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.
Achieving high ion conductivity in
glass-based Na-ion conducting
materials for their applications as solid electrolytes in batteries
is still challenging owing to the vague knowledge on the factors governing
Na-ion dynamics. In the present study, an attempt has been made to
identify the factors affecting the sodium-ion dynamics through structure
and conductivity property correlation for the 37.5Na2O–37.5P2O5–15Al2O3–10NaF
(FS-0; mol %) glass system with varied concentrations of Na2SO4. 31P, 27Al, and 23Na MAS NMR (magic-angle spinning nuclear magnetic resonance) and
Raman spectroscopy are employed to assess the structural modifications,
and impedance spectroscopy is used to measure the variations in ionic
conductivity on the addition of Na2SO4 in the
FS-0 glass. Raman spectra and MAS NMR analysis indicate that the quantity
of P–O–Na bonds and sulfate (SO4
2–) units surrounded by sodium increase with increasing Na2SO4 concentration. Impedance analysis reveals that the
conductivity of FS-0 glass enhances by 1 order with the addition of
6 mol % Na2SO4. We identify from the ac-conductivity
spectral analysis that the concentration of charge carriers and the
critical hopping length of mobile cations increase with the addition
of 6 mol % Na2SO4. Overall, we reveal that the
structural modifications, Na-ion concentration, and the shallower
potential well that is created for sodium due to its interaction with
the nearest neighboring cations affect the Na-ion dynamics. The information
obtained from the present study certainly helps to optimize the chemical
composition of glasses demonstrating high ionic conductivity.
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