Review of Hybrid Ion Capacitors: From Aqueous to Lithium to Sodium

, sodium super ionic conductor (NASICON), etc. The Ragone chart characteristics of HIC devices critically depend on their anode-cathode architectures. Combining electrodes with the flattest capacity versus voltage characteristics, and the largest total voltage window, yields superior energy. Unfortunately "flat voltage" materials undergo significant volume expansion/contraction during cycling and are frequently lifetime limited. Overall more research on HIC cathodes is needed; apart from high surface area carbon, very few positive electrodes demonstrate the necessary 10 000 or 100 000 plus cycle life. It remains to be determined whether its lithium ion capacitors (LICs) or sodium ion capacitors (NICs) are superior in terms of energy-power and cyclability. We discuss unresolved issues, including poorly understood fast-charge storage mechanisms, prelithiation and presodiation, solid electrolyte interface (SEI) formation, and high-rate metal plating.