Surface film formation on LiNi0.8Co0.15Al0.05O2 cathodes using attenuated total reflection IR spectroscopy

The surface films formed on commercial LiNi0.8Co0.15Al0.05O2 cathodes (ATD Gen2) charged from 3.75 to 4.2 V vs. Li/ Li+ in ethylene carbonate: diethyl carbonate-1 M LiPF6 were analyzed using ex situ Fourier transform IR spectroscopy with the attenuated total reflection technique. A surface layer of Li2CO3 is present on the virgin cathode, probably from reaction of the active material with air during the cathode preparation procedure. The Li2CO3 layer disappeared even after soaking in the electrolyte, indicating that the layer dissolved into the electrolyte possibly even before potential cycling of the electrode. IR features only from the binder (poly(vinylidene difluoride)) and a trace of polyamide from the Al current collector were observed on the surfaces of cathodes charged to below 4.2 V, i.e., no surface species from electrolyte oxidation. However, some new IR features were found on the cathode charged to 4.2 V and higher. An electrolyte oxidation product was observed that appeared to contain dicarbonyl anhydride and (poly)ester functionalities. The reaction appears to be an indirect electrochemical oxidation with overcharging (removal of >0.6 Li ion) destabilizing oxygen in the oxide lattice, resulting in oxygen transfer to the solvent molecules. (C) 2004 The Electrochemical Society.