When spatially resolved models are applied to describe the behavior of lithium-ion pouch cells, the surface temperature of a cell as well as its terminal voltage are usually considered for validation purposes. Examining a cell’s behavior whilst being placed within a climate chamber, the influential heat transfer from the cell to its surroundings can only be estimated due to undirected, turbulent air flow and unknown heat flux along the attached cables.

In the work presented here, a laboratory test setup for infrared thermography measurements on lithium-ion cells is introduced which allows for well-defined thermal test conditions and a controlled, directed air flow of 1 m/s to 3 m/s, whereby an overall temperature measurement accuracy of ±0.1 K is achieved. By monitoring the temperature gradient along additional copper bars mounted on the cell’s terminals, the conductive heat flux between the cell and the attached cables can be calculated. The convective heat transfer from the cell’s surface to the surrounding air can be estimated by calculating characteristic dimensionless groups (i.e. Reynolds, Prandtl and Nusselt numbers) from empirical correlations resembling the studied geometry and flow condition. By combining the two approaches, the cell’s energy balance can be calculated for purposes of both cell characterization and model validation. It turns out that the temperature distribution within the cell can be shifted dramatically depending on the electrical and thermal contact conditions prevailing at the cell’s terminals (see Fig. 1, left). We used additional layers of nickel plated steel pads for provoking a resistance increase between the test bench terminals (copper bars) and the cell terminals (tabs). Further, the positive tab’s response towards the artificial resistance increase differs substantially from the negative one (see Fig. 1, right). In literature, a characteristic higher positive tab temperature is often explained as being dependent on the difference in electrical resistance of the positive and negative current collector and electrode layers based on their electrical conductivity and thickness.¹ However, the difference in resistance of the positive and negative electrodes seems to be superimposed by an additional resistance arising from a varying goodness of electrical contact between the cables and the positive and negative tabs.^2,3

In this work, we extend this approach and investigate four different mechanisms which we assume to be dominating the observed variation in temperature distribution: the contact resistance between the cables and the cell’s tabs, the tab’s electrical bulk resistance, the contact resistance between the tabs and current collectors, and the electrical bulk resistance of the electrodes. To prove our presumptions, a spatially resolved model is prepared for determining the main influencing factors of the studied characteristic inhomogeneous temperature distribution within lithium-ion pouch cells. With the aid of experimental studies carried out by using the presented test setup, the impact of the four mechanisms on the spatial temperature distribution within lithium-ion pouch cells is discussed.

Figure 1. Infrared thermographic measurement data (left) and cut lines along the positive and negative terminal (right) of a 40 Ah pouch cell during a 4C discharge rate at an ambient temperature of 25 °C and an air flow speed of 1 m/s flowing parallel to the cell’s planar surface. Varying the electrical contact resistance by including two (2H) and four nickel-plated steel strips (4H) shows to have a dominant influence on the location of the maximum surface temperature being shifted clearly towards the positive cell terminal, however, not on the absolute value of the measured temperature maximum.

1. U. S. Kim, C. B. Shin, and C. S. Kim, J Power Sources, 180, 909 (2008).

2. J. Yi, U. S. Kim, C. B. Shin, T. Han, and S. Park, J Electrochem Soc, 160, A437 (2013).

3. B. Wu, Z. Li, and J. B. Zhang, J Electrochem Soc, 162, A181 (2015).