Lattice Contraction-Induced V 3d Orbital Splitting for Achieving High-Energy-Density Na₃V₂(PO₄)₃ Cathodes

Transition metal (TM) doping can activate the high-voltage V⁴⁺/V⁵⁺ redox couple in Na₃V₂(PO₄)₃ (NVP), yet the atomistic origin governing this process remains obscured, impeding the rational design of high-energy-density Na-ion batteries. Herein, we reveal that Cr doping induces progressive lattice contraction in NVP, directly triggering V 3d orbital splitting near the Fermi level. This electronic reorganization enhances the electron-donating propensity of V, enabling reversible V⁴⁺/V⁵⁺ redox activity. The activated 4.0 V plateau intensifies with Cr concentration, elevating energy density to 430.6 W h kg^-1-surpassing pristine NVP (340.3 W h kg^-1). More importantly, the generality of this behavior is confirmed through Fe doping, which similarly contracts the lattice, reproduces the orbital splitting, and activates high-voltage redox activity (409.11 W h kg^-1). In contrast, Mn or Co dopants cause lattice expansion and fail to trigger either orbital splitting or redox activity. We thus establish lattice contraction-mediated orbital splitting as a universal electronic descriptor for high-voltage activation in NVP, thereby establishing a general principle for the rational design of high-energy-density electrodes through targeted orbital engineering.