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The Impact of Thermal Gradients on the Aging Behavior of Lithium-Ion Batteries
Cell characterization

Temperature is a dominant parameter in both the performance and aging behavior of lithium-ion batteries. Aging mechanisms in lithium-ion batteries correspond to unwanted side reactions, often associated with film formation on the electrodes. Like all chemical reactions, high temperatures generally accelerate these temperature-dependent aging processes. At low temperatures, on the other hand, the general performance and efficiency of the cells decrease due to, among other things, the increase in internal resistance. In addition, relatively low temperatures favor lithium plating when charging with higher currents. For these reasons, each lithium-ion battery needs a specific temperature window for optimal operation in an application.

However, batteries rarely undergo homogeneous temperatures in applications. Instead, thermal gradients usually form under load, varying in intensity depending on the exact boundary conditions and influencing the cells‘ performance and aging behavior. The dimension-dependent effect of those thermal gradients still needs to be investigated in more detail, but data indicates that especially through-plane gradients can negatively influence battery aging.

This work shows measurement results illustrating the effect of thermal gradients on the aging behavior of a commercial 11,6 Ah nickel-rich lithium-ion pouch cell. Using a carefully designed test setup, we aged multiple battery cells under different thermal gradient conditions and electrical loads. This test setup allows us to precisely control the temperatures on the two large surfaces of the studied pouch cell and, thus, apply a through-plane thermal gradient normal to the electrode stack within the battery cell. This thermal gradient causes an inhomogeneous internal current distribution that leads to different local discharge depths during a discharging and charging routine and may even cause plating in cold areas locally.

Batteries were aged with 1C and 0.5C at homogeneous 25 °C and with thermal through-plane gradients of 5, 10, 20 and 30 K. We present the aging behavior of these aged commercial battery cells by discussing electrical key characteristics like capacity fade and impedance changes and results from post-mortem analyses. Overall, all batteries aged quicker than the datasheet suggests, which might be caused by plating due to the strict thermal boundaries. Nevertheless, the electrical data indicates that the aging behavior of the battery cells accelerates with an increasing thermal gradients. Furthermore, the post-mortem analysis shows that the main aging processes occur on the anode while the nickel-rich cathode largely remain unchanged.

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Dirk Uwe Sauer

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