The largest capacity fade occurs during the first weeks of storage and the overall trend exhibits a square-root of-time behavior. This trend is in good correlation with SEI growth, for which a linear increase with the square root of time is generally assumed.

This means that if a cell loses 2.5% of its capacity due to SEI thickening in the first 10 months of calendar age, it should lose 8.6% of its capacity after 10 years of life due to calendar ageing alone.

Part of SEI layer on anode.

Related:

- Time, not cycle count is the dominant factor of SEI thickening
- Storing a cell at a high temperature and state-of-charge accelerates capacity fade