Excellent energy storage density and superior discharge properties of NBT–NN–ST/xHfO2 ceramics via 0–3 type heterogeneous structure designing

Abstract

Na_0.5Bi_0.5TiO₃ (NBT) based relaxor ferroelectrics (RFEs) have been considered one of the most promising candidates for developing superior performance dielectric capacitors due to their high polarization and low energy hysteresis loss. However, the contradiction between high polarization and high breakdown strength hinders their progression for energy storage performance. In this work, HfO₂ was introduced into 0.75Na_0.5Bi_0.5TiO₃–0.24NaNbO₃–0.01SrTiO₃ (NBT–NN–ST) RFE ceramics to form 0–3 type heterogeneous NBT–NN–ST/HfO₂ composite ceramics to overcome the contradictory relationship between high polarization and high breakdown strength. Some HfO₂ particles are embedded in the NBT–NN–ST grain boundaries, which increases the activation energy of conduction (E_a) and suppresses the development of a local electric branch in NBT–NN–ST/HfO₂ composites, enhancing the breakdown strength (E_b) of NBT–NN–ST/HfO₂ composites. In addition, some Hf⁴⁺ ions diffuse into the NBT–NN–ST lattice and lead to an increase in Ti–O bond length and Bi–O bond length, which results in a larger difference in electronegativity between Bi/Ti and O atoms and stronger polar Ti–O/Bi–O bonds compared to pure NBT–NN–ST ceramics. Therefore, HfO₂ modified NBT–NN–ST RFE ceramics maintain high saturation polarization above 47 μC cm⁻². With an increment in E_b while maintaining the polarization, NBT–NN–ST/xHfO₂ ceramics with x = 7 wt% exhibit an excellent recoverable energy storage density of 5.3 J cm⁻³ with a charge–discharge efficiency of 85%, as well as favorable under-damped charge–discharge properties with a maximum current, current density and power density of 24 A, 2600 A cm⁻², and 280 MW cm⁻³, respectively. These characteristics make them a promising candidate for developing high-performance dielectric capacitors.