Bi‐Based Lead‐Free Dielectrics for Energy Storage: From High‐Polarization Potential to Reversible Device Responses

Bi‐based lead‐free dielectric materials have emerged as a promising materials platform for pulsed power capacitors owing to their high polarization potential, rich local structural diversity, and broad tunability across multiple length scales. The 6s 2 lone pair electrons of Bi 3+ favor large polar displacements, while cation disorder, local structural distortion, and phase competition promote the formation of nanodomains, polar nanoregions, and relaxor states. These characteristics provide a structural basis for achieving low remanent polarization and reversible energy‐storage behavior. However, their practical performance is often limited by Bi volatilization, oxygen vacancies, and defect‐assisted conduction, which degrade breakdown strength, efficiency, and reliability. This review summarizes recent progress in four representative Bi‐based systems: BiFeO 3 ‐based materials, BNT‐based materials, BMT‐related BiMeO 3 systems, and Aurivillius‐type layered compounds. We highlight how each system addresses the key trade‐off between high polarization and high insulation with low hysteresis. We further discuss major optimization strategies, including compositional engineering, high‐entropy engineering, defect and bandgap engineering, nanodomain and local structure engineering, grain and densification engineering, and interface and heterostructure engineering. Finally, future opportunities are discussed for application‐oriented development in low‐/medium‐field, high‐temperature, multilayer, and thin‐film capacitors.