Li⁺ intercalation chemistry on 2D transition metal dichalcogenides towards phase evolution, scalable production, and application

Li⁺ intercalation chemistry is a powerful tool to induce phase transitions in transition metal dichalcogenides (TMDs), but only the transition of 2H-to-1T/1T’ in group Ⅵ TMDs (MoS₂ and WS₂) is well-known and widely explored for applications in areas such as transistors, memristors, catalysis, and batteries. Here, we develop a fully documented landscape of phase evolution in group IV-Ⅵ TMDs induced by electrochemical Li⁺ intercalation through in-situ X-ray diffraction (XRD) and Raman techniques. We found emerging structural phase evolutions that had never been noticed before, including 1T-to-1T (transition-free) in group IV TMDs (TiS₂ and ZrS₂), 2H-to-3R in group Ⅴ TMDs (NbS₂), as well as 1T-to-2H in group Ⅴ TMDs (VS₂ and TaS₂). Theoretical calculations uncovered the crucial role played by lithium intercalation in facilitating electron transfer from the s orbital of lithium to the d orbital of the transition metal center and clarified the reasons of the difference of phase transitions for different families of TMDs. Furthermore, we discovered that phase transitions also occur in the subsequent exfoliation process for scalable preparation of TMD atomically thin sheets, embodying 1T-to-1T (transition-free) in TiS₂, 1T-to-amorphous in ZrS₂, 3R-to-H in NbS₂, and 2H-to-1T in VS₂ and TaS₂. Our developed Li⁺ intercalation chemistry enriches phase transition nanotechnology, which facilitates not only the understanding of the mechanism of phase transition but also its control, opening up new possibilities for phase-dependent TMD-based nanoelectronic, photonic, and thermoelectric devices. As a proof-of-concept application, we developed a thermoelectric device using our exfoliated TiS₂ nanosheets, achieving a maximum power density of 458.6 W·m⁻² at a 53 K temperature difference.