The internal battery pressure increases at high charging capacities and at high charging speeds, while a negative internal battery pressure occurs when the charging state goes towards zero, and discharging too quickly.
That's why batteries should be long-term-conserved at around 50% of charging level, to ensure minimum internal pressure.
I don't think this is correct unless you can point to a paper on it.
Pouch cells do swell as the SOC increases but this is mechanical and caused by the electrodes themselves swelling, it is not due to internal gas pressure.
I found this paper where they used neutron imaging to see this process happening. The change is tiny and not terribly linear though, pretty much every other method of measuring SOC would be better! They describe their 'dilatometer' measuring device in the paper.
Cells only become pressurised when abused and that's caused by vaporisation and/or decomposition of the electrolytes, vaporisation might be reversible when the cell cools (if the cell doesn't rupture) but decomposition is not.
