Sodium ion capacitors: materials, mechanism and challenges.

Sodium ion capacitors (SICs), as designed to deliver high energy density, rapid energy delivery, and long lifespan, have attracted much attention because of their comparable performance to lithium ion capacitors (LICs), albeit with abundant sodium sources. The conventional SICs design is based on battery-like anode and capacitive cathode, in which the battery-like anode materials involve various reactions such as inserted reaction, alloying reaction and conversion reaction, and the capacitive cathode materials usually depend on activated carbon (AC). However, researchers attempted to construct SICs based on battery-like cathode and capacitive anode or a combination of both in recent years. Here, the charge storage mechanism and material design strategies in SICs are summarized, with a focus on battery-like anode materials from inorganic to organic materials. Additionally, the challenges in the fabrication of SICs and future research directions are discussed.