Ceramic　dielectrics　are　reported　with　superior　energy　storage　performance　for　applications,　such　as　power　electronics　in　electrical　vehicles.　A　recoverable　energy　density　(Wrec)　of　∼4.55　J　cm-3　with　η　∼　90%　is　achieved　in　lead-free　relaxor　BaTiO3-0.06Bi2/3(Mg1/3Nb2/3)O3　ceramics　at　∼520　kV　cm-1.　These　ceramics　may　be　co-fired　with　Ag/Pd,　which　constitutes　a　major　step　forward　toward　their　potential　use　in　the　fabrication　of　commercial　multilayer　ceramic　capacitors.　Compared　to　stoichiometric　Bi(Mg2/3Nb1/3)O3-doped　BaTiO3　(BT),　A-site　deficient　Bi2/3(Mg1/3Nb2/3)O3　reduces　the　electrical　heterogeneity　of　BT.　Bulk　conductivity　differs　from　the　grain　boundary　only　by　1　order　of　magnitude　which,　coupled　with　a　smaller　volume　fraction　of　conducting　cores　due　to　enhanced　diffusion　of　the　dopant　via　A-site　vacancies　in　the　A-site　sublattice,　results　in　higher　breakdown　strength　under　an　electric　field.　This　strategy　can　be　employed　to　develop　new　dielectrics　with　improved　energy　storage　performance.