Confined Space in Hollow Micro/Nano Structures: Boosting Supercapacitor Performance to New Heights

High-performance electrode materials are key to advancing supercapacitor technology. Hollow micro- and nanostructured materials with confined space effects act as precise "nanoreactors." These materials effectively regulate ion transport kinetics, enhance interfacial interactions, and stabilize the electrode/electrolyte interface, achieving rapid ion storage and efficient diffusion. This review systematically summarizes the latest advances in the application of such materials in supercapacitors. We first elucidate the definition, classification, and preparation methods of hollow micro-nano structures, then explore the advantages of confined space effects in ion restriction and selection, reaction kinetics optimization, and other aspects. Furthermore, structural design and functionalization modifications can further enhance specific capacitance, rate performance, and cycling stability. We also discuss the physicochemical mechanisms involved in promoting charge storage, suppressing ion aggregation, and mitigating volume changes. Notably, machine learning has shown great potential in guiding the precise and controllable synthesis of complex hollow micro/nano structures, providing new insights and strategies for material design. Despite potential, challenges like precise synthesis, mass production of complex structures, in-depth ionic behavior study in confined spaces, and long-term stability remain. Finally, we outline future research directions to offer theoretical and technical guidance for designing next-generation high-performance supercapacitor electrodes based on confinement effects.