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

The Chemical composition of the solid electrolyte interphase (SEJ) layer formed on the surface of lithium metal electrodes cycled in phosphonium bis(florosulfonyl)imide ionic liquid (IL) electroytes 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 reamin relatively unchanged during extended cycling (up to 250 cycles at 1.5 mA.cm(-2), 3 mA h.cm(-2), 50 degrees C). The main components detected by MAS NMR and XPS agter several hundreds of cycles are LiF and breakdown products from the bis(flourosufonyl)imide anion including Li2S. Similarities in chemical composition are observed in the case of the dilute (0.5 mol.kg(-1) of Li salf in IL) and the highly concentrated (3.8 mol.kg(-1) 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 rendency for dendrite formation.