Role of Sodium-Ion Dynamics and Characteristic Length Scales in Ion Conductivity in Aluminophosphate Glasses Containing Na₂SO₄

Achieving high ion conductivity in glass-based Na-ion conducting materials for their applications as solid electrolytes in batteries is still challenging owing to the vague knowledge on the factors governing Na-ion dynamics. In the present study, an attempt has been made to identify the factors affecting the sodium-ion dynamics through structure and conductivity property correlation for the 37.5Na₂O–37.5P₂O₅–15Al₂O₃–10NaF (FS-0; mol %) glass system with varied concentrations of Na₂SO₄. ³¹P, ²⁷Al, and ²³Na MAS NMR (magic-angle spinning nuclear magnetic resonance) and Raman spectroscopy are employed to assess the structural modifications, and impedance spectroscopy is used to measure the variations in ionic conductivity on the addition of Na₂SO₄ in the FS-0 glass. Raman spectra and MAS NMR analysis indicate that the quantity of P–O–Na bonds and sulfate (SO₄^2–) units surrounded by sodium increase with increasing Na₂SO₄ concentration. Impedance analysis reveals that the conductivity of FS-0 glass enhances by 1 order with the addition of 6 mol % Na₂SO₄. We identify from the ac-conductivity spectral analysis that the concentration of charge carriers and the critical hopping length of mobile cations increase with the addition of 6 mol % Na₂SO₄. Overall, we reveal that the structural modifications, Na-ion concentration, and the shallower potential well that is created for sodium due to its interaction with the nearest neighboring cations affect the Na-ion dynamics. The information obtained from the present study certainly helps to optimize the chemical composition of glasses demonstrating high ionic conductivity.