Bistriflimide, systematically known as bis(trifluoromethane)sulfonimide and colloquially known as TFSI is a non-coordinating anion

Non-coordinating anion

Anions that interact weakly with cations are termed non-coordinating anions, although a more accurate term is 'weakly coordinating anion'. Non-coordinating anions are useful in studying the reactivity of electrophilic cations. They are commonly found as counterions for cationic metal complexes...

 with the chemical formula of [(CF₃SO₂)₂N]^-. The anion is widely used in ionic liquid

Ionic liquid

An ionic liquid is a salt in the liquid state. In some contexts, the term has been restricted to salts whose melting point is below some arbitrary temperature, such as . While ordinary liquids such as water and gasoline are predominantly made of electrically neutral molecules, ILs are largely made...

s, since it is less toxic and more stable than more "traditional" counterions such as tetrafluoroborate

Tetrafluoroborate

Tetrafluoroborate is the anion BF4−. This tetrahedral species is isoelectronic with tetrafluoromethane, CF4 and tetrafluoroammonium NF4+, and is valence isoelectronic with many stable and important species including the closely related anion perchlorate, ClO4−...

. Developing a IUPAC name for bistriflimide that indicates the structure and reactivity is challenging, and changes to current names have been proposed.

This anion is also of importance in lithium-ion and lithium metal batteries because of its high dissociation and conductivity. It has the added advantage of suppressing crystallinity in poly(ethylene oxide), which increases the conductivity of that polymer below its melting point at 50C.

The conjugate acid of bistriflimide is a superacid. It is a crystalline compound, but is extremely hygroscopic and deliquescent. Its pK_a value in water cannot be accurately determined but in acetonitrile

Acetonitrile

Acetonitrile is the chemical compound with formula . This colourless liquid is the simplest organic nitrile. It is produced mainly as a byproduct of acrylonitrile manufacture...

 it has been estimated as 0.3 and in 1,2-dichloroethane

1,2-Dichloroethane

The chemical compound 1,2-dichloroethane, commonly known by its old name of ethylene dichloride , is a chlorinated hydrocarbon, mainly used to produce vinyl chloride monomer , the major precursor for PVC production. It is a colourless liquid with a chloroform-like odour...

 -11.9 (relative to the pK_a value of 2,4,6-trinitrophenol, arbitrarily taken as zero).