Aqueous Solution‐Processable Small Molecular Metal‐Chelate Complex Electrolyte for Flexible All‐Solid State Energy Storage Devices

A small molecular metal‐chelate complex, tris(8‐hydroxyquinoline‐5‐sulfonic acid) aluminum (AlQSA 3 ), that has three sulfonic acid groups per molecule leading to an excellent solubility in water is reported as a liquid‐free perfect solid‐state electrolyte for flexible film‐type all‐solid‐state energy storage devices. The AlQSA 3 material is synthesized by one‐step reaction of aluminum triisopropoxide and 8‐hydroxyquinoline‐5‐sulfonic acid. The aqueous solutions of AlQSA 3 are applied to fabricate flexible film‐type all‐solid state electric double layer capacitors with indium‐tin oxide thin film electrodes. The ion conductivity of the AlQSA 3 film reaches 0.116 mS cm −1 , while a pronounced hysteresis are obtained in the cyclic voltammetry measurement. The AlQSA 3 film capacitors exhibit an output voltage of 1.5 V at 20 μA cm −2 , which is considerably stable by the repeated operation. In particular, the peak output voltage is well kept even after 180° bending for 500 times in the case of the flexible AlQSA 3 film capacitors.