Recent Development of Flexible and Stretchable Supercapacitors Using Transition Metal Compounds as Electrode Materials

Flexible and stretchable supercapacitors (FS‐SCs) are promising energy storage devices for wearable electronics due to their versatile flexibility/stretchability, long cycle life, high power density, and safety. Transition metal compounds (TMCs) can deliver a high capacitance and energy density when applied as pseudocapacitive or battery‐like electrode materials owing to their large theoretical capacitance and faradaic charge‐storage mechanism. The recent development of TMCs (metal oxides/hydroxides, phosphides, sulfides, nitrides, and selenides) as electrode materials for FS‐SCs are discussed here. First, fundamental energy‐storage mechanisms of distinct TMCs, various flexible and stretchable substrates, and electrolytes for FS‐SCs are presented. Then, the electrochemical performance and features of TMC‐based electrodes for FS‐SCs are categorically analyzed. The gravimetric, areal, and volumetric energy density of SC using TMC electrodes are summarized in Ragone plots. More importantly, several recent design strategies for achieving high‐performance TMC‐based electrodes are highlighted, including material composition, current collector design, nanostructure design, doping/intercalation, defect engineering, phase control, valence tuning, and surface coating. Integrated systems that combine wearable electronics with FS‐SCs are introduced. Finally, a summary and outlook on TMCs as electrodes for FS‐SCs are provided. The growing demand for wearable and portable electronic devices has resulted in the need for flexible and stretchable supercapacitors (FS‐SCs) with reliable performance when deformed. A comprehensive comparison of promising transition metal compounds used for FS‐SCs is provided here, and some key design strategies are highlighted. Additionally, current challenges and future prospects are discussed.