Sulfur-doped graphite (S-DG) was synthesized using a sustainable method. Low-cost cellulose fiber and carbon disulfide (CS2) were used as precursors for carbon and sulfur (S) sources, respectively, and a multistep chemical and thermochemical synthesis process was employed to obtain S-DG. Advanced microscopic and spectroscopic techniques and porosimetry analyses were performed to probe the surface morphology, surface area, microstructures, and chemical bonding states of S-DG. Interestingly, X-ray photoelectron spectroscopy revealed that the S loads were 3.81 and 7.02 atom % when reactions were carried out in aqueous and dimethyl sulfoxide (DMSO) media, respectively. The specific surface areas were calculated as 522.77 and 693.27 m2g-1for the two materials, respectively. To our best knowledge, the 7.02 atom % S load is the highest S doping reported so far in a graphite material where laboratory chemicals have been used as precursors. Both materials, i.e., S-DGWaterand S-DGDMSO, were tested as supercapacitor electrodes in a portable electrochemical cell with a three-electrode system to check their potential and stability for the fabrication of renewable energy storage devices. Cyclic voltammetry and galvanostatic charging-discharging techniques were performed within a potential window of 1 V in the presence of 1 M Na2SO4as an electrolyte. The maximum specific capacitances (CSp) of S-DGWaterand S-DGDMSOwere obtained as 155.61 and 261.43 F g-1(at a scan rate of 10 mV s-1), respectively. The result concluded that the extent of S doping appeared to be the key factor for control of the peak current. Importantly, our investigation suggests that the S load in graphite facilitates a diffusion-driven storage mechanism and a higher amount of S may enhance surface-controlled storage as well. Both materials show excellent cyclic stability with >98% retention of the initial specific capacitance over 10000 cycles of charging-discharging. © 2022 American Chemical Society. All rights reserved.