Defect engineering for high-performance lithium/sodium-ion capacitor electrodes: mechanisms, advances, and future perspectives

The increasing need for sophisticated energy storage solutions that integrate high energy density, high power density, and extended cycle life has propelled the advancement of hybrid devices like lithium/sodium-ion capacitors (LICs/SICs). These devices combine battery-type and capacitor-type electrodes to address the performance disparity between traditional batteries and supercapacitors. However, the fundamental kinetic imbalance and interfacial instability between Faradaic and non-Faradaic electrodes continue to pose significant challenges to realizing their complete potential. In this context, this review thoroughly outlines the latest advancements in defect engineering approaches aimed at improving the electrochemical performance of LICs and SICs electrode materials. Specifically, through the systematic tailoring of vacancies (such as oxygen, cation, and anion vacancies), heteroatom doping (including N, F, S), and interfacial defects, significant improvements have been made in electronic conductivity, ion diffusion, the introduction of additional active sites, and structural stability. Furthermore, the review examines the essential mechanisms by which these defects influence charge storage behavior and the interactions between electrodes and electrolytes, thereby highlighting their significance in enhancing pseudocapacitive contributions and reducing degradation. Finally, the review underscores ongoing challenges in defect control, scalability, and mechanistic understanding, while delineating future directions focused on achieving precise defect manipulation and practical application. By offering a comprehensive examination of defect-enabled material design, this review intends to steer the advancement of high-performance LICs and SICs that can satisfy the demands of future energy storage systems.