It becomes easier to tear the separator (via punches or cell deformation) of a cycled (aged) cell than of a new cell.

This happens because reaction products accumulate in cell's pores.

Teared separator leads to a short-circuit.

Part of Cell degradation and Battery safety.

See also:

• Primary risk of overcharge of a cell is catching fire

References

https://pubs.rsc.org/en/content/articlelanding/2017/RA/C7RA11585G