Orbital reconfiguration and a solid-solution mechanism in MXene-templated NASICON cathodes

NASICON-type NaTi₂(PO₄)₃ (NTP) cathodes hold immense promise for use in sodium-ion batteries but are fundamentally limited by their single-electron Ti⁴⁺/Ti³⁺ redox chemistry and intrinsically sluggish electronic conductivity. Introducing multi-electron redox centers (e.g., vanadium) is a theoretical ideal, yet this is practically hindered by a "synthetic paradox" where mismatched precursor kinetics cause severe cation segregation. Herein, we introduce a two-dimensional (2D) structural chaperone strategy, that is, utilizing a dual-transition-metal (Ti_1-xV_x)₃C₂ MXene, to translate atomic-level cationic homogeneity directly into three-dimensional (3D) Na₂Ti_1-xV_x(PO₄)₃ polyanions. Beyond circumventing phase segregation, this atomic-precision synthesis triggers profound orbital reconfiguration. Strong V 3d-O 2p orbital hybridization collapses the band gap from 1.66 to 0.22 eV and reduces the Na⁺ migration barrier from 0.57 to 0.31 eV, establishing a synergistic electronic-ionic transport network. Consequently, structural optimization of Na₂Ti_0.5V_0.5(PO₄)₃ (NTVP) transforms its sodium storage mechanism from a sluggish biphasic reaction into a highly reversible solid-solution process, unlocking sequential V³⁺/V⁴⁺/V⁵⁺ multi-electron redox chemistry. NTVP delivers exceptional rate capability (128 and 81 mAh g^-1 at 0.05 and 10 A g^-1, respectively) and enables a full-cell energy density of 248 Wh kg^-1. This work enriches polyanion design, establishing a general MXene-templated platform for atomically precise multi-cation modulation.