An experimental comparison between commercial lithium-ion pouch and experimental cells

Summary:

The dynamic performance of lithium-ion cells depends strongly on the underlying loss processes such as contact resistances, charge transfer and diffusive processes. The combination of these processes defines 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).
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 independently in view of performance and also impedance behaviour. They are therefore a valuable tool to determine the limiting electrode and processes in various situations and to predict behaviour under a broad spectrum of operating conditions of lithium-ion cells.
In order to draw reliable conclusions from experimental cell measurements about the commercial cell behaviour, the experimental cell’s behaviour must be representative and the observations must be transferable to the commercial pouch cell scale. 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 electrode materials. A standard experimental cell set-up from El-Cell with a 220 μm PP/PE separator and 80 μl LP30 as electrolyte was chosen. Subsequently, equivalent measurements were carried out on the built experimental cells.
In the overall comparison of these measurements, a higher ohmic resistance for experimental cells and a different diffusion behaviour as well as increasing differences for lower SoCs and temperatures was found. These impedance observations are confirmed by the measurements with different currents and current profiles.
In summary we can draw conclusions from experimental to pouch cells for low C-rates and moderate temperatures. At higher C-rates and lower temperatures the differences through to varying thicknesses of the separator and different electrolytes dominate and therefore no reliable conclusions can be drawn from experimental to pouch cell.