Investigation of Electrochemical Stability of ALD Grown Li₃PO₄ Thin Films and Its Application in High‐Voltage PEO‐based All‐Solid‐State Lithium Batteries

Abstract In lithium batteries, the interface between electrolytes and electrodes with optimized structural stability and fast charge transfer kinetics plays a pivotal role in achieving overall cycle and rate performance. Introducing a uniform interfacial layer that conducts Li⁺ ions while insulating electrons, through atomic layer deposition (ALD) of lithium compounds, has shown promise in facilitating lithium ion transport and enhancing interface stability. However, synthesizing lithium compounds with both robust chemical stability and a broad electrochemical window using ALD poses challenges due to the difficulty of obtaining desired products under specific ALD reaction conditions. In this study, this challenge by investigating Li₃PO₄ is addressed, a compound grown by ALD that exhibits a noteworthy property: the in situ formation of a more stable LiPO₃ phase under high applied voltage. The LiPO₃ phase possesses an extended theoretical stability window (LiPO₃: 2.48‐5.00 V versus Li₃PO₄: 0.69‐4.21 V), and its emergence substantially enhances the electrochemical performance of all‐solid‐state lithium batteries based on polyethylene oxide (PEO). This work not only presents experimental evidence of the in situ transformation of the interfacial layer but also offers a pragmatic approach to realizing all‐solid‐state lithium batteries with elevated energy density, emphasizing safety and stability considerations.