Decomposition Mechanism of Lithium Methyl Carbonate with Electrolyte in the Solid Electrolyte Interphase to Elucidate the Early Stages of Thermal Runaway in Lithium-Ion Batteries

Abstract

With the growing demand for electric vehicles, the safety of lithium-ion batteries has been greatly emphasized. Thermal runaway caused by the continuous side reactions inside the battery makes it difficult to extinguish fires caused by abnormal activity. The temperature at which a typical thermal runaway begins is around 100°C, the temperature at which several elements in the solid electrolyte interphase (SEI) layer on the anode surface experience thermal decomposition reactions (causing gas generation and heat production). Therefore, to understand the first stage of thermal runaway and increase battery safety, it is necessary to describe the thermal decomposition process of the SEI layer.To investigate the thermal decomposition process of the SEI layer, it is necessary to understand the various organic components. During the initial charging phase, the reduction reaction between the lithium ions and the electrolyte produces a series of components that make up the SEI layer. On the surface of the anode active material, the SEI layer is a thin layer with an overall thickness of 10-50 nm. It is composed of various organic (inner layer) and inorganic (outer layer) elements. Under thermal stress, the unstable organic SEI layer decomposes into an inorganic SEI layer due to its low stability in the presence of heat and gas generation, which is mostly caused by the decomposition of lithium carbonate (ROCOOLi). It is important to note that the most used electrolytes in batteries today are ethylene carbonate and ethyl methyl carbonate (EMC) dissolved in salts based on lithium hexafluorophosphate. Due to the application of EMC, lithium methyl carbonate (LMC) accounts for more than 50% of the SEI layer in the ROCOOLi compounds.This study used both theoretical and experimental techniques to fully elucidate the pyrolysis process of high-purity synthesized LMC with the electrolyte. First, LMC was chemically synthesized using a nucleophilic substitution reaction. Then, in-situ thermogravimetry analysis (TGA)-mass spectrometer(MS), attenuated total reflectance (ATR)- fourier transform infrared (FTIR), and nuclear magnetic resonance (NMR) results along with bond dissociation energy (BDE)/spectral analysis/energy diagrams obtained by density functional theory (DFT) calculations were analyzed to propose the elementary reaction steps involved in the pyrolysis reaction. We propose six elementary reaction steps for the pyrolysis reaction of LMC. To verify the validity of the proposed basic reaction mechanisms, chain reactions involving radicals generated during pyrolysis were finally calculated using GRI-MECH 3.0, and the calculated results were compared with the composition of the actual gas. We hope to utilize the techniques used in this study to obtain the reaction steps of additional SEI components (LEMC, Li2CO3, etc.). The precise mechanisms obtained through modeling studies can be used to predict the self-heating phase of thermal runaway and to develop materials that can stop the SEI decomposition reaction.