<p>High-voltage layered oxide cathodes have received extensive attention for sodium-ion batteries owing to their potential high energy and power densities, but their stabilization remains a universal challenge. Herein, a stable high-voltage K_xNa_5-xV₁₂O₃₂ cathode is designed by synergistically tuning the irreversible phase and oxygen vacancies through the substitution of Na with K. The functional mechanism regulating the electronic structure of K_xNa_5-xV₁₂O₃₂ is elucidated: K substitution strengthens V 3<i>d</i>–O 2<i>p</i> hybridization and V 3<i>d</i>–orbital electron delocalization in the K_0.147Na_4.853V₁₂O₃₂ structure, enriching charge distribution and reinforcing the V₃O₈ structure. This promots electron transfer kinetics, suppresses irreversible phase transition, and lowers the Na⁺ diffusion energy barrier. Moreover, the reversible redox reaction of V⁵⁺/V⁴⁺ is significantly enhanced, delivering 250.1 mA h g⁻¹ (1.5–4.3 V vs. Na/Na⁺), which increases the average operating voltage from 4.0 to 4.3 V and boosts the overall energy density. Consequently, the K_0.147Na_4.853V₁₂O₃₂ electrode significantly enhances cycling performance, retaining 98.2% of the capacity after 1000 cycles at 1300 mA g⁻¹ and enabling stable cycling with 98.7% retention after 300 cycles in a hard carbon | | K_0.147Na_4.853V₁₂O₃₂ pouch cell. This strategy of electronic structure modulation offers avenues for developing high energy density stable vanadium-based cathode materials for sodium-ion batteries.</p>