Tailoring Surface Chemistry of Ti₃C₂N_x MXene for Superior H⁺, Li⁺, and Na⁺ Storage via Molten Salt-Derived N-Termination

2D transition metal carbides and nitrides (MXene) are promising candidates for next-generation electrode materials due to their high electrical conductivity, large specific capacity/capacitance, and tunable surface chemistry. Nitrogen-doped MXene, in particular, have shown excellent electrochemical energy storage performance. However, the low and uneven nitrogen content has hindered both their performance and understanding of how N-terminal groups affect cation storage. This study successfully synthesizes N-terminated Ti₃C₂Nx via ion-exchange reactions in a hydrogen-containing argon atmosphere and investigates its energy storage behavior for H⁺, Li⁺, and Na⁺ ions. Ti₃C₂Nx shows outstanding H⁺ storage with a capacitance of 471 F g⁻¹, while Li⁺ and Na⁺ storage exhibit a prominent intercalation mechanism. The Ti₃C₂Nx electrode delivers stable capacities of 209 mAh g⁻¹ for Li⁺ and 79 mAh g⁻¹ for Na⁺ after rate cycling, indicating good rate capability and reversibility. Combining density functional theory calculations with experimental data, this study reveals the correlations between adsorption energy, binding energy, and electronic density of states, highlighting the importance of real-gap distance in efficient cation intercalation, offering guidance for the design of MXene for H⁺, Li⁺, and Na⁺ storage.