AbstractDiffusion processes at the electrode/electrolyte interphase drives the performance of lithium‐sulfur batteries, and activated carbon (AC) can remarkably vehicle ions and polysulfide species throughout the two‐side liquid/solid region of the interphase. We reveal original findings such as the values of the diffusion coefficient at various states of charge of a Li−S battery using a highly porous AC, its notable dependence on the adopted techniques, and the correlation of the diffusion trend with the reaction mechanism. X‐ray photoelectron spectroscopy (XPS) and X‐ray energy dispersive spectroscopy (EDS) are used to identify in the carbon derived from bioresidues heteroatoms such as N, S, O and P, which can increase the polarity of the C framework. The transport properties are measured by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic intermittent titration technique (GITT). The study reveals Li+‐diffusion coefficient (DLi+) depending on the technique, and values correlated with the cell state of charge. EIS, CV, and GITT yield a DLi+ within 10−7–10−8 cm2 s−1, 10−8–10−9 cm2 s−1, and 10−6–10−12 cm2 s−1, respectively, dropping down at the fully discharged state and increasing upon charge. GITT allows the evaluation of DLi+ during the process and evidences the formation of low‐conducting media upon discharge. The sulfur composite delivers in a Li‐cell a specific capacity ranging from 1300 mAh g−1 at 0.1 C to 700 mAh g−1 at 2C with a S loading of 2 mg cm−2, and from 1000 to 800 mAh g−1 at 0.2C when the S loading is raised to 6 mg cm−2.