Lead-free　piezoelectric　energy　harvesters　(PEHs)　with　long-term　stable　power　generation　capability　is　an　important　solution　for　realizing　reliable　self-power　supply　of　microelectronic　devices　such　as　wireless　sensors.　As　the　core　of　PEHs,　piezoceramics　with　practical　value　require　not　only　high　figure　of　merit　(FOM)　to　achieve　high　power　generation,　but　also　strong　fracture　toughness　(KIC)　to　prevent　the　fracture　failure　caused　by　long-term　cyclic　vibration,　which　poses　a　challenge　to　the　material　design.　To　address　these　concerns,　K0.5Bi0.5TiO3　was　rationally　introduced　into　Na0.5Bi0.5TiO3　matrix,　and　the　synergistic　effect　of　grain　refinement　and　polarization　enhancement　was　realized　through　composition　design　and　grain　size　regulation　to　obtain　excellent　performance　with　KIC　of　1.57　MPa　&　sdot;m1/2　and　FOM　of　1.94　x　10-12　m2/N.　The　cantilever　beam-type　PEH　with　the　optimal　sample　achieved　a　high　output　power　density　of　395.5　mu　w/cm3,　in　particular,　under　different　acceleration　conditions　(1g-3g),　after　106　cycles,　the　PEH　can　still　maintain　a　stable　power　generation　output,　showing　a　super-strong　service　capability.　This　work　not　only　provides　a　class　of　NBT-based　piezoceramics　with　excellent　electromechanical　properties,　but　more　importantly,　the　design　strategy　associated　with　double　synergy　effect　is　expected　to　be　extended　to　the　construction　of　more　high-quality　lead-free　piezoelectric　devices.