Stabilizing Semi-Crystalline Phase in CsPbBr₃ Nanocrystals for Supercapacitor Application

Halide perovskite-based energy storage devices have gained much attention due to their high electronic and ionic conductivity. However, poor performance and cyclic stability of hybrid halide perovskite supercapacitors have become the bottleneck for commercialization. Typical electrolytes used so far in halide perovskite-based supercapacitors are nonaqueous tetrabutylammonium tetrafluoroborate, or tetrabutylammonium perchlorate having large cations. We demonstrated that inorganic halide perovskite-based supercapacitors with Li-ion electrolytes are highly efficient with a specific energy density of ∼250 W h/kg. There are no structural changes in the two-dimensional tetragonal phase of the CsPb₂Br₅ lattice. However, the orthorhombic phases of the three-dimensional CsPbBr₃ crystal structure disappear due to Li-ion intercalation/conversion. A quasi-reversibility is observed during the discharging cycles. We have also shown that introducing perovskite nanocrystals can stabilize the quasi-reversible orthorhombic to amorphous phase transition in CsPbBr₃. This is mainly because of the nanocrystals’ finite size (∼10 nm), where Li-ion intercalation results in a semicrystalline phase. Therefore, no such structural changes are observed in CsPbBr₃ nanocrystals during charging or discharging cycles in Li-ion electrolytes. We have further fabricated solid-state supercapacitors by introducing quasi-solid-state gel electrolytes between symmetric electrodes having an energy density over 4.58 μW h/cm². These devices are stable over 5000 GCD (galvanostatic charge–discharge) cycles with more than 89% capacity retention.