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 X2BC (X = Mg, Ca, Sr, Ti, V, Mo) sheets as anode materials for Li-ion and Na-ion batteries (LIBs and NIBs) using first-principles calculations based on density functional theory. The X2BC sheets are found to be intrinsically metallic with good stability. Besides, X2BC (X = Mg, Ca, Sr) sheets are auxetic 2D materials. More interestingly, the highest predicted specific capacities are calculated to be 1502, 1042, and 905 mAh g(-1) for Mg2BC, Ca2BC, and Ti2BC, respectively. The activation energy barrier of Na on Ti2BC (0.105 eV) is the lowest among all the considered Li and Na X2BC structures. The calculated open-circuit voltage values of 0.040 V (0.153 V) and 0.353 V (0.200 V) for Li (Na) of Ca2BC and Ti2BC, 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 X2BC sheets are promising candidates for both LIBs and NIBs.