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

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 Na3Fe0.6Al0.4V(PO4)3 (ME-NF0.6A0.4VP) 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 V4+/V5+ 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-NF0.6A0.4VP 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.