Motivated by the presence of metallic conductivity in the group VI transition metal dichalcogenides, the supercapacitive performance of the 1T phase of molybdenum (MoS2x Se2(1–x)) sulfoselenides is explored. Ultrahigh frequency response (∼1.4 kHz) along with excellent electrochemical stability are exhibited by 1T MoS2x Se2(1–x)-based supercapacitors. The S:Se ratio in the mixed chalcogenide is shown to influence the rate performance of the device. High areal capacitances of ∼450 μF cm–2 with short RC time constants of ∼1.3–3.2 ms at 120 Hz are achieved in the case of 1T MoSSe phase. The i–v characteristics are almost rectangular at very high scan rates of ∼1000 V s–1. Very stable gravimetric specific capacitance of ∼36 F g–1 is retained for several thousands of charge–discharge cycles leading to an energy density of ∼12.1 Wh kg–1 at a specific power of ∼842 W kg–1. The observed specific power of ∼50 kW kg–1 at ∼1.5 Wh kg–1 energy density makes the sulfoselenide phase capacitors attractive for high power applications without compromising on the energy density.

We report on a systematic study of the structural, magnetic and transport properties of highpurity 1T-VS 2 powder samples prepared under high pressure. The results differ notably from those previously obtained by de-intercalating Li from LiVS 2 . First, no Charge Density Wave (CDW) is found by transmission electron microscopy down to 94 K. Though, ab initio phonon calculations unveil a latent CDW instability driven by an acoustic phonon softening at the wave vector q CDW ≈ (0.21,0.21,0) previously reported in de-intercalated samples. A further indication of latent lattice instability is given by an anomalous expansion of the V-S bond distance at low temperature.Second, infrared optical absorption and electrical resistivity measurements give evidence of non metallic properties, consistent with the observation of no CDW phase. On the other hand, magnetic susceptibility and NMR data suggest the coexistence of localized moments with metallic carriers, in agreement with ab initio band structure calculations. This discrepancy is reconciled by a picture of electron localization induced by disorder or electronic correlations leading to a phase separation of metallic and non-metallic domains in the nm scale. We conclude that 1T-VS 2 is at the verge of a CDW transition and suggest that residual electronic doping in Li de-intercalated samples stabilizes a uniform CDW phase with metallic properties.