Department of Chemical Engineering and Department of Materials Science and Engineering, Texas A&M University, College Station, TX 77843, USA.
Phys Chem Chem Phys. 2014 Aug 28;16(32):17091-8. doi: 10.1039/c4cp01948b.
Solid-electrolyte interphase (SEI) layers are films deposited on the surface of Li-ion battery electrodes during battery charge and discharge processes. They are due to electrochemical instability of the electrolyte which causes electron transfer from (to) the anode (cathode) surfaces. The films could have a protective passivating role and therefore understanding the detailed reduction (oxidation) processes is essential. Here density functional theory and ab initio molecular dynamics simulations are used to investigate the reduction mechanisms of vinylene carbonate (VC) and fluoroethylene carbonate (FEC) on lithiated silicon surfaces. These species are frequently used as "additives" to improve the SEI properties. It is found that on lithiated Si anodes (with low to intermediate degrees of lithiation) VC may be reduced via a 2e(-) mechanism yielding an opened VC(2-) anion. At higher degrees of lithiation, such a species receives two extra electrons from the surface resulting in an adsorbed CO(2-)(ads) anion and a radical anion ˙OC2H2O(2-). Additionally, in agreement with experimental observations, it is shown that CO2 can be generated from reaction of VC with the CO3(2-)anion, a product of the reduction of the main solvent, ethylene carbonate (EC). On the other hand, FEC reduction on LixSiy surfaces is found to be independent of the degree of lithiation, and occurs through three mechanisms. One of them leads to an adsorbed VC(2-) anion upon release from the FEC molecule and adsorption on the surface of F(-) and one H atom. Thus in some cases, the reduction of FEC may lead to the exact same reduction products as that of VC, which explains similarities in SEI layers formed in the presence of these additives. However, FEC may be reduced via two other multi-electron transfer mechanisms that result in formation of either CO2(2-), F(-), and ˙CH2CHO(-) or CO(2-), F(-), and ˙OCH2CHO(-). These alternative reduction products may oligomerize and form SEI layers with different components than those formed in the presence of VC. In all cases, FEC reduction also leads to formation of LiF moieties on the anode surface, in agreement with reported experimental data. The crucial role of the surface in each of these mechanisms is thoroughly explained.
固体电解质界面（SEI）层是在锂离子电池电极的充放电过程中沉积在电极表面的薄膜。它们是由于电解质的电化学不稳定性引起的，导致电子从（到）阳极（阴极）表面转移。这些薄膜可能具有保护钝化作用，因此理解详细的还原（氧化）过程是至关重要的。在这里，我们使用密度泛函理论和从头算分子动力学模拟来研究乙烯碳酸酯（VC）和氟代碳酸乙烯酯（FEC）在锂化硅表面上的还原机制。这些物质经常被用作“添加剂”来改善 SEI 的性能。研究发现，在锂化硅阳极（低至中等程度的锂化）上，VC 可能通过 2e(-) 机制还原，生成开环的 VC(2-)阴离子。在更高程度的锂化时，这种物质从表面获得两个额外的电子，导致吸附的 CO(2-)(ads)阴离子和自由基阴离子 ˙OC2H2O(2-)。此外，与实验观察结果一致，研究表明 VC 与主要溶剂碳酸乙烯酯（EC）的还原产物 CO3(2-)阴离子反应可以生成 CO2。另一方面，在 LixSiy 表面上，FEC 的还原发现与锂化程度无关，并且通过三种机制发生。其中一种机制导致 FEC 分子释放后吸附在表面上的 F(-)和一个 H 原子上，形成吸附的 VC(2-)阴离子。因此，在某些情况下，FEC 的还原可能会导致与 VC 完全相同的还原产物，这解释了在这些添加剂存在下形成的 SEI 层的相似性。然而，FEC 可能通过另外两种多电子转移机制还原，导致形成 CO2(2-)、F(-)和 ˙CH2CHO(-)或 CO(2-)、F(-)和 ˙OCH2CHO(-)。这些替代的还原产物可能会聚合，并形成与 VC 存在时形成的不同成分的 SEI 层。在所有情况下，FEC 的还原也会导致在阳极表面形成 LiF 部分，这与报道的实验数据一致。在这些机制中的每一种机制中，表面都起着至关重要的作用。
