Model Studies on Solid Electrolyte Interphase Formation on Graphite Electrodes in Ethylene Carbonate and Dimethyl Carbonate II: Graphite Powder Electrodes

As part of a systematic study on the formation and composition of the solid electrolyte interphase (SEI) in lithium-ion batteries (LIBs), going stepwise from highly idealized electrodes such as highly oriented pyrolytic graphite and conditions such as ultrahigh vacuum conditions to more realistic materials and reaction conditions, we investigated the decomposition of simplified electrolytes (ethylene carbonate (EC)+1 M LiPF6 and dimethyl carbonate (DMC)+1 M LiPF6) at binder-free graphite powder model electrodes. The results obtained from cyclic voltammetry and ex situ X-ray photoelectron spectroscopy half-cell measurements - in particular on the effect of cycling rate, solvent and electrode - are explained in terms of a mechanistic model where electrolyte decomposition occurs at the SEI | electrode interface and where transport of solvent and salt species through the growing SEI plays an important role for explaining the observed change from preferential salt decomposition to solvent decomposition with increasing cycling rate.