… we present in this work the synthesis of small, uniformly distributed Li₂S, particles homogeneously coated by a nitrogen-doped carbon layer. The preparation procedure is based on the carbothermal reduction of Li₂SO₄ in the presence of a carbon precursor, added as the nanoparticle stabilizing amphiphilic polymer. … We demonstrate that the direct conversion of Li₂S to sulfur occurs if a high overvoltage is applied, while formation of polysulfides only occurs as a back-reaction of the Li₂S with solution based sulfur species. The direct conversion of Li₂S into sulfur allows proposing a novel paradigm towards more stable Li-S batteries, namely systems where by the choice of solvent and carbon enwrapment the dissolution of the sulfur species is avoided to a maximal possible extent.

—Vizintin

The process for producing the cathode material is simple and scalable. The carbothermal reduction of Li₂SO₄ leads to the formation of Li₂S active material while the simultaneously generated excess carbon is nanoporous. This porosity is necessary for the controlled contact of the ion-containing electrolyte with the active material—i.e., the “ionic wiring”.

Proposed reaction mechanism. Reaction I corresponds to the direct conversion of Li2S to S8, following by the dissolution of S8 in the electrolyte (reaction II). In reaction III the S8 reacts with Li2S, forming long chain polysulfides, which are partially reduced to S8 (reaction IV). It is the choice of the solvent and the structure of the encapsulating carbon shell which determines the amount of sulfur dissolved in equilibrium. If the quantity of solvent as well as its ability to dissolve sulfur is low, reaction I is favored, while all other reactions are kinetically hindered. Consequently we have direct conversion of Li2S into sulfur where polysulfides can potentially act as redox mediators and that explains the higher potential required for oxidation. —Vizintin et al.

They determined that the as-generated special carbon shell, together with the employment of PVdF as binder, led to direct sulfur formation at the higher over-potentials. Experiments with different amount of electrolyte underlined the role of ionic wiring and further showed the possibility of influencing the charging mechanism.

This work encourages looking into possibilities to also obtain direct Li₂S formation throughout discharge, which would lead to the ideal situation of polysulfide-free cycling.

—Vizintin et al.

Resources

• Alen Vizintin, Laurent Chabanne, Elena Tchernychova, Iztok Arčon, Lorenzo Stievano, Giuliana Aquilanti, Markus Antonietti, Tim-Patrick Fellinger, Robert Dominko (2017) “The mechanism of Li₂S activation in lithium-sulfur batteries: Can we avoid the polysulfide formation?” Journal of Power Sources, Volume 344, Pages 208-217 doi: 10.1016/j.jpowsour.2017.01.112