Advancements in Metal-Ion Capacitors: Bridging Energy and Power Density for Next-Generation Energy Storage

Metal-ion capacitors (MICs) have emerged as advanced hybrid energy storage devices that combine the high energy density of batteries with the superior power density and long cycle life of supercapacitors. By leveraging a unique configuration of faradaic and non-faradaic energy storage mechanisms, MICs offer a balanced performance that meets the diverse requirements of modern applications, including renewable energy systems, electric vehicles, and portable electronics. MICs employ diverse ions such as lithium, sodium, and potassium, which provide flexibility in material selection, scalability, and cost-effectiveness. For instance, lithium-ion capacitors (LICs) excel in compact and high-performance applications, while sodium-ion (NICs) and potassium-ion capacitors (KICs) provide sustainable and affordable solutions for large-scale energy storage. This review highlights the advancements in electrode materials, including carbon-based materials, transition metal oxides, and emerging candidates like MXenes and metal–organic frameworks (MOFs), which enhance MIC performance. The role of electrolytes, ranging from organic and aqueous to hybrid and solid-state systems, is also examined, emphasizing their influence on energy density, safety, and operating voltage. Additionally, the article discusses the environmental and economic benefits of MICs, including the use of earth-abundant materials and bio-derived carbons, which align with global sustainability goals. The review concludes with an analysis of practical applications, commercialization challenges, and future research directions, including AI-driven material discovery and integration into decentralized energy systems. As versatile and transformative energy storage devices, MICs are poised to play a critical role in advancing sustainable and efficient energy solutions for the future.