Two-dimensional transition metal dichalcogenides (TMDs) are known as the “versatile players" in the materials field, with abundant crystal phases and phase-specific physical and chemical properties, showing great application potential in electronics, optoelectronics, and energy conversion. Lithium intercalation is a core technology for regulating the crystal phase and achieving efficient exfoliation of TMDs, but previous research has only focused on a small number of group VI TMDs. The lithium intercalation phase evolution law of a large number of group IV-V TMDs has long been unknown, which has become a major barrier to their industrialization from laboratory research.
Recently, a research team led by Prof. Zhiyuan Zeng and Prof. Xiao Cheng Zeng from City University of Hong Kong , in collaboration with Prof. Kian Ping Loh from the National University of Singapore and Dr. Xiaodong Wang from Harbin Institute of Technology (Shenzhen) , achieved a critical breakthrough in this field. Using in-situ X-ray diffraction, in-situ Raman spectroscopy combined with density functional theory calculations, the team for the first time drew a "comprehensive navigation map" of the electrochemical lithium intercalation phase evolution of IV-VI TMDs, breaking the traditional cognition of a single phase transition mode (2H→1T/1T').
The team selected group IV (TiS 2 /ZrS 2 ), group V (NbS 2 /VS 2 /TaS 2 ) and group VI (MoS 2 ) TMDs as research objects, and captured the dynamic structural changes during lithium intercalation with in-situ characterization techniques. Four novel phase transition modes were discovered: TiS 2 /ZrS 2 stably maintains the 1T phase during intercalation; NbS 2 transforms from 2H to 3R phase by only adjusting the layer stacking mode; VS 2 /TaS 2 undergoes the most complex phase transition from 1T to 2H phase through multiple intermediate phases; the phase transition start and end points of MoS 2 were also accurately located, providing a precise basis for quantitative phase regulation.
Interestingly, the team also found that TMDs undergo "secondary phase transition" during the exfoliation process after lithium intercalation. Chemical effects, mechanical stress and local thermal effects during exfoliation will further affect the crystal phase structure, opening up new ideas for precise regulation of material phases.
The team further uncovered the universal mechanism of lithium intercalation-induced phase transition from three dimensions: structure, energy and electron distribution. Materials tend to select more stable crystal phases, lithium intercalation reduces the phase transition energy barrier, and electron transfer from lithium atoms triggers crystal phase transformation, which provides a solid theoretical basis for phase regulation of TMDs.
Based on the deciphered phase evolution law and more than a decade of technical accumulation, the team optimized a mild electrochemical intercalation and exfoliation strategy, successfully realizing the mass production of atomically thin sheets of IV-VI TMDs. The prepared nanosheets have good dispersibility, controllable phase structure and are compatible with various deposition processes, breaking the shortcomings of low efficiency and small scale of traditional preparation methods, and laying a core foundation for the large-scale device integration of 2D materials.
The value of the technology is ultimately reflected in practical applications. Taking TiS 2 as the model material, the team prepared a flexible TiS 2 nanosheet film with a diameter of 10 cm. The film has excellent flexibility and metallic luster, with a room temperature electrical conductivity of 427 S·cm -1 , which is 2-4 times that of the reported similar materials.
Furthermore, the team assembled an 8-leg flexible thermoelectric generator by combining the n-type TiS 2 film with the p-type Bi 0.4 Sb 1.6 Te 3 film. The device exhibits outstanding performance: the maximum power density reaches 458.6 W·m -2 at a temperature difference of 53 K, the open-circuit voltage increases linearly with the hot-end temperature, and the voltage has almost no attenuation after continuous operation for 6 hours at a temperature difference of 30 K, with only 8% power loss caused by interface resistance. It shows strong stability and practical application potential, providing a new solution for the self-power supply of wearable electronic devices.
This research fills the knowledge gap in the phase engineering of group IV-V TMDs, and the mass production technology breaks the technical barrier between laboratory small-batch preparation and industrial production. In the future, this achievement is expected to promote the industrial development of 2D materials in electronics, thermoelectricity, energy storage and other fields, making the "versatile players" in the materials field truly enter daily life.
National Science Review
Experimental study