Solvent oligomerization pathways facilitated by electrolyte additives during solid-electrolyte interphase formation

Abstract

The solid-electrolyte interphase (SEI) layer formation is known to play an important role in determining the lifetime of lithium-ion batteries. A thin, stable SEI layer is linked to overall improved...

Effect of Fluoroethylene Carbonate Additive on the Initial Formation of Solid Electrolyte Interphase on Oxygen Functionalized Graphitic Anode in Lithium Ion Batteries

“…, C, O, H, and F A previous computational study from our group showed through reaction network analysis that PEC and PEO are the most thermodynamically favorable products of EC decomposition via SN1 and SN2 reaction pathways, respectively. 36 As can be seen from Table 1, the SN2 oligomerization pathway is exothermic for all surfaces examined, while the SN1 pathway is only exothermic for pristine graphite, hydroxyl, and ketonic surfaces. The most exothermic SN2 decomposition for EC is on the carbonyl surface, which in combination with the highly endothermic SN1 reaction energies, implies that EC decomposition on the carbonyl surface would result in high PEO content.…”

“…In vacuum, both SN1 and SN2 oligomerization pathways have been found to be exothermic. 36 Coupled with CO2 generation, oligomerization through an SN2 pathway results in alkoxide regeneration, which is responsible for continual polymerization reactions along the electrode surface, as is observed in experiment. 34 In addition to the polarity of the electrolyte species, SEI formation is also found to be highly sensitive to the surface structure of electrodes.…”

“…Eventually, ethoxide will react with FEC molecule. Nucleophilic attack of ethoxide on the FEC molecule results in aldehyde formation rather than ethoxide reformation, 36 thus the presence of FEC molecules in the electrolyte terminates oligomer growth along the interface. The ability of FEC to suppress alkoxide regeneration reduces the size of oligomers in the SEI, which based on experimental studies, is responsible for the formation of a thinner and more flexible SEI layer.…”

“…35 Previous work included a reaction network analysis of EC and FEC decomposition in vacuum and confirmed that PEC and PEO are the most thermodynamically favorable products of EC decomposition via SN1 and SN2 mechanisms, respectively. 36 In addition to oligomer species, decomposition through SN2 mechanism also releases gaseous CO2 which may then undergo further reaction with another EC molecule to form an inorganic Li2CO3 species. [37][38][39] The oligomerization products obtained through SN1 and SN2 reaction pathways for both EC and FEC are summarized in Figure 1.…”

Quantum chemical calculations of lithium-ion battery electrolyte and interphase species

“…However, even very reactive molecules can often be studied using first-principles quantum chemical methods such as density functional theory (DFT). Such computational simulations can therefore fill a gap and potentially provide a more fundamental understanding of the reactive chemistry of the SEI -for instance through the calculation of reaction free energies and energy barriers [8][9][10][11][12] . DFT can additionally be used to generate reference spectra that can be compared to experiment 13 .…”