Why Bis(fluorosulfonyl)imide Is a “Magic Anion” for Electrochemistry

Abstract

Ionic liquids consisting of bis(fluorosulfonyl)imide (FSI − ) anion show promise as electrolytes for Li-ion-based electric storage devices, as they exhibit relatively low viscosity, high chemical stability, and form robust solid−electrolyte interphase (SEI) protecting liquid electrolyte from further breakdown on the electrode. These ionic liquids have been reported to inhibit dendrite formation on lithium metal and lithiated graphite electrodes, which also relates to the unusual SEI properties. In this study, we examine the chemistry aspects that may account for this behavior. Radiolysis was used to induce redox reactions of FSI − anions in model systems, and matrix isolation electron paramagnetic resonance was used to identify radical (ion) intermediates generated in these reactions. Our results suggest that qualitative differences between such ionic liquid electrolytes versus common carbonate electrolytes reflect ease of mineralization of the reduced anion without the concurrent generation of organic radicals and/or elimination of gaseous products in side reactions of the corresponding radical intermediates.

Towards safer sodium-ion batteries via organic solvent/ionic liquid based hybrid electrolytes

“…S2, Supplementary Information) showed the same trends as for pure IL both in terms of ESW and absence of sodium plating/stripping. The absence of reversible sodium deposition could be due to the TFSI anion as FSI based IL electrolytes have been shown to reversibly deposit Li þ and Na þ upon cycling [105]. In addition, corrosion of the aluminium plunger working electrode was observed coupled to the introduction of the organic solvents.…”

Electrochemical and in-situ scanning tunneling microscopy studies of bis(fluorosulfonyl)imide and bis(trifluoromethanesulfonyl)imide based ionic liquids on graphite and gold electrodes and lithium salt influence

“…Such a LiF layer has been known as an effective component of SEI in common electrolytes for lithium ion batteries. Besides, the reduction of FSI can produce the radical anion intermediate $SO 2 NX À that lacks of reactivity so that gaseous SO 2 species cannot be released, which is important to the lithium ion batteries from safety point of view [28]. However, the breaking of the SeC bond in TFSI mainly produces the gaseous trifluoromethane and forms a variety of Li salts on the surface [8,9].…”

“…Li salts can be dissociated in ILs and the released Li þ ions can be solvated with three FSI anions or two TFSI anions at room temperature, which may promote Li þ ion transport [27,28]. In addition, AC impedance spectroscopy measurements reveal that presence of FSI anions in ionic liquids can lower the charge transfer resistance of electrode reactions and relieve transport limitation through the surface film [29e31].…”

“…In addition, AC impedance spectroscopy measurements reveal that presence of FSI anions in ionic liquids can lower the charge transfer resistance of electrode reactions and relieve transport limitation through the surface film [29e31]. An explanation based on the chemistry of SEI is also proposed: The SEI structure formed in the FSI based ILs could ease the mineralization of the products during further reduction of the FSI anions, which suppresses the concurrent generation of organic radicals and gaseous products in side reactions [28].…”

Solid polymer electrolyte comprised of lithium salt/ether functionalized ammonium-based polymeric ionic liquid with bis(fluorosulfonyl)imide

“…The bis(fluorosulfonyl)imide ([N(SO 2 F) 2 ] − , FSI − ) anion is a homologue of TFSI − anion, and in recent years both lithium salt (i.e., lithium bis(fluorosulfonyl)imide, LiFSI) and ionic liquids (ILs) with FSI − anion have been intensively studied as electrolyte materials for Li and Li-ion battery [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38], because of its excellent compatibility with various electrodes, such as Li metal [25,29], graphitized carbon [26][27][28], and silicon [33] anodes and LiFePO 4 and LiCoO 2 cathodes in conventional liquid carbonate electrolyte [31,32], room temperature ionic liquids [25][26][27][28][29], ambient temperature molten salt [30,34,35], and polymer electrolyte systems [23,24,36,37]. Additionally, FSI − anion displays better plasticizing effect than TFSI − anion, which could significantly reduce the glass transition temperature (T g ) and increase the ionic conductivities of polymer electrolytes (e.g., lower T g and higher conductivities for both…”