Advanced　dielectric　film　capacitors　with　high　energy　density　and　charge-discharge　efficiency　are　urgently　needed　with　ever-increasing　demand　of　energy　storage　devices　for　electronics　and　power　systems.　Herein,　polyetherimide　(PEI)/boron　nitride　nanosheets　(BNNSs)　nanocomposites　with　homogeneous　structures　prepared　by　facile　solution-processing　are　demonstrated.　It　is　found　that　the　incorporation　of　BNNSs　enhances　multiple　dielectric　properties　of　polymer　matrix　including　DC　dielectric　breakdown　strength　and　volume　resistivity.　Compared　with　pristine　PEI　film,　the　PEI/BNNSs　nanocomposite　films　exhibit　concomitant　enhancements　in　discharged　energy　density　and　charge-discharge　efficiency　which　is　owing　to　the　increase　of　breakdown　strength　and　the　decrease　of　dielectric　loss.　Additionally,　excellent　high-temperature　energy　storage　capabilities　also　have　been　achieved　in　the　PEI/BNNSs　nanocomposites,　e.g.　2.83　J/cm3　of　discharged　energy　density　measured　at　450　MV/m　and　150　oC.　This　composite　approach　paves　a　way　of　flexible　polymeric　dielectrics　for　high-temperature　energy　storage　applications.　©　2019　IEEE.