Competing Ethylene Carbonate Reactions on Carbon Electrode in Li-Ion Batteries

Ethylene carbonate (EC) is the archetype solvent in Li-ion batteries. Still, questions remain regarding the numerous possible reaction pathways of EC. Although the reaction pathway involving direct EC reduction and SEI formation is most commonly discussed, EC ring-opening is often observed, but seldomly addressed, especially with respect to SEI formation. By applying Online Electrochemical Mass Spectrometry, the EC ring-opening reaction on carbon is found to start already at ∼2.5 V vs Li + /Li as initiated by oxygenic carbon surface groups. Later, OH − generated from H 2 O reduction reaction at ∼1.6 V further propagates EC to ring-open. The EC reduction reaction occurs <0.9 V but is suppressed depending on the extent of EC ring-opening at higher potentials. Electrode/electrolyte impurities and handling conditions are found to have a significant influence on both processes. In conclusion, SEI formation is shown to be governed by several kinetically competing reaction pathways whereby EC ring-opening can play a significant role.