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).

Studies have shown fluorinated electrolyte solvents can form desirable solid electrolyte interphase (SEI) in lithium metal batteries. In this study, we develop a detailed mechanistic understanding of two high performing electrolytes, Fluoroethylene Carbonate (FEC) and Difluoroethylene Carbonate (DFEC) to demonstrate minimal structural variations can lead to different decomposition products, and thereby the nature of the SEI. Using density functional theory (DFT) calculations, we find different initial bond-breaking mechanisms between FEC and DFEC. We develop free energy diagrams for the decomposition pathways including both electrochemical and chemical steps. Using the computational Li electrode, we identify the largest limiting potential of 1.77 V for FEC decomposition, associated with the formation of lithium fluoride, lithium oxide and FEC oligomers, and 1.53 V for DFEC, which correspond to the formation of polymerized vinylene carbonate and lithium fluoride. We suggest the formation of oligomers in the case of FEC instead of long polymers may lead to better SEI compactness. We also demonstrate the SEI components of FEC and DFEC are not stable on typical cathode voltage (3.87 V). This study presents a unified electrocatalytic perspective on SEI formation and decomposition.

No abstract

Fluorinated linear organic solvents have great potential in improving the safety and lifetime of next-generation Li metal batteries. However, this group of solvents is underexplored. Here, we investigate the molecular and interfacial reactivity properties of seven partially and fully fluorinated linear carbonates designed based on conventional solvents. Using density functional theory, we find the highest occupied molecular orbital levels decrease with increasing substitution of the fluorinated functional groups, implying that fluorination, to a first approximation, improves the stability toward high voltage cathodes. On the basis of the simulated decomposition mechanisms and statistical analyses, we find that a fluorinated linear carbonate with partial fluorination at the methyl component is more accessible in terms of degradation and LiF nascence formation, leading to a potentially LiF-rich solid electrolyte interphase (SEI). The molecular design concepts and the computational techniques presented are transferable to ester and ether systems, facilitating the navigation in a large chemical design space.

Ionic conductivity in liquid electrolytes depends on molecular interactions dictating the relative populations and behaviors of stoichiometric ion solvation clusters. However, the connections from molecular interactions to bulk ionic conductivity are not well-established, limiting the fast in silico evaluation of liquid electrolytes before experimental synthesis. To illustrate a bottom-up approach to predicting ionic conductivity, we outline a method using a chemical physics formalism with parameters computed by classical molecular dynamics (MD) simulations. The method is demonstrated on two liquid electrolyte chemistries with salts of differing electrolyte strengths. Using the proposed approach without empirical fitting, we achieve quantitative and qualitative prediction agreements with respect to conductivity measurements for strong and weak electrolytes, respectively. This approach provides the basis for closing the structure-based design computational loop to aid emerging high-throughput electrolyte discovery frameworks.