Relaxor nature and superior energy storage performance of Sr2Ag0.2Na0.8Nb5O15-based tungsten bronze ceramics through B-site substitution

A series of Sr2Ag0.2Na0.8Nb5-xSbxO15 (SANNS) tungsten bronze ceramics were synthesized by traditional solid-phase methods to investigate the impact of Sb replacement on the structure, relaxor nature, and energy storage properties. The study on the relationship between structure and relaxor behavior revealed two main conclusions: (1) as the Sb5+ content increased, the crystal structure transformed from an orthorhombic Im2a phase to Bbm2, and then to a tetragonal paraelectric P4/mbm phase, accompanied by a decrease in both the cell volume V and the tetragonality of c/a; (2) switching from ferroelectric to relaxor behavior at room temperature was realized through the doping of B-sites, which could be attributed to incommensurate local structural modulations caused by orthogonal distortion of the P4bm structure. Finally, we obtained a superior recoverable energy storage density (2.27 J/cm3) and an ultrahigh efficiency (93.3%) in Sr2Na0.8Ag0.2Nb4.7Sb0.3O15. The charging-discharging performance was also evaluated: a high current density (1166.56 A/cm2) and a large powder density (118.54 MW/cm3) were achieved simultaneously.