Suppressing charge injection and preventing the extension of electrical trees of polymer-based composites through two-dimensional metal–organic frameworks nanosheets

•Ultralow loading two-dimensional metal–organic framework in nanocomposites is proposed.•Excellent Ud of 21.16 J/cm3 accompany with high η of 67.9 % are achieved.•The Ud and η with no evidence of discernable degradation over 104 charge/discharge cycles.