Preintercalation Induced Transition from Double Layer to Redox Calcium‐Ion Storage in Vanadium Carbide MXene

MXenes are a versatile class of two‐dimensional (2D) materials that exhibit proton‐induced pseudocapacitance, however, intercalation capacitive behavior in neutral electrolytes limits the charge storage capacities. In this work, the design of pseudocapacitive vanadium carbide (V ₂ CT _x ) MXene electrodes in calcium‐ion electrolytes is reported. By incorporation of room temperature ionic liquid, 1‐ethyl‐3‐methylimidazolium bis(trifluoromethanesulfonyl)imide (EMIM‐TFSI), as a preintercalant, a transition from electrical double layer to redox charge storage is observed. Facile diffusion of high charge density Ca‐ions to inaccessible surface‐active sites is enabled by the confined fluid effect of ionic liquid, results in a specific calciation capacity of 116 mAh g ⁻¹ at a current density of 0.05 A g ⁻¹ . However, inevitable capacity loss limits the long‐term stability, which is possibly due to spontaneous intercalation of water leading to plausible (electro)chemical dissolution of V ₂ CT _x MXene in aqueous Ca‐ion electrolyte as confirmed by in situ X‐ray diffraction studies. By the optimal choice of a hybrid electrolyte consisting of propylene carbonate and water, the charge storage capacity is improved up to 180 mAh g ⁻¹ at a current density of 0.09 A g ⁻¹ with an enlarged potential window of 1.6 V and capacity retention of 70% over 2000 cycles. This study opens avenues for exploring hybrid electrolytes toward long‐term cycling stability of MXene electrodes.