Origin of the Two-Plateaued or One-Plateaued Open Circuit Voltage in Li/S Batteries from First Principle Calculation

Li/S battery is widely considered as one of the next generation energy storage devices because of its high specific energy density. Most of the discharging curves for Li/S batteries can be categorized into two types: with two-plateaus or one-plateau. Two-plateaued discharging curves are typically observed in Li/S batteries with a liquid electrolyte, and they are signatures for the generation of soluble polysulfide (PS). One-plateaued discharging curves have been observed in Li-S batteries with solid electrolytes, indicating direct transformation from S to solid state Li 2 S 2 and Li 2 S, without the formation of PS. More interestingly, some Li/S batteries with liquid electrolyte also displayed one-plateaued discharging curves, which seems related to the nanopores in cathode materials or the limited solubility of PS in the electrolyte. The fundamental understanding of the mechanism and the conditions for the one-plateaued discharging curve will provide guidelines for Li/S battery design in order to improve its cycle life by preventing the PS dissolution. In this article, we systematically investigated the influence of temperature and solvation on the predicted free energy and the open circuit voltage (OCV) curves for Li/S batteries. First, we confirmed that S, Li 2 S and Li should be modeled as crystal structures; while PS, such as Li 2 S 4 should be modeled as monomer under solvation. Secondly, it was shown that the effect of temperature is negligible for predicting OCV curves associated only with solid-phase transformation, but plays a deterministic role in the free energy of dissolved molecules, such as PS. Thirdly, the solvation energy calculations are deemed to be the most accurate with a combined implicit and explicit model, which means the first solvation shell is modeled explicitly with an implicit dielectric continuum media. With the appropriate treatment of temperature and solvation effect, we demonstrated that the two-plateaued discharging curves occur when the PS is fully solvated (by DOL in our model) and one-plateaued discharging curve occurs when PS is not solvated, in agreement with experimental observation in typical Li/S battery with liquid or solid electrolytes. Furthermore, if PS is only partially solvated, its formation energy increases, leading to a transition from the two-plateaued to the one-plateaued discharging curves, as the PS changes from fully solvation to partial or no-solvation. Partial solvation can be created with a highly concentrated solution or by varying pore size and volume in the carbon matrix to confine S and limit the number of solvents transported into the pores, as practical methods to mitigate the PS shuttle problem from its root cause.