Superior charge storage performance of WS2 quantum dots in a flexible solid state supercapacitor

The fabrication of flexible solid state supercapacitor devices using WS2 nanocrystals is reported in this paper. The charge storage performance of WS2 quantum dots (QDs) and nanosheets is compared in a solid state symmetric two electrode system for the first time. A lithium bromide-assisted lithium intercalation and sonication method has been employed to synthesize WS2 nanosheets and QDs. The formation of defect-rich QDs of size varying from 1 to 3 nm by fragmentation of WS2 nanosheets is confirmed through different spectroscopic and microscopic techniques. The fabricated solid state devices using WS2 QDs show a high areal specific capacitance (28 mF cm−2) with high energy density (1.49 μW h cm−2), at a current density of 0.1 mA cm−2. The superior performance of the QD-based devices as compared to that of the nanosheets is attributed to the large number of defect states present in the WS2 QDs. Fabricated solid state devices exhibit good flexibility and superior retention of specific capacitance (80% after 10 000 cycles), indicating WS2 as a potential candidate for future unconventional energy storage devices.