Ab Initio Insights into the Solid Electrolyte Interphase Formation in Lithium Metal Batteries with Carbonate Electrolytes

Abstract

As the research endeavors push the performance of widely used Li-ion battery to its theoretical limit, there are strong urges to develop next-generation energy storage devices. Li metal is a promising candidate battery anode material which can bring the energy density to a higher level. There are safety and cycling efficiency issues associated with the Li-metal battery (LMB), and the metal dendrite nucleation and lithium whisker formation are largely responsible for them. The solid electrolyte interphase (SEI) generated from the reactions between lithium metal and battery electrolyte components can regulate the lithium-ion diffusion and deposition which protects anode materials from undesired consumptions. According to the experimental evidence, the SEI from carbonate electrolytes may contain inorganic components including Li2O, Li2CO3, LiF, as well as organic structures such as polymers and oligomers with carbonate and carbonyl structures. However, the exact composition and ionic diffusion properties of SEI are challenging to be directly characterized via experimental techniques.In this work, we initiated the elucidation of the SEI composition by investigating plausible reaction pathways. The inorganic structures are mainly resulted from the degradation of electrolyte solvent and salt species upon contact with the anode material. Ab initio molecular dynamics (AIMD) simulation was implemented to track the interfacial reduction of ethylene carbonate (EC), vinylene carbonate (VC) and lithium hexafluorophosphate (LiPF6). Bader charge analysis demonstrated that the LiPF6 reduction requires consecutive uptake of outside electrons, no matter such reaction happens right on the metallic surface or in electrolyte phase. The final product of the LiPF6 degradation was determined to be Li3P, and the reaction thermochemistry was computed based on density functional theory (DFT). The experimentally measured battery electrochemical performance enhancement of the VC additive has been reported while the chemical mechanism behind such phenomenon was less discussed. We modeled the degradation and polymerization reactions of EC and VC molecules in the LMB environment. VC has significantly smaller energy barrier than EC to generate the lithium-carbonyl complexes and the oligomer/polymer species which takes longer time scale to form. In addition, the solvated lithium cation was found to have significant contribution to the polymerization reaction thermodynamics. The most kinetically favored pathways and products for EC and VC polymerization were identified via DFT calculations. Multiscale simulation based on the computed thermodynamic information was able to provide a glimpse of the microscopic morphology inside the SEI.