Battery Management Systems for Large Lithium-Ion Battery Packs
Section 1.2 Addenda
There are two mechanisms that result in cell expansion:
- During charging, the active material in the anode expands slightly as it receives Lithium ions (through intercalation); this process is reversed during discharge. This expansion is proportional to the SOC.
- When the cell is nearly full, any further charging will result in part of the electrolyte to be converted to a gas; that gas is converted back to a liquid electrolyte when the cell SOC drops below 100 %. How much gas is produced depends on the chemistry of the cell, and varies considerably from manufacturer to manufacturer. The presence of this gas increases the pressure inside the cell, which may result in cell expansion (which can be quite large). Overcharging will result in this conversion of electrolyte into gas to continue, often resulting in permanent deformation of the cell, and in release of the gas, either in a controlled manner, or in an explosion.
Note that that second mechanism of expansion only occurs when the STATE of the cell is around 100 % SOC. This point is very important. The expansion is NOT due to heat, and it is NOT due to the act of charging. The expansion is the same regardless of cell temperature (cooling won't prevent the expansion) and cell current (at a given SOC level, the expansion is the same, regardless of whether the cell is charging, discharging or sitting on a shelf).
Cylindrical cells will inherently contain that expansion. Pouch cells won't do so, so they must be contained. Prismatic cells will contain expansion to a small extent, so they too need to be contained. How much containment is required is up for debate, and I discuss it in the addenda to section 18.104.22.168.2. The jest of it is that you need to allow for some expansion (for mechanism 1) , but not too much (for mechanism 2), and the answer depends on the particular model of cell.