Reduction of Carbonate Electrolytes and the Formation of Solid-Electrolyte Interface (SEI) in Lithium-Ion Batteries. 2. Radiolytically Induced Polymerization of Ethylene Carbonate

Extensive polymerization of ethylene carbonate (EC) leading to the formation of oligomers with masses up to 1 to 2 kDa in electron beam radiolysis is demonstrated using electrospray ionization mass spectrometry and nuclear magnetic resonance. This polymer has a different structure and morphology than the linear chain copolymer of ethylene oxide and EC that is generated in anionic polymerization of intact EC molecules. This radiolytically generated polymer exhibits chain branching and pendant carbonate groups, and it can form a 3D organic network that is additionally cross-linked through lithium ions. Such a morphology is consistent with the occurrence of anionic and radical polymerization that involve the products of recombination and disproportionation of secondary radicals generated in one-electron reduction of EC. Our examination of this chemistry suggests that the same polymer is likely to occur in electrochemical reduction of EC. The formation of this polymeric network qualitatively accounts for some of unexplained properties of the solid-electrolyte interface (SEI) occurring in electrochemical cells with EC-based electrolyte, including common lithium batteries.