Potential of Na3AlF6 as solid electrolyte for all-solid-state Na batteries

Na3AlF6 (cryolite) has traditionally been used as an additive to molten Al2O3 to lower its melting temperature. In this study, we explored the potential of Na3AlF6 as a battery material. Since the electrochemical properties of Na3AlF6-Al2O3 melt, such as ion conductivity and transference number, are directly related to the efficiency of Al production, the ion dynamics of Na3AlF6 have been extensively investigated in the liquid state. We examined the ion dynamics of Na3AlF6 in the solid state and demonstrated its potential as a solid electrolyte in all-solid-state Na batteries. The Na+ conductivity of Na3AlF6 was enhanced by forming a Na3AlF6-Na2SiF6 solid solution, achieving a maximum conductivity of 7.1 x 10-6 S cm-1 at 30 degrees C. The conductivity improvement was attributed to Na+ vacancies, presumably introduced as charge compensation for the substitution of Al3+ with Si4+. Molecular dynamics simulation results indicated that the dominant conductive ion in Na3AlF6-Na2SiF6 was Na+, and long-range hopping of F- was still minor even in the presence of F- vacancies. The oxidation stability of Na3AlF6 was excellent, without decomposition even up to 5 V vs. Na+/Na. In contrast, Na3AlF6-Na2SiF6 underwent reductive decomposition below 1 V vs. Na+/Na. The charge-discharge cycle of the all-solid-state Na/NaCrO2 battery was confirmed; however, the cycle was unstable. A Na-rich layer formed at the interphase after polarisation at 0.5 V, which partially explained the poor cycling performance. Owing to the Na+ conductivity and excellent high voltage tolerance, Na3AlF6-based materials can be utilized as coating materials for high voltage cathodes in Na batteries.