LATP and LiCoPO4 thin film preparation - Illustrating interfacial issues on the way to all-phosphate SSBs

Thin film solid-state batteries are suitable model systems for the study of degradation phenomena in bulk solid-state batteries. As a necessary prerequisite for the construction of thin film batteries, high quality thin films of electrode materials and solid electrolytes have to be deposited. In this study, phosphate-based cathode (LiCoPO4) and solid electrolyte (Li1.5Al0.5Ti1.5(PO4)(3)) thin films were successfully deposited by pulsed laser deposition on silicon substrates. By temperature dependent grazing incidence X-ray diffraction measurements, the high quality of the deposited thin films was shown as well as crystallization temperatures were determined. Moreover, atomic force microscopy and scanning electron microscopy measurements were carried out for surface analysis, highlighting the surface smoothness of the films and unraveling the microstructure of the deposited LATP films, which contain grain boundaries. Further, a model interface composed of a thin layer of LiCoPO4 of varying thickness on top of Li1.5Al0.5Ti1.5(PO4)(3) was used to corroborate information about changes in the chemical state of the materials as well as to track the inter-diffusion across the interface of all corresponding ionic species. This was done by X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry depth profiling. While well-defined interfaces were observed for unheated interfaces, significant inter-diffusion of transition metal ions was observed between heated LATP and LCP films. Despite inter-diffusion, no changes in the chemical states were observed at the interface, excluding significant phase transformations to compounds with altered oxidation states. Although stable phosphate-based materials were chosen for, both, electrolyte and active material to diminish known instabilities of the interface in this study, the need for interfacial layers arises to suppress the inter-diffusion, which may affect lithium ion transport across the interface. This work provides detailed insight into the complex problems of interfacial stability in all-solid-state batteries.