Layered molybdenum disulfide (MoS₂) shows a unique combination of valuable structural, electronic, optical, mechanical, chemical, and thermal properties that have been studied for decades. The inexpensiveness and availability have offered the uses for lubricant, optoelectronic, catalytic, and clean energy storage applications. 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 reaction mechanism of MoS₂ has important significance, both in fundamental studies and practical application.

In their continuous effort to develop in-situ transmission electron microscopy (TEM) characterization methods for nanomeasurements and device mechanism (JACS 2009, 131, 62-63; JACS 2010, 132, 4197-4201; and Adv. Mater. 2012, 24, 4676-4682, etc.), a group led by Prof. BAI Xuedong from the Institute of Physics (IOP), Chinese Academy of Sciences recently studied the dynamic electrochemical lithiation process of MoS₂ nanosheets by construction of an in-situ TEM electrochemical cell. They found that MoS₂ undergoes a trigonal prismatic (2H)-octahedral (1T) phase transition upon lithium intercalation. The in-situ real-time 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 their 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, as shown in the figure below. Furthermore, with the capability of characterizing the phase boundary, the shear mechanism of the 2H-1T phase transition has been confirmed.

In addition, 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.

The fundamental understanding of lithium ion intercalation behavior can allow for development in other transition metal dichalcogenides (TMDCs)-related energy devices and should enable a wide range of studies, such as TMDCs-based composition and optoelectronics.

This work has been published recently on J. Am. Chem. Soc. (JACS 2014, 136, 6693-6697].

This work was supported the National Basic Research Program (Grant Nos. 2012CB933003 and 2013CB93200), the Natural Science Foundation  (Grant No. 11004230), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB07030100) of China.