Honeycomb Borophene Fragment Stabilized in Polyanionic Sandwich Lithium Salt: A New Type of Two-Dimensional Material with Superconductivity

For seeking new types of two-dimensional (2D) materials based on honeycomb borophene, honeycomb borophene fragments stabilized by Li atoms in sandwich molecules are found theoretically. This is based on our understanding on the hexagonal B-6 ring with two-electron deficiency and Li atoms as suitable electron donors. In these sandwich molecules, each hexagonal B-6 ring unit of honeycomb borophene fragments is sandwiched between two Li atoms, forming stable sandwich lithium salts due to electron transfer from Li atoms to the borophene fragment. In the sandwich lithium salts, the middle planar layer of honeycomb borophene fragment polyanions is sandwiched between two Li+ layers. Owing to the large size effect, the lithium salt (7Li)B-7+(24)14-(7Li)(7+) including the planar polyanion sheet B-24(14-) with a high valence of -14 can serve as an electron reservoir unusually exhibiting two different behaviors of the electron donor and electron acceptor. Furthermore, material property calculations show that this 2D sandwich lithium salt (Li2B2 sheet) is demonstrated to be thermally and mechanically stable. Remarkably, the Li2B2 sheet is not only a metal 2D material with a Dirac cone but also an intrinsic superconductor with the superconducting transition temperature T-c approximate to 6.2 K. This work provides a new potential two-dimensional material for practical application.