Investigation of surface film formation on LiNi0.5Mn1.5O4 cathodes in carbonate based liquid electrolytes using the secondary ion mass spectrometry

摘要

The scope of the present work was the investigation of the interface film formed between LiNi0.5Mn1.5O4, a cathode material for lithium ion batteries, and liquid carbonate based electrolytes. Main characterization tool was the secondary ion mass spectrometry. Thin film model electrodes, which are in contrast to technical electrodes free of conductive carbon and binder, were studied in order to exclude the influence of these additives on the surface film formation and to simplify the system. By usage of thin film electrodes in addition the electrode roughness could be distinctly reduced. This in turn increases the depth resolution of the secondary ion mass spectrometry. Main aim during thin film optimization was the reduction of the electrode roughness. Mainly by increasing the deposition temperature values of less than 10 nm could be reached. It could be revealed that the interface film shows a stacked structure. The region close to the electrolyte is mainly composed of organic species, while mainly inorganic ones are located close to the cathode surface. The thickness of the interface film increases with cycle number, storage time and temperature. Both transition metals contained in the cathode show significant dissolution in the electrolyte and diffuse through the electrolyte towards the anode. A model concerning the surface film formation was developed.