Compositional Tuning of Bilayer Hf _{1− <i>x</i>} Zr <i> _x </i> O ₂ for Enhanced Negative Capacitance and Energy Storage Performance

Electrostatic capacitors based on hafnium–zirconium oxide (HZO) have attracted considerable attention as scalable, CMOS‐compatible candidates for next‐generation energy storage. Despite their fast charge–discharge capability and high power density, their application remains limited by low energy storage density (ESD) and efficiency, mainly constrained by polarization saturation and hysteresis losses. Although compositional engineering has been explored to enhance the antiferroelectric (AFE) or negative capacitance (NC) behavior of HZO, understanding how the Hf:Zr ratio governs NC onset voltage and its correlation with energy storage remains incomplete. Here, we systematically investigate the role of Hf:Zr composition in bilayer HZO capacitors to identify the optimal ratio balancing performance and reliability. Increasing the Zr fraction from 1:1 to 1:3 in both layers improves recoverable ESD from 62.7 to 82.1 J cm −3 and efficiency from 88% to 91%, enabled by delayed NC onset and enhanced charge amplification. This enhancement originates from a higher tetragonal‐phase fraction, elevating the NC onset voltage and widening the stable NC window while suppressing charge injection and hysteresis losses. These findings establish a compositional design rule for HZO‐based NC supercapacitors, providing a pathway toward high‐efficiency, fatigue‐resistant, and energy‐autonomous electronic systems.