LiPON Layer Effect for Reduction of Interfacial Resistance of LLZO/Li for All-Solid-State Battery

Cubic garnet-type solid electrolytes for all-solid-state batteries (ASSBs) have attracted attention due to their stability over a wide electrochemical window and high ionic conductivity. However, current challenges for garnet-type solid electrolytes include overcoming poor wettability and high interfacial resistance with lithium metal anodes. When a lithium metal anode contacts an oxide solid electrolyte, a passivation layer of Li₂CO₃ forms on the surface of the solid electrolyte, which can lead to increased resistance. Additionally, non-uniform Li⁺ intercalation/extraction can lead to increased resistance and electrochemical instability at the solid electrolyte/anode interface. To improve contact resistance, interfacial modification methods have been introduced. One such method is to coat the surface of the solid electrolyte with a ceramic material that is highly reactive with lithium. This can stabilize the interfacial chemistry and improve lithium wettability and charge transport. In this study, LiPON coating was deposited on the surface of LLZO pellets using an RF sputtering process. The interface between the coating layer and the LLZO pellet was analyzed using transmission electron microscopy-energy dispersive spectroscopy (TEM-EDS) to confirm the X-ray photoelectron spectroscopy (XPS) depth profile spectrum. Molten Li contact angle measurements were also performed to confirm wettability with metallic lithium. The results showed that the LiPON coating forms a Li-LiPON layer when in contact with molten Li and exhibits a low contact angle of less than 90°. This indicates that the LiPON coating can improve lithium wettability and reduce interfacial resistance. These results suggest that the chemical properties of the lithium electrode and the properties of the interface play an important role in the interfacial resistance and lithium wettability, compared to the microstructure of the LLZO solid electrolyte. This discovery will have implications for the design of LLZO/Li interfaces for ASSBs containing LLZO as the electrolyte.