Lead-free halide perovskites for high-performance thin-film flexible supercapacitor applications

Abstract

The growing interest in smart portable electronic devices demands a flexible and lightweight power supply. Among all the rechargeable energy storage devices thin film-based supercapacitors are the best alternatives for high-power and high-energy-density applications. Very recently hybrid halide perovskites have gained a lot of attention in energy harvesting and storage applications due to their superior electronic and ionic conductivity. However, one of the challenges in these materials is lead toxicity, which may be a bottleneck for commercialization in portable and wearable electronics. In this article, we have demonstrated that the lead-free double perovskite could be the best choice for thin-film-based supercapacitors due to their higher energy and power density compared to lead-based perovskite supercapacitors. We have compared the electrochemical performance of organic–inorganic hybrid perovskites such as methylammonium lead trichloride (MAPbCl₃), inorganic perovskites such as cesium lead trichloride (CsPbCl₃), and lead-free perovskites such as cesium bismuth chloride (Cs₃Bi₂Cl₉). Lead-free perovskite-based asymmetric thin film supercapacitor devices fabricated from quasi-solid-state gel electrolyte show an areal capacitance of 64 mF cm⁻², which is 8–10 times higher than that of lead-based supercapacitors due to the higher specific surface area (26.4 m² g⁻¹) and ionic conductivity in the Cs₃Bi₂Cl₉ electrode. The estimated energy density is over 6.6 μW h cm⁻², which is also 3–4 times higher than the lead-based devices. X-Ray diffraction analysis after the first charging cycle reveals the reappearance of the orthorhombic peak of Cs₃Bi₂Cl₉, while lead-based perovskites do not show any change during charging or discharging cycles. Lead-free perovskite supercapacitors also show improved stability over lead-based perovskite supercapacitors. These devices are also flexible over wide angles for bending and twisting with ∼90% capacity retention.