Enhancing the low-potential capacity of anode materials is significant in boosting the operating voltage of full-cells and constructing high energy-density energy storage devices. Graphitic carbons exhibit outstanding low-potential potassium storage performance, but show a low K⁺ diffusion kinetics. Herein, in-situ defect engineering in graphitic nanocarbon is achieved by an atomic self-activation strategy to boost the accessible low-voltage insertion. Graphitic carbon layers grow on nanoscale-nickel to form the graphitic nanosphere with short-range ordered microcrystalline due to nickel graphitization catalyst. Meanwhile, the widely distributed K⁺ in the precursor induces the activation of surrounding carbon atoms to in-situ generate carbon vacancies as channels. The graphite microcrystals with defect channels realize reversible K⁺ intercalation at low-potential and accessible ion diffusion kinetics, contributing to high reversible capacity (209 mAh g^-1 at 0.05 A g^-1 under 0.8 V) and rate capacity (103.2 mAh g^-1 at 1 A g^-1). The full-cell with Prussian blue cathode and graphitic nanocarbon anode maintains an obvious working platform at ca. 3.0 V. This work provides a strategy for the in-situ design of carbon anode materials and give insights into the potassium storage mechanism at low-potential for high-performance full-cells.