Sensorless online temperature measurement of lithium-ion cells based on electrochemical impedance spectroscopy (EIS) measurements is introduced and applied to a commercial 2 Ah pouch cell. The method's sensitivity on temperature and state of charge (SOC) is investigated, taking a closer look to the selection of measurement frequency. The internal temperature is determined as precise as ±0.17 K with SOC status known and ±2.5 K with SOC status unknown at isothermal conditions. Furthermore, the influence of a temperature gradient, as it will develop during cell operation, is studied in detail and the simulation results are validated experimentally. Finally, the method's ability to monitor the temperature is demonstrated during a thermal cool down.