How does the choice of cell format influence battery pack mass?
The pouch cell needs the most support and fixing, followed by the prismatic and the most self contained mechanically is the cylindrical cell.
We analysed the data in our Battery Pack Database to limit it to just NCA and NMC chemistries. Battery pack mass versus total energy. Below you can see datapoints for each format and a best straight line fit.
OK, there isn’t a great amount of data, but you can see that for the larger battery packs the prismatic cell results in a heavier battery pack. For a 100kWh battery pack the prismatic cell results in the pack being 50kg heavier than a cylindrical or pouch based pack of the same capacity.
For smaller battery packs the lightest option appears to be those based on a cylindrical cell format.
The prismatic module has a very similar construction to that of the pouch cell module.
This means that there is some duplication between the cell case and module structure.
The cylindrical cell is self contained in terms of the mechanics required to apply pressure to the active layers.
Hence this is perhaps the simplest module mechanically.
In conclusion the cylindrical cell results in the lightest battery pack across the battery pack energy range.
As the cell is charged lithium ions move into the graphite anode and the cell will increase in thickness. Silicon in the anode will increase this swelling significantly. The layers of the cell are likely to fatigue and fracture over a lifetime of charging and discharging. The external pressure can help to maintain the contact of the layers over time.
Also, gas generation can cause the active layers to delaminate, hence reducing the active working area of the cell and reducing capacity and power capability. Applying a pressure normal to the active planes will keep the layers working together.