LiNi x Co y Mn z O2 cathode materials play a vital role in next-generation lithium-ion batteries because of their high energy density, but they suffer capacity decay at elevated temperatures. Improving the cycling stability of LiNi x Co y Mn z O2 requires a basic comprehension of its capacity fade mechanisms. Here, we investigate the electrochemical performance of LiNi0.6Co0.2Mn0.2O2 at 55 °C under various cutoff voltages and examine the corresponding degradation mechanisms. The discharge capacity of LiNi0.6Co0.2Mn0.2O2 increases from 176.0 mAh g−1 to 218.1 mAh g−1 at 1 C as the cutoff voltage increases from 4.3 V to 4.7 V, whereas the capacity retention decreases from 96.3% to 78.9% after 50 cycles, respectively. Ex situ analyses via high-resolution transmission electron microscopy illuminate structural transformations of cathode materials after electrochemical cycling. This accounts for the increase in charge transfer resistance and capacity loss. In addition, X-ray photoelectron spectroscopy analyses highlight the correlations between the electrochemical performance and interfacial compositions of the cathode/electrolyte interface. These data provide another explanation for the capacity degradation. Our results underscore the value of improving the surface structural stability of cathode materials and reducing the charge transfer resistance of the electrode/electrolyte interface.