It is widely accepted that achieving high energy storage density and efficiency in ferroelectric ceramics depends on their ability to exhibit high saturation polarization and low residual polarization, which are essential for excellent energy storage performance. In addition, improving energy storage density in ferroelectric relaxor ceramics also requires higher breakdown field strength, an important factor that should not be overlooked.

Ferroelectric ceramics with tungsten bronze (TB) structure possess unique structural features, numerous composition control mechanisms, and the ability to easily induce relaxation behavior. These properties enable them to achieve significantly higher polarization values ​​at lower electric field strengths, which greatly improves their energy storage performance and expands their application possibilities in the field of energy storage.

Therefore, it is imperative to investigate strategies to achieve exceptional energy storage performance in ferroelectric ceramics with tungsten bronze structure.

A team of researchers has now published a paper on this topic in Magazine for modern ceramics.

“In this report we have Tetragonal TB Sr_4.5-xBa_XSm_0.5Zr_0.5No._9.5O₃₀ (x = 2.5, 3, 3.5, 4, 4.5)(SBSZN) ceramics using the Ba co-doping strategy²⁺Mister²⁺Sm³⁺ in the A-location and Zr⁴⁺ instead of Nb⁵⁺ in the B site to break the long-range ferroelectric order and confirmed the presence of polar nanoregions (PNRs) by high-resolution transmission electron microscopy. It was confirmed that the small PNRs inherently induce the ferroelectric relaxation behavior of SBSZN ceramics,” said Changzheng Hu, a professor at the College of Materials Science and Engineering, Guilin University of Technology (China), who is mainly engaged in research on the ferroelectric and tribocatalytic properties of structured tungsten bronze ceramics.

“At the same time, SEM observed that the grain size decreased with increasing doping amount. Combined with the resistance test, it was confirmed that the reduction in grain size led to a significant increase in resistance, which in turn led to an increase in the electric breakdown field strength,” said Prof. Hu.

The dielectric properties tests showed the typical temperature-dependent relaxation behavior of the dielectric constant and dielectric loss. It is noteworthy that the ceramic with x = 3.5 exhibits a Curie peak at room temperature, which is attributed to its continuous composition modulation. This unique property contributes significantly to improved energy storage performance.

“By combining the empirical Vogel-Fulcher formula with the modified Curie-Weiss law and the Maxwell-Boltzmann distribution, it can be analyzed from the dipole dynamics behavior that it is easier to cross the energy barrier due to the small size of the PNRs, and the weak coupling between the dipoles facilitates the dipole switch, which leads to the relaxation behavior of SBSZN ceramics,” added Prof. Hu.

The extended hysteresis loops observed in all ceramics during the ferroelectric tests can be attributed to the disturbance of the long-range ferroelectric order, which induces a relaxation-type ferroelectric behavior.

“The ceramic with x = 3.5 exhibits high polarization due to the Curie temperature close to room temperature, while the small size of the PNRs gives them low residual polarization and hence they exhibit excellent energy storage performance by achieving an energy storage density of 4.31 J cm.^-3 and an efficiency of 93.8 % at a breakdown field strength of 310 kV·cm^-1,” said Hu.

Due to the significant reduction in grain size, which leads to an increase in resistivity and thus to a significant improvement in breakdown field strength, the ceramic with x = 4.5 also exhibits excellent energy storage properties, resulting in a high energy storage density of 5.3 J cm.^-3 at 460 kV·cm^-1 and a high efficiency of 81.5%.

“The significant difference in the size of the PNRs is responsible for the difference in efficiency between the two ceramic components, with the ceramic with x = 3.5 having PNR sizes between 11 and 23 nm and the ceramic with x = 4.5 having PNR sizes around 150 nm,” said Hu.

In addition, under overdamping conditions at a field strength of 400 kV·cm^-1The ceramic with x = 3.5 showed excellent discharge performance with a discharge energy density of up to 2.27 J cm.^-3 within an extremely short discharge time (t_0.9 ~ 34 ns). In addition, the charge/discharge performance is determined by C_D = 713.38 A·cm^-2 and P_D = 87.51 MW·cm^-3 was achieved under high attenuation conditions at a field strength of 240 kV·cm^-1Therefore, this work provides valuable insights into achieving high-performance energy storage properties even under weak electric fields.

However, for tungsten bronze structural ceramics to achieve a combination of superior energy storage properties, the potential of combining high polarization with low residual polarization at high electric field strengths needs to be explored. In this context, Hu also suggests several possible directions for future investigations, including the development of composites with polymers and chalcogenides with high polarization values.

Other contributors include Yuejun Dan, Liupan Tang, Wenzhi Ning, Yingzhi Meng, Laijun Liu and Liang Fang from the College of Materials Science and Engineering, Guilin University of Technology.