Spectroscopic Characterization of the SEI Layer Formed on Lithium Metal Electrodes in Phosphonium Bis(fluorosulfonyl)imide Ionic Liquid Electrolytes

Abstract

The chemical composition of the solid electrolyte interphase (SEI) layer formed on the surface of lithium metal electrodes cycled in phosphonium bis(fluorosulfonyl)imide ionic liquid (IL) electrolytes are characterized by magic angle spinning nuclear magnetic resonance (MAS NMR), X-ray photoelectron spectroscopy (XPS), fourier transformed infrared spectroscopy, and electrochemical impedance spectroscopy. A multiphase layered structure is revealed, which is shown to remain relatively unchanged during extended cycling (up to 250 cycles at 1.5 mA·cm, 3 mA h·cm, 50 °C). The main components detected by MAS NMR and XPS after several hundreds of cycles are LiF and breakdown products from the bis(fluorosulfonyl)imide anion including LiS. Similarities in chemical composition are observed in the case of the dilute (0.5 mol·kg of Li salt in IL) and the highly concentrated (3.8 mol·kg of Li salt in IL) electrolyte during cycling. The concentrated system is found to promote the formation of a thicker and more uniform SEI with larger amounts of reduced species from the anion. These SEI features are thought to facilitate more stable and efficient Li cycling and a reduced tendency for dendrite formation.

Ionic Liquids – Further Progress on the Fundamental Issues

“…As an example of the more fundamental approach that is needed, Girard et al used various techniques, including magic angle spinning nuclear magnetic resonance (MAS NMR) and X-ray photoelectron spectroscopy (XPS), to study the solidelectrolyte interphase (SEI) layer on the lithium metal anode on cycling in [P 1,1,1,i4 ][fsi]. [27] The main breakdown products were found to be reduced species of the [fsi] À anion and LiF, enhancing the formation of a uniform SEI layer on the lithium anode. The structural degradation of LiCoO 2 beyond 4.2 V versus Li/Li þ in the presence of [C 3 mpyr][NTf 2 ]-based hybrid electrolyte was also examined by Theivaprakasam et al through XRD analysis.…”

Safe, Stable Cycling of Lithium Metal Batteries with Low‐Viscosity, Fire‐Retardant Locally Concentrated Ionic Liquid Electrolytes

“…[ 3 ] They are thus one of the few classes of materials that may meet the stringent requirements of new LMB electrolytes. Indeed, IL electrolytes containing bis(fluorosulfonyl)imide (FSI) anions enhance compatibility with Li metal [ 4 ] building an interface enriched with a fluorinated compound (i.e., LiF) between the electrolyte and Li metal. [ 4a,5 ] Nevertheless, Li‐salt‐containing IL electrolytes still suffer from sluggish Li transport owing to the complicated Li solvation structure and dynamics.…”

“…Indeed, IL electrolytes containing bis(fluorosulfonyl)imide (FSI) anions enhance compatibility with Li metal [ 4 ] building an interface enriched with a fluorinated compound (i.e., LiF) between the electrolyte and Li metal. [ 4a,5 ] Nevertheless, Li‐salt‐containing IL electrolytes still suffer from sluggish Li transport owing to the complicated Li solvation structure and dynamics. For instance, even at moderate Li salt concentration (≈1 m ), ILs significantly increase the electrolyte viscosity, thereby decreasing ionic conductivity tenfold.…”

Simultaneous Stabilization of Potassium Metal and Superoxide in K–O2 Batteries on the Basis of Electrolyte Reactivity

“…The chemical difference in the SEIs is further related to the reduction of anions on the K metal surface. The weak N−S bond in TFSI − anions is cleaved, which leads to a KF‐lean SEI, whereas the reduction of FSI − anions occurs through cleavage of the S−F bond as well as the N−S bond, thus resulting in fluoride‐abundant fragments . Owing to the low solubility (in DME), good chemical stability, and mechanical strength of alkali metal fluorides, the KF‐rich SEI is expected to reduce electrolyte decomposition and suppress K dendrite formation.…”