Mixed -N/-O Surface Terminations Enable Fast Ammonium-Ion Storage in Ti3C2Tx MXene

Aqueous ammonium-ion batteries are promising for safety, low cost, and fast energy storage. However, the influence of electrode surface chemistry on NH4+ storage remains poorly understood. Two-dimensional Ti3C2Tx MXene, with metallic conductivity, tunable interlayer spacing, and abundant surface terminations, serves as an ideal platform to elucidate NH4+ storage mechanisms. Herein, the role of MXene surface termination in regulating NH4+ storage behavior is systematically investigated using density functional theory calculations and experimental validation. The calculations reveal that the -N terminations offer moderate NH4+ adsorption and high electronic conductivity for rapid charge transfer and ion transport, whereas the -O terminations exhibit stronger ion affinity and enhanced surface stability. Importantly, the mixed -N/-O terminations effectively balance adsorption strength and transition-state stabilization, resulting in the lowest NH4+ migration barrier among the modeled pathways. The calculations employ idealized vacuum-separated surfaces with isolated NH4+ and do not explicitly include solvation or electrolyte effects. Guided by these findings, a mixed -N/-O terminated Ti3C2Tx MXene is constructed via a urea-assisted hydrothermal strategy. The optimized NTC-20 electrode delivers 107 mAh g-1 at 1 A g-1, retains 67% capacity at 10 A g-1, and maintains 91% capacity after 10,000 cycles. Ex situ XRD, XPS, and FTIR analyses are consistent with reversible NH4+-associated changes in the local chemical environment and possible storage involving Ti-N···H-N and Ti-O···H-N interactions. Furthermore, the NTC-20//MnO2/CNTs full cell achieves an energy density of 26.7 Wh kg-1 at 1.3 kW kg-1. The results support a dominant role for mixed -N/-O terminations in regulating NH4+ adsorption and diffusion, providing insights into surface-termination engineering for high-performance AAIBs.