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

Abstract

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 × 10⁻⁶ S cm⁻¹ at 30 °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.