AbstractLithium metal is a promising anode material for the development of advanced all-solid-state batteries (ASSBs) with high energy density. Among the various solid electrolytes, lithium phosphorus oxynitride glass electrolyte (LiPON) is notable for facilitating stable Li plating-stripping reactions in ASSBs employing Li metal. The aim of this study is to examine the Li/LiPON interface, with a specific emphasis on the reductive decomposition of LiPON near this interface. We employed time-of-flight elastic recoil detection analysis (TOF-ERDA) to assess changes in Li concentration around the Cu/LiPON interface immediately prior to the Li plating reaction. Our electrochemical measurements indicate that critical decomposition of LiPON occurs when the voltage at the Cu electrode is reduced to 0.1 V vs. Li/Li⁺ at 25 °C, resulting in the in situ formation of Li₃P operating at 0.7 V vs. Li/Li⁺ as an anode material. The TOF-ERDA findings reveal that this decomposition reaction results in a layer with partial decomposition (ranging from 5 to 25% on average) extending up to approximately 30 nm from the Cu/LiPON interface. This insight is vital for enhancing the design and performance of ASSBs. Graphical abstract