Correlation of structure and ionic-conductivity in phosphate glass using MAS-NMR and impedance spectroscopy: Influence of sodium salt

In the process of diminishing the safety concerns of sodium-ion batteries, the development of glass-based solid electrolyte materials has received adequate interest. Nevertheless, achieving a high ionic-conductivity at room temperature for glass materials remains a challenging task because of the poor correlation between the conductivity and the glass structure. Here, we attempt to understand the effective influence of NaCl on the structure and ionic-conductivity of the phosphate-based glass network. For this study, $x\mathrm{NaCl}\text{\ensuremath{-}}(100\text{\ensuremath{-}}x$) ($31.725\phantom{\rule{4pt}{0ex}}{\mathrm{Na}}_{2}\mathrm{O}\text{\ensuremath{-}}12.69\phantom{\rule{0.16em}{0ex}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}\text{\ensuremath{-}}31.725\phantom{\rule{0.16em}{0ex}}{\mathrm{P}}_{2}{\mathrm{O}}_{5}\text{\ensuremath{-}}8.46\phantom{\rule{0.16em}{0ex}}\mathrm{NaF}\text{\ensuremath{-}}5.40\phantom{\rule{0.16em}{0ex}}{\mathrm{Na}}_{2}{\mathrm{SO}}_{4}\text{\ensuremath{-}}10\phantom{\rule{0.16em}{0ex}}{\mathrm{MoO}}_{3}$) glass systems (mol %) were selected, where $x=0$, 5, 10, 15, and 20 mol %. To investigate structural changes with the addition of different NaCl concentrations, $^{27}\mathrm{Al}, ^{23}\mathrm{Na}, ^{31}\mathrm{P}$ magic angle spinning nuclear magnetic resonance (MAS-NMR), $^{31}\mathrm{P}$ two-dimensional (2D) phase-adjusted spinning sideband (PASS), and $^{31}\mathrm{P}$ 2D J-resolved NMR techniques and Raman spectroscopic techniques were utilized. Impedance spectroscopy and ac conductivity spectra were used to assess ionic-conductivity and sodium-ion dynamics, respectively. Impedance spectral analysis reveals that the ionic-conductivity of the base glass is increased by 2.4 times (from 1.85 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}7}$ to 4.44 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}7}$ S/cm at 373 K) with the addition of 20 mol % of NaCl. Raman spectra confirm the presence of P\ensuremath{-}O\ensuremath{-}Mo and the absence of Mo\ensuremath{-}O\ensuremath{-}Mo bonds in these glass systems, and $^{31}\mathrm{P}$ 2D J-resolved spectra indicate the absence of P\ensuremath{-}O\ensuremath{-}P bonds. Upon increasing the NaCl concentration, significant changes in the shapes of $^{31}\mathrm{P}$ and $^{27}\mathrm{Al}$ MAS-NMR spectra were observed, indicating the effective influence of NaCl on the distribution of alumina and phosphorus structural units. Irrespective of the temperature, sodium-ion dynamic studies show that the mean-square displacement $\ensuremath{\langle}{R}^{2}({t}_{p})\ensuremath{\rangle}$ decreases with increasing NaCl concentration up to 10 mol % and then increases with a further increase in NaCl concentration. This investigation aids in understanding the sodium-ion dynamics and the structural information of a multicomponent glass system to enhance the room-temperature conductivity.