Origin of Stability in the Solid Electrolyte Interphase formed between Lithium and Lithium Phosphorus Oxynitride

Area of Science:

• Materials Science
• Electrochemistry
• Solid-state Batteries

Background:

• Lithium phosphorus oxynitride (LiPON) is a solid electrolyte known for forming a passivating solid electrolyte interphase (SEI) with metallic lithium.
• Understanding the Li-LiPON SEI's stability is crucial for developing stable interfaces in next-generation solid-state batteries.
• Previous studies have identified key compounds in the SEI but lacked detailed insight into their spatial distribution and formation mechanisms.

Purpose of the Study:

• To investigate the formation and evolution of the solid electrolyte interphase (SEI) between lithium metal and LiPON using in situ techniques.
• To elucidate the chemical and structural inhomogeneity of the Li-LiPON SEI.
• To identify the factors contributing to the exceptional stability of the Li-LiPON SEI.

Main Methods:

• In situ lithium plating X-ray photoelectron spectroscopy (XPS) was employed to monitor SEI formation during lithium deposition.
• Analysis focused on the chemical states and spatial distribution of species within the SEI.
• Electrochemical potentials were correlated with SEI composition and stability.

Main Results:

• The Li-LiPON SEI is chemically and structurally inhomogeneous.
• Fully reduced compounds (Li2O, Li3N, Li3P) are concentrated near the lithium metal, while partially lithiated species (e.g., Li P) are closer to the LiPON.
• The LiPON reduction potential (0.68 V vs Li+/Li) is lower than the oxidation potentials of the reduced SEI compounds, and the graded SEI structure prevents direct contact between reduced species and LiPON.

Conclusions:

• The stability of the Li-LiPON SEI is attributed to its graded structure and the electrochemical potential of LiPON.
• The graded SEI prevents electrical contact between the LiPON electrolyte and the highly conductive, reduced SEI components.
• These findings offer insights for designing stable SEIs in other solid electrolyte systems, potentially using Li3P-like species.