Charge-Carrier Dynamics and Relaxation in ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ 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 Å 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 $3.77\times {10}^{-5}\mathrm{S}{\mathrm{cm}}^{-1}$ 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 ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ as the electrode has delivered a specific capacitance of 3830 F ${\mathrm{g}}^{-1}$ at a current density of 2 A ${\mathrm{g}}^{-1}$. A quasi-solid-state asymmetric supercapacitor device was also fabricated, which delivered an energy density of 51 Wh ${\mathrm{kg}}^{-1}$ and a power density as high as 852 W ${\mathrm{kg}}^{-1}$ at a current density of 1 A ${\mathrm{g}}^{-1}$. We believe this work will open up the avenue to another generation of lead-free, perovskite-based, sustainable energy-storage systems.

Figure 1

(a) XRD profile with Rietveld refinement with crystal structure at inset, (b) FESEM image, (c) EDX spectrum, (d)–(g) EDX elemental mapping of ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$, (h),(i) HRTEM image, (j) SAED pattern of ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$, and (k) lattice (d-spacing) pattern.

Figure 2

(a) XPS survey scan and core level XPS spectra of ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$, (b) $\mathrm{Cs},$ (c) $\mathrm{Sn},$ (d) $\mathrm{I}$.

Figure 3

(a) Electronic band structure, (b) total and projected DOS with orbital contributions, (c) phonon dispersion and (d) density of phonon states for the cubic phase of the ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ double perovskite.

Figure 4

(a),(b) Complex impedance plots of ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ double perovskite at different temperatures. Inset figures show the equivalent circuit model for analyzing the impedance data, (c),(d) temperature dependence of resistance and constant phase element values obtained from equivalent circuit model fitting of experimental data, (e) reciprocal temperature dependence of dc conductivity for ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$. Frequency dependence of (f) real impedance spectra (Z′) and (g) imaginary impedance spectra (−Z^″).

Figure 5

(a) ac conductivity for ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ at different temperatures and solid line is the best fits to Eq. (2). Reciprocal temperature dependence of (b) dc conductivity and (c) hopping frequency for ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ perovskite. The inset of (c) represents the temperature variation of the power-law exponent.

Figure 6

Frequency dependence of (a) dielectric constant ε′ and (b) dielectric loss ε^″ at different temperatures, (c) real and (d) imaginary part of complex electric modulus spectra at different temperatures, (e) frequency dependence of imaginary part of complex modulus with fitting of Kohlrausch-Williams-Watts function at T = 193 K and (f) reciprocal temperature dependence of inverse relaxation time (1/τ) and the solid lines are the best fits to Eq. (6).

Figure 7

(a) Schematic model representing energy storage in ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ electrode, (b) CV curves at different scan rates, (c) plot of specific capacitance values derived from CV as a function of scan rate, (d) GCD curves for ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ as a function of current density, (e) specific capacitance values derived from GCD at different current densities, (f) Nyquist plot with equivalent circuit for ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ perovskite electrode and (g) capacitance retention (%) at current density 8 A g^−1.

Figure 8

(a) Digital image and schematic diagram of a single asymmetry supercapacitor device, CV curves of (b) activated carbon and ${\mathrm{Cs}}_2{\mathrm{Sn}\mathrm{I}}_6$ perovskite at 2 mV s⁻¹ in three-electrode configuration, (c) asymmetric supercapacitor device at different voltage ranges, (d) asymmetric supercapacitor at different scan rates, (e) galvanostatic charge-discharge profiles of the asymmetric supercapacitor at different current densities, (f) specific capacitances of the asymmetric supercapacitor at different current densities, (g) Ragone plot of asymmetric supercapacitor device, (h) capacity retention at current density 5 A g⁻¹, (i) Nyquist plots with equivalent circuit model fits of the device for initially and after 2000 cycles, and (j) snapshots of the glowing LEDs by the fabricated device.