Hexagonal Mg 2 B 2 and Ca 2 B 2 monolayers as promising anode materials for Li-ion and Na-ion batteries.

Exploring new high-performance anode materials is of great significance for next generation renewable energy technologies. Two-dimensional (2D) transition metal borides, named as MBenes, are receiving increasing attention in energy storage fields due to their structure-and-composition diversities and high electrical conductivities. Expanding the family of hexagonal MBenes to non-transition metal borides, we report two hexagonal IIA metal M 2 B 2 -typed MBenes and demonstrate the feasibility of exfoliation and application as anode materials for Li-ion batteries (LIBs) and Na-ion batteries (SIBs) using ab initio calculations. The predicted 2D Mg 2 B 2 and Ca 2 B 2 monolayers exhibit robust thermal and mechanical stabilities, inherent metallic properties, electrical conductivities, and excellent electrode performance. The outstanding theoretical specific capacities of Mg 2 B 2 and Ca 2 B 2 for Li/Na ions reach up to 764/527 and 859/527 mA h g -1 , respectively. Furthermore, the average open-circuit voltages (OCVs) of Mg 2 B 2 and Ca 2 B 2 range from 0.275 to 0.395 V, indicating their suitability for anode materials. Additionally, the low diffusion energy barriers of Mg 2 B 2 and Ca 2 B 2 for Li/Na ions at delithiated/desodiated states (27/14 and 16/12 meV) and lithiated/sodiated states (187/58 and 52/34 meV) illustrate their excellent charge-discharge capabilities. These intriguing findings demonstrate the superior performance of the predicted IIA metal boride monolayers as promising anode materials for LIBs and SIBs and break through the limitations of transition metal MBenes as anode materials in terms of overall performance. Our study opens the door to the application of the main group metal MBenes in ion energy storage.