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Researchers suggest approach for boosting Li-S performance; conversion of Li2S to sulfur without polysulfides

6 February 2017

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”.

Lis
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

February 6, 2017 in Batteries, Li-Sulfur | Permalink | Comments (1)

Comments

I figured Lithium sulfate would work.

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