▎ 摘　　要

One of the biggest challenges for polyimide (PI) insulators is to achieve high breakdown strength compatible with low dielectric loss, which would significantly affect the operating power, energy loss and electric equipment ageing during application. Here, we successfully realized enhancement of breakdown strength of PI film from 2.37 MV/cm to 2.99 MV/cm by in-situ compositing fluorinated graphene (FG), and this is accompanied with a reduction of dielectric loss from 0.017 to 0.006. By combining solubility testing in concentrated sulfuric acid and in-situ Fourier transform infrared (FTIR) characterization, the "cross-linking" structure of PI/FG film was experimentally confirmed. Meanwhile, it was found that space charge accumulation of cross-linked PI film under an electric field was suppressed and uniformed, and the mobility of its molecular chain under an alternating current (AC) electric field was found to be significantly constrained. Therefore, it was supposed that the cross linking structure plays a determining role in optimizing the dielectric performance of PI/FG films, which provides a feasible strategy to optimize the overall insulating/dielectric performance of insulators and dielectrics.