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 Cs2SnI6 double perovskite using dielectric spectroscopy and assessed its electrochemical response. The cubic phase (Fm3 over bar m) with a lattice constant of 11.644 angstrom 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 Cs atoms and dynamical rotation of SnI6 octahedra. Complex impedance spectra have provided details of the contributions of grain boundaries, grains, and anharmonic rattling to charge-carrier dynamics. The Cs2SnI6 exhibits electrical conductivity of 3.77 x 10-5 S cm-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 Cs2SnI6 as the electrode has delivered a specific capacitance of 3830 F g-1 at a current density of 2 A g-1. A quasi-solid-state asymmetric supercapacitor device was also fabricated, which delivered an energy density of 51 Wh kg-1 and a power density as high as 852 W kg-1 at a current density of 1 A g-1. We believe this work will open up the avenue to another generation of lead-free, perovskite-based, sustainable energy-storage systems.

Automated summary

The charge-carrier dynamics and relaxation properties of lead-free Cs2SnI6 perovskite were investigated using dielectric spectroscopy and electrochemical response. The research revealed the lattice structure and phonon dispersion, as well as the contributions of grain boundaries, grains, and anharmonic rattling to charge-carrier dynamics.