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Increased Li+ ion conductivity in phosphate glasses achieved by the addition of WO3 and MoO3

In development of applicable oxide glass electrolyte for solid-state batteries, the high ionic conductivity represents the ultimate challenge. One way to increase the ionic conductivity in these materials is to substitute the glass-forming oxide by second glass former to induce so-called mixed glass former effect. However, we have recently showed that a similar effect on the Na+ ion conductivity can be also achieved by the addition of the conditional glass forming oxides such as WO3 and MoO3. In this study, we report a huge enhancement of lithium ion conductivity as a result of structural changes in two series of phosphate glasses, 40Li2O–xMoO3–(60- x)P2O5 and 40Li2O–xWO3–(60-x)P2O5 ; x=0-50 mol%. Glasses were prepared by melt quenching technique and were electrically characterized by impedance spectroscopy in a wide frequency (0.01 Hz – 1 MHz) and temperature (-90 °C – 240 °C) range. Their structural features were evaluated by 31P MAS-NMR and Raman spectroscopies. The results showed that the addition of WO3 and MoO3 in the phosphate network significantly increases DC conductivity for five orders of magnitude in the case of tungsten and more than three orders in the case of molybdenum. The increase in conductivity in both glass series was found to be continuous up to 40 mol% of WO3 and MoO3, while for x=50 mol% the conductivity remains nearly the same. Based on the structural analysis we found that WO6 and MoO6 octahedra incorporate in the phosphate network and form mixed tungstate/molybdate-phosphate units with the addition of WO3/MoO3. Induced structural changes consequently lead to a more disturbed and opened structure that strongly facilitates the transport of Li+ ions. However, at 50 mol% of WO3 and MoO3, these polyhedra start to condensate into clusters forming more ordered glass structure. This in turn hinders the transport of lithium ions and limits the ionic conductivity.

Lithium phosphate glasses ; Ionic conductivtiy ; Glass structure

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