Solvent Co-intercalation: An Emerging Mechanism in Li-, Na-, and K-Ion Capacitors

Solvated-ion intercalation or co-intercalation reactions make graphite a versatile anode for Na-ion chemistry and beyond. This alternate intercalation mechanism could overcome the difficulties faced by conventional intercalation reactions with graphite. The proper choice of the solvent molecule could co-intercalate Na-, Li-, and K-ions with high capacity and power density values, which are tailor-made for metal-ion capacitor (MIC, M = Li, Na, and K) applications. This review summarizes significant advances in co-intercalation chemistry, research progress in MICs with a graphite anode, and activated carbon cathodes in glyme family solutions. Also, we compare the advantages and challenges of MICs with the co-intercalation-based mechanism in place of conventional graphite anodes with bare-ion intercalation. The progress indicates high-performance hybrid-ion capacitors with high power capability and fast reaction kinetics. At the same time, it is essential to find methods to improve the energy-storage capability of such MICs to realize their commercial reality.