Electrochemical behavior of current collectors for lithium batteries in non-aqueous alkyl carbonate solution and surface analysis by ToF-SIMS

Several metals (Cu, Fe, Al, Ti, and Cr) as current collector for lithium-ion battery were investigated to understand their electrochemical behavior and passivation process in a non-aqueous alkyl carbonate solution containing LiPF6 salt. From cyclic voltammetric study, it was found that Cu and Fe metals were dissolved into the electrolyte below 4 V vs. Li/Li+. Alternatively, Al and Ti were stable up to 5 V vs. Li/Li+. Their scratched surfaces at 5 V vs. Li/Li+ were polarized in a transient mode and it was found that the surfaces were passivated during the polarization test. Formed passive film was composed of two hybrid layers: outer layer by metal (Al and Ti) fluoride and inner by metal oxide, as confirmed by time-of-flight secondary ion mass spectroscopy. Presence of HF in the electrolyte was indispensible to form the metal fluoride layer on the oxide layer. The outer fluoride layer would protect the inner oxide layer and metal substrate from HF attack, bringing about satisfactory corrosion resistance under lithium-ion battery environment.