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In pursuit of novel and cost-effective materials for the development of solid-state batteries as energy storage systems characterized by high safety, high energy density, and long cycling, sodium phosphate-based glasses have emerged as potential candidates for use as solid-state electrolytes1. However, simple binary sodium phosphate glass is limited by its poor chemical durability and low conductivity, nonetheless, these limitations can be easily overcome by changing its composition, e.g. by incorporating an intermediate oxide (Al2O3) and/or another (conditional) glass-forming oxide (Nb2O5) that strengthen the glass network by cross-linking phosphate groups via mixed P–O–M (M = Al, Nb) bonds2. What's more, the formation of a mixed network can have a beneficial effect on sodium ion transport, thereby increasing ionic conductivity, which is referred to as the mixed-glass forming effect (MGFE)3. The aim of this study is to investigate the effects of structural changes, brought by stepwise substitution of P2O5 by Nb2O5, on the electrical properties of two series of glasses from system Na2O–(Al2O3)–P2O5–Nb2O5. The glass-forming region (GFR) is determined by the conventional melt- quenching technique, and the amorphous character of the obtained glasses is confirmed by PXRD analysis. Partially crystallized samples formed at the edge of the GFR due to spontaneous crystallization at high Nb2O5 content are additionally qualitatively analysed. The thermal behaviour of prepared glass-(ceramics) is investigated by DTA, while their (micro)structural properties are evaluated by SEM-EDS, Raman, and IR-ATR spectroscopy. Solid-state impedance spectroscopy (SS-IS) over a broad frequency range (0.01 Hz to 1 MHz) and temperature range (–90 °C to 240 °C) is employed for detailed study of electrical transport. Findings of this research show a significant improvement in ionic conductivity, reaching a maximum at 20 mol% Nb2O5 in both glass series confirming the MGFE. The influence of structural changes induced by introduction of Nb2O5 and Al2O3 on the physical and chemical properties of these glass-(ceramics) is comprehensively studied and explained. This work is supported by the Croatian Science Foundation under the project IP-2018-01-5425. 1. S. Gandi et al., J. Power Sources 521 (2022) 230930. 2. Z. Teixeira et al., J. Am. Ceram. Soc. 90 (2007) 256–263. 3. S. Renka et al., Phys. Chem. Chem. Phys. 23 (2021) 9761–9772.

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.

Sodium phosphate-based glasses have emerged as safer, cheaper and environmentally friendly materials for the development of solid electrolytes for solid-state batteries. Significant improvement in their electrical conductivity can be achieved by the addition of a second oxide, such as Al2O3 and/or Nb2O5, that causes a maximum conductivity at a certain glass composition, a phenomenon known as the mixed glass former effect (MGFE). The aim of this research is to verify the MGFE and to study the relationship between the composition, structure and electrical properties of the glass system Na2O-(Al2O3)-P2O5-Nb2O5.

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 families of 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. This phenomenon is well-known as the 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 (0.01 Hz – 1 MHz) and temperature (-90 °C – 250 °C) 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, being five and four orders of magnitude for WO3 and MoO3, respectively. On the other hand, the enhancement of sodium ion conductivity was four orders of magnitude in the case of WO3 and three in the case of MoO3. Such difference is related to a smaller ionic radius and therefore higher mobility of lithium ions which can be more easily facilitated by the incorporation of tungsten and molybdenum units in the glass network. 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.

Oxide glasses containing transition metal oxides (TMO) such as V2O5, MoO3, WO3, and Fe2O3 exhibit electronic conductivity with a small polaron hopping mechanism. The polaronic conductivity in these glasses arises from the transfer of electrons between TM ions in different oxidation states and depends on the amount of TMO, the fraction of reduced TM ions, and the average distance between TM ions. In this study, we investigate the influence of the addition of a second TMO on the polaronic conductivity of vanadium phosphate glasses. For that purpose, a series of ternary xFe2O3–(60–x)V2O5–40P2O5 glasses with a wide range of compositions, x=10-45 mol%, were prepared by the melt quenching method. The structure and electrical properties of the prepared glasses were studied by Raman and impedance spectroscopy, whereas Differential Thermal Analysis investigated thermal properties. The sum fraction of transition metal ions in a lower oxidation state was determined from the temperature dependence of magnetization measured using a SQUID magnetometer. With the addition of Fe2O3, conductivity shows a deep minimum at 27 mol% of Fe2O3 followed by a substantial increase. While a decrease in conductivity is related to the decrease in the number density of vanadium ions which govern the polaronic transport in this compositional range, the increase at higher Fe2O3 fractions originates from two effects: i) active contribution of iron ions to the conduction process via polaron hopping between various transition metal ions (Fe2+−Fe3+, Fe2+−V5+ and V4+ −Fe3+) in the glass network and ii) nanocrystallization of mixed-valence Fe7(PO4)6 which occurs spontaneously during glass preparation. In particular, it was found that a high polaronic conductivity of glass-ceramic containing 45 mol% of Fe2O3 results from the transport of polarons along and through the nanocrystalline Fe7(PO4)6 which contains both Fe2+ and Fe3+ ions.

Electronic conduction has been demonstrated in various oxide glasses containing transition metal oxides (TMO) such as V2O5, MoO3, WO3, and Fe2O3. The electronic conductivity in these glasses arises from small polaron hopping between TM ions in different oxidation states and depends on the amount of TMO, the proportion of TM ions in different oxidation states, and the average distance between these ions. In this study, we investigate the influence of the addition of the second TMO on the electronic conductivity in vanadate-phosphate glasses. For that purpose, two series of ternary (60-x)V2O5–xTMO–40P2O5 glasses with a wide range of compositions: x = 0-60 mol% for TMO=WO3 and MoO3, were prepared by the melt quenching method. The structure and electrical properties of the prepared glasses were studied by Raman and impedance spectroscopy, and the thermal properties by Differential Thermal Analysis. The sum fraction of transition metal ions in the lower oxidation state was determined from the temperature dependence of magnetization. Interestingly, glass series show different behavior in the gradual substitution of V2O5 by TMO. While the addition of MoO3 conductivity continuously decreases over three orders of magnitude, it shows a deep minimum at 53 mol% WO3. The observed conductivity decrease over the entire range of mixed compositions in the case of MoO3 and WO3 is directly related to the decrease in the concentration of vanadium ions which play a dominant role in the polaron transport. It seems that molybdenum and tungsten ions in these glasses do not contribute to the electronic transport as much as vanadium and hence can not compensate for the decrease in V2O5 content. The high conductivity of binary WO3-P2O5 glass is related to the structural peculiarities of this glass system evidenced in the tendency of tungsten units to form clusters within which the transport of polarons is facilitated.

Oxide glasses containing transition metal oxides (TMO) such as V2O5, MoO3, WO3, and Fe2O3 exhibit electronic conductivity with a small polaron hopping mechanism. The polaronic conductivity in these glasses arises from the transfer of electrons between TM ions in different oxidation states and depends on the amount of TMO, the fraction of reduced TM ions, and the average distance between TM ions. In this study, we investigate the influence of the addition of a second TMO on the polaronic conductivity of vanadium phosphate glasses. For that purpose, three series of ternary xTMO–(60–x)V2O5–40P2O5 glasses with a wide range of compositions, x=10-60 mol% for TMO=WO3 and MoO3 and x=10-45 mol% for TMO=Fe2O3, were prepared by the melt quenching method. The structure and electrical properties of the prepared glasses were studied by Raman and impedance spectroscopy, whereas Differential Thermal Analysis investigated thermal properties. The sum fraction of transition metal ions in a lower oxidation state was determined from the temperature dependence of magnetization measured using SQUID magnetometer. Each glass series shows different electrical behaviour upon gradual substitution of V2O5 by TMO. While with the addition of MoO3, conductivity continuously decreases over three orders of magnitude, it shows a deep minimum at ~53 mol% of WO3 and ~27 mol% of Fe2O3. The observed decrease in conductivity over the entire range of mixed MoO3-V2O5-P2O5 and WO3-V2O5-P2O5 compositions is directly related to a decrease in the concentration of vanadium ions which have a determining role in the electrical processes. It seems that molybdenum and tungsten ions do not participate in the polaron transport in these glasses and hence cannot compensate for the decrease in V2O5 content. However, the high conductivity of binary 40WO3-60P2O5 glass is related to the structural peculiarities of this glass system manifested by the tendency of tungsten units to form clusters within which the transport of polarons is facilitated. On the other hand, a substantial increase in conductivity above 27 mol% of Fe2O3 suggests that iron ions actively contribute to the conduction process. Since the binary Fe2O3-P2O5 glasses exhibit significantly lower polaronic conductivity (

Owning to the good thermal and electrochemical stability, lithium phosphate glasses are nowadays investigated as potential cathode and electrolyte materials in solid-state batteries. However, most of the glassy materials suffer from relatively low electrical conductivity which has been, in some cases, found to be greatly improved by the structural modification induced by the addition of second conventional or conditional glass-forming oxide. In this study, we report a huge enhancement of lithium ion conductivity arising with the replacement of P2O5 with MoO3 and WO3 in two glass series prepared by melt quenching technique: 40Li2O–xMoO3–(60-x)P2O5 and 40Li2O–xWO3–(60- x)P2O5 ; x=0-50 mol%. The electrical properties of these glasses were studied by the impedance spectroscopy (in a range of 0.01 Hz to 1 MHz and -90 °C to 240 °C) while the structural properties were evaluated by 31P MAS-NMR and Raman spectroscopies. As shown in Figure 1., the addition of WO3 and MoO3 significantly increases DC conductivity for five orders of magnitude in the case of tungsten and more than three in the case of molybdenum oxide. Also, the increase is continuous up to 40 mol% of WO3 and MoO3 while the conductivity remains nearly the same value for the highest content. The obtained electrical properties were found to be structurally driven in a way that incorporation of WO6 and MoO6 octahedra in phosphate glass network forms a more disordered and opened structural network which eases up the mobility of Li+ ions. However, at the 50 mol% structural network is dominated by three- dimensional clusters of WO6 and MoO6 octahedra which tends to limit the mobility of ions and hence their ionic conductivity.

Electronic conduction has been demonstrated in various oxide glasses containing transition metal oxides (TMO) such as V2O5, MoO3, WO3, and Fe2O3. The electronic conductivity in these glasses arises from small polaron hopping between TM ions in different oxidation states and depends on the amount of TMO, the fraction of reduced TM ions, and the average distance between TM ions. In this study, we investigate the influence of the addition of a second TMO on the polaronic conductivity of vanadium phosphate glasses. For that purpose, three series of ternary xTMO–(60–x)V2O5–40P2O5 glasses with a wide range of compositions, x=10-60 mol% for TMO=WO3 and MoO3 and x=10-45 mol% for TMO=Fe2O3, were prepared by the melt quenching method. The structure and electrical properties of the prepared glasses were studied by Raman and impedance spectroscopy whereas thermal properties were investigated by Differential Thermal Analysis. The sum fraction of transition metal ions in a lower oxidation state was determined from the temperature dependence of magnetization measured using SQUID magnetometer. Interestingly, each glass series shows different electrical behavior upon gradual substitution of V2O5 by TMO. While with the addition of MoO3 conductivity continuously decreases over three orders of magnitude, it shows a deep minimum at 53 mol% of WO3 and 27 mol% of Fe2O3. The observed decrease in conductivity over the entire range of mixed MoO3-V2O5-P2O5 and WO3-V2O5-P2O5 compositions is directly related to a decrease in the concentration of vanadium ions which play a major role in the polaron transport. It seems that molybdenum and tungsten ions do not participate in the polaron transport in these glasses and hence cannot compensate for the decrease in V2O5 content. The high conductivity of binary 40WO3-60P2O5 glass, however, is related to the structural peculiarities of this glass system manifested by the tendency of tungsten units to form clusters within which the transport of polarons is facilitated. On the other hand, a strong increase in conductivity above 27 mol% of Fe2O3 suggests that iron ions actively contribute to the conduction process. Since the binary Fe2O3-P2O5 glasses exhibit significantly lower polaronic conductivity (10-9 (Ω cm)-1) than our mixed Fe2O3-V2O5-P2O5 compositions it can be inferred that a high conductivity of the latter glasses originates from the polaron hopping between various transition metal ions (V4+−V5+, Fe2+−Fe3+, Fe2+−V5+ and V4+−Fe3+).

Nowadays, sodium phosphate glasses are widely investigated as a cheap and easily accessible material for application in electrochemical devices. However, these glasses usually suffer from low electrical conductivity and a significant research effort has been undertaken to improve this property. In this study we show that the increase in sodium mobility can be achieved in a simple manner, by the addition of WO3 and MoO3 which depolymerize condense phosphate glass network and hence facilitate transport of Na+ ions thus improving overall electrical conductivity. The research was conducted on two glass series, 40Na2O–xMoO3–(60-x)P2O5 and 40Na2O–xWO3–(60- x)P2O5 ; x=0-50 mol% prepared by melt-quenching. The electrical properties were studied by impedance spectroscopy in a wide frequency (0.01 Hz – 1 MHz) and temperature (-90 °C – 250 °C) range. The results showed a significant increase in electrical conductivity with the addition of transition metal ions, being four orders of magnitude in the case of WO3 and three in the case of MoO3. However, the trend in conductivity was not linear, yet exhibited the maximal value at 30 mol% of MoO3 and 40 mol% of WO3 (Figure 1). In order to follow structural evolution, 31P MAS-NMR spectroscopy was used where the compositional dependence of P-O-Mo/W linkages exhibited a similar trend as the electrical conductivity. This led us to conclude that the maximally interconnected phosphate and molybdenum/tungsten units form a favourable environment for fast sodium ion transport in contrast to the predominantly phosphate or molybdenu/tungsten glass network. Furthermore, tungsten oxide was found to be more interconnected with the various P2O5 units inducing therefore a higher electrical conductivity of the Na2O–WO3–P2O5 glasses.

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.