Capacitors for energy storage based on ceramic dielectrics are commonly used in circuits for noise filtering and voltage stabilization. However, they often have a lower energy density compared to supercapacitors. The energy density of a ceramic capacitor is typically described by two factors: polarization (P), which largely depends on the permittivity (ε) of its dielectric component, and the maximum electric field strength (E_max), above which the dielectric breaks down. Most studies have focused on refining the compositions of non-linear dielectrics — materials that exhibit complex responses to electric fields — to maximize energy density by optimizing P and E_max. Now, Ian Reaney, Ge Wang, and colleagues in the UK and China have devised an approach in which the energy density is increased by inducing a linear or quasi-linear dielectric behaviour — where the derivative of P to E, dP/dE, is constant or shows small deviations — in the ceramic.

The researchers chose NaNbO₃ as the starting composition because it has a large bandgap, indicative of a high breakdown strength, and the potential to achieve high ε via dopants. In this composition, the material is in a ferroelectric state, where electric dipoles are arranged over a micron-scale length. Reaney and the team then induced a relaxor ferroelectric state by altering the composition, leading to shorter-range polar interactions. Further element substitutions reduced the range of these interactions to just a few unit cells. The resultant material, 0.88NaNb_0.9Ta_0.1O₃-0.10SrTiO₃-0.02La(Mg _1/2Ti_1/2)O₃, exhibits a near-constant dP/dE, typical of quasi-linear dielectric behaviour, and high ε, showcasing exceptionally high energy density. Reaney and the team suggest that dP/dE could serve as a third critical factor, in addition to P and E_max, for high-performance ceramic capacitor design.