Lithium Bis(fluorosulfonyl)imide (LiFSI)

Lithium bis(fluorosulfonyl)imide (LiFSI) is the lithium salt of the bis(fluorosulfonyl)imide anion, (FSO₂)₂N⁻, often written FSI⁻. It is used as the conducting salt of electrolytes for lithium-ion and lithium-metal batteries, where it is studied as an alternative to lithium hexafluorophosphate (LiPF₆), the usual salt of commercial lithium-ion cells.

Carbonate electrolytes for lithium-ion cells. In a mixture of ethylene carbonate and dimethyl carbonate, the ionic conductivity, viscosity and lithium-ion diffusion of LiFSI electrolytes have been compared with those of LiPF₆ electrolytes. In a study of anodes with a high content of micron-sized silicon, carbonate electrolytes based on LiFSI gave better capacity, cycling stability and cycle life than LiPF₆. The difference was attributed to the solid electrolyte interphase (SEI): with LiFSI it had a thin, mainly inorganic inner layer and appeared more homogeneous, more flexible and less resistive.

Corrosion of current collectors. LiFSI promotes the dissolution of aluminium and stainless steel, which limits its use in lithium-ion cells that operate above 4 V versus Li/Li⁺. For stainless steel, a recent study proposes that trace chloride impurities start local pitting, and that the FSI⁻ anion then forms soluble complexes with the dissolved iron, which extends the dissolution. Adding lithium difluoro(oxalato)borate suppressed it.

Concentrated electrolytes for lithium metal. Highly concentrated solutions of LiFSI in 1,2-dimethoxyethane allow a lithium metal anode to cycle at high rate and high Coulombic efficiency without dendrite growth. The authors attribute this to the larger share of solvent bound to lithium ions, which leaves little free solvent, and to the higher concentration of available lithium ions. Concentrated LiFSI–ethylene carbonate electrolytes have been tested with lithium metal, silicon/graphite and LiNi₀.₃₃Mn₀.₃₃Co₀.₃₃O₂ electrodes, but were less compatible with the high-voltage spinel LiNi₀.₅Mn₁.₅O₄.

Localised high-concentration electrolytes. Diluting a concentrated LiFSI–dimethyl carbonate electrolyte with a hydrofluoroether antisolvent gives a localised high-concentration electrolyte. In a study of this system, the antisolvent did more than dilute: it lowered the dielectric constant of the medium, strengthened the inductive effect on the C=O group of dimethyl carbonate, and weakened the binding of the lithium ion to the solvent and to the anion. In lithium–sulfur cells with a localised high-concentration electrolyte based on LiFSI and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), a higher share of LiFSI improved ionic conductivity, interphase formation and compatibility with lithium metal, but lowered sulfur utilisation through side reactions with long-chain polysulfides. There, LiFSI worked best as a minor co-salt.

Ionic liquid electrolytes. LiFSI is dissolved in ionic liquids with the same anion, for example N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide, to give ionic liquid electrolytes that have been studied against lithium metal. LiFSI is also used to prepare FSI⁻ ionic liquids. In one study, alkyl- and (2-alkoxyethyl)trimethylphosphonium bromides were converted into the phosphonium FSI⁻ salts by anion metathesis with LiFSI in dry acetone. The product was taken up in dichloromethane and washed with small portions of water, and the absence of halide was checked with silver nitrate. The ionic liquids were then mixed with LiFSI to give electrolytes with a common anion.

Polymer and solid electrolytes. In dry solid polymer electrolytes based on poly(ethylene oxide), LiFSI gave a higher Coulombic efficiency for lithium plating than earlier dry polymer electrolytes. The improvement was linked to a lithium-rich, highly conductive interphase. Poly(ethylene oxide) has also been combined with a concentrated solution of LiFSI in N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide for all-solid-state lithium-metal cells. With succinonitrile, LiFSI forms the molecular crystal Li{N(SO₂F)₂}(NCCH₂CH₂CN)₂, a solid lithium-ion conductor that has been used as the electrolyte of an all-solid-state lithium battery.