Measuring inhomogeneous pressure distribution in lithium-ion pouch cells during long-term ageing

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Summary:

Several studies have shown that applying the right amount of mechanical pressure on lithium-ion batteries can reduce their aging drastically. In research, this pressure is usually applied using stiff plates that apply either a nearly constant force by using springs or a constant strain with fixed rods. In contrast, the industry uses cushioning pads in their battery modules to absorb reversible and irreversible expansions of the battery cell. Designing cushioning pads with an optimized stress strain curve for the module application could increase battery lifetime significantly. Furthermore, most cells show locally different current densities causing inhomogeneous degradation.

This presentation focuses on a low cost, multi-point spatially resolved pressure measurement in a custom cell tensioning construction while using cushioning foams. Those measurements are carried out while aging cells in long-term storing and cycling experiments. The setup described also attempts to homogenize the aging of the cell by applying targeted inhomogeneous pressures. In addition to an evaluation of the tensioning process using the sensor data, this work shows the behavior of the pressure distribution for changing SOC of the cell.

Finally, methods are presented to compare sensor data from the long-term aging tests with post-mortem analyses of the battery cells. Optical electrode images from a flatbed scanner, as well as, spatially resolved thickness measurements of the aged electrode sheets are correlated with anomalies in the long-term pressure measurement.

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