Diffusional Features of a Lithium-Sulfur Battery Exploiting Highly Microporous Activated Carbon

Abstract

Diffusion 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.