Pascal Tod A, Wujcik Kevin H, Wang Dunyang Rita, Balsara Nitash P, Prendergast David
Molecular Foundry, Lawrence Berkeley National Lab, Berkeley, CA 94720, USA.
Department of Chemical and Biomolecular Engineering, University of California, Berkeley, California 94720, USA and Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
Phys Chem Chem Phys. 2017 Jan 4;19(2):1441-1448. doi: 10.1039/c6cp06889h.
An understanding of the complex solution phase chemistry of dissolved lithium polysulfides is critical to approaches aimed at improving the cyclability and commercial viability of lithium sulfur batteries. Experimental measurements are frustrated by the versatile sulfur-sulfur bond, with spontaneous disproportionation and interconversion leading to unknown equilibrium distributions of polysulfides with varying lengths and charge states. Here, the solubility of isolated lithium polysulfides is calculated from first-principles molecular dynamics simulations. We explore the associated changes in the dissolution free energy, enthalpy and entropy in two regimes: liquid-phase monodentate solvation in dimethylformamide (DMF) and polymer-like chelation in bis(2-methoxyethyl) ether (diglyme). In both of these technologically relevant solvents, we show that the competition between enthalpy and entropy, related to specific interfacial atomic interactions, conspires to increase the relative stability of long chain dianionic species, which exist as Li-LiS contact-ion-pairs. Further, we propose a mechanism of radical polysulfide stabilization in simple solvents through the reorientation of the 1 shell solvent molecules to screen electrostatic fields emanating from the solute and explain nonmonotonicity of the dissolution entropy with polysulfide length in terms of a three-shell solvation model. Our analysis provides statistical dynamics insights into polylsulfide stability, useful to understand or predict the relevant chemical species present in the solvent at low concentrations.
了解溶解态多硫化锂复杂的溶液相化学对于旨在提高锂硫电池循环性能和商业可行性的方法至关重要。实验测量受到多功能硫-硫键的困扰，自发歧化和相互转化导致不同长度和电荷状态的多硫化物存在未知的平衡分布。在此，通过第一性原理分子动力学模拟计算了分离出的多硫化锂的溶解度。我们在两种体系中探索了溶解自由能、焓和熵的相关变化：在二甲基甲酰胺（DMF）中的液相单齿溶剂化以及在双（2-甲氧基乙基）醚（二甘醇二甲醚）中的类聚合物螯合。在这两种具有技术相关性的溶剂中，我们表明，与特定界面原子相互作用相关的焓和熵之间的竞争共同作用，增加了以Li-LiS接触离子对形式存在的长链双阴离子物种的相对稳定性。此外，我们提出了一种在简单溶剂中通过第一溶剂层分子重新定向以屏蔽溶质产生的静电场来稳定自由基多硫化物的机制，并根据三溶剂层模型解释了溶解熵随多硫化物长度的非单调性。我们的分析提供了关于多硫化物稳定性的统计动力学见解，有助于理解或预测低浓度下溶剂中存在的相关化学物种。
