New eddy-current sensor setup for high-resolution lithium-ion cell
dilation measurements
Abstract
Lithium-ion cells exhibit a dilation during charge and discharge cycles
and over their lifetime. Quantifying this dilation is a reliable method
to characterize materials and observe degradation mechanisms. Currently,
such experiments base on dial gauges or laser scanners, which are quite
expensive and either unable to measure spatial differences, to apply
pressure simultaneously or take up a lot of space.
In this work, a new sensor and measurement setup is presented that
significantly improves the state of the art. The first priority is a
high resolution to measure the dilation accurately. While the absolute
accuracy of the setup is around 7 µm, the resolution of the sensor is
lower than 4 nm if the cell to sensor distance is less than 3 mm. The
second priority is the design of a space saving jig, which still allows
the application of a uniform pressure. A setup based on
well-characterized foams, which can be compressed to a defined force are
combined with an integrated sensor in the compression plates. Finally,
the third priority is to use only off-the-shelf parts in order to
achieve a cost-efficient product.
The sensor is thoroughly explained and simulated under different
circumstances. A calibration function is developed to map the nonlinear
eddy-current sensor output to the corresponding distance. In the last
part, measurements on lithium-ion cells are conducted and the results
were compared to earlier literature studies.