Vanadium coordination environment in phospho-vanadate glass for improving water durability

Understanding the mechanism behind the water durability improvement of phospho-vanadate glass is crucial for enabling the practical use of low-melting glass in lead-free sealing and secondary ion batteries. We demonstrate that Fe₂O₃ additive is a key component for improving the water durability while suppressing the rise of the glass transition temperature of V₂O₅P₂O₅ (VP) glass, and characterize the local structural change by X-ray absorption fine structure measurements. In VP glass, the addition of Fe₂O₃ increases the coordination number of vanadium ion without changing the oxidation state. The water durability is relatively sensitive to the vanadium valence and coordination environment compared to the glass transition temperature and hardness. Minimizing V⁵⁺O₄ units in phospho-vanadate glass is the key to improving the water durability.