A commonly used material for the pouch cell structure is aluminium laminated film. This pouch material typically consists of a layer of aluminium (Al), coated on the outside with a layer of nylon and/or polyethylene terephthalate (PET) and on the inside with a layer of polypropylene (PP). There are not many configurations of this material out there, most fall under a ~85, ~110, or ~150 µm thickness, depending on thickness of the components and whether PET is present as an outermost layer.
An uncommon, but very challenging condition to root cause is cell pouch corrosion. The aluminium layer of the cell pouch appears to be “rusted” away. Additionally, if the outer nylon & PET layers are no longer intact due to damage to the underlying aluminium, then electrolyte leak will occur in addition to corrosion.
The mechanism of cell pouch corrosion is two-fold:
- Contact between the anode electrode (including anode tab) and the Al portion of the pouch through the internal PP layer.
- Contact between electrolyte and the Al portion of the pouch through the internal PP layer.
When (1) occurs, the Al portion of the pouch can be considered to be an extension of the anode itself because it is at anode potential. If (2) then occurs, the lithium ions contained within the electrolyte become reduced at the contact point and alloy with the Al, forming Li-Al alloy. Because the lithium ions intercalate into the Al and disrupt the crystal lattice structure of the Al, the result is structurally unstable and considered corrosion.
Over time, the corrosion area expands and causes the cell to lose electrolyte, which may cause arcing, loss of isolation in the pack, and a “dead” cell.
To design a cell against cell pouch corrosion, execute the design in a way that reduces the risk of either mechanism (1), mechanism (2), or both, based on the cell design concept.
An example of reducing risk of (1) through tape design is to increase the thickness of tab protection films or burr protection tapes, which provides additional protection against anodic contact as the cell undergoes vibration and shock.
An example of reducing risk of (2) through pouch design is to reduce/optimize pouch cup forming depths wherever possible, which reduces strain and microcracking in the PP layer. However, it should be noted that microcracks in the PP layer naturally develop over the life of a cell as the pouch structure swells/shrinks during charge/discharge, so some degree of electrolyte contact with the Al portion of the pouch over time may be inevitable.
References
- Eric Darcy, Li-Ion Pouch Cell Designs; Are They Ready for Space Applications?, NTRS – NASA Technical Reports Server
See also: Pouch Cell Edge Seal Integrity