Swelling

Example

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Compression pad

A foam sheet between pouch or prismatic cells that keeps a defined pressure on them as they breathe on charge and swell with age. It also provides some thermal separation between neighbours.

Gas generation

Formation of gases — ethylene, hydrogen, carbon monoxide and carbon dioxide — by electrolyte decomposition, mostly during formation and later under high voltage or temperature. In a pouch cell it shows as swelling; in a can it raises pressure until the vent opens.

Pouch cell

A cell sealed in a flexible bag of aluminium laminated with plastic, holding stacked electrode sheets in larger cells or a flattened wound roll in many small ones. It is light and easy to cool, but needs outside support and can visibly swell if gas forms inside.

Silicon anode

A negative electrode that uses silicon, which alloys with lithium and stores roughly ten times more charge per gram than graphite but swells by up to 300 %. Today it is blended into graphite at a few percent to limit the swelling.

Stack pressure

Pressure applied to a cell stack, in production or in use, to keep its layers in contact. Solid-state cells depend on it, because ions can only cross where two solids touch.

Swelling test

Measuring how a cell grows during cycling or storage in one of two fixtures: constant pressure, where springs or a pneumatic press hold a set force and the thickness change is logged, or constant volume, where rigid plates fix the gap and the rising force is logged. Module designers need both to size end plates and compression pads.

References & further reading

Technical references

1. Lithium ion battery degradation: what you need to know

   Physical Chemistry Chemical Physics · 2021

   Technical reference · Degradation mechanisms, modes, operational effects and their interactions.