Material Science and Engineering

Biography

Biography: Xuedong BAI

Abstract

The weak van der Waals interaction between the MoS2 layers allows alkali ions to intercalate without a significant volume expansion, which enables MoS2 to be an alternative as an electrode material for high capacity lithium ion batteries. Research on the electrochemical lithiation mechanism of MoS2 has important significance, both in fundamental studies and practical applications. We recently studied the dynamic electrochemical lithiation process of MoS2 nanosheets by construction of an in-situ TEM electrochemical cell. It is found that MoS2 undergoes a trigonal prismatic (2H)-octahedral (1T) phase transition upon lithium intercalation. The in-situ characterization at atomic scale provides a great leap forward in the fundamental understanding of the lithium ion storage mechanism in MoS2. MoS2 by its nature is a semiconductor with trigonal (2H) structure, where the S atoms locate in the lattice position of a hexagonal close-packed structure. Planes of Mo atoms are sandwiched between two atomic layers of S, such that each Mo is coordinated to six S atoms in a trigonal prismatic geometry (2H). Another MoS2 polytype based on tetragonal symmetry is the octahedral phase (1T) with one MoS2 layer per repeat unit. A structural transformation of 2H-1T corresponds to the electronic structure change from semiconducting to metallic. In our work, a systematical study has been performed on the structural properties of MoS2 nanosheets during the lithiation process using an in-situ electrochemical TEM holder. The results demonstrate the existence of a phase transition of 2H-MoS2 to 1T-LiMoS2 and structural modulation in the 1T-LiMoS2 in the first lithiation process. The time-lapse migration of lithiation reaction boundary is shown in the figure below. Furthermore, utilizing the in-situ measurements, the electrochemical reaction in each stage has been studied, which can also be correlated with the ex-situ performance of MoS2 coin-type cells. After the phase transition of 2H-MoS2 to 1T-LiMoS2, there follows a conversion reaction during the lithiation of MoS2. So the structural mechanism corresponding to the electrochemical property of MoS2 during lithiation can be clearly understood. In this talk, our studies on the dynamic lithiation processes of SnO2 nanowires and silicon nanowires will be also included.