The Chemistry of Lithium Alkyl Carbonates in the Graphite SEI of Lithium-Ion Batteries

The properties of the graphite solid-electrolyte interphase (SEI) strongly affect the performance of modern lithium-ion batteries, but the understanding of its formation and evolution is limited. It is known that the major decomposition products in the SEI of an organic carbonate electrolyte are lithium alkyl carbonates and alkoxides. This work focuses on the electrolyte system of 1 M LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC). These solvents mainly decompose to lithium ethylene dicarbonate (LEDC), lithium ethylene monocarbonate (LEMC), lithium ethyl carbonate (LEC), and lithium ethoxide. These compounds have been synthesized and characterized in the past, but some inconsistencies in the literature led to recent discussions about potential misidentification of LEDC and LEMC, and the question whether LEDC occurs in a battery at all. Here, these lithium alkyl carbonates are thoroughly characterized with 2D-NMR spectroscopy, and their chemical reactivity is discussed. Then, cycled graphite anodes are analyzed with NMR spectroscopy and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) to investigate the formation and evolution of these compounds in Graphite||LiFePO4 coin cells under different conditions. Evidence is found for an early SEI composed of mainly LEDC, which evolves with further cycling into an SEI of mainly LEC and lithium ethoxide. LEMC was found to be a hydrolysis product of LEDC, and a product of hydroxide-induced ring-opening of EC.