THE FILM

The solid electrolyte interphase

At the anode's low potential, the electrolyte breaks down on the surface on the first charge. The products build a thin film that blocks electrons but lets lithium ions through.

The SEI stops the electrolyte from being eaten forever.

WHAT IT IS

Made of broken electrolyte

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Thickness A few to a few tens of nanometres

Inner layer Dense inorganic salts like LiF, Li₂O and Li₂CO₃

Outer layer Porous organic lithium salts from the solvents

Job Pass Li⁺, block e⁻, stay stable

A good SEI is thin, even, tough and ion-conducting.

THE PRICE

First-cycle loss

The lithium that goes into the SEI never comes back. A graphite cell typically loses around 5–10% of its lithium on the first charge, which is why the first discharge gives back less than the first charge put in.

Cell makers design in extra lithium to pay for the SEI.

BUILDING IT WELL

Additives and formation

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Vinylene carbonate (VC)

Breaks down first and leaves a tougher film

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FEC

Adds LiF, especially helpful on silicon

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Slow first charge

Formation cycling grows the SEI gently and evenly

A cell's lifetime is largely decided in its first few charges.

WHEN IT GOES WRONG

SEI growth is aging

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The SEI keeps growing slowly for the life of the cell, faster when it is hot or kept full. Each bit of growth eats lithium and adds resistance. On silicon, swelling keeps cracking it open so it regrows again and again.

Much of a cell's slow capacity fade is the SEI quietly thickening.

UNIT 9 STUDY COMPLETE

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Ready for the Fab Challenge?

You've watched the SEI form, seen what it costs on the first charge, and learned why a good SEI means a long-lived cell.