In many practical cases, the thermal performance is evaluated by the changes of internal resistance (IR), open circuit voltage (OCV) and thickness caused by the thermal tests, especially for the assessment of commercial lithium ion pouch cells. The IR increase, OCV decrease as well as thickness increase are usually observed after the thermal tests. However, these changes could not provide enough information for understanding and improving the battery performance. Here in this report, the electrochemical impedance spectroscopy (EIS) is utilized to further investigate the resistance of commercial lithium ion pouch cells under some practical testing conditions. Two commonly used thermal tests were employed, namely thermal shock and high temperature storage. For the thermal shock test, the cells underwent twenty thermal cyclings, each of which consists of two hours aging at 85 °C and -40 °C, respectively. For the high temperature storage test, the cells were placed in the environment of 60 °C for two weeks.
The cells tested in this work were manufactured with two different processing techniques, namely baking process and hot pressing process. The cells were charged to 4.25V, 4.35V or 4.4V to investigate the effect of aging voltage on the thermal performance. The impedances both before and after the thermal tests were measured and compared. The equivalent circuit was used to fit the Nyquist plot in order to analyze the solid electrolyte interface resistance (Rsei) and charge transfer resistance (Rct). And the sum of Rsei and Rct was used as an indicator of the total resistance (Rt).
It is revealed that Rct increased after both the thermal tests, as a result of the degradation of conduction properties of electrodes and electrolyte due to high temperature duration. However, the changes of Rsei were different through the two thermal tests. Rsei decreased after thermal shock test, while it increased after high temperature storage test. It is widely acknowledged that Rsei is mainly controlled by the composition and structure of the solid electrolyte interface (SEI). Thus, the two thermal tests exhibited quite different effects on SEI. During the thermal shock test, the high temperature could facilitate the grow of SEI, but rapidly lowering the temperature could probably cause damage to SEI, such as stress concentration or even micro cracks. The structural degradation could accumulate with the thermal cycling, and a decreased Rsei was observed after the test. On the other hand, SEI could continuously grow without structural degradation during the high temperature storage, thus leading to an increase of Rsei.
In addition, the increase of Rt is highly dependent on the cell processing technique and voltage. For the thermal shock test, voltage had a pronounced effect on the Rt increase. For example, the Rt increase at 4.4V is about three times larger of that at 4.25V, regardless of the processing techniques. And at the same voltage level, cells manufactured with hot pressing exhibited an increase about twice of those with baking technique. In contrast, for the high temperature storage test, Rt increases were not so distinct between different processing techniques or voltages. Rt increases at 4.4V and 4.35V were nearly the same with baking technique, while it was a bit higher at 4.4V than at 4.35V with hot pressing technique. Like the trend for the same voltage exhibited in the thermal shock test, the cells with hot pressing had a larger Rt increase than those with baking technique. These results imply the differences of the underlying mechanisms of the two thermal tests, and could be used as a reference for proper evaluating the thermal performance against lithium ion cells with different processing and voltage.
The results and discussions in this report are a further analysis, compared to the commonly used method, on the two practical thermal tests. This work is supposed to provide some extra information and guidelines for the lithium ion battery designers, manufactures and researchers, based on which a more detailed investigation work is needed to further understand and improve the thermal performances of lithium ion battery.