Materials exhibiting high energy/power density are currently needed to meet the growing demand of portable electronics, electric vehicles and large-scale energy storage devices. The highest energy densities are achieved for fuel cells, batteries, and supercapacitors, but conventional dielectric capacitors are receiving increased attention for pulsed power applications due to their high power density and their fast charge-discharge speed. The key to high energy density in dielectric capacitors is a large maximum but small remanent (zero in the case of linear dielectrics) polarization and a high electric breakdown strength. Polymer dielectric capacitors offer high power/energy density for applications at room temperature, but above 100 °C they are unreliable and suffer from dielectric breakdown. For high-temperature applications, therefore, dielectric ceramics are the only feasible alternative. Lead-based ceramics such as La-doped lead zirconate titanate exhibit good energy storage properties, but their toxicity raises concern over their use in consumer applications, where capacitors are exclusively lead free. Lead-free compositions with superior power density are thus required. In this paper, we introduce the fundamental principles of energy storage in dielectrics. We discuss key factors to improve energy storage properties such as the control of local structure, phase assemblage, dielectric layer thickness, microstructure, conductivity, and electrical homogeneity through the choice of base systems, dopants, and alloying additions, followed by a comprehensive review of the state-of-the-art. Finally, we comment on the future requirements for new materials in high power/energy density capacitor applications.

Ultrahigh discharge energy density (W_dis = 10.5 J cm⁻³) and efficiency (η = 87%) have been obtained in doped BiFeO₃–BaTiO₃ ceramic multilayers by achieving an electrically rather than chemically homogeneous microstructure. Back scattered scanning and transmission electron microscopy combined with energy dispersive X-ray spectroscopy mapping of (0.7 − x)BiFeO₃–0.3BaTiO₃–xNd(Zn_0.5Zr_0.5)O₃ (0.05 ≤ x ≤ 0.10) ceramics revealed a core–shell grain structure which switched from a bright to dark contrast as x increased. Compositions with x = 0.08 were at the point of cross over between these two manifestations of core–shell contrast. Dielectric measurements together with the absence of macrodomains in diffraction contrast TEM images suggested that compositions with x = 0.08 exhibited relaxor behaviour within both the core and shell regions. Impedance spectroscopy demonstrated that, despite being chemical dissimilar, the grains were electrically homogeneous and insulating with little evidence of conductive cores. Multilayers of x = 0.08 had enhanced breakdown strength, E_BDS > 700 kV cm⁻¹ and a slim hysteresis loop which resulted in large W_dis and high η which were temperature stable to <15% from 25 to 150 °C.

Tailored dopant strategies are proposed to optimise the energy density of BiFeO ₃ –SrTiO ₃ , which can be adapted for other high polarisability oxide-based systems.

Ceramic dielectrics are reported with superior energy storage performance for applications, such as power electronics in electrical vehicles. Recoverable energy density (Wrec) of ~ 4.55 J cm-3 with η~90% is achieved in a lead-free relaxor BaTiO3-0.06Bi2/3(Mg1/3Nb2/3)O3 ceramics at ~ 520 kV cm-1. These ceramics may be co-fired with Ag/Pd, which constitutes a major step forward towards their potential use in the fabrication of commercial multilayer ceramic capacitors. Compared to stoichiometric Bi(Mg2/3Nb1/3)O3 doped BaTiO3 (BT), A-site deficient Bi2/3(Mg1/3Nb2/3)O3 reduces the electrically heterogeneity of BT. Bulk conductivity differs from grain boundary only by one order of magnitude which, coupled with a smaller volume fraction of conducting cores due to enhanced diffusion of the dopant via A-site vacancies in the A-site sublattice, results in higher breakdown strength under electric field. This strategy can be employed to develop new dielectrics with improved energy storage performance.

Electrical homogeneity is vital in optimising energy storage performance in BiFeO₃–BaTiO₃–xBi(Li_0.5Nb_0.5)O₃ ceramic multilayer capacitors, giving rise to high recoverable energy density ∼13.8 J cm⁻³ under electric field ∼950 kV cm⁻¹ and fatigue resistance behaviour.