Simultaneously achieving high performance of energy storage and transparency via A-site non-stoichiometric defect engineering in KNN-based ceramics

[Display omitted] •A novel defect engineering is proposed to significantly refine the grain size of energy storage ceramics.•Achieving simultaneously high energy storage density, a rapid discharge time and superior transparency.•In-depth explanations are supported by the DFT calculations and the finite element analysis.•Bright-field TEM revealed the presence of ultrafine grains, PNRs and highly symmetric phase. Ceramic-based transparent dielectric materials are regarded as the best candidates for advanced energy storage and conversion materials because of their outstanding optical and electric properties. Nevertheless, due to the presence of low density, low band gap energy and large grain size, it is difficult to simultaneously obtain high energy storage density and high optical transmittance in lead-free based ceramics, which limiting their further development of practical applications. In this work, the relaxor ferroelectric ceramics of x mol% Er3+-doped 0.91 K0.5Na0.5NbO3-0.09BamSrnTiO3 (xEr-SrmBan) were constructed via A-site non-stoichiometric defect engineering. It is worth noting that excessive addition of Sr and Ba, especially Sr, can significantly refine the grain size and domain size on account of vacancy-related defect pinning. Finally, high energy storage density (W = 6.39 J/cm3, Wrec = 3.42 J/cm3) together with high optical transmittance (∼72% at 900 nm) can be achieved simultaneously in 0.25Er-Sr1Ba0.5 due to the exist of dense structure, ultrafine grain size (