Achieving high energy density/efficiency in light-metal-element-rich relaxor ferroelectric ceramics by annihilating volatile Schottky defects

Abstract

With the rapid growth of energy storage capacitors in advanced power systems, the pursuit of lightweight devices is gradually gaining importance despite the urgent need for device miniaturization. To reduce the mass of dielectric energy storage materials, certain light metal elements, such as potassium (K) and sodium (Na), offer promising prospects for achieving low material density. However, the intrinsic formation of point defects, including alkali metal vacancies and oxygen vacancies (Schottky defects), is inevitable during high-temperature sintering because of the volatilization of alkali metal oxides. These defects can lead to high conductivity under high electric fields, thereby deteriorating energy storage performance (ESP). In this work, we focus on a potassium-rich composition, K₂La_0.75Gd_0.25Nb₅O₁₅, which is characterized by a tetragonal tungsten bronze (TTB) structure, to investigate the impact of volatile point defects. Different types of defects were designed by changing the K₂O content, and energy storage properties were investigated. Based on leakage current measurements, impedance spectra, and DFT calculation results, it could be concluded that K–O defects with a shallow energy level cause conduction loss, thereby influencing polarization and ESP. Consequently, a high recoverable energy storage density (W_rec) of 5.02 J cm⁻³ and an energy efficiency of 85.6% were simultaneously achieved for the composition with the lowest defect content. This work reveals a mechanism in which volatile defects dominate conduction behavior and relevant ESP, stresses the significance of suppressing alkaline Schottky defects, and provides a feasible solution to developing lightweight dielectric energy storage materials.