Modulating the Open-Circuit Voltage of Two-Dimensional MoB MBene Electrode via Specific Surface Chemistry for Na/K Ion Batteries: A First-Principles Study

Two-dimensional (2D) materials have been widely investigated as potential electrode materials for Na and K ion batteries because of their high specific surface area, favorable metallic conductivity, and desirable space to accommodate large metal ions. However, most of the 2D materials show limited affinity and low open-circuit voltage (OCV) toward Na and K ions, which could only be used as anodes. Development of appropriate technology to tune the OCVs of 2D materials and make them suitable for cathode application remains a great challenge. Herein, motivated by the substantial advance in experiment to control the surface atoms of 2D materials (Kamysbayev et al. Science 2020, 369 (6506), 979-983), we investigated the possibility of using oxygen group atoms (X = O, S, Se, and Te) to modulate the OCV and electrochemical performance of an experimentally available 2D MoB electrode. A strong combination between X atoms and MoB was identified in the resulting MoBX, retaining favorable metallic conductivity and excellent mechanical performance. The OCVs of MoBX compounds exhibit obvious surface-atom-type dependency, where Na0.5MoBO and K0.5MoBO showed average OCVs of 2.7 and 2.7 V, respectively, suitable for cathode application. In contrast, the OCVs of MoBS, MoBSe, and MoBTe are significantly smaller (0.08-0.38 V) and are only favorable for anode material application. The capacities of Na0.5MoBO and K0.5MoBO cathodes are 110 and 110 mAh/g, respectively, and the theoretical specific capacities of Na2MoBS, Na2MoBSe, and Na2MoBTe anodes are 386, 289, and 229 mAh/g, respectively. Moreover, shallow and steady intercalation/deintercalation resistance of the Na and K ions in MoBX at the dilute limit indicates excellent rate performance and high cyclic stability. These results open a new avenue and broaden the fields of endeavor to ameliorate the performance of 2D materials for cathode application.