The dynamic performance of lithium-ion cells depends strongly on the underlying loss processes such as contact resistances, charge transfer and diffusive processes. The total of these processes define the frequency depending impedance behaviour. A well-known characterisation method for the impedance of electrochemical systems, such as lithium-ion cells, is the electrochemical impedance spectroscopy (EIS). Combined with equivalent circuit modeling, EIS offers the possibility to quantify the different processes as a function of temperature and state-of-charge (SoC). In further combination with open-circuit voltage measurements, models, describing the complete cell behaviour, can be derived.
However, in commercial cells, only the full cell behaviour in terms of impedance and performance can be studied and characterised. Therefore no statement about the limiting processes in the individual electrode in certain operating situations, e.g. fast charging at different temperatures, is possible. In contrast, experimental cells enable examining the two electrodes separately in view of performance and also impedance behaviour. They are therefore a valuable tool to determine the limiting electrode in various situations and to predict behaviour under diverse operating conditions of lithium-ion cells.
In order to draw reliable conclusions from experimental cell measurements about the commercial pouch cell behaviour, the experimental cell’s behaviour must be representative and the observations must be transferable. For this purpose, a specific investigation must be carried out for different operating points of both cell types. In a next step an evaluation has to be made, to what extent a reliable transfer of results from experimental cells to pouch cells is possible.
In this work we show a systematic comparison between the performance and impedance behaviour of a commercial automotive high-energy pouch cell with 60 Ah capacity and experimental cells. The pouch cell was first subjected to thorough examinations: Beside different constant current charge and discharge cycles also different pulse profiles were performed. Additionally, we measured impedance spectra for a broad spectrum of different SoCs and temperatures. The pouch cell was then opened in a glovebox and experimental cells were built from the extracted materials.
Subsequently, equivalent measurements were carried out on the built experimental cells. In the final step an overall comparison of these measurements is done, whereby different aspects like area-related capacities for different C-rates and area-related impedances are taken into account. The results of the impedance comparisons are then put into context to the results of the performances of both cell types.