Camacho-Forero Luis E, Balbuena Perla B
Department of Chemical Engineering, Texas A&M University, College Station, TX 77843, USA.
Phys Chem Chem Phys. 2017 Nov 22;19(45):30861-30873. doi: 10.1039/c7cp06485c.
The lithium metal anode is one of the key components of the lithium-sulfur (Li-S) batteries, which are considered one of the most promising candidates for the next generation of battery systems. However, one of the main challenges that have prevented Li-metal anodes from becoming feasible to be used in commercial batteries is the continuous decomposition of the electrolyte due to its high reactivity, which leads to the formation of solid-electrolyte interphase (SEI) layers. The properties of the SEI can dramatically affect the performance of the batteries. Thus, a rigorous understanding of the electrolyte decomposition is crucial to elucidate improvements in performance of the Li-S technology. In this work, using density functional theory (DFT) and ab initio molecular dynamics simulations (AIMD), we investigate the effect of electron-rich environments on the decomposition mechanism of electrolyte species in pure 1,2-dimethoxyethane (DME) solvent and 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide (LiFSI) salt solutions. It is found that systems with pure DME require an average environment of at least ∼0.9 |e| per molecule for a DME to decompose into CHO and CHvia a 4-electron transfer. In the case of mixtures, the salts are very prone to react with any excess of electrons. In addition, DME dehydrogenation due to reactions with fragments coming from the salt decompositions was detected. Formation of oligomer anionic species from DME and salt fragments were also identified from the AIMD simulations. Finally, the thermodynamics and kinetics of the most relevant electrolyte decomposition reactions were characterized. DME decomposition reactions predicted from the AIMD simulations were found to be thermodynamically favorable under exposure to Li atoms and/or by reactions with salt fragments. In most cases, these reactions were shown to have low to moderate activation barriers.
锂金属负极是锂硫（Li-S）电池的关键组件之一，锂硫电池被认为是下一代电池系统最有前景的候选者之一。然而，阻碍锂金属负极在商业电池中可行应用的主要挑战之一是电解质因其高反应活性而持续分解，这导致固体电解质界面（SEI）层的形成。SEI的性质会显著影响电池性能。因此，深入了解电解质分解对于阐明Li-S技术性能的提升至关重要。在这项工作中，我们使用密度泛函理论（DFT）和从头算分子动力学模拟（AIMD），研究富电子环境对纯1,2 - 二甲氧基乙烷（DME）溶剂以及1 M双（三氟甲磺酰）亚胺锂（LiTFSI）和双（氟磺酰）亚胺锂（LiFSI）盐溶液中电解质物种分解机制的影响。研究发现，对于纯DME体系，每个分子平均需要至少约0.9 |e|的环境才能使DME通过4电子转移分解为CHO和CH；对于混合物体系，盐极易与任何过量的电子发生反应。此外，检测到DME因与盐分解产生碎片的反应而发生脱氢。从AIMD模拟中还识别出由DME和盐碎片形成的低聚物阴离子物种。最后，对最相关的电解质分解反应的热力学和动力学进行了表征。发现AIMD模拟预测的DME分解反应在暴露于锂原子和/或与盐碎片反应时在热力学上是有利的。在大多数情况下，这些反应显示出低到中等的活化能垒。
