The　oxygen　hopping　through　oxygen　defect　site　plays　an　extremely　important　role　in　cathode　catalysts　of　solid　oxide　fuel　cells　(SOFC)　application.　Herein,　a　dual　Ni2+/Ni3+　and　Mn2+/Mn3+/Mn4+　redox　pairs　strategy　is　developed　to　construct　a　series　of　defective　spinel　Mg0.4NixMn2.6−xO4+δ　(abbreviated　MN(x)MO)　to　gain　insights　in　terms　of　oxygen　nonstoichiometry.　By　regulating　the　stoichiometric　proportion　of　Ni　and　Mn,　it　is　possible　to　optimize　electronic　conductivity　and　oxygen-vacancy　concentration.　The　optimized　MN(1.4)MO　provides　electrical　conductivity　as　high　as　68　S·cm−1　at　800　°C,　2.72　folds　that　of　MN(1.0)MO.　Based　on　oxygen　transport　performance,　the　surface　exchange　coefficient　of　MN(1.4)MO　at　900　°C　is　162　folds　that　of　commercial　La0.7Sr0.3MnO3-δ　(LSM).　When　a　MN(1.4)MO　cathode　was　used,　the　resulted　SOFC　exhibited　extraordinarily　high　maximum　power　density　of　0.34　W·cm−2　at　600　°C　and　2.02　W·cm−2　at　800　°C.　To　the　best　of　our　knowledge,　the　performance　is　the　best　among　the　spinel-based　cathodes　ever　reported　for　SOFC　application.　Endowed　with　optimal　properties,　MN(1.4)MO-based　SOFC　displays　peak　power　density　which　is　2.27　and　1.44　folds　that　of　LSM-based　SOFC　at　600　°C　and　800　°C,　respectively.　A　test　of　50　h　revealed　the　MN(1.4)MO-based　SOFC　is　remarkably　stable　at　800　°C,　continuously　offering　2.02　W·cm−2　at　0.5　V.　The　excellent　performance　and　stability　of　MN(1.4)MO-based　SOFC　suggests　that　MN(1.4)MO　is　a　promising　cathode　material　for　the　development　of　intermediate　temperature　SOFC　technology.　©　2023