Low-Temperature Characteristics and Film-Forming Mechanism of Elemental Sulfur Additive on Graphite Negative Electrode

Abstract

We report here that the low-temperature performance of graphite electrode is improved by adding a small amount of elemental sulfur (S8) into the graphite negative electrode. The reversible capacity at −30°C is much larger for the sulfur-added graphite electrode. The origin of this beneficial feature is examined through impedance analysis, which illustrates that the charge transfer resistance is much smaller in the sulfur-added graphite electrode at low temperatures. In the first lithiation step, the elemental sulfur is electrochemically reduced to be lithium polysulfide (Li2S8), which is soluble in the working solvent (carbonate-based ones). Organic thiocarbonates are generated by the chemical reaction between the lithium polysulfide and carbonate solvents. The as-generated thiocarbonates are then electrochemically decomposed to form the sulfur-containing surface film. The superior low-temperature performance of the sulfur-added graphite is thus attributed to the presence of sulfur-enriched surface film, which seems to facilitate the charge transfer reaction between the graphite and lithium.