Visualizing The Chemical Incompatibility of Halide and Sulfide-Based Electrolytes in Solid-State Batteries

Because of the ongoing demand on energy consumption, high energy density batteries are needed. Solid-state batteries can lead to higher energy densities compared to commonly used liquid lithium batteries and avoid the flammable organic solvents. The class of halide electrolyte (Li₃MX₆, M = metal, X = Cl, Br, I) provide promising characteristics in solid-state batteries due to their reasonable conductivity values and their larger electrochemical stability window compared to sulfides. Due to the good conductivity value of 1 mScm^-1, the broad theoretical electrochemical stability window and the water-based synthesis possibility the electrolyte Li₃InCl₆ is a good candidate for the use with high potential cathode materials like LiNi_0.8Mn_0.1Co_0.1O₂. It was shown that the Li₃InCl₆ is not stable at the lithium metal anode, therefore a bilayer setup with a sulfide and a halide layer is commonly used to study halides at the cathode side. Omitting the halide layer and only using a sulfide layer, like Li₆PS₅Cl, lead to a capacity loss over the cycling and decomposition products.<br/><br/>Based on this literature we studied three different cell setups with the halide electrolyte Li₃InCl₆ and sulfide electrolyte Li₆PS₅Cl in combination with the cathode material LiNi_0.8Mn_0.1Co_0.1O₂. Cycling of the cells over 100 cycles revealed the fast capacity fading of the cell setup with the LiNi_0.8Mn_0.1Co_0.1O₂:Li₃InCl₆ cathode composite in direct contact to the Li₆PS₅Cl, whereas the cells with avoiding this triple phase boundary cycle more stable. Within the cycling process, electrochemical impedance spectroscopy was performed and showed a significant increase of the interfacial resistance in the fast capacity fading cell, which was not the case for the pure sulfide and the bilayer cell setup.<br/>For further analyzation of the fast capacity fade, time-of-flight secondary ion mass spectrometry and focused ion beam scanning electron microscopy analysis were performed to understand the interfacial resistance increase. At the buried interface of the cathode composite to the sulfide separator an indium-sulfide species could be detected, which were also slightly less present at the separator-separator interface of the halide and sulfide electrolyte. This indicates the incompatibility of the Li₃InCl₆ and Li₆PS₅Cl electrolytes, which is more pronounced in the presence of LiNi_0.8Mn_0.1Co_0.1O₂.<br/><br/>This work highlights that in the search for new battery components the chemical stability of the materials to each other should not be overseen. Adding more components in a battery cell, like bilayers, increases the number of interfaces and possible degradation areas. Therefore, studies of solid-state batteries should also consider more properties besides electrochemical stability windows of single components.