Pouch cells look like an aluminium jiffy bag with +ve and -ve terminals protruding from the edge.
They need to be supported mechanically and need a controlled pressure applied to the surface to deliver the power and energy over their lifetime. This is normally achieved by mechanically fixing and supporting the pouches in a well constructed module.
- pouch cell with tab at each end offers one of the lowest internal resistance cell geometries
- very high energy density at cell level
- easy to damage as cell case offers little protection
- maximum thickness of active material stack limited to ~15mm
- difficult to seal around +ve and -ve tabs and hence limits tab thickness
A module will be required that can mechanically support the cells. This needs to maintain the required pressure, support electrical interconnections and manage the venting / failure of the cells in a controlled manner.
A strong thermal interface to the cell is difficult to design. There are examples in industry of every possible cooling arrangement for pouch cells.
The best overall option comes out as Edge Cooling and this is the most common pouch cell cooling system that you will see in battery electric vehicle applications.
As we pursue faster charging and we solve the electrical isolation and thermal conductivity in more cost effective ways the tab cooling approach is likely to become more accepted.
The design of the pouch cell and the optimisation of the cooling system have gone hand in hand. The cell has been optimised to reduce internal resistance and to improve the pathway to the cooling plate. This optimisation of the edge cooled design has also reduced cost, space requirements and mass.
In order to keep the cell working over the long term it is necessary to apply a pressure to the main faces of the pouch cell. Thus keeping the active materials in “contact”. Example:
- LG Chem 51Ah pouch cell: NMC with dimensions of 290 x 160 x 10.6mm
- Compression force (assembly, 30%-75% SoC) 1,200N ~0.25Bar
- Maximum compression force (EOL) 15,000N ~3.2Bar
Stacking versus Winding
Some of the smaller pouch cells have traditionally used a jelly roll of active layers that are then flattened before being wrapped in the pouch cell case. This is low cost, but is not optimal for energy or power density.
- 2021 Audi e-tron GT quattro – LG Chem pouch cell in an LG Chem module
- Nissan Leaf – since the release of the very first Nissan Leaf it has used an AESC pouch cell supported in a “sardine tin” module case.
- Dai, F., Cai, M. Best practices in lithium battery cell preparation and evaluation. Commun Mater 3, 64 (2022)