Flexible Polypyrrole@Fe₂O₃@Stainless Steel Yarn Composite Electrode for Symmetric Thread‐Like Supercapacitor with Extended Operating Voltage Window in Li₂SO₄‐Based Aqueous Electrolyte

Abstract A flexible symmetric thread‐like supercapacitor (TSC) with high operating voltage window and good electrochemical performance is manufactured, that fulfills the requirements of wearable electronics. First, iron oxide (Fe₂O₃) nanosheets are directly grown in situ on the surface of stainless steel yarn (SSY) by an acid corrosion and self‐oxidation process, and then a polypyrrole (PPy) coating is covered on the surface of Fe₂O₃ nanosheets via chemical oxide method to obtain the thread‐like PPy@Fe₂O₃@SSY composite electrode. The electrode achieves a wide potential range of −1.0–0.8 V (versus Ag/AgCl) and a high areal specific capacitance of 667.8 mF cm⁻² with outstanding galvanostatic charge/discharge cycling stability of 87.5% capacitance retention after 10 000 cycles in 1 m lithium sulfate (Li₂SO₄) aqueous solution. The symmetric TSC based on the PPy@Fe₂O₃@SSY electrode and carboxymethyl cellulose sodium salt/Li₂SO₄ gel electrolyte exhibits a wide operating voltage window of 1.8 V, and its maximum energy density can reach 39.3 μWh cm⁻². Additionally, the TSC demonstrates excellent electrochemical performance under various bending states and a bending cycling test. The preparation method and raw materials are low‐cost and scalable, and the overall performance of the TSC is excellent, which makes this device a promising energy storage device for wearable electronics.