Intercalation of Metal Ions into Ti3C2Tx MXene Electrodes for High‐Areal‐Capacitance Microsupercapacitors with Neutral Multivalent Electrolytes

Abstract Microsupercapacitors (MSCs) with neutral multivalent electrolytes are safer, cheaper, and exhibit higher theoretical energy densities compared with the MSCs with acidic and alkaline electrolytes. Multivalent charge carriers (e.g., Mg 2+ , Zn 2+ ) in the MSCs with Ti 3 C 2 T x MXene electrodes have not been demonstrated, which could theoretically achieve higher specific capacitances and energy densities. However, because of the larger size of multivalent charge carriers, the MXene electrodes require further modifications to facilitate reversible electrochemical reactions. Herein, through spontaneous intercalation of various metal ions into MXene multilayers, twelve metal ion intercalated MXene electrodes (M n + ‐MXene) are fabricated and demonstrate improved electrochemical performance. Different nanopillar effects are observed between divalent Be 2+ and trivalent Al 3+ intercalants, which are systematically investigated by electrochemical impedance spectroscopy and molecular dynamics simulation. Among all M n + ‐MXene electrodes, the Be 2+ ‐MXene electrode largely facilitates the charge‐transfer process with minimal disturbance of electrolyte diffusion rates, showing improved specific capacitances and high rate performance in univalent (Li 2 SO 4 , Na 2 SO 4 , K 2 SO 4 ) and multivalent electrolytes (BeSO 4 , MgSO 4 , ZnSO 4 ). Finally, flexible Be 2+ ‐MXene MSCs with neural ZnSO 4 gel electrolytes are fabricated, demonstrating superior areal capacitances (77.2 mF cm −2 ) and high energy density (3.86 μWh cm −2 at 0.12 mW cm −2 ) together with high user safety.