/3B2O2 monolayers not only can accommodate alkaline (earth) metal ions, but also form stable multi-layer adsorption structures for most of the studied metal ions (Li+, Na+, K+, Mg2+ and Ca2+). The predicted gravimetric capacities of the bare Mo4/3B2 monolayer (Mo4/3B2O2) are 625.9 mA h g-1 (357.3 mA h g-1), 247.20 mA h g-1 (392.1 mA h g-1), 101.8 mA h g-1 (206.4 mA h g-1), 667.0 mA h g-1, and 413.0 mA h g-1 (485.4 mA h g-1) for Li+, Na+, K+, Mg2+ and Ca2+ ions, respectively. The bare Mo4/3B2 exhibits lower onset charging open circuit voltages for alkaline (earth) metal ions than that of Mo4/3B2O2. The diffusivities of the metal ions were revealed to be high on the boridene monolayer especially for the outer fully stable adsorption layers, where the migration energy barriers were found to be less than 0.10 eV. Similar to that of MXenes, the negative electron cloud (NEC) also plays a vital role in stabilizing the observed multi-layer adsorption structures for various metal ions on either the bare Mo4/3B2 or Mo4/3B2O2 monolayer.