(A) study of enhancing the ionic conductivity of a layered solid sodium ion electrolyte by substituting $Fe^3+$

Abstract

Solid electrolytes, although generally exhibit lower ionic conductivity compared to liquid counterparts, have garnered attention in devices where safety is paramount, such as in electric vehicles and Energy Storage Systems (ESS), given the significantly enhanced safety compared to the explosive risk associated with liquid electrolytes. Among solid electrolytes, oxide-based materials offer high oxidative and electrochemical stability. Particularly, layered oxide-based solid electrolytes require lower synthesis sintering temperatures than other oxide-based solid electrolytes and demonstrate decent ionic conductivity along with a straightforward Na-ion path. This work aimed to replace $Zn^2+$ in $Na_2Zn_2TeO_6$(NZTO), which is recognized as having the highest ionic conductivity among layered oxide solid electrolytes, with Fe. By substituting $Fe^3+$ for $Zn^2+$, this substitution attempted to increase ionic carriers, create vacancies at Na sites, and improve ionic conductivity. The effect of the $Fe^3+$ substitution was investigated at various compositions (0.05, 0.1, and 0.15). X-ray diffraction and inductively coupled plasma-mass spectrometry analysis were used to confirm the substitution into NZTO. Ionic conductivity changes were investigated via electrochemical impedance spectroscopy (EIS), which confirmed that ionic conductivity increased when $Fe^3+$ replaced $Zn^2+$ in NZTO. Ionic conductivity has been observed to significantly increase from $0.3961 mS/cm$ in pristine NZTO to $0.6914 mS/cm$ at $20^\circ C$, especially at a 0.1 substitution of Fe. This showed the highest increase in ion conductivity compared to previous studies aiming to enhance ion conductivity in $Na_2Zn_2TeO_6$(NZTO) through the substitution of $Ca^2+$ ions or $Ga^3+$ ions. Above a composition of 0.15 substitution of $Fe^3+$, an alternative crystal structure, $Na_2.9Zn_2.9Fe_1.1Te_2O_12$, was synthesized. This resulted decrease in ionic conductivity. Thus, the highest ionic conductivity can only be obtained by modifying the $Fe^3+$ substitution ratio while taking trade-offs into account. Furthermore, it is crucial to create real cells to determine variations in $Fe^3+$ substitution performance in real cell conditions.