Recent research on piezoelectrically driven self-charging supercapacitor power cells (SCSPCs) aims to increase their self-charging performance and understand the charge storage/delivery mechanism. Here, we fabricated a novel SCSPC using a Nafion solid polyelectrolyte separator coated with graphene sheets as supercapacitor electrodes, which exhibited a self-charging process due to the piezo-ionic (PI) effect. The graphene PI-SCSPC can self-charge up to 341 mV under a mechanical stimulus that is superior to many of the reported SCSPCs. The self-charging process is explained based on the creation of the Donnan potential and Nernst's theory of the 2D system. Electrochemical gating coupled with mechanical stimulation was used for understanding the charging dynamics of the graphene PI-SCSPC via examining the critical role of (i) electrochemical doping, and variation in the (ii) voltage of the charge-neutrality point, and (iii) current amplitudes. These findings open a new paradigm for developing novel SCSPCs and understanding their charging dynamics that is of great interest for the realization of next-generation integrated energy cells.