Solid electrolytes in combination with a lithium anode are a promising approach for the development of future batteries with high energy density and stability, but the realisation of solid state cells poses a major challenge. In the past, different types of solid electrolytes with high conductivity have been developed, such a NaSICON-type electrolytes, garnets and perowskites. However, next to high conductivity, also other properties are relevant, such as interface formation and mechanical behavior. To be able to tailor the properties of future electrolytes for a particular application, the combination of different materials as hybrid electrolytes is a promising approach. To realise this, especially the interface between different phases, as a zone of usually complicated reactions, has to be well understood.
A present example for such hybrid electrolytes is the protection of highly conductive, LATGP glass ceramic sheets by sputtered LiPON coatings to increase the chemical stability. The company Ohara Inc. offers two types of NaSICON based Li-Ion conductors: LATGP glass ceramics with NaSICON type crystal phase and LATP type partially sintered plates, also containing the highly conductive NaSICON phase. None of these materials is stable in direct contact to metallic Lithium. A state of the art Li Ion conductor that is applicable in direct contact to Li metal anodes is LiPON: A nitridated Li-phosphate glass, offering Li conductivity in the range of 10-6 S/cm. Due to the relative simple thin film deposition process of reactive sputtering from a Li3PO4 target in nitrogen plasma, LiPON is widely used in the field of thin film batteries. West et al. showed the principle possibility of protecting LATGP glass ceramics against Lithium by thin film LiPON coating. Shimada et al. were able to reduce the interface resistance by applying a thin Ti film between the two electrolytes.
In this contribution, the interface between commercial LATGP/LATP and LiPON is studied by photoelectron spectroscopy with regards to chemical reactions taking place. Nitridation of the phosphate component and a partial reduction of transition metal oxides was observed. Chemical reactions at the interface can be attributed to the standard LiPON deposition process of reactive rf-magnetron sputtering. Results of this interface study help to improve the fundamental understanding of Li-electrolyte interfaces and give way to tailoring properties of future solid electrolytes.