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-LiNbxP1- xO3) 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.

Design of Cathode Coating Using Niobate and Phosphate Hybrid Material for Sulfide-Based Solid-State Battery.

Coating the surface of the cathode active material of all-solid-state batteries with sulfide-based solid electrolytes is key for improving and enhancing the battery performance. Although lithium niobate (LiNbO3) is one of the most representative coating materials, its low durability at a highly charged potential and high temperature is an impediment to the realization of high-performance all-solid-state batteries. In this study, we developed new hybrid coating materials consisting of lithium niobate (Li-Nb-O) and lithium phosphate (Li-P-O) and investigated the influence of the ratio of P/(Nb + P) on the durability performance. The cathode half-cells, using a sulfide-based solid electrolyte Li6PS5Cl/cathode active material, LiNi0.5Co0.2Mn0.3O2, coated with the new hybrid coating materials of LiPxNb1-xO3 (x = 0-1), were exposed to harsh conditions (60 °C and 4.55 V vs Li/Li+) for 120 h as a degradation test. P substitution resulted in higher durability and lower interfacial resistance. In particular, the hybrid coating with x = 0.5 performed better, in terms of capacity retention and interfacial resistance, than those with other compositions of niobate and phosphate. The coated cathode active materials were analyzed using various analytical techniques such as scanning electron microscopy/energy-dispersive X-ray spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy (XAS) to elucidate the improvement mechanism. Moreover, the degraded cathodes were observed using time-of-flight secondary-ion mass spectrometry, TEM/electron diffraction, and XAS. These analyses revealed that the Nb-O-P coordination in the hybrid coating material captured O by P. The coordination suppressed the release of O from the coating layer as a decomposition side reaction to realize a higher durability than that of LiNbO3.