The impact of CO₂ evolved from VC and FEC during formation of graphite anodes in lithium-ion batteries

Additives such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are commonly added to lithium-ion battery electrolytes in order to form a solid electrolyte interphase (SEI) on the anode, suppressing continuous solvent reduction. In this work, we directly compare VC and FEC by analyzing the SEI with FTIR and XPS, and the evolved gases with on-line electrochemical mass spectrometry (OEMS) in different model systems. Since both additives evolve mainly CO₂ during formation, the effect of CO₂ as an additive is compared to the addition of VC and FEC. While Li₂CO₃ is as expected the main SEI compound found due to the added CO₂, surprisingly no CO was detected in the gas phase of such cells. Based on FTIR, NMR and OEMS analyses of cells filled with ¹³C labeled CO₂, we suggest a mechanism explaining the beneficial effects of CO₂ and hence also of CO₂ evolving additives in lithium-ion battery cells. While the generation of polycarbonate from FEC or VC reduction is observed, the generation of Li₂CO₃ may be as important as the generation of polycarbonate.