Reduction mechanisms of additives on Si anodes of Li-ion batteries
Reduction mechanisms of additives on Si anodes of Li-ion batteries
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DOI：
10.1039/c4cp01948b
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发表时间：
2014-08-28
影响因子：
3.3
通讯作者：
Balbuena, Perla B.
中科院分区：
文献类型：
--
作者：
de la Hoz, Julibeth M. Martinez;Balbuena, Perla B.
固体电解质界面层是电池充放电过程中沉积在锂离子电池电极表面的薄膜。这是由于电解液的电化学不稳定导致电子从阳极(阴极)表面转移到阳极(阴极)表面。薄膜可以起到保护性的钝化作用，因此了解详细的还原(氧化)过程是至关重要的。本文用密度泛函理论和从头算分子动力学模拟方法研究了碳酸亚乙酯(VC)和碳酸氟乙烯(FEC)在锂化硅表面的还原机理。这些物种经常被用作“添加剂”来改善SEI性能。发现在锂化阳极(锂化度低到中等)上，VC可以通过2E(-)机制被还原，得到一个开放的VC2-离子。在较高的锂化度，这种物种从表面获得两个额外的电子，导致吸附的CO((ADS))(2-)阴离子和自由基阴离子中心点OC2H2O2-。此外，与实验观察一致的是，VC与CO(3)(2-)阴离子反应可生成CO2，CO(3)(2-)阴离子是主溶剂碳酸乙烯(EC)还原的产物。另一方面，LixSiy表面上的FEC还原与锂化度无关，并通过三种机制发生。其中一种导致VC2-离子从FeC分子中释放并吸附在F-和一个H原子的表面。因此，在某些情况下，FeC的还原可能导致与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可以通过另外两种多电子转移机制被还原，这两种机制导致形成CO22-、F-和中心点CH2CHO-或CO2-、F-和中心点OCH2CHO-。这些替代还原产物可以齐聚并形成SEI层，其组成不同于在VC存在下形成的那些SEI层。在所有情况下，FeC还原也导致在阳极表面形成LiF部分，这与报道的实验数据一致。这些机制中表面的关键机械都得到了彻底的解释。
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 VC2- anion. At higher degrees of lithiation, such a species receives two extra electrons from the surface resulting in an adsorbed CO((ads))(2-)anion and a radical anion center dot OC2H2O2-. Additionally, in agreement with experimental observations, it is shown that CO2 can be generated from reaction of VC with the CO(3)(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 VC2- 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 CO22-, F-, and center dot CH2CHO- or CO2-, F-, and center dot 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 rote of the surface in each of these mechanisms is thoroughly explained.