Sandwich Structure Design for Coordinated Enhancement of Polarization and Breakdown in Lead-Free Energy Storage...

High-energy-density ceramic capacitors that do not rely on lead critical for mitigating environmental pollution and addressing the growing energy demand. However, electromechanical coupling often prevents the simultaneous enhancement of maximum polarization (P_max) and breakdown field strength (E_b), which limits further advances in energy storage performance (ESP). To overcome this constraint, we adopt a macroscopic structural design strategy and fabricate sandwich-structured ceramics using a tape-casting process. This architecture enables concurrent regulation of polarization and breakdown strength, effectively alleviating their intrinsic trade-off and substantially increasing the energy density. Consequently, lead-free ceramics with a sandwich configuration deliver a recoverable energy density (W_rec) of 6.83 J·cm^-3 accompanied by a high efficiency (η) of 92.0% under 487 kV·cm^-1. The η remains above 93.3% over frequencies from 1 to 100 Hz and temperatures from 20°C to 140°C, while the variation in W_rec stays within ±5.2%. In addition, the ceramics exhibit a high power density (P_D) of 78.52 MW·cm^-3. These results highlight sandwich-structured lead-free ceramics as promising candidates for high-performance energy-storage capacitors.