Enhancing device performance and stability of lead-free quasi-2D halide perovskite supercapacitor through Ag+/Bi3+ cation interaction

Compared to their three-dimensional (3D) counterparts, low-dimensional layered perovskite (2D) structures using bulky organic ammonium cations (PEA⁺) have significantly improved stability but generally worse performance. 3D perovskites with significant ion migration, one of the major concerns for structural instability, show better charge storage capacity. In contrast, strong van der Waals contacts and bulky spacer ligands in 2D perovskites inhibit the migration of halide ions. Mixed properties of 2D and 3D or quasi-2D layered perovskite demonstrate more efficient, tuneable optoelectronic properties and long-term stability. The performance and stability of the electrochemical supercapacitor may be significantly influenced by ion migration, as we have shown by fabricating porous electrodes from 3D-Cs₂AgBiBr₆ bulk perovskite, 2D/3D or quasi-2D PEA-Cs₂AgBiBr₆, and layered perovskite 2D PEA₄AgBiBr₈. The quasi-2D electrodes were found to have an energy density ∼1.75 times higher than the 3D perovskite electrodes and ∼4.5 times higher than that of pure 2D halide electrodes. Compared to 2D and 3D electrodes, quasi-2D has a maximum capacitance retention of around 93 % after 2000 operation cycles. Ex-situ X-ray diffraction was conducted to examine further structural changes in the quasi-2D, 2D, and 3D perovskite electrode materials. It was determined that the ordering arrangement of Ag⁺/Bi³⁺ cation improves the crystallinity of the structure, which enhances the device performance and stability of the quasi-2D electrode. Also, Ag³⁺ is essential for improving the strength of quasi-2D and 2D electrodes, as evidenced by X-ray photoelectron spectroscopy (XPS). A symmetric solid-state supercapacitor was fabricated and analyzed using a two-electrode method, demonstrating that the quasi-2D configuration has the highest energy density compared to the pure 2D and 3D perovskite electrode materials.