Single-to-Trilayer MXenes Enabling Kinetically Enhanced High-Energy-Density Li-Ion Capacitors

A dual faradaic lithium-ion capacitor (LIC) promises high energy density but commonly suffers from low-power characteristics. The reason causing this deficiency is attributed to bulk-phase mass-transfer-induced sluggish dynamics, especially in the anode. Two-dimensional MXenes are promising to solve this issue because of their open structure and low ion-migration energy barrier. However, the self-stacking phenomenon of MXenes greatly diluted these advantages. Here we develop a biothermochemistry method to produce single-to-trilayer Nb₂C and Ti₃C₂ MXenes with a high ratio of >95%. The optimized Nb₂C MXene with wider ion transport channels and a larger electrode/electrolyte contact area facilitates lower diffusion resistance and a higher diffusion coefficient. When assembled with a LiNi_0.8Co_0.1Mn_0.1O₂ (NCM) cathode, dual faradaic Nb₂C|LiPF₆|NCM LIC delivers simultaneously a high energy density of 107 Wh kg^–1 and a power density of 870 W kg^–1. A 300 mAh soft-packaged Nb₂C|LiPF₆|NCM LIC drives a toy racing car over 400 m and still works even after bending-cutting-needling processes.