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 Na₃SbS₄ and Na_2.4Er_0.4Zr_0.6Cl₆ 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 Na₃SbS₄ with the catholyte Na_2.4Er_0.4Zr_0.6Cl₆. In conclusion, cathodes with Na_2.4Er_0.4Zr_0.6Cl₆ show a higher stability at low C-rates, resulting in lower interfacial resistance and improved cycling performance.