A significant enhancement of sodium ion conductivity in phosphate glasses by addition of WO3 and MoO3: the effect of mixed conventional–conditional glass-forming oxides

Abstract

Ion conducting oxide glasses are attractive materials for application in various electrochemical devices and an understanding of the structure–transport properties relationship is crucial for their development. An interesting effect of glass structure on the dynamics of mobile ions is the mixed glass-former effect which causes a non-linear change of ionic conductivity when glass-forming oxides get gradually substituted. Here, we report a strong, positive effect of structural changes on the conductivity of sodium ions in two glass systems 40Na₂O–xMoO₃–(60−x)P₂O₅ and 40Na₂O–xWO₃–(60−x)P₂O₅; x = 0–50 mol% where a conventional glass-forming oxide (P₂O₅) is gradually replaced by WO₃/MoO₃ which are conditional ones. In both glass systems, the compositional change in DC conductivity is non-linear, with the maximal increase of four orders of magnitude in the case of WO₃ and three orders of magnitude in the case of MoO₃. This significant enhancement of ionic conductivity is related to the formation of mixed phosphate–tungstate and phosphate–molybdate units in the glass network. The facilitating effect of these structural units on sodium ion dynamics is also observed in the changes of the shape of frequency-dependent conductivity and in the values of typical spatial extent of diffusion of sodium ions known as the Sidebottom length.