Origin of O2 Generation in Sulfide‐Based All‐Solid‐State Batteries and its Impact on High Energy Density

Abstract The cathode surface of sulfide‐based all‐solid‐state batteries (SBs) is commonly coated with amorphous‐LiNbO 3 in order to stabilize charge–discharge reactions. However, high‐voltage charging diminishes the advantages, which is caused by problems with the amorphous‐LiNbO 3 coating layer. This study has investigated the degradation of amorphous‐LiNbO 3 coating layer directly during the high‐voltage charging of SBs. O 2 generation via Li extraction from the amorphous‐LiNbO 3 coating layer is observed using electrochemical gas analysis and electrochemical X‐ray photoelectron spectroscopy. This O 2 leads to the formation of an oxidative solid electrolyte (SE) around the coating layer and degrades the battery performance. On the other hand, elemental substitution (i.e., amorphous‐LiNb x P 1‐ x O 3 ) reduces O 2 release, leading to stable high‐voltage charge–discharge reactions of SBs. The results have emphasized that the suppression of O 2 generation is a key factor in improving the energy density of SBs.