Medium‐Entropy‐Vacancy Engineering Unlocks Energy‐Power Trade‐Off Reconciliation in Sodium‐Ion Hybrid Capacitors

ABSTRACT The intrinsic asymmetry between the high‐capacity but kinetically sluggish battery‐type Na + storage and the fast but low‐capacitance charge storage of capacitors has fundamentally constrained sodium‐ion hybrid capacitors (SIHCs), precluding the simultaneous achievement of high energy density, high power output and long‐term durability. Here, we report a thermodynamically guided entropy‐vacancy engineering strategy to reconcile this fundamental asymmetry by constructing a medium‐entropy Na 3 Fe 0.6 Al 0.4 V(PO 4 ) 3 (ME‐NF 0.6 A 0.4 VP) cathode. Machine‐learning potential calculations reveal that selective iron incorporation induces abundant oxygen vacancies, which construct continuous Na + diffusion pathways and accelerate interfacial charge transfer, while aluminium incorporation modulates the local coordination environment to stabilize highly redox‐active sites. The enthalpy‐entropy coupling activates the high‐voltage V 4+ /V 5+ redox couple, unlocks previously inaccessible Na1 sites, and enables a highly reversible high‐voltage solid‐solution reaction with a minimal lattice expansion of only 6.28%. Consequently, ME‐NF 0.6 A 0.4 VP cathode delivers an exceptional cyclability over 10 000 cycles at an ultrahigh rate of 100C, and robust wide‐temperature application from ‐30°C to 60°C. The full SIHCs device achieves a maximum energy density of 223 Wh kg −1 (at 44 W kg −1 ) and retains 51 Wh kg −1 at 5940 W kg −1 . This work delineates a viable thermodynamic design pathway for cathode design, providing a blueprint for practical and high‐performance sodium‐ion energy‐storage systems.