Areal　power　density　is　one　of　the　core　indicators　determining　how　large　areas　a　microbattery　need　to　occupy　when　integrated　directly　with　microelectronic　devices　for　the　Internet　of　Things.　Unfortunately,　the　low　power　density　of　microbatteries　hinders　their　applications,　because　microelectronic　devices　only　provide　finite　areas　for　integration.　Herein,　we　show　that　sputtered　iron　oxysulfide　(FeOxSy)　thin　films　subjected　to　in　situ　plasma　pretreatment　display　ultrahigh　power　density.　This　in　situ　plasma　pretreatment　can　be　regarded　as　a　universal　interface　optimization　strategy　for　suppressing　mechanical　degradation　upon　extended　cycling.　The　synergistic　effects　of　high　structural　integrity　(robust　interfacial　adhesiveness　and　stress-relieving　islands),　perfect　electrochemical　reversibility,　and　near-surface　charge　exchanges　(pseudocapacitive　lithium　storage　mechanism)　result　in　extremely　high　power　density　and　stable　cycling　performance.　The　pretreated　FeOxSy　thin　films　can　output　an　areal　power　density　as　high　as　14.6　mW　cm(-2)　and　a　considerable　volumetric　energy　density　of　291　mu　W　h　cm(-2)　mu　m(-1).　Such　a　high-power　density　constitutes　a　new　state-of-the-art　level　for　sputtered　thin-film　materials　with　comparative　areal　capacity.　This　work　provides　an　efficient　and　simple　pretreatment　method　for　achieving　ultrahigh-power　and　stable　thin-film　electrodes　for　microbatteries.