High energy-storage density of lead-free (Sr_1-1.5xBi_x)Ti_0.99Mn_0.01O₃ thin films induced by Bi³⁺-V_Sr dipolar defects

Capacitors with high energy storage density, low cost, ultrafast charge-discharge capability, and environmental friendliness are in high demand for application in new energy vehicles, modern electrical systems, and high-energy laser weapons. Here, lead-free (Sr_1-1.5xBi_x)Ti_0.99Mn_0.01O₃ (x = 0.01, 0.05, 0.1) thin films grown on Pt/Ti/SiO₂/Si substrates were obtained by a sol-gel method. All the thin films have a relatively high dielectric breakdown strength (BDS) due to the added 1% Mn and pinched polarization hysteresis loops can be observed in 5 and 10 mol% Bi-doped SrTiO₃ thin films. The ferroelectric behaviors of the Bi-doped SrTiO₃ thin films come from the rotation of the TiO₆-octahedra induced by the formation of Bi³⁺-V_Sr dipolar defects. With the increase of doping concentration, the P_max-P_r values of the Bi-doped SrTiO₃ thin films increased dramatically and can reach 34.3 μC cm^-2 upon doping with 10 mol% Bi. A high recoverable energy-storage density of 24.4 J cm^-3 with excellent temperature stability was obtained for the 10 mol% Bi-doped ST thin film, which shows that the (Sr_0.85Bi_0.1)Ti_0.99Mn_0.01O₃ thin film is a promising candidate for high-power energy storage applications. This finding demonstrates an improved energy density of SrTiO₃-based thin film systems and a reasonable explanation for the source of the ferroelectricity based on first-principles calculations is given.