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A facile fabrication strategy is reported to obtain N/O codoped porous carbon nanosheets for purpose of ameliorating the charge transfer and accumulation in the concentrated LiTFSI (lithium bis(trifluoromethane sulfonyl)imide) electrolyte. By tunning the feed ratio of comonomers, the porous nanosheet structure is endowed with a significant ion-adsorption surface area (1630 m2/g) and interconnected hierarchical porosity; meanwhile, high-level N/O dopants (N: 3.58 at%, O: 12.91 at%) increase the effective contact area for electrolyte ions, and further facilitate rapid ion/electron transfer. Benefiting from the advantageous features, carbon nanosheets electrode reveal an enhanced specific capacitance (375 F/g) in three-electrode configuration and the H2SO4-based device yields a high gravimetric energy density of 11.4 Wh/kg. Particularly, the ion-diffusion highways in porous carbon nanosheets contribute to the 2.25 V LiTFSI-based symmetric device with a high energy delivery up to 33.1 Wh/kg. This work offers an inspiring strategy for facile fabrication of carbon nanosheets, and demonstrates their promising application in “water-in-salt” electrolyte-based supercapacitor systems.