Two-dimensional materials for lithium/sodium-ion capacitors

Lithium-ion capacitors (LICs), constructed with a battery-type electrode and capacitor-type electrode in electrolytes containing a Li-salt, are designed to bridge the gap between lithium-ion batteries (LIBs) and supercapacitors (SCs). Such a configuration gives LICs a high energy density, high power density and long-term cycling stability. Hence, LICs are regarded as one of the most promising alternatives to present electrochemical energy storage (EES) devices. As the most important components of LICs, extensive efforts have been made to develop novel electrode materials during the past two decades. However, some critical issues including a kinetic imbalance between a battery-type electrode and a capacitor-type electrode, unsatisfactory energy and power densities and cycling stability still need to be effectively addressed. Two-dimensional (2D) materials, because of the unique advantages, including a high specific surface area, excellent electrical conductivity, a tunable layered structure, rich electrochemical active sites and mechanical flexibility, have been used as electrode materials and additives for LICs and great progress has been made in recent years. In this review, we summarize the recent progress in the use of 2D materials, including graphene, transition metal dichalcogenides (TMDs) and MXenes, as battery-type electrode materials, capacitor-type electrode materials and additives in LICs. The typical application of 2D materials in sodium-ion capacitors (NICs) is also briefly reviewed. Finally, an outlook for the future researches on achieving higher-performance LICs and NICs is presented.