Low‐Content 2D Al ₂ O ₃ Nanosheets for Concurrent Electrical Insulation and Thermal Management Toward High‐Temperature Dielectric Energy Storage

Dielectric capacitors for pulsed power require stable operation at elevated temperatures, but commercial biaxially oriented polypropylene (BOPP) shows sharp performance degradation above ~105°C. Polyetherimide (PEI) is a promising polymer with high glass transition temperature, yet its low dielectric constant and poor thermal management limit high‐temperature energy storage. Here, low‐loading 2D Al 2 O 3 nanosheets (AO NSs) are introduced into PEI to simultaneously regulate charge transport and heat dissipation while avoiding the high‐loading drawbacks of conventional 0D/1D ceramic fillers. The optimal 0.6 wt.% AO/PEI composite exhibits a high breakdown strength of 571.2 MV·m −1 at 25°C and retains 540.7 MV·m −1 at 100°C and 513.6 MV·m −1 at 150°C. Consequently, a maximum recoverable energy density of 7.9 J·cm −3 ( η > 90%) is achieved at 25°C, and 5.5 J·cm −3 ( η > 80%) at 150°C. Experiments combined with electric‐treeing and temperature‐distribution simulations verify suppressed field concentration and reduced heat accumulation, enabling stable performance over 10 4 cycles. These results demonstrate that low‐content two‐dimensional fillers provide an effective strategy for high‐temperature dielectric energy storage.