ORGANIC ELECTROLYTES

Solvents that don't split

TEABF₄ in acetonitrile

Organic electrolytes dissolve a salt such as tetraethylammonium tetrafluoroborate (TEABF₄) in a solvent like acetonitrile or propylene carbonate. With no water to split, the window reaches about 2.5 to 2.7 V. This is the electrolyte in most commercial carbon supercapacitors.

More than double the voltage of water means several times the energy.

THE PRICE OF VOLTAGE

Organic: strengths and catches

Strengths

A wide window near 2.7 V, good low-temperature behavior, and a mature commercial supply chain

Catches

Lower conductivity than water, flammable and sometimes toxic solvents, and trace moisture that causes gas and aging, so cells need dry assembly and tight seals

Bulkier ions and lower conductivity also mean more resistance per cell than in water.

IONIC LIQUIDS

Salts that are liquid on their own

EMIM⁺ TFSI⁻: no solvent

An ionic liquid is a salt that stays molten at or near room temperature. Its bulky, lopsided ions (imidazolium or pyrrolidinium cations paired with anions such as BF₄⁻ or TFSI⁻) don't pack into a crystal easily. No solvent is needed: the liquid is the ions.

Ions with no solvent shell, at very high concentration.

WIDE WINDOW, THICK SYRUP

Ionic liquids: what you gain and give up

Voltage window Often 3 to 4 V, the widest of the liquids

Safety and stability Negligible vapor pressure, non-flammable, and stable at high temperature

Speed Viscous, with conductivity often around 10 mS/cm or lower at room temperature

Cold They thicken sharply as it cools, so power drops

Cost and ion size Expensive, and their large ions need large enough pores

A trade of speed and price for voltage and safety.

LIQUIDS COMPARED

Three liquid families at a glance

Aqueous About 1 V (up to about 3 V water-in-salt) · highest conductivity · cheapest · non-flammable

Organic About 2.5–2.7 V · moderate conductivity · flammable, needs dry assembly

Ionic liquid About 3–4 V · lowest conductivity · costly · non-flammable

Families can be mixed: an ionic liquid diluted in acetonitrile, for example, cuts viscosity while keeping a wide window.

UNIT 22 STUDY COMPLETE

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Ready for the Fab Challenge?

You've covered wider windows from solvents that don't split and salts that stay liquid.