Material Design of Quaternary Halide Electrolytes

The future of electrochemical energy storage can be considered to be within the development of all-solid-state batteries (ASSBs). The key to enabling this future resides in the result of solid-state electrolytes (SSEs). For lithium-ion battery systems, halide electrolytes have been reported with room temperature ionic conductivities &gt;10-3 S cm-1, good stability against oxidation, and good stability against cathode materials1. We find that the space explored for sodium halide electrolytes has been somewhat limited within the literature and primarily focused on ternary sodium chloride SSEs with the limited investigation of sodium bromide and iodide SSEs 2,3,4. Previous studies have demonstrated that Li-halide systems such as Li3YCl6 (P-3m1) and Li3InCl6 (C2/m) form layered structures with high ionic conductivity. In contrast, ternary halides such as Na3YCl6 often form double perovskite structures (P21/n) with intrinsically low ionic conductivity. Discovering new layered Na-halide systems is crucial to improving the ionic conductivity of this class of materials. This study provides a high-throughput materials design workflow to provide insight into the development of quaternary sodium metal halide SSEs through theoretical calculations of thermodynamic stability, electrochemical stability, stability against common sodium cathodes, transport properties as well as the synthesis of Na6M’M’’X12 SSEs. 3710 compositions are considered in four different space groups,ps, P21/n, P-3m1, P31c, C2/m. Our candidate pool is screened to 25 candidates by filtering for a ground state phase in the C2/m structure, on the convex hull (Ehull=0), no radioactive elements and an insulator (PBEsol bandgap &gt;2 eV). High oxidation potentials are often observed for the candidates indicating stability against cathodes. Further interface thermodynamics stability analysis revealed that there is a driving force for reactions at the interface between the SSEs and cathodes. On-the-fly machine-learning molecular dynamics (MLMD) was used to probe the ionic conductivity of the candidates. Synthesis of promising candidate material Na6CaZrBr12 revealed the existence of a sodium halo spinel phase not initially considered in the initial high-throughput screening. The high-throughput screening and synthesis performed in this study can provide design principles for the development of new SSEs and can be extrapolated to other conducting-ion systems as well as providing a method to expand the search space of known SSE chemistries. <br/><br/>References: <br/>[1] ACS Energy Lett. 2022, 7, 1776-1805 <br/>[2] J. Mater. Chem. A, 2021,9, 23037-23045 <br/>[3] Nat. Commun. 2021, 12, 1256 <br/>[4] J. Phys. Chem. Lett. 2020, 11, 3376−3383