Several high voltage cathode materials such as Li-excess layered oxide Li(Li_xNiCoMn)O₂ and spinel LiNi_0.5Mn_1.5O₄ are considered to be promising candidates for high-energy electrode material for Li-ion batteries, particularly for electric vehicle applications. Nevertheless, many of these high voltage cathodes suffer from poor cycle life and capacity degradation, especially at elevated temperatures. In this work new findings on surface and interface stability affecting the electrochemical longevity of the high voltage cathode material are investigated using a combination of in situ and ex situ imaging and spectroscopic tools, including: transmission X-ray microscopy, synchrotron X-ray absorption spectroscopy, and double-aberration-corrected scanning transmission electron microscopy. These tools unveil that cation migration and subsequent surface structural changes at the atomic levels are majorly responsible for the degradation. Combining the DFT + U calculations with our experimental observations, a correlation between these interface structural instability and the capacity degradation can be established. On the other hand, with the development of solid-state electrolytes, the solid/solid electrode/electrolyte interfaces present some major challenges in diagnosis methods. We will discuss some of the new diagnosis method development as well.