Reduction of Carbonate Electrolytes and the Formation of Solid-Electrolyte Interface (SEI) in Lithium-Ion Batteries. 1. Spectroscopic Observations of Radical Intermediates Generated in One-Electron Reduction of Carbonates

Whereas there are numerous experimental and computational studies of electrochemical reduction leading to the formation of solid-electrolyte interface (SEI) in lithium-ion batteries, so far there have been no direct spectroscopic observations of radical intermediates involved in the SEI formation. In Part 1 of this series, radiolysis and laser photoionization of carbonate electrolytes are used to observe and identify these reaction intermediates using electron paramagnetic resonance spectroscopy. Our study indicates that the suggested scenarios for electrolyte reduction require elaboration. In particular, we establish the occurrence of efficient H abstraction and 1,2-migration involving radicals generated through the reductive ring-opening. Instead of the primary radicals postulated in the current models, secondary and tertiary radicals are generated, biasing the subsequent chemistry to radical disproportionation. The consequences of this bias for radical and anionic polymerization are examined, and it is suggested that branching and the formation of a polymer network is favored. We argue that this chemistry accounts for some of the heretofore unexplained properties of SEI, including the dramatic difference in solvent permeability for SEIs derived from ethylene carbonate and propylene carbonate.