External Electric Field Enhanced Ti₃C₂ MXene Surface Passivation for Realizing Ultra-Long Cycling Stability

External electric field (EEF), as a stimulating factor, is an effective method for optimizing the surface composition and structure of materials. Ti₃C₂ MXene surface enriched with negatively charged functional groups (─OH, ─O, etc.) will exhibit high sensitivity to EEF. However, the impact of EEF on the interaction mechanisms between the guest ions and MXene surface remains unclear and requires further investigation. Herein, the density functional theory (DFT) is employed to simulate the adsorption energies between butyl trimethylammonium ion (BTA⁺) and MXene surfaces under different intensities of EEFs (±0.9, ±0.7, ±0.5, ±0.3, ±0.1, and 0 V Å^-1), indicating EEF can effectively regulate adsorption. It will increase the encapsulation degree of BTA⁺ on the MXene surface, thereby enhancing surface passivation. Based on theoretical predictions, quaternary-ammonium ions with different chain-lengths (BTA⁺, DTA⁺, STA⁺) are selected as guest ions to unveil the mechanism of EEF on MXene surface passivation. The applied-EEF promotes the formation of Ti─O─N bonds between ─OH and ammonium groups to construct more-denser protective layer, leading to enhancement of surface passivation and obviously increasing the capacitance retention after 100,000 cycles (50.8% to 97.5%). This work provides a new pathway and theoretical support for the surface passivation of MXene.