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

The cathode surface of sulfide-based all-solid-state batteries (SBs) is commonly coated with amorphous-LiNbO₃ in order to stabilize charge-discharge reactions. However, high-voltage charging diminishes the advantages, which is caused by problems with the amorphous-LiNbO₃ coating layer. This study has investigated the degradation of amorphous-LiNbO₃ coating layer directly during the high-voltage charging of SBs. O₂ generation via Li extraction from the amorphous-LiNbO₃ coating layer is observed using electrochemical gas analysis and electrochemical X-ray photoelectron spectroscopy. This O₂ 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_xP_{1- x}O₃) reduces O₂ release, leading to stable high-voltage charge-discharge reactions of SBs. The results have emphasized that the suppression of O₂ generation is a key factor in improving the energy density of SBs.