Synergically improved energy storage performance and stability in sol–gel processed BaTiO₃/(Pb,La,Ca)TiO₃/BaTiO₃ tri-layer films with a crystalline engineered sandwich structure

Achieving an excellent energy storage performance, together with high cycling reliability, is desirable for expanding technological applications of ferroelectric dielectrics. However, in well-crystallized ferroelectric materials, the concomitant high polarizability and low polarization-saturation field have led to a square-shaped polarization–electric field loop, fatally impairing both recoverable energy density (W_rec) and efficiency (η). Nanocrystalline ferroelectric films with a macroscopically amorphous structure have shown an improved W_rec and η, but their much lower polarizability demands an extremely high electric field to achieve such performances, which is undesirable from an economic viewpoint. Here, we propose a strategy to boost the energy storage performances and stability of ferroelectric capacitors simultaneously by constructing a tri-layer film in which a well-crystallized ferroelectric layer was sandwiched by two pseudo-linear dielectric layers with a dominant amorphous structure. In sol–gel-derived BaTiO₃/(Pb,La,Ca)TiO₃/BaTiO₃ (BTO/PLCT/BTO) tri-layer films, we show that the above design is realized via rapid thermal annealing which fully crystallized the middle PLCT layer while left the top/bottom BTO cap layers in a poor crystallization status. This sandwiched structure is endowed with an enhanced maximum polarization while a small remnant one and a much-delayed polarization saturation, which corresponds to large W_rec ≈ 80 J/cm³ and high η ≈ 86%. Furthermore, the film showed an outstanding cycling stability: its W_rec and η remain essentially unchanged after 10⁹ electric cycles (ΔW/W < 4%, Δη/η < 2%). These good energy storage characteristics have proved the effectiveness of our proposed strategy, paving a way for the utilization of sandwiched films in applications of electric power systems and advanced pulsed-discharge devices.