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 Na2.4Er0.4Zr0.6Cl6 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 Na2.4Er0.4Zr0.6Cl6. In conclusion, cathodes with Na2.4Er0.4Zr0.6Cl6 show a higher stability at low C-rates, resulting in lower interfacial resistance and improved cycling performance.

Protective NaSICON Interlayer between a Sodium-Tin Alloy Anode and Sulfide-Based Solid Electrolytes for All-Solid-State Sodium Batteries.

This paper presents a suitable combination of different sodium solid electrolytes to surpass the challenge of highly reactive cell components in sodium batteries. The focus is laid on the introduction of ceramic Na3.4Zr2Si2.4P0.6O12 serving as a protective layer for sulfide-based separator electrolytes to avoid the high reactivity with the sodium metal anode. The chemical instability of the anode|sulfide solid electrolyte interface is demonstrated by impedance spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. The Na3.4Zr2Si2.4P0.6O12 disk shows chemical stability with the sodium metal anode as well as the sulfide solid electrolyte. Impedance analysis suggests an electrochemically stable interface. Electron microscopy points to a reaction at the Na3.4Zr2Si2.4P0.6O12 surface toward the sulfide solid electrolyte, which does not seem to affect the performance negatively. The results presented prove the chemical stabilization of the anode-separator interface using a Na3.4Zr2Si2.4P0.6O12 interlayer, which is an important step toward a sodium all-solid-state battery. Due to the applied pressure that is mandatory for battery cells with sulfide-based cathode composite, the use of a brittle ceramic in such cells remains challenging.