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Abstract

2D transition metal carbides and/or nitrides, so-called MXenes, are noted as ideal fast-charging cation-intercalation electrode materials, which nevertheless suffer from limited specific capacities. Herein, it is reported that constructing redox-active phosphorus−oxygen terminals can be an attractive strategy for Nb₄C₃ MXenes to remarkably boost their specific capacities for ultrafast Na⁺ storage. As revealed, redox-active terminals with a stoichiometric formula of PO₂- display a metaphosphate-like configuration with each P atom sustaining three P−O bonds and one P=O dangling bond. Compared with conventional O-terminals, metaphosphate-like terminals empower Nb₄C₃ (denoted PO₂-Nb₄C₃) with considerably enriched carrier density (fourfold), improved conductivity (12.3-fold at 300 K), additional redox-active sites, boosted Nb redox depth, nondeclined Na⁺-diffusion capability, and buffered internal stress during Na⁺ intercalation/de-intercalation. Consequently, compared with O-terminated Nb₄C₃, PO₂-Nb₄C₃ exhibits a doubled Na⁺-storage capacity (221.0 mAh g^-1), well-retained fast-charging capability (4.9 min at 80% capacity retention), significantly promoted cycle life (nondegraded capacity over 2000 cycles), and justified feasibility for assembling energy−power-balanced Na-ion capacitors. This study unveils that the molecular-level design of MXene terminals provides opportunities for developing simultaneously high-capacity and fast-charging electrodes, alleviating the energy−power tradeoff typical for energy-storage devices.