Piezoelectric energy harvesters (PEHs) fabricated using piezoceramics could convert directly the mechanical vibration energy in the environment into electrical energy. The high piezoelectric charge coefficient (d₃₃) and large piezoelectric voltage coefficient (g₃₃) are key factors for the high-performance PEHs. However, high d₃₃ and large g₃₃ are difficult to simultaneously achieve with respect to $g_{33}=d_{33}/({\epsilon }_0{\epsilon }_{\text{r}})$ and $d_{33}=2Q{\epsilon }_0{\epsilon }_{\text{r}}{P}_{\text{r}}$. Herein, the energy harvesting performance is optimized by tailoring the CaZrO₃ content in (0.964-x)(K_0.52Na_0.48)(Nb_0.96Sb_0.04)O₃ -0.036(Bi_0.5Na_0.5)ZrO₃-xCaZrO₃ ceramics. First, the doping CaZrO₃ could enhance the dielectric relaxation due to the compositional fluctuation and structural disordering, and thus reduce the domain size to ~30 nm for x = 0.006 sample. The nanodomains switch easily to external electric field, resulting in large polarization. Second, the rhombohedral-orthorhombic-tetragonal phases coexist in x = 0.006 sample, which reduces the polarization anisotropy and thus improves the piezoelectric properties. The multiphase coexistence structures and miniaturized domains contribute to the excellent piezoelectric properties of d₃₃ (354 pC/N). Furthermore, the dielectric relative permittivity (ε_r) reduces monotonously as the CaZrO₃ content increases due to the relatively low ion polarizability of Ca²⁺ and Zr⁴⁺. As a result, the optimized energy conversion coefficient (d₃₃ × g₃₃, 5508 × 10^-15 m²/N) is achieved for x = 0.006 sample. Most importantly, the assembled PEH with the optimal specimen shows the excellent output power (~48 μW) and lights up 45 red commercial light-emitting diodes (LEDs). This work demonstrates that tailoring ferroelectric/relaxor behavior in (K,Na)NbO₃-based piezoelectric ceramics could effectively enhance the electrical output of PEHs.

The growing demand for high-power-density electric and electronic systems has encouraged the development of energy-storage capacitors with attributes such as high energy density, high capacitance density, high voltage and frequency, low weight, high-temperature operability, and environmental friendliness. Compared with their electrolytic and film counterparts, energy-storage multilayer ceramic capacitors (MLCCs) stand out for their extremely low equivalent series resistance and equivalent series inductance, high current handling capability, and high-temperature stability. These characteristics are important for applications including fast-switching third-generation wide-bandgap semiconductors in electric vehicles, 5G base stations, clean energy generation, and smart grids. There have been numerous reports on state-of-the-art MLCC energy-storage solutions. However, lead-free capacitors generally have a low-energy density, and high-energy density capacitors frequently contain lead, which is a key issue that hinders their broad application. In this review, we present perspectives and challenges for lead-free energy-storage MLCCs. Initially, the energy-storage mechanism and device characterization are introduced; then, dielectric ceramics for energy-storage applications with aspects of composition and structural optimization are summarized. Progress on state-of-the-art energy-storage MLCCs is discussed after elaboration of the fabrication process and structural design of the electrode. Emerging applications of energy-storage MLCCs are then discussed in terms of advanced pulsed power sources and high-density power converters from a theoretical and technological point of view. Finally, the challenges and future prospects for industrialization of lab-scale lead-free energy-storage MLCCs are discussed.

It is a very difficult work to sinter K_0.5Na_0.5NbO₃ (KNN)-based materials with good reduction resistance in strong reducing atmosphere. 0.945K_0.48Na_0.52Nb_0.96Ta_0.04O₃-0.055BaZrO₃ + 0.03ZrO₂ + y mol%MnO (KNNT-0.055BZ + 0.03Zr + yMn) ceramics sintered in reducing atmosphere were prepared successfully by conventional solid-state reaction methods. MnO dopant increases grain size at y = 5-8 due to strong lattice distortion and then decreases grain size at y = 9 due to much Mn₄Nb₂O₉ accumulated at the grain boundary. MnO dopant as an excellent sintering aid can effectively reduce volatilization of alkali metal by decreasing the sintering temperature (T_sinter). Reducing alkali metal volatilization can greatly reduce oxygen vacancies and improve piezoelectric properties. MnO dopant can improve the anti-reduction properties. The KNNT-0.055BZ + 0.03Zr + yMn ceramics at y = 6-9 show outstanding anti-fatigue of unipolar piezoelectric strain under the synergistic effect of reduced oxygen vacancies due to reduced volatilization and increased grain size. Piezoelectric properties and temperature stability of KNNT-0.055BZ + 0.03Zr ceramics sintered in reducing atmosphere are improved simultaneously by MnO dopant. Optimum inverse piezoelectric coefficient (d^*₃₃) of ceramics at y = 8 reaches up to 480 pm/V under low driving electric field E = 20 kV/cm at room temperature, and its temperature stability of d^*₃₃ reaches 158 ℃. It will be an excellent lead-free material candidate for the preparation of multilayer piezoelectric actuators co-fired with nickel electrode.

Multilayer ceramic capacitors (MLCCs) for energy storage applications require a large discharge energy density and high discharge/charge efficiency under high electric fields. Here, 0.87BaTiO₃-0.13Bi(Zn_2/3(Nb_0.85Ta_0.15)_1/3)O₃ (BTBZNT) MLCCs with double active dielectric layers were fabricated, and the effects of inner electrode and sintering method on the energy storage properties of BTBZNT MLCCs were investigated. By using the pure Pt as inner electrode instead of Ag_0.6Pd_0.4 alloys, an alternating current (AC) breakdown strength (BDS) enhancement from 1047 to 1500 kV/cm was achieved. By investigating the leakage current behavior of BTBZNT MLCCs, the Pt inner electrode and two-step sintering method (TSS) were confirmed to enhance the Schottky barrier and minimize the leakage current density. With relatively high permittivity, dielectric sublinearity, and ultra-high BDS, the Pt TSS BTBZNT MLCCs exhibited a surprisingly discharge energy density (U_dis) of 14.08 J/cm³. Moreover, under an operating electric field of 400 kV/cm, the MLCCs also exhibited thermal stability with U_dis variation < ±8% over a wide temperature (t) range from -50 to 175 ℃ and cycling reliability with U_dis reduction < 0.3% after 3000 charge-discharge cycles. These remarkable performances make Pt TSS BTBZNT MLCCs promising for energy storage applications.