Today’s conventional lithium ion batteries comprise organic liquid electrolytes, which allow high performance of the batteries, however, also cause problems due to their limited stability. A promising approach to solve these issues is to replace the liquid electrolyte by a ceramic lithium ion conductor thus creating a so-called solid-state battery. Ceramics in general offer a superior chemical and thermal stability compared to any organic material.    

A challenge in solid-state batteries is the processing of a fully inorganic battery cell because of thermally or field activated high-energy treatments that are necessary to create an inherent contact between the functional layers. This work focuses on the interaction and diffusion phenomena of the cathode material LiFePO₄ and typical current collector materials like aluminum, titanium, and carbon as a conductive additive. It turned out that the current collector species exhibited high diffusion into and chemical reaction with the cathode material during processing and significantly changed the electrochemical performance of the cathode material. Planar and compact thin-film batteries manufactured by physical vapor deposition (PVD) were used to study this behavior in more detail by scanning electron microscopy, X-ray diffraction, cyclic voltammetry and secondary ion mass spectrometry (SIMS). Especially SIMS benefits from the thin-film approach on planar geometry with smooth surfaces and interfaces, along with pure LiFePO₄ cathodes, to identify for example interdiffusion phenomena or chemical reactions.

The capacity of the pure LiFePO₄ cathode layers was significantly lower than the theoretical capacity of 170 mAh/g.  LiFePO₄ is well known to be a material with extremely low electronic conductivity, and the lower capacity has been also attributed to this issue. Carbon is usually added to this active material to circumvent this drawback, as done with all cathode materials applied in conventional lithium ion batteries. After PVD processing of the cathode material with additional carbon, where carbon was already added to the sputter target, the cathodes had a capacity in the range of the theoretical one. This is not only accounted for a higher electronic conductivity – surprisingly, the microstructure completely changed from an apparently dense layers of LiFePO₄ without carbon to a highly porous morphology consisting fibers with dimensions of the order of 100 nanometers with clear spacings of about 100 nanometer between them. As an outlook, these layers could also be used in lithium batteries with liquid electrolyte, featured by apparently continuous LiFePO₄ wires instead of LiFePO₄ powder stuck together with polymeric binders.