Unraveling the Degradation Mechanism of LiNbO3‐Coated NMC Cathode at High Potential in All‐Solid‐State Batteries Using 10 K Extended X‐ray Absorption Fine Structure Analysis

All solid‐state batteries (ASSBs) utilizing sulfide‐based solid electrolytes hold promise for enhancing battery energy density while mitigating safety concerns, thus meeting the stringent requirements for electric vehicle applications. For practical application of ASSBs, it is important to stabilize the interface between the solid electrolyte and the cathode. Although cathode coated with a thin layer of LiNbO3 allows higher interface stability, which significantly improves charge‐discharge and cycle performance, degradation at high potentials has also been noted. In this study, we focused on the degradation mechanism of LiNbO3‐coated LiNi0.5Co0.2Mn0.3O2 cathode active materials at high potentials by using 3 electrode system for ASSBs which allows separating the impedance measurement of the interface between cathode and solid electrolyte. We performed X‐ray absorption spectroscopy (XAS) measurements at low temperature (10 K) to analyze the local structure around Nb and correlating these findings with impedance measurements. Our results indicate that the impedance of LiNbO3 increased rapidly due to the oxygen desorption reaction at high potentials. This study aims to elucidate the dynamic change and degradation mechanism of LiNbO3‐coated LiNi0.5Co0.2Mn0.3O2 in ASSBs and provide new ideas for the design of interfacial coating materials.