Cell Configurations and Electrode Materials for Nonaqueous Sodium-Ion Capacitors: The Current State of the Field

Nonaqueous sodium-ion capacitors are a new type of hybrid energy storage cells that combine the advantages of sodium-ion batteries and supercapacitors. Unlike their commercialized lithium-ion analogues, sodium-ion capacitors use sodium ions in the cell operation. A classic configuration of a sodium-ion capacitor includes a battery-type anode with a capacitive cathode within the same cell, thus optimizing both the energy and power densities of the device. This is commonly achieved by using hard carbon anodes and activated carbon cathodes, although other suitable anode materials can be used. Meanwhile, the energy storage community has moved beyond this classic concept, with a number of cell configurations deviating from the classic sodium-ion capacitor design having been described. These include the use of titanate and TiO2 anodes with sloping charge profiles, the use of other nonstandard battery-type anodes, an upside-down sodium-ion capacitor design, and the application of new materials (e.g., MXenes) as alternatives to the traditional capacitive electrodes. This review summarizes the reported cell configurations and electrode materials described by various researchers, and provides an analysis of the current state of the field of nonaqueous sodium-ion capacitors.