Page 6 - Read Online

Page 2 of 11 Liu et al. Microstructures 2023;3:2023009 https://dx.doi.org/10.20517/microstructures.2022.29

has gradually become the focus of research. Currently, energy storage devices are mainly divided into four
[1-3]
categories: lithium-ion batteries, fuel cells, electrochemical super-capacitors, and dielectric capacitors .
Solid-state dielectric capacitors, compared with other energy storage devices, possess high power density
and ultrafast charge-discharge rates, which are widely used in advanced high power and pulse power
electronic devices, such as hybrid electric vehicles, distributed power systems, and directional energy
[4,5]
weapons . However, the low recoverable energy storage density (W ) limits their energy storage
rec
development.
In the context of energy saving and environmental protection, to effectively improve the W of dielectric
rec
capacitors, lead-free perovskite energy storage ceramics have become a research hotspot . The total energy
[6,7]
storage density (W ), W , and efficiency (η) are the main parameters to evaluate energy storage
total
rec
performance which can be calculated based on the following formula:

where P , P , and E are the maximum polarization, remanent polarization, and applied electric field,
max
r
respectively. As a result, large ∆P (P -P ) and high E are indispensable for materials with high W rec [8] . The
b
r
max
researches on lead-free energy storage materials generally focus on linear dielectrics, ferroelectrics,
antiferroelectrics, and relaxor ferroelectrics. Linear dielectrics possess ultrahigh η and E but low W due to
rec
b
their low polarization characteristic . Both high P and P can be found in ferroelectrics, resulting in
[9]
max
r
highly inferior W and η. Similarly, antiferroelectrics also own unsatisfactory energy storage properties with
rec
low η and poor cycle stability because of irreversible antiferroelectric to ferroelectric phase transition under
applied electric field and comparatively significant difference between E and E A [10-12] . Relaxor ferroelectrics
F
are characterized by a diffuse phase transition over a broad temperature range, from the Burns temperature
(T ) at which nanodomains appear, to the intermediate temperature (T ) at which nanodomains grow and
B
m
the permittivity reaches the maximum, and finally to the freezing temperature (T) at which nanodomains
f
become frozen(T < T < T ) [13,14] . In particular, relaxor ferroelectrics located at the temperature range of
B
f
m
T -T can be defined as superparaelectrics, in which the size of nanodomains is further decreased, and the
m
B
[14]
domain interaction is further weakened . Therefore, Relaxor ferroelectrics, especially for
superparaelectrics [15,16] , show excellent performance superiority for achieving both high W and η
rec
simultaneously [17-20] .
NaNbO (NN) is one of the typical lead-free ferroelectrics with complex crystal structure and phase
3
transition under various temperatures. Despite the remaining controversies, it is commonly agreed that NN
adopts seven major phases with the sequence of U→T2→T1→S→R→P→N on cooling, where the common P
and R phases are antiferroelectrics [21,22] . The complex temperature-driven structure also means great
potential for performance regulation. NN ceramic exhibits antiferroelectric P phase structure with Pbma
space group at room temperature [23,24] . Generally, an effective strategy to improve the energy storage of NN
ceramics focuses on stabilizing their antiferroelectric phase. For example, ultrahigh W of 12.2 J/cm was
3
rec
obtained in 0.76NaNbO -0.24(Bi N )TiO ceramics due to stable relaxor antiferroelectric phase, however,
3
0.5
3
0.5