Ultrafast Sodium Intercalation Pseudocapacitance in MoS₂ Facilitated by Phase Transition Suppression

Sodium-ion intercalation pseudocapacitance promises fast energy storage that is cheaper than lithium-ion-based systems. MoS2 is an attractive sodium-ion host due to its large van der Waals gaps, high Na+ mobility, and high electronic conductivity in the 1T phase. In this paper, we have quantified high levels (>90%) of pseudocapacitive charge storage in 30 μm thick MoS2 nanocrystal-based composite electrodes, which can be charged to almost 50% of their 1C capacity in just under 40 s. In addition, very little decay is observed in the delivered capacity (retention of 97%) after 1800 cycles at a rate of 20C. Synchrotron-based operando X-ray diffraction shows that the pseudocapacitive performance is enabled through suppression of the trigonal 1T-MoS2 to triclinic NaxMoS2 phase transition in MoS2 nanocrystals during charge–discharge.