Halide and Sulfide Electrolytes in Cathode Composites for Sodium All-Solid-State Batteries and their Stability.

Sodium all-solid-state batteries may become a novel storage technology overcoming the safety and energy density issues of (liquid-based) sodium ion batteries at low cost and good resource availability. However, compared to liquid electrolyte cells, contact issues and capacity losses due to interface reactions leading to high cell resistance are still a problem in solid-state batteries. In particular, sulfide-based electrolytes, which show very high ionic conductivity and good malleability, exhibit degradation reactions at the interface with electrode materials and carbon additives. A new group of solid electrolytes, i.e., sodium halides, shows wider potential windows and better stability at typical cathode potentials. A detailed investigation of the interface reactions of Na 3 SbS 4 and Na 2.4 Er 0.4 Zr 0.6 Cl 6 as catholytes in cathodes and their cycling performance in full cells is performed. X-ray spectroscopy, time-of-flight spectrometry, and impedance spectroscopy are used to study the interface of each catholyte with a transition metal oxide cathode active material. In addition, impedance measurements were used to study the separator electrolyte Na 3 SbS 4 with the catholyte Na 2.4 Er 0.4 Zr 0.6 Cl 6 . In conclusion, cathodes with Na 2.4 Er 0.4 Zr 0.6 Cl 6 show a higher stability at low C-rates, resulting in lower interfacial resistance and improved cycling performance.