THE RECIPE

A salt in a mix of solvents

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Salt LiPF₆, about 1 mol per litre: supplies the lithium ions

Cyclic carbonate Ethylene carbonate (EC): dissolves the salt and helps build the SEI

Linear carbonates DMC, EMC or DEC: thin the mix so ions move fast

Additives A few percent of VC, FEC and others to protect the electrodes

No water: lithium-ion voltages would split it apart.

THE SEPARATOR

A plastic film full of holes

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Thin

About 10–25 µm of polyethylene and/or polypropylene

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Porous

About 40% holes, so the electrolyte and ions pass through

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Shutdown

Polyethylene melts near 130 °C and closes its pores

The separator is the cell's most important safety part.

THE FOILS

Current collectors

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Aluminium (cathode)

About 10–15 µm, light and cheap, stable at high potential

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Copper (anode)

About 6–10 µm; aluminium would alloy with lithium at low potential

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Tabs

Uncoated strips welded to the cell's terminals

Ions take the electrolyte; electrons take the foils.

WHO DOES WHAT

Which part does this job?

Electrolyte, separator, current collector or binder: every part of the stack has one job.

A cell only works when every layer does its part.

THE WEAK SPOT

Why the electrolyte burns

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Carbonate solvents are flammable, and LiPF₆ reacts with traces of water to make hydrofluoric acid. That is why cells are built in dry rooms and why safer electrolytes, from flame-retardant additives to solids, are a hot research topic.

Much of a lithium-ion cell's risk lives in its liquid.

UNIT 8 STUDY COMPLETE

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Ready for the Fab Challenge?

You've looked at everything between and behind the electrodes: the electrolyte, the separator, the foils and the binder.