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Enhanced mobility of lithium and sodium ions in phosphate glasses obtained by WO3 and MoO3 addition

Increased demand for stable and efficient electrolytes in batteries puts the focus of recent studies on the improvement of relatively low ionic conductivity in various oxide glasses containing alkali ions. A popular approach in achieving this goal is to add the second glass-forming oxide to the glass in order to induce structural changes which have a facilitating effect on the ionic transport (mixed glass-former effect). In this study, we show that a significant increase in ionic conductivity can be achieved by the replacement of glass-forming oxide P2O5 with WO3 and MoO3 which are not conventional glass-formers but conditional ones. For that purpose, four series of glasses 40Na2O–xMoO3–(60-x)P2O5, 40Na2O– xWO3–(60-x)P2O5, 40Li2O–xMoO3–(60-x)P2O5 and 40Li2O–xWO3–(60-x)P2O5 ; x=0-50 mol% were prepared by melt-quenching. The electrical characterization was performed by impedance spectroscopy in a wide frequency and temperature range while the structural changes were followed by the 31P MAS- NMR and Raman spectroscopy. Comparing the phosphate glasses with different conducting ions, it was found that an increase in DC conductivity with the addition of transition metal oxides was more pronounced in lithium series as a result of a smaller ionic radius and therefore higher mobility of lithium ions in contrast to the sodium ones. The trend in conductivity of sodium phosphate glasses was found to be non-linear, which is typical for the mixed glass former effect, exhibiting the maximal value at 30 mol% of MoO3 and 40 mol% of WO3. Similar compositional dependence was also found in the fraction of mixed molybdate/tungstate-phosphate units and the number of P-O-Mo/W linkages. These findings indicate that the fastest transport of sodium ions occurs at the maximally interconnected phosphate and molybdenum/tungsten units. On the other hand, lithium containing phosphate glasses did not show the typical effect of mixed glass formers on the electrical conductivity, even though the addition of WO3 and MoO3 produced similar network modification as in sodium containing glasses. In both series, the enhancement of ionic conductivity was found to be continuous for glasses containing up to 40 mol% of WO3 and MoO3 but retained for a higher fraction of these oxides. Here, similarly as in the sodium phosphate series, the formation of mixed molybdenum/tungsten-phosphate units strongly facilitates the transport of ions. However, at the highest amounts of WO3 and MoO3, smaller Li+ ions are less affected by the hindering nature of the glass network composed predominantly of molybdate and tungstate units, so the characteristic maximum in conductivity was not observed rather high values were retained.

Lithium phosphate glass ; Sodium phosphate glass ; Ionic conductivity ; Glass structure

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