Instantaneous internal temperature monitoring of a commercial 18650 LiCoO2 lithium-ion battery was performed using a single-point EIS measurement. A correlation between the imaginary impedance, –Zimag , and internal temperature at 300Hz was developed that was independent of the battery’s state of charge. An Arrhenius-type dependence was applied, and the activation energy for SEI ionic conductivity was found to be 0.13eV. Two separate temperature-time experiments were conducted with different sequences of temperature, and single-point impedance tests at 300Hz were performed to validate the correlation. Limitations were observed with the upper temperature range (68°C< T <95°C), and consequently a secondary, empirical fit was applied for this upper range to improve accuracy. Average differences between actual and fit temperatures decreased around 3-7°C for the upper range with the secondary correlation. The impedance response at this frequency corresponded to the anode/SEI layer, and the SEI is reported to be thermally stable up to around 100°C, at which point decomposition may occur leading to battery deactivation and/or total failure. It is therefore of great importance to be able to track internal battery temperatures up to this critical point of 100°C, and this work demonstrates an expansion of the single-point EIS diagnostic to these elevated temperatures.