Two-dimensional graphene-based materials are of great interest in electrochemical energy storage devices owing to their high theoretical specific capacitance; however, it is a big challenge to approach the theoretical limit. Here, we report a facile strategy for the selective production of N-doped graphene nanosheets (NGNS) in quasi-closed montmorillonite (MMT), which act as a nanoreactor and open only along the perimeter to enable the entrance of aniline (AN) monomer. As a proof of concept applications, as-synthesized NGNS are used as the electrode material for a symmetric supercapacitor (SSC). The NGNS supported on carbon paper (NGNS@CP) exhibits excellent capacitive performance in an aqueous neutral electrolyte (LiCl) by achieving a high capacitance of 480 and 472 F g(-1) at 1 A g(-1) for negative and positive electrodes, respectively. The NGNS@CP electrode also shown outstanding rate capabilities and cycling stabilities in positive and negative voltage windows by retaining the 98.07, and 99.44 % of the original value, respectively. Moreover, the assembled flexible symmetric supercapacitor (NGNS@CP//NGNS@CP) using LiCl/PVA hydro-gel electrolyte can operate at high voltage window of 0.0-2.0 V and achieved an ultra-high energy density of 82.72 Wh kg(-1) at a moderate power density of 425.68 W kg(-1) with remarkable cycling stability of 96.91 % over 10000 charge/discharge cycles. Furthermore, the NGNS@CP//NGNS@CP demonstrates excellent flexibility and one device can power the two light-emitting diodes brightly. The achieved energy density of the NGNS@CP//NGNS@CP is much superior to the previous graphene-based SSCs and suggests a new generation of ultra-fast symmetric supercapacitor as novel high-performance energy storage devices. (C) 2019 Elsevier Ltd. All rights reserved.