Heteroatom Doping Strategy of Advanced Carbon for Alkali Metal-Ion Capacitors

Alkali metal-ion capacitors (AMICs) combine the advantages of the high specific energy of alkali metal-ion batteries (AMIBs) and the high power output of supercapacitors (SCs), which are considered highly promising and efficient energy storage devices. It is found that carbon has been the most widely used electrode material of AMICs due to its advantages of low cost, a large specific surface area, and excellent electrical conductivity. However, the application of carbon is limited by its low specific capacity, finite kinetic performance, and few active sites. Doping heteroatoms in carbon materials is an effective strategy to adjust their microstructures and improve their electrochemical storage performance, which effectively helps to increase the pseudo-capacitance, enhance the wettability, and increase the ionic migration rate. Moreover, an appropriate heteroatom doping strategy can purposefully guide the design of advanced AMICs. Herein, a systematic review of advanced heteroatom (N, S, P, and B)-doped carbon, which has acted as a positrode and negatrode in AMICs (M = Li, Na, and K) in recent years, has been summarized. Moreover, emphasis is placed on the mechanism of single-element doping versus two-element doping for the enhancement in the performance of carbon positrodes and negatrodes, and an introduction to the use of doped carbon in dual-carbon alkali metal-ion capacitors (DC-AMICs) is discussed. Finally, an outlook is given to solve the problems arising when using doped carbon materials in practical applications and future development directions are presented.