Theoretical Screening of Metal Borocarbide Sheets for High-Capacity and High-Rate Li- and Na-Ion Batteries

Two-dimensional (2D) materials with intrinsic metal properties are promising for battery applications. Herein, we theoretically design a series of ${X}_{2}\mathrm{BC}$ ($X=$ $\mathrm{Mg}$, $\mathrm{Ca}$, $\mathrm{Sr}$, $\mathrm{Ti}$, $\mathrm{V}$, $\mathrm{Mo}$) sheets as anode materials for $\mathrm{Li}$-ion and $\mathrm{Na}$-ion batteries (LIBs and NIBs) using first-principles calculations based on density functional theory. The ${X}_{2}\mathrm{BC}$ sheets are found to be intrinsically metallic with good stability. Besides, ${X}_{2}\mathrm{BC}$ ($X=$ $\mathrm{Mg}$, $\mathrm{Ca}$, $\mathrm{Sr}$) sheets are auxetic 2D materials. More interestingly, the highest predicted specific capacities are calculated to be 1502, 1042, and $905\phantom{\rule{0.2em}{0ex}}{\mathrm{mAh g}}^{\ensuremath{-}1}$ for ${\mathrm{Mg}}_{2}\mathrm{BC}$, ${\mathrm{Ca}}_{2}\mathrm{BC}$, and ${\mathrm{Ti}}_{2}\mathrm{BC}$, respectively. The activation energy barrier of $\mathrm{Na}$ on ${\mathrm{Ti}}_{2}\mathrm{BC}$ (0.105 eV) is the lowest among all the considered $\mathrm{Li}$ and $\mathrm{Na}$ ${X}_{2}\mathrm{BC}$ structures. The calculated open-circuit voltage values of 0.040 V (0.153 V) and 0.353 V (0.200 V) for $\mathrm{Li}$ ($\mathrm{Na}$) of ${\mathrm{Ca}}_{2}\mathrm{BC}$ and ${\mathrm{Ti}}_{2}\mathrm{BC}$, respectively, fall within the acceptable range of 0.1--1.0 V for anode materials. They are found to be smaller than or comparable with those of commercial graphite anode. The results show that ${X}_{2}\mathrm{BC}$ sheets are promising candidates for both LIBs and NIBs.