engleski

The improvement of ionic conductivity of sodium phosphate glasses by addition of WO3 and MoO3

In recent years, sodium phosphate glasses have gained considerable interest as potential components of various electrochemical devices. Their chemistry makes them particularly interesting as a replacement for conventional and high cost lithium materials used in solid- state batteries. Although sodium glasses show lower electrical conductivity than lithium ones, the ionic transport can be greatly facilitated by the addition of another conventional or in our case conditional WO3/MoO3 glass-former (so-called mixed glass former effect, MGFE). In this study, we report enhancement of ionic conductivity as a result of compositional and structural changes occurring with the gradual replacement of P2O5 with WO3/MoO3 in 40Na2O– xMoO3–(60-x)P2O5 and 40Na2O–xWO3–(60-x)P2O5 ; x=0-50 mol% glass systems. The electrical properties were studied by impedance spectroscopy in a wide frequency (0.01 Hz – 1 MHz) and temperature (-90 °C – 250 °C) range while the structural features were evaluated by 31P MAS-NMR spectroscopy. From the frequency-independent part of the conductivity spectra, the values of DC conductivity, DC, at various temperatures and activation energy, EDC, for all glasses were determined. Interestingly, the results show non-linear dependence of DC conductivity with increase in WO3 and MoO3 content, with the highest value at 30 mol% of MoO3 and 40 mol% of WO3. The observed increase is more pronounced in tungsten containing glasses resulting in significantly higher electrical conductivity than molybdenum ones. 31P MAS NMR spectroscopy revealed that the changes in glass network i.e. formation of mixed phosphate- molybdate/tungstate units and number of P-O- Mo/W linkages are directly linked to the mobility of sodium ions. Precisely, the compositional dependence of P-O-Mo/W linkages follows similar compositional dependence as does the electrical conductivity, with the same position of maximal values at 30 and 40 mol% of MoO3 and WO3, respectively. Based on this, we assume that the maximally interconnected P2O5 and WO3/MoO3 units provide favourable structural environment for faster sodium transport than predominantly phosphate or predominantly molybdenum/tungsten network. Comparing the two glass series, a higher degree of interconnection with various phosphate units was also found in tungsten containing glasses exhibiting therefore higher electrical conductivity. In the next step, we investigated the frequency-dependent conductivity where the validity of Summerfield scaling for all glasses confirmed time-temperature superposition (TTS) and temperature invariant mechanism of ionic conductivity. More importantly, characteristic hopping length of sodium ions, evaluated from the crossover frequency between DC and AC conductivity, was found to be strongly correlated to the structural features, once again, reaching the highest value for maximally interconnected glass network (30 and 40 mol% of MoO3 and WO3, respectively). This confirmed that the formation of mixed units in our materials significantly enhances the dynamic of sodium ions on both microscopic and macroscopic level.

Phosphate glasses ; Electrical properties ; Mixed glass former effect

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