Pouch cell testing with precision
The CompreDrive with CompreCell Pouch allows highly precise pressure and temperature control of pouch cells during testing. The plates of the pouch holder are smooth, planar, level, and do not flex under pressure. This is important in order to avoid that the pressure is preferentially applied to one side, or to the edges of the pouch cell, instead of evenly across the surface. In two previous application notes, we showed how to use this system as a dilatometer, and for impedance spectroscopy studies. In this application note, we employ TekScan pressure mapping sensors to find out how the force is distributed over the pouch cell surface, in a collaboration with CMV Hoven.
Mapping the pressure
By placing pressure mapping sensors from TekScan directly below a pouch cell in the CompreCell Pouch, it is possible to map out the force distribution. To the left, the results for a 2.8 Ah multilayer lithium-ion battery pouch cell under 1 kN load are shown.
Smoothing out hot spots
Since pouch cells are not perfectly flat, pressing them between hard metal plates will focus the pressure onto the tallest points of the pouch cell, e.g. where the tabs are welded to the current collector. Using soft compression pads (compliance layers) can help to smooth out those pressure points. In the lower figure, polyurethane foam pads sandwiched the pouch cell and sensor mat, smoothing out the pressure hot spots.
Uniform force distribution
The fact that the plates of the CompreCell Pouch are very flat and parallel leads to a uniform pressure distribution over the whole surface, without significant deviation left/right, front/back, or center/edge.
Cycling under Constant Volume or Constant Pressure
In pouch cell R&D, there are two common ways of constricting the pouch cell during cycling:
- Constant volume – a stiff cell fixture restricts the pouch thickness, leading to pressure increase as the cell is charged.
- Constant pressure – a flexible cell fixture allows more breathing of the cell.
In reality, if springs are used to keep the pressure constant, there will still be a significant pressure increase during charging, according to the spring constant of the springs and the entire jig. This is the case in the CompreFrame cell fixture, as shown in a previous application note.
To really achieve constant pressure, a force sensor can be employed in a closed-loop feedback control, as for the CompreDrive. As a result, the applied pressure the overall pressure changes are completely avoided, as shown to the right.
In operando pressure distribution monitoring
In addition to the overall force applied to the pouch cell during cycling shown in the plot above, using the pressure mapping sensor we can see exactly how the force is distributed while cycling in either constant volume or constant pressure mode.
To cite this application note, please use: “Karlsson et al, rhd instruments GmbH & Co. KG, Application Note: Mapping Pouch Cell Pressure Distribution in Operando: CompreCell Pouch × TekScan sensor, June 2024, https://docs.rhd-instruments.de/appnotes/application-note_Pouch_Cell_Pressure_Mapping.pdf”.