Researchers suggest approach for boosting Li-S performance; conversion of Li2S to sulfur without polysulfides
Lithium-sulfur batteries are one of the most promising alternatives for next-generation high-energy-density batteries; however, one of the main obstacles to widespread commercialization that still needs to be addressed is the polysulfide shuttle mechanism between the two electrodes. The polysulfide shuttle—the migration of lithium polysulfides formed during charge and discharge from cathode to anode—leads to serious self-discharge, poor efficiency and limited cycle life. (E.g., earlier post.)
Now, an international team of researchers in Europe is suggesting a possible approach to convert Li2S into sulfur without the detectible formation of polysulfides. A paper on their work is published in the Journal of Power Sources.
… 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”.
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.
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