In Situ Measurement of Buried Electrolyte-Electrode Interfaces for Solid State Batteries with Nanometer Level Precision

Many technologies including high-energy solidstate batteries rely on high quality solid-solid interfaces. Solid-solid interfaces are particularly difficult to study as their nature requires the interface to be buried. In this work we demonstrate the use of a combination of neutron reflectometry and in situ electrochemistry to measure the interface between Li metal and the solid electrolyte LiPON across an 8 cm2 area. Neutron reflectometry measurements confirm the interphase to be less than 7 nm thick. The neutron reflectometry data further emphasize that the interphase that forms is a chemical gradient consisting of a Li-rich layer that gradually decreases in Li content until it blends into pure LiPON. Experimental confirmation that we can make ideal solid-solid interphases less than 10 nm thick will help facilitate the adoption of high efficiency next generation solid state batteries. Further this combination of complementary techniques provides a more general methodology for studying buried solid-solid interfaces across applications.