Chemical Framework to Design Linear-like Relaxors toward Capacitive Energy Storage

Abstract

ABO 3 -type perovskite relaxor ferroelectrics (RFEs) have emerged as the preferred option for dielectric capacitive energy storage. However, the compositional design of RFEs with high energy density and efficiency poses significant challenges owing to the vast compositional space and the absence of general rules. Here, we present an atomic-level chemical framework that captures inherent characteristics in terms of radius and ferroelectric activity of ions. By categorizing A/B-site ions as host framework, rattling, ferroelectrically active, and blocking ions and assembling these four types of ions with specific criteria, linear-like relaxors with weak locally correlated and highly extendable unit-cell polarization vectors can be constructed. As example, we demonstrate two new compositions of Bi 0.5 K 0.5 TiO 3 -based and BaTiO 3 -based relaxors, showing extremely high recoverable energy densities of 17.3 and 12.1 J cm −3 , respectively, both with a high efficiency of about 90%. Further, the role of different types of ions in forming heterogeneous polar structures is identified through element-specific local structure analysis using neutron total scattering combined with reverse Monte Carlo modeling. Our work not only opens up new avenues toward rational compositional design of high energy storage performance lead-free RFEs but also sheds light on atomic-level manipulation of functional properties in compositionally complex ferroelectrics.

Strong Local Polarization Fluctuations Enabled High Electrostatic Energy Storage in Pb-Free Relaxors

“…This is an effective way to differentiate the polar displacement vectors along the R , T , and O directions (Figure c). Bi 3+ is predominantly displaced along the T directions­(Figure d), which is similar to some other Bi-based compounds. ,, The M ion shows a preference for the R direction, together with a relatively weak O direction preference (Figure e), while Ti 4+ exhibits a broad and diffuse distribution (Figure f). The combination of such differentiated orientations of the polar displacement vectors leads to a strong orientation fluctuation in local polarization vectors.…”

“…Subsequently, the inherent characteristics of B-site ions in terms of their theoretical atomic displacement ( D B ) and ionic radius ( R B ) are analyzed to facilitate the selection of foreign ions (Figure b). ,, Surprisingly, only Zn 2+ exhibits a displacement comparable to that of Ti 4+ , which is why most B-site substitutions within ATiO 3 perovskite have been observed to substantially reduce polarization in previous studies . Nb 5+ , Ta 5+ , and Fe 3+ possess a relatively large D B , although it is lower than that of Ti 4+ .…”

“…Dielectric capacitors store electrical energy as an electrostatic field, offering the highest power density of up to 10 8 MW kg –1 versus 10 2 W kg –1 for batteries and 10 5 W kg –1 for electrochemical capacitors due to their fastest charge–discharge rates at the microsecond scale. − Electrostatic energy-storage ceramic capacitors represent the core components in modern pulsed power systems and are utilized in a wide variety of applications, including electric vehicles, electronic gadgets, and power grids. , As technology continues to advance, there is an increasing need to develop next-generation dielectric ceramic capacitors to meet the demands of cutting-edge applications. These capacitors are anticipated to have a high energy-storage density ( W rec ) to support miniaturization and integration, as well as high energy-storage efficiency (η) to minimize energy loss and ensure reliability. − Hence, a vast range of Pb-free perovskite-structured dielectric ceramics, including BaTiO 3 (BT)-, , (Bi 0.5 Na 0.5 )­TiO 3 (BNT)-, − (Bi 0.5 K 0.5 )­TiO 3 (BKT)-, − NaNbO 3 (NN)-, − and AgNbO 3 (AN)- − based systems have been explored for electrostatic energy storage, with W rec values advancing to reach typically around 10 J cm –3 at η above 80%. − Despite these substantial advances, there is an ongoing challenge to boost W rec while maintaining a high η, given the trade-off relationship between these two merits.…”

“…In general, the value of P m observed in the P – E loops is derived from either the reorientation or extension of the unit-cell polarization vectors under external E . Hence, it is important to have a structure that offers large unit-cell polarization vectors and provides sufficient room for extension in order to achieve a high P m value . Interestingly, recent studies have shown that strong fluctuations in both the magnitude and orientation of unit-cell polarization vectors can lead to the emergence of a set of synergistic features, including a relatively large P m , low H , and high E B , which are critical for electrostatic energy storage.…”

Achieving outstanding energy storage behaviors via combinatorial optimization design in BNT-based relaxor ferroelectric ceramics under medium–low electric fields

“…The presence of Ba 2+ and Zr 4+ with larger ionic radii can expand the [AO 12 ] cube-octahedral and [BO 6 ] octahedral main frameworks, providing space for ion displacement while partially disrupting the long-range ordered ferroelectric domains. 11,12 The BNT–BZT ceramic exhibits high P max , but the energy storage efficiency is still low due to the large P r . Upon the introduction of NN, the macroscopic ferroelectric domains will be further disrupted, leading to significant reductions in P r and the coercive field ( E c ).…”