Tailored sulfur and nitrogen co-doped carbon quantum dot interfacial layer on copper foil for highly stable and ultrafast lithium-ion capacitors

Lithium-ion capacitors (LICs) are promising energy storage devices offering the advantages of lithium-ion batteries (LIBs) and supercapacitors (SCs). However, the interfacial structure between the electrode and current collector significantly affects their cycling performance and stability during ultrafast cycling. Therefore, we propose a tailored sulfur and nitrogen co-doped carbon quantum dot (SNCQD) interfacial layer on a copper foil (SNCQD/Cu) using an ultrasonic spray-coating system. A half-cell LIC electrode, fabricated with the SNCQD/Cu as the current collector, exhibited a superior specific capacity of 80.8 mAh/g and capacity retention of 90.1 % after 500 cycles at an ultrafast current density of 3000 mA/g. Furthermore, the electrode demonstrated excellent ultrafast cycling stability of 84.7 % at a capacity rate of 5C in full-cell LICs. These results are attributed to the synergistic effect of the SNCQD interfacial layer, which prevents oxidation and corrosion, enhances interfacial adhesion, and increases electrical conductivity. Therefore, the introduction of an SNCQD interfacial layer can help achieve highly stable, ultrafast LICs.