Sodium storage in a promising MoS-carbon anode: elucidating structural and interfacial transitions in the intercalation process and conversion reactions.

Sodium-ion batteries and capacitors are considered as low-cost energy storage devices, compared to Li-ion counterparts. However, most anodes for sodium-ion devices show sluggish kinetics and poor structural stability caused by the large radius (1.02 Å) of Na+. One candidate anode is MoS2, a 2D atomic layered material with a large interlayer spacing of 6.2 Å, that can take up and release Na+via two working principles: a two-electron intercalation process and a four-electron conversion reaction. Herein, we report a facile method to synthesize a MoS2-amorphous carbon (MoS2-AC) nanocomposite and further study the effect of the two working principles on the structure, interphase, and charge storage properties of MoS2-AC. The two-electron intercalation reaction enables the MoS2-AC electrode to have a higher rate capability and superior stability than that via the four-electron Na+ conversion reaction. This favorable Na+ charge storage performance of MoS2-AC via the two-electron intercalation process is attributed to its pseudocapacitive behavior, a stable solid electrolyte interphase and robust stability of the structure, which enables us to fabricate a sodium-ion capacitor that can deliver high energy density at a high rate. This work underscores the potential importance of realizing fast Na+ charge storage via an intercalation process as a strategy for the fabrication of high-performance sodium-ion capacitors and batteries.