The effect of mechanical grinding on the electrochemical properties of P100 and P75 fibers and natural graphite UF4 was studied. The electrochemical experiments were conducted in the NaClO4–ethylene carbonate (EC) electrolyte. Graphite did not allow significant sodium intercalation intrinsically but P100 and P75 fibers, which display lower graphitization level lead to richer compounds. A moderate mechanical grinding of P75 fibers resulted in the appearance of an extra reversible capacity corresponding to sodium insertion in the amorphous and porous parts of the material. Such a treatment allowed to change the composition from Na0.22C8 to Na0.31C8. With graphite, efficient grinding allowed this material to accommodate large amounts of sodium. Thus, the composition Na0.06C8 obtained with as-received graphite was changed to Na0.67C8 after grinding.