High-power two-dimensional molybdenum boride MBene electrodes for lithium-ion batteries and capacitors

Abstract

There is a need for new electrochemical energy storage materials that can handle high cycling rates (high power) for rapid charging without compromising high energy density, such as high-power Li-ion batteries (LIBs) and Li-ion capacitors (LICs). Electrically conductive and redox-active two-dimensional (2D) materials, such as transition metal carbides and borides, are promising candidates for these applications. Tailoring in-plane chemically ordered MAB phases (i-MAB) has facilitated the synthesis of their 2D derivatives (i-MBenes), which possess ordered vacancies at the metal sites. The first reported i-MBene paper is Mo_4/3B₂T_x, which is derived from the parent i-MAB phase (Mo_2/3Y_1/3)₂AlB₂ by the selective etching of Al and Y. In this study, we report on the synthesis of 2D Mo_4/3B₂T_x aerogel and its electrochemical performance as an electrode material for LIBs. Our aerogel exhibits remarkable stability during life-cycling testing at high applied specific currents, maintaining a specific capacity of 260 mAh g⁻¹ even after completing 500 cycles under a high specific current of 2 A g⁻¹. At a moderate specific current of 100 mA g⁻¹, it delivers an energy density of 363 Wh kg⁻¹, while at a high specific current of 2 A g⁻¹, it achieves a specific power of 1300 W kg⁻¹. Complementary density functional theory calculations further reveal that Li preferentially occupies hexagonal Mo sites in Mo_4/3B₂T_x, supporting the observed stable lithiation behavior and excellent high-rate capability. These results suggest that 2D Mo_4/3B₂T_x aerogel is a promising candidate for high-power LIBs and LICs.