Analysis of Temperature Gradients in Lithium-Ion Batteries By Electrothermal Impedance Spectroscopy (ETIS)

Abstract

While the surface temperature (Tsurf) of a lithium-ion cell is measurable by an external temperature sensor, the internal temperature (Tint) remains unknown. The difference between the surface temperature and the internal temperature increases with charging/discharging rate and with cell volume. A thermal model describing the correlation between load and cell temperature is necessitated, and requires reliable parameter settings. The electrothermal impedance spectroscopy (ETIS) is an in-situ method, which determines the thermal impedance and parameterizes a thermal model. ETIS is comparable with electrochemical impedance spectroscopy (EIS), but in ETIS the excitation signal is a sinusoidal heat flow q(t), which leads to a sinusoidal temperature response (Tint(t) and Tsurf(t)). From the input signal q(t) and output signal T(t) the thermal impedance can be calculated. Using a simple thermal model (Figure 1 b)) and the measured thermal impedance (Figure 1 a)), the temperature change and the difference between the internal temperature and the surface temperature can be calculated as a function of the load current. Consequently, the ETIS-Method was applied to a high energy (KOKAM 560 mAh) and to a high power (KOKAM 350 mAh) lithium-ion pouch cell. Both are made of a NCA-LCO blend cathode and a graphite anode, but differ in electrode thickness. The study shows that the simple model from Figure 1 b) sufficiently describes the thermal behavior of both commercial cell types. The parametrized model describes fairly well the thermal gradient from Tsurf to Tint and is capable to quantify the thermal gradients in lithium-ion cells. The thermal resistance Rth of the high energy pouch cell is more than doubled (2.5 K/W) comparing to the high power cell (1 K/W), thus reflecting the influence of electrode thickness to the thermal behavior of a lithium-ion cell. Figure 1