(491e) Interfacial Electrolyte Structure with Anti-Reductive Solvent at Electrode for Practical Lithium-Sulfur Batteries: A Molecular Dynamics Study

The emergence of Li-metal batteries, with the high-energy-density Li metal anode, has energized the rechargeable battery industry. Lithium-sulfur (Li-S) battery emerges as a promising candidate due to the theoretically high specific energy and low material cost. Soluble Li polysulfide (PS) intermediates, e.g., Li₂S_4-8, which are generated from solid S₈, can be reductively transformed into solid Li₂S₂ or Li₂S during a discharge process. While the dissolved PSs can facilitate the high specific capacity and high discharge voltage of S/carbon cathode, they induce daunting challenges for stable Li metal anode due to the inevitable parasitic reactions between Li and PSs.

In our previous work (Angew. Chem. Int. Ed. 2021, 60, 2-9), we found that by introducing isopropyl ether (DIPE), which is more stable than conventional ether solvents against Li metal, the reactivity of PSs is greatly suppressed and the cycling performance of working Li-S batteries is significantly improved. Based on molecular dynamics (MD) simulations and nuclear magnetic resonance, the electrolyte structure of PSs with anti-reductive solvent shells is revealed, in which DIPE molecules, as a coslvent, tend to distribute in the outer solvent shell due to poor solvating power. While this study provided new insights into the working mechanism of DIPE in terms of improving Li-S battery lifespan, molecular distributions near the anode might be drastically different from that in the bulk.

Herein, we report a MD study on the molecular distributions of anti-reductive electrolyte (DIPE) at electrodes. Graphene sheets are employed to represent the electrodes. Two systems (System PSE containing DOL/DME=1/1 by vol.; System EPSE containing DIPE/DOL/DME=3/3.5/3.5 by vol.) with varying potentials are studied. We demonstrate that DIPE can be adsorbed at the anode driven by the excluded volume effect (Soft Matter 2011, 7(23), 11222-11231), which might mitigate the oxidation-reduction chemical reaction between the Li anode and other electrolytes and salts. Due to the presence of DIPE, the adsorption of DOL at the anode is largely suppressed. There exists optimum electric double layer (EDL) potential for the highest DIPE adsorption at electrode, which is around -0.5 V. In addition, DIPE distributes at the outer shell of TFSI^- and PSs, both in bulk and at the near electrode. As a result, the protection of TFSI^- and PSs from DIPE from oxidation-reduction chemical reaction might also take place near the electrodes. In fact, XPS spectra from experiment (Angew. Chem. Int. Ed. 2021, 60, 2-9) proved that SO₄^2- and SO₃^3- functional group (the product of TFSI^- and PS oxidation-reduction reaction between Li anode) is decreased in the EPSE systems. This work sheds lights on anti-reductive solvent and salt distributions near the electrodes and provides fundamental understanding and important insights into the design of electrolyte formulas for stable Li-S batteries.