Vacancy Ordered Lead Free Double Perovskite Based Flexible Supercapacitor for Sustainable Energy Applications

Due to their extraordinary photophysical properties, lead halide perovskites are widely used in optoelectronics and energy applications. These distinctive materials have exceptional ionic responses because of halide ions and A-site cations. However, the lead toxicity along with ion kinetics in halide perovskites leads to structural deformation and instability, preventing these materials from commercialization. In this article, we have explored the possible application of vacancy-ordered tin-based lead-free double perovskite (A2BX6) in energy storage devices. We have further tuned the X-site anion (X = I, Br, Cl) to see the effect of halide ions on the supercapacitor properties. The Cs2SnI6-based electrode has the highest specific capacitance (∼420 F/g) and energy density (∼200 W h/kg) as estimated from the three-electrode half-cell measurement system. Solid-state devices were also fabricated using gel electrolytes. We have achieved areal capacitance over 10 mF/cm2, while energy density and power density are 3.2 μW h/cm2 at 0.2 mA/cm2 and 670 μW/cm2 at 0.75 mA/cm2 current density, respectively. We also achieved a maximum voltage window over 1.8 V for the Cs2SnI6 sample. Various fundamental characterizations such as ex situ XRD, FTIR, and XPS measurements are carried out to understand the charge storage mechanism in vacancy-ordered perovskite structures.