Two‐Dimensional Titanium Carbide MXene as a Capacitor‐Type Electrode for Rechargeable Aqueous Li‐Ion and Na‐Ion Capacitor Batteries

Abstract To bridge the gap of the alkali‐metal‐ion (Li or Na) batteries and supercapacitors, hybrid devices called capacitor batteries are being designed, which are adaptive for large‐scale energy‐storage systems. Two‐dimensional transition‐metal carbide materials (MXene) with high electronic conductivities have become capable electrode materials for supercapacitors. Herein, Ti 3 C 2 T x synthesized by selectively etching the Al layer from the Ti 3 AlC 2 presents a remarkable electrochemical performance as an electrode material for the supercapacitor with Li 2 SO 4 or Na 2 SO 4 aqueous electrolyte. It displays a capacitance of 243 F g −1 at a low current density of 100 mA g −1 with a capacitance retention of 95 % in the Li 2 SO 4 electrolyte, and a capacitance of 150 F g −1 at a low current density of 200 mA g −1 with a capacitance retention of 95 % in the Na 2 SO 4 electrolyte, which shows its potential application for capacitor batteries. Moreover, the LiMn 2 O 4 //Ti 3 C 2 T x Li‐ion capacitor battery and MnO 2 //Ti 3 C 2 T x Na‐ion capacitor battery are designed and assembled for the first time. Such rechargeable aqueous capacitor batteries display capable capacities, remarkable rate abilities, and excellent cycling performances, which further demonstrates that Ti 3 C 2 T x is an outstanding capacitor‐type electrode material for capacitor batteries.