Spectroscopic Ellipsometry Study of TiO₂/LiPON and LNMC/LiPON Solid Electrode/Electrolyte Interfaces of Solid-State Batteries

In all-solid-state thin-film batteries, the interface properties between solid electrolytes and solid electrodes are the major factors limiting their performances; however, knowledge of the interface properties is limited in most of the cases. Using the magnetron sputtering technique, thin films of Li3+xPO4–xNx (LiPON) solid-state electrolyte along with LiPON/TiO2 (anode) and LiPON/LiNi0.33Mn0.33Co0.33O2 (LNMC) (cathode) thin-film bilayers have been prepared under different sputtering conditions and characterized by spectroscopic ellipsometry to study the properties of electrolyte/anode and electrolyte/cathode interfaces of Li-ion solid-state batteries. Spectroscopic ellipsometry, X-ray photoelectron spectroscopy, and secondary ion mass spectrometry studies of single-layer LiPON showed that radiofrequency (RF) power and nitrogen pressure used during deposition control the nitrogen and lithium contents of the film. Films deposited at a higher RF power showed a lower refractive index, which signifies a lower nitrogen content and a more Li3PO4-like film; impedance measurements also confirmed this result. The interface width found from the ellipsometric study of the LiPON/TiO2 and LiPON/LNMC bilayer films showed that the nitrogen content of the LiPON layer has a very strong effect on the interface as well as on the lithiation of the TiO2 layer during LiPON sputtering; higher nitrogen content in the LiPON layer increases both. This finding has also been supported by cross-sectional FESEM results. Hybrid half-cells of the TiO2 anode and LNMC cathodes have been prepared using a higher nitrogen content LiPON electrolyte. The stable electrochemical performance of the cells manifests that higher interface width favors the formation of a stable solid electrolyte interface layer at the anode–electrolyte interface.