The crystal chemistry and reactivity of ternary Na2Fe3Cl8 from the NaCl-FeCl2 system and its potential application as coating layer for cathode in sodium ion batteries

This report explores theoretical and experimental methods to characterize the ternary phases arising from the NaCl + FeCl2 system at both low (150 °C) and high temperatures (550 °C), through milling and evaporation processing techniques. We found that Na2Fe3Cl8 is the only metastable ternary compound produced in either case and only at high temperatures, which is in good qualitative agreement with density functional theory calculations performed with the recent r2SCAN metaGGA functional. The elementary, crystallographic, and grain structure information on Na2Fe3Cl8 collected using a combination of x-ray diffraction, scanning electron microscopy, energy dispersive and Mössbauer spectroscopy is described and discussed in detail. Na2Fe3Cl8 is determined to have a layered trigonal structure in the R3̅m space group. Other ternary stable or metastable ternary phases such as Na6FeCl8 and Na2FeCl4 were not observed, which is likely the result of decomposition occurring beyond 400 °C. While the structure of Na2Fe3Cl8 makes it inadequate as a potential cathode in Na-ion batteries, Na2Fe3Cl8 may operate as suitable coating layer to regulate the passage of ions from electrolytes to active electrode materials without interfacial degradation.