Not All Fluorination Is the Same: Unique Effects of Fluorine Functionalization of Ethylene Carbonate for Tuning Solid-Electrolyte Interphase in Li Metal Batteries

Abstract

Li metal batteries (LMBs) are crucial for electrifying transportation and aviation. Engineering electrolytes to form desired solid-electrolyte interphase (SEI) is one of the most promising approaches to enable stable long-lasting LMBs. Among the liquid electrolytes explored, fluoroethylene carbonate (FEC) has seen great success in leading to desirable SEI properties for enabling stable cycling of LMBs. Given the many facets to desirable SEI properties, numerous descriptors and mechanisms have been proposed. To build a detailed mechanistic understanding, we analyze varying degrees of fluorination of the same prototype molecule, chosen to be ethylene carbonate (EC) to tease out the interfacial reactivity at the Li metal/electrolyte. Using density functional theory (DFT) calculations, we study the effect of mono-, di-, tri-, and tetra-fluorine substitutions of EC on its reactivity with Li surface facets in the presence and absence of Li salt. We find that the formation of LiF at the early stage of SEI formation, posited as a desirable SEI component, depends on the F-abstraction mechanism rather than the number of fluorine substitution. The best illustrations of this are cis- and trans-difluoro ECs, where F-abstraction is spontaneous with the trans case, while the cis case needs to overcome a nonzero energy barrier. Using a Pearson correlation map, we find that the extent of initial chemical decomposition quantified by the associated reaction free energy is linearly correlated with the charge transferred from the Li surface and the number of covalent-like bonds formed at the surface. The effect of salt and the surface facet have a much weaker role in determining the decompositions at the immediate electrolyte/electrode interfaces. Putting all of this together, we find that tetra-FEC could act as a high-performing SEI modifier as it leads to a more homogeneous, denser LiF-containing SEI. Using this methodology, future investigations will explore −CF3 functionalization and other backbone molecules (linear carbonates).

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

“…A recent literature on lithium metal batteries also supports our finding of the relation between LiF formation and surface F-abstraction mechanism. 70 This finding also consolidates the differences in LiF formation reaction mechanisms along with various oligomerization species detected in experiments: EC•, PFEC oligomer, vinoxyl radical, and PFEO. The chemical structure of EC•, PFEC oligomer, vinoxyl radical, and PFEO as products of FEC decomposition and oligomerization as obtained from our AIMD simulations are shown in Figure 6.…”

Design Rules for Selecting Fluorinated Linear Organic Solvents for Li Metal Batteries

“…Additionally, we find that the presence of LiPF 6 may have an impact on the solvent decomposition, which is quite different from our previous study involving fluorinated cyclic carbonates. 59 The simulation setups and the computational methods, including the implementation of the statistical analyses, are explained in detail in the Supporting Information.…”

Similarities in Lithium Growth at Vastly Different Rates

“…This is plausible because the SEI varies depending on the orientation of the underlying lithium. [29] As already observed by Yamaki et al, [21] crystalline defects (tips and kinks) can control later stages of growth. They and others [22,30] attribute the early growth of the whiskers to the release of mechanical stresses and the extrusion of metal.…”