Formation Mechanism and Stability of Solid Electrolyte Interphases in Linear Carbonate-Based Electrolytes

Abstract

Broad application of lithium batteries, necessitates improvements in their safety, cycle life, durability and energy density. Clear understanding of the electrode/electrolyte interfaces is one of the important factors in designing batteries with enhanced performance. Linear organic solvents are extensively used in electrolytes of lithium batteries to reduce viscosity and melting temperature of cyclic carbonates. However, these organic compounds are not stable in contact with anode, leading to decomposition of the electrolyte and formation of Solid Electrolyte Interphase (SEI). Although SEI is proven to be one of the most important parts of batteries, its structural complexity, mechanism of formation, and properties are not completely understood yet. In this work, we consider mechanism of formation and stability of SEIs composed of redox products of linear carbonate solvents. So far, the main focus of research was on decomposition products of cyclic carbonates. In case of linear carbonates, the SEI stability consideration is limited to the systems being in DMC solvent and to the best of our knowledge, there are no theoretical investigations on linear carbonate-based SEIs formation mechanism and properties.In this work, we study Lithium Methyl Carbonate (LMC), Lithium Ethyl Carbonate (LEC) and Lithium Propyl Carbonate (LPC), which are single-electron reduction products of carbonate solvents in contact with the electrode, in their corresponding solvents being Dilithium Methyl Carbonate (DMC), Dilithium Ethyl Carbonate (DEC) and Dilithium Propyl Carbonate (DPC), respectively (Figure 1). Our aim is to chemically, kinetically, and thermodynamically study the aggregation, formation and stability of SEI layer in these systems from experimental, Molecular Dynamics (MD) simulation and Density Functional Theory (DFT) calculation points of view. Our study indicates that in LMC/DMC mixture LMC molecules have strong tendency to self-assemble and have low barriers to undergo second-electron reduction to form stable Li2CO3. Stable SEI was also observed in LPC/DPC as well, which is believed to be a result of slow dynamics of larger molecules and therefore increased possibility of the second-electron reduction near electrode surface. In the LEC/DEC system, however, we did not observe a stable SEI formation due to low tendency of LEC molecules to self-assemble, low electronic affinity coupled with high energy barrier for possible second electron reduction reaction. Therefore, our work illustrates that the selection of the linear carbonate as a blending partner for cyclic carbonates, might have an important impact on the ability of electrolyte to form stable and compact SEI layer. Figure 1. Carbonate solvents and components studied in this work. Figure 1