Charge-Carrier Dynamics and Relaxation in Cs2SnI6 Perovskite for Energy Storage: Existence of Anharmonic Rattling-Assisted Polaron Dynamics

We have explored different aspects of charge-carrier dynamics and relaxation in lead-free ${\mathrm{Cs}}_{2}{\mathrm{Sn}\mathrm{I}}_{6}$ double perovskite using dielectric spectroscopy and assessed its electrochemical response. The cubic phase $(Fm\overline{3}m)$ with a lattice constant of 11.644 \AA{} is confirmed for synthesized perovskite. The phonon dispersion illustrated by density-functional theory indicates the existence of soft optical modes triggered by anharmonic rattling of $\mathrm{Cs}$ atoms and dynamical rotation of ${\mathrm{Sn}\mathrm{I}}_{6}$ octahedra. Complex impedance spectra have provided details of the contributions of grain boundaries, grains, and anharmonic rattling to charge-carrier dynamics. The ${\mathrm{Cs}}_{2}{\mathrm{Sn}\mathrm{I}}_{6}$ exhibits electrical conductivity of $3.77\phantom{\rule{0.25em}{0ex}}\ifmmode\times\else\texttimes\fi{}\phantom{\rule{0.25em}{0ex}}{10}^{\ensuremath{-}5}\phantom{\rule{0.2em}{0ex}}\mathrm{S}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ at ambient conditions. The values of the power-law exponent for all temperatures suggest superlinear power-law (SPL) behavior of the ac conductivity. The relaxation time and the stretched exponent in the Kohlrausch-Williams-Watts (KWW) function of the electric modulus are caused by charge-carrier short-range mobility and the hopping of rattling-assisted polarons. The supercapacitor fabricated with ${\mathrm{Cs}}_{2}{\mathrm{Sn}\mathrm{I}}_{6}$ as the electrode has delivered a specific capacitance of 3830 F ${\mathrm{g}}^{\ensuremath{-}1}$ at a current density of 2 A ${\mathrm{g}}^{\ensuremath{-}1}$. A quasi-solid-state asymmetric supercapacitor device was also fabricated, which delivered an energy density of 51 Wh ${\mathrm{kg}}^{\ensuremath{-}1}$ and a power density as high as 852 W ${\mathrm{kg}}^{\ensuremath{-}1}$ at a current density of 1 A ${\mathrm{g}}^{\ensuremath{-}1}$. We believe this work will open up the avenue to another generation of lead-free, perovskite-based, sustainable energy-storage systems.