Scalable Production of Freestanding Few-Layer β-Borophene Single Crystalline Sheets as Efficient Electrocatalysts for Lithium-Sulfur Batteries.

Two-dimensional (2D) borophene has attracted tremendous interest due to its fascinating properties, which have potential applications in catalysts, energy storage devices, and high-speed transistors. In the past few years, borophene was theoretically predicted as an ideal electrode material for lithium-sulfur (Li-S) batteries because of its low-density, metallic conductivity, high Li-ion surface mobility, and strong interface bonding energy to polysulfide. But until now, borophene-based Li-S batteries have not yet been achieved in experiments due to the absence of a large-scale synthetic method of freestanding borophene nanostructures with a high enough structural stability, conductivity, and uniformity. Herein, we developed a low-temperature liquid exfoliation (LTLE) method to synthesize freestanding few-layer β-borophene single-crystalline sheets with a symmetry in tens of milligrams. The as-synthesized 2D sheets were used as the polysulfide immobilizers and electrocatalysts of Li-S batteries. The resulting borophene-based Li-S battery delivered an extralarge areal capacity of 5.2 mAh cm at a high sulfur loading of 7.8 mg cm, an excellent rate performance of 8 C (@721 mAh g), and an ultralow capacity fading rate of 0.039% in 1000 cycles, outperforming commercial Li-ion batteries and many other 2D material-based Li-S batteries. Based on the density functional theory model, the excellent electrochemical performances of the borophene-based Li-S batteries should originate from the enormous enhancement of β-borophene sheets for both the surface migration of the Li-ions and the adsorption energy of LiS clusters. Our results thus demonstrate a great potential for scalable production of freestanding β-borophene single-crystalline sheets in future high-performance Li-S batteries.