In this work an experimental setup for in-operando temperature measurements across the interfaces anode – separator/electrolyte – cathode of a lithium-ion battery cell is developed to get a better understanding of the heat generating mechanisms. The results show differences in the heat evolution rates of the anode, the separator and the cathode according to the electrochemical reactions, the state of charge, the overvoltage and the electric current density. The LiCoO2 cathode was identified as the most decisive component for the heat evolution in the investigated battery stack. Changes of the ohmic resistance and the entropy of LiCoO2 with the state of charge were reflected by the temperature measurements.