Putting cells under pressure
Controlling the pressure applied to pouch cell batteries is crucial to improving their performance and cycle life. One of two modes of pressure application is typically used: Constant volume (CV) or constant pressure (CP).
Using a stiff frame to inhibit cell expansion does not work completely in practice, since any cell fixture will expand under the high forces developed during cell cycling. Here we call this mode “pseudo-constant volume”.
True constant volume
In this application note, we introduce the possibility of true constant volume pouch cell cycling, by actively controlling the cell thickness in the CompreDrive using a confocal distance sensor. In the new so-called “Direct drive” mode, the cell thickness can be kept constant even when the cell would normally expand as a result of charging, by increasing the pressure accordingly.
Keeping the pressure constant
Cell fixtures usually include springs to level out pressure variation (like in the CompreFrame). However, the pressure will not be completely constant, since the springs will be compressed as a result of the cell expansion during charging, hence increasing the pressure. Here we call this “Pseudo-constant pressure”.
On the other hand, the CompreDrive is designed to actively control the pressure based on the measured force applied to the cell. This enables true constant pressure application, since the servo motor will actively drive up and down to allow for the cell expansion and contraction during cycling.
In the application note below, the four different pressure application modes shown in the figure above are compared by cycling a LiCoO2/graphite pouch cell and monitoring the resulting cell thickness and pressure.
Four pressure modes – one experiment
During true constant pressure mode, the actual measured pressure was completely constant, as expected, while the cell thickness increased by 115 µm as a result of charging. In pseudo-constant pressure mode, however, the pressure was in fact not constant, but increased from 100 kPa to 179 kPa during charging. A more detailed investigation of cell cycling at various constant pressures was published previously.
Perhaps surprisingly, cycling the cell in a stiff cell fixture (pseudo-constant volume mode) resulted in a cell expansion of 95 µm during charging, almost the same as for the spring-loaded system (pseudo-constant pressure mode). Clearly, a passive cell fixture is not enough to suppress the volume changes of a cycling pouch cell through passive means alone.
The exact amount of pressure increase will be very sensitive to the total spring constant of the cell fixture. This is true for all passive setups, both with and without springs. Inconsistent pouch cell cycle life might be caused by variation in the stiffness of the cell fixture.
The pressure rises
The true constant volume mode in the CompreDrive was able to keep the cell thickness completely constant throughout the charge and discharge procedure. The standard deviation of the measured cell thickness during this step was only 32 nm. The pressure required to keep the cell constrained at 100% SOC was 4.2 MPa, an increase by over 4000%!
Read the full application note below for all the details.
To cite this application note, please use: “Karlsson et al., rhd instruments GmbH & Co. KG, Application Note: Introducing: True Constant Volume Pouch Cell Cycling, January 2025, https://docs.rhd-instruments.de/appnotes/application-note-pressure-application-modes.pdf”.