No abstract

The solid electrolyte interphase (SEI) on the graphite anode of lithium ion batteries plays a crucial role for the battery performance. It is believed that the SEI blocks electrons and solvent molecules, while Li + can easily migrate across the SEI. However, quantitative measurements of transport coefficients for these species in the SEI are problematic due to the complex structure of graphite composite anodes. Here we have grown model SEIs on glassy carbon electrodes and have characterized them by a combination of scanning electron microscopy, AFM-based scratching experiments, impedance spectroscopy and redox probe experiments. SEM and AFM experiments reveal a dual-layer structure of the SEI. The redox probe experiments with ferrocene molecules provide strong indication that the diffusion of the redox molecules across pores in the inner SEI layer is faster than electron transport across the SEI. Remarkably, the effective diffusion coefficient of ferrocene in the SEI is virtually identical to the effective diffusion coefficient of Li + obtained from the SEI semicircle in the impedance spectra. Moreover, both diffusion coefficients show the same temporal evolution after SEI formation. This suggests that in our model SEIs, Li + is primarily transported in the liquid electrolyte phase inside the pores of the inner layer.

During the production of commercial lithium-ion batteries, the solid electrolyte interphase (SEI) on the graphite particles of the negative electrode is typically formed through galvanostatic protocols with low current densities. Consequently, SEI formation is a time-consuming and rather expensive production step. In order to better understand the influence of the formation current density on the transport of ions and molecules across the SEI, we formed model-type SEIs on planar glassy carbon electrodes under galvanostatic control. In accordance with the expectations from electrochemical nucleation and growth theory, we find that the transport of both ions and molecule becomes slower with increasing formation current density. However, it is remarkable that the ion transport is slowed down more strongly than the molecule transport. We show that at high formation current densities of about À 71 μA cm À 2 , our model-type SEIs clearly exceed the area-specific resistance tolerable in commercial lithium-ion cells.

We have carried out a basic study on the charge transfer across the interface between LiNi 0.5 Mn 1.5 O 4 (LNMO) thin-film cathodes and different solid electrolyte coatings, namely LiNbO 3 , ZrO 2 and Li 4 Ti 5 O 12 . In contact to LNMO, the spinel material Li 4 Ti 5 O 12 should act as a solid electrolyte, since it exhibits a very low electronic conductivity in the fully oxidized state. The thin films were prepared by means of sol-gel chemistry and spin-coating. The electrochemical and interfacial properties were studied by combining electrochemical impedance spectroscopy (EIS), time-of-flight secondary-ion mass spectrometry (ToF-SIMS) and cross-sectional SEM imaging. The results show that the LNMO / LiNbO 3 interface combines a low charge transfer resistance with a high stability.