Intermediate phases in sodium intercalation into MoS₂ nanosheets and their implications for sodium-ion batteries

Alkali metal ion intercalation into layered transition-metal dichalcogenide structures is a promising approach to make next generation rechargeable batteries for energy storage. It has been noted that the number of Na-ions which can be reversibly intercalated and extracted per MoS₂ is limited, and the chemical and electrochemical processes/mechanisms remain largely unknown, especially for nano-sized materials. Here, sodiation of MoS₂ nanosheets are studied by in-situ electron diffraction and the phase transformations in sodiation are identified with the aid of DFT calculations to reveal the reaction mechanism. Several thermodynamically stable/metastable structures are identified in the sodiation pathway of MoS₂ nanosheets, previously unnoticed in bulk MoS₂. The gradual reduction of Mo⁴⁺ upon Na-ion intercalation leads to a transition of the Mo-S polyhedron from a trigonal prism to an octahedron around 0.375 Na per MoS₂ inserted (i.e. Na_0.375MoS₂). When the intercalated Na-content is larger than 1.75 per MoS₂ structural unit (i.e. Na_1.75MoS₂), the MoS₂ layered structure collapses and the intercalation reaction is replaced by an irreversible conversion reaction with the formation of Na₂S and metal Mo nanoparticles. The calculated sodiation pathways reproduce the experimental sodiation voltages. The current observations provide useful insights in developing sodium-ion batteries with high cycling stability.