Excellent energy storage and mechanical performance in hetero-structure BaTiO3-based relaxors via local phase boundary design

•A record-high Wrec of 9.04 J cm−3 in BT-based relaxors with a large η of 87.2% are realized.•Coexisting multiphase ergodic polar nanoregions is successfully designed by phase boundary adjustment.•Excellent comprehensive performance of energy storage and mechanical is realized.•Structural correlation between energy storage and mechanical is established.