Laboratory-size LiNi0.8Co0.15Al0.05O2/graphite lithium-ion pouch cells were cycled over 100% DOD at room temperature and 60°C in order to investigate high-temperature degradation mechanisms of this important technology. Capacity fade for the cell was correlated with that for the individual components, using electrochemical analysis of the electrodes and other diagnostic techniques. The high-temperature cell lost 65% of its initial capacity after 140 cycles at 60°C compared to only a 4% loss for the cell cycled at room temperature. Cell ohmic impedance increased significantly with a elevated temperature cycling, resulting in some of loss of capacity at the C/2 rate. However, as determined with slow rate testing of the individual electrodes, the anode retained most of its original capacity, while the cathode lost 65%, even when cycled with a fresh source of lithium. Diagnostic evaluation of cell components including X-ray diffraction (XRD), Raman, CSAFM and suggest capacity loss occurs primarily due to a rise in the impedance of the cathode, especially at the end-of-charge. The impedance rise may be caused in part by a loss of the conductive carbon at the surface of the cathode and/or by an organic film on the surface of the cathode that becomes non-ionically conductive at low lithium content.