As the commercial market is perpetually asking for smaller and lighter batteries, energy density (both gravimetric and volumetric) had become a very important criterion to select a cathode material for both automotive and mobile application. Nowadays, one of the most effective ways to elevate energy density is to raise the cutting off voltages of NMC or LiCoO2 cathodes. On the other hand, the capacity decay rate is much faster when the cutting voltage is higher than 4.5V vs. graphite, as illustrated in Fig. 1. In order to fully understand the mechanism, we’ve conduct multiple characterizations including TEM/SAED, XRD, hard/soft XAS, XPS Spectroscopy, combining with simulation work (first principle calculation), to reveal the degradation procedure of NMC and LiCoO2 during high voltage cycling. It turns out both surface (majorly) and bulk degradation contributes to the capacity fast decay. We hope the results might be helpful to tackle the problem of short cycling life of cells using high voltage NMC and LiCoO2 cathodes.