Following　increased　environmental　concerns　on　the　toxicity　of　lead,　the　discovery　of　ultrahigh　piezoelectricity　in　lead-free　piezoelectric　materials　is　critical　for　the　substitution　of　commercial　lead　zirconate　titanate　(PZT)　ceramics　in　numerous　electronic　devices.　In　this　work,　a　synergistic　design　strategy　is　proposed　to　enhance　the　piezoelectricity　in　lead-free　piezoelectric　materials　by　flattening　the　Gibbs　free　energy　density　profile,　via　the　coexistence　of　multiple　phases　and　local　structural　heterogeneity.　This　strategic　material　design　approach　is　based　on　first-principles　calculations　combined　with　Landau　phenomenological　theory　and　phase　field　simulations.　Sustainable　Stannum-doped　BaTiO3　lead-free　ferroelectric　ceramics　are　prepared　to　validate　our　proposed　mechanism,　and　a　giant　piezoelectric　coefficient　d33　＞　1100　pC/N　is　achieved,　being　the　highest　value　reported　in　lead-free　piezoceramics.　The　mechanism　and　paradigm　of　the　excellent　piezoelectricity　achieved　here　provides　a　feasible　solution　for　replacing　lead-based　piezoelectrics　by　lead-free　counterparts.　©　2020