The formation of a solid electrolyte interphase (SEI) at the electrode/electrolyte interface substantially affects the stability and lifetime of lithium-ion batteries (LIBs). One of the methods to improve the lifetime of LIBs is by the inclusion of additive molecules to stabilize the SEI. To understand the effect of additive molecules on the initial stage of SEI formation, we compare the decomposition and oligomerization reactions of a fluoroethylene carbonate (FEC) additive on a range of oxygen functionalized graphitic anode to those of an ethylene carbonate (EC) organic electrolyte. A series of density functional theory (DFT) calculations augmented by ab-initio molecular dynamics (AIMD) simulations reveal that EC decomposition on an oxygen functionalized graphitic (1120) edge facet through an SN2 mechanism is spontaneous, even in an uncharged cell. Decomposition of EC through an SN2 reaction pathway results in alkoxide species regeneration which is responsible for continual oligomerization along the graphitic surface. In contrast, FEC prefers to decompose through an SN1 pathway, which does not promote alkoxide regeneration. The ability of FEC as an additive to suppress alkoxide regeneration results in a smaller and thinner SEI layer that is more flexible towards lithium intercalation during the charging/discharging process. In addition, the presence of different oxygen functional groups at the surface of graphite dictates the oligomerization products and LiF formation in the SEI.

中文翻译：

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碳酸氟乙烯酯对锂离子电池中氧功能化石墨阳极上固态电解质中间相初始形成的影响

在电极/电解质界面处形成固态电解质中间相（SEI）会严重影响锂离子电池（LIB）的稳定性和寿命。改善LIB寿命的方法之一是通过添加添加剂分子来稳定SEI。为了了解添加剂分子对SEI形成初期的影响，我们比较了一系列氧官能化石墨阳极上的氟代碳酸亚乙酯（FEC）添加剂与碳酸亚乙酯（EC）有机电解质的分解和低聚反应。通过从头算分子动力学（AIMD）模拟得到的一系列密度泛函理论（DFT）计算表明，通过SN2机制在氧官能化石墨（1120）边缘刻面上的EC分解是自发的，即使在未充电的电池中 EC通过SN2反应途径分解会导致醇盐物种再生，这是导致石墨表面持续低聚的原因。相反，FEC倾向于通过SN1途径分解，这不会促进醇盐的再生。FEC作为添加剂抑制醇盐再生的能力可导致更小更薄的SEI层，在充电/放电过程中，SEI层对于锂的嵌入更具灵活性。另外，在石墨表面上存在不同的氧官能团决定了SEI中的低聚产物和LiF的形成。FEC倾向于通过SN1途径分解，这不会促进醇盐的再生。FEC作为添加剂抑制醇盐再生的能力可导致更小更薄的SEI层，在充电/放电过程中，SEI层对于锂的嵌入更具灵活性。另外，在石墨表面上存在不同的氧官能团决定了SEI中的低聚产物和LiF的形成。FEC倾向于通过SN1途径分解，这不会促进醇盐的再生。FEC作为添加剂抑制醇盐再生的能力可导致更小更薄的SEI层，在充电/放电过程中，SEI层对于锂的嵌入更具灵活性。另外，在石墨表面上存在不同的氧官能团决定了SEI中的低聚产物和LiF的形成。