Abstract
Identifying the factors influencing the movement of sodium cations (Na+) in glasses accelerates the possible options of glass-based solid electrolyte materials for their applications as a promising electrolyte material in sodium-ion batteries. Nevertheless, due to the poor correlation between the structure and conductivity in glass materials, identifying the factors governing the conductivity still exists as a challenging task. Herein, we have investigated the DC-conductivity variations by correlating the structure and conductivity in sodium superionic conductor (NASICON) based Na3Al2P3O12 (NAP) glass (mol%: 37.5 P2O5—25.0 Al2O3—37.5 Na2O) due to the successive substitution of Na2SO4 for Al2O3. Structural variations have been identified using the Raman and magic-angle spinning nuclear magnetic resonance (MAS-NMR) (for 31P, 23Na, and 27Al nuclei) and conductivity measurements have been done using the impedance spectroscopy. From the ac-conductivity spectra, the correlations between mean square displacement (MSD) and dc-conductivity and between the Na+ concentration and dc-conductivity have also been evaluated. Raman spectra reveal that the increase in the Na2SO4 concentration increases the number of isolated SO42− sulfate groups that are charge compensated by the Na+ cations in the NAP glass. MAS-NMR spectra reveal that the increase in Na2SO4 concentration increases the concentration of non-bridging oxygens and further neither S-O-P nor S-O-Al bonds are formed. Impedance spectroscopy reveals that, at 373 K, the DC conductivity of the NAP glass increases with increasing the Na2SO4 up to 7.5 mol% and then decreases with the further increase. In the present study, we have shown that the mobility of sodium cations played a significant role in enhancing the ionic-conductivity. Further, we have shown that inter-ionic Coulombic interactions and the structural modification with the formation of SO42− units significantly influence the critical hopping length < R2 (tp)> of the sodium cations and consequently the mobility and the ionic conductivity. The present study clearly indicates that, based on the compositions, glass materials can also be treated as strong-electrolyte materials.
Highlights
Owing to the slow progress in the pace of battery development, the unprecedented development in the field of electronics over the past few decades has not had a full impact on new technologies (Armand and Tarascon, 2008)
The present study clearly suggests that Na2SO4 has a significant effect on the ionic conductivity, which increases from 3.75 × 10−8 S cm−1 to 1.39 × 10−7 S cm−1 with the 7.5 mol% substitutions for Al2O3 in NAP glass at 100 C
magic-angle spinning nuclear magnetic resonance (MAS-NMR) indicates that the addition of Na2SO4 significantly reduces the connectivity of the aluminophosphate network within the AlPO4 groups through the formation of Q0(2Al) units over Q0(3Al) units and increases the connectivity within the phosphate units through the formation of Q1(0Al) units
Summary
Owing to the slow progress in the pace of battery development, the unprecedented development in the field of electronics over the past few decades has not had a full impact on new technologies (Armand and Tarascon, 2008). It is clearly understood that the factors governing the enhanced ionic conductivity in glass materials absolutely depend on the concentration, mobility, and on network structure of glass. NASICON based Na3Al2P3O12 (NAP) glass is studied for its structure and electrical properties and found that the conductivity can be enhanced by modifying its chemical compositions (Keshri et al, 2021; Mandal et al, 2021). Na2SO4 has been selected for the substitution of Al2O3 in the NAP glass: (1) increases the concentration of sodium cations, (2) decreases the activation energy due to weaker bond strength of Na+ with SO42− units, and (3) modify the structure of NAP glass. This experiment was performed in the air atmosphere using two platinum probes in a temperature range of 323–473 K to collect temperaturedependent EIS data
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