The SEI layer (film) is porous so it can pass small Lithium ions, but at the same time the holes are small enough to prevent further reaction of larger solvent molecules with the active material.
SEI film consumes Lithium, thus reducing the capacity of the cell.
The goal of Cell formation is crating a thin but uniform SEI layer on the anode particles.
SEI layer can break (crack), and new SEI is formed in the cracks.
SEI layer grows because of Coulombic efficiency less than 1.
The SEI formation is attributed to the electrochemical reduction of the solvent and electrolyte salt to generate a complex mixture of surface species including lithium fluoride, lithium carbonate, (CH2OCO2Li)2, ROLi, and ROCO2Li. The compositions of the SEI depend on the electrolyte, and higher temperature leads to thicker and more resistive SEI films with higher concentrations of lithium hexafluoro phosphate decomposition products.
SEI consumes anode material itself (apart from Lithium), creating silicon carbonate.
SEI layer is exponential: the power is proportional to the negated potential difference between the electrode and the electrolyte and is proportional to the temperature.
See Tafel equation for more.
SEI layer grows faster when we at a high state-of-charge (because the potential difference between the anode and the electrolyte is smaller), and also faster when we are charging at high rate.
In practical terms, this means that Both NMC and LFP cells lose 2-3% of capacity when stored at 50% SoC for 10 months and about 5% of capacity when stored at 80-90% SoC at 25 °C:
Time, not cycle count is the dominant factor of SEI thickening.
SEI thickness grows as the square root of the calendar age of the cell
SEI growth is one of the main mechanisms of the Loss of lithium inventory.