Commercially available 18650 Li-ion batteries are considered for high-energy-density storage and usage in mobile applications as well as to store energy from intermittent energy sources. This has triggered intense research for suitable electrode and electrolyte materials, while their current state-of-the-art, temperature-dependent performance is hardly described in detail. The fatigue process in two brands of rechargeable commercial high-energy Li-ion batteries (18650-type, 3500 mAh LiNi0.83Mn0.07Co0.11O2 (NMC-811) and LiNi0.86Co0.11Al0.03O2 (NCA)) as a function of cycling temperature has been investigated using in situ neutron powder diffraction (NPD) and electrochemical impedance spectroscopy (EIS). The batteries (∼140) were cycled at conditions specified by the manufacturer and simulated realistic user conditions with good statistics. Cycling temperature (25, 35, and 45 °C) had a significant influence on capacity fade, with 35 °C showing the highest capacity retention. The NCA cells reached the end-of-life condition of 70% of the initial capacity after only 200 cycles when cycling at 25 °C and 1/0.5C discharging/charging rates. EIS showed that the largest increase in impedance came from the charge-transfer resistance for both cell brands, while one brand also showed a sudden increase in the ohmic resistance, which coincides with a sudden capacity drop. From in situ NPD, the decrease in Li content in the cathode and the anode could be traced as a function of cycle number, and this was found to correlate well with the observed discharge capacity. In addition, Rietveld refinement allowed for the detection of changes in the lattices of the anode and cathode, which were linked to trapping of Li ions in the electrodes.