Tailoring the Lithium Solid Electrolyte Interphase for Highly Concentrated Electrolytes with Direct Exposure to Halogenated Solvents

We investigate the effect of pretreatment of Li metal electrodes with chloroethylene carbonate (CEC). In comparison with either untreated or fluoroethylene carbonate (FEC)-treated Li surfaces, the CEC-treated electrodes exhibit smaller overpotentials and greater stability in symmetric cell cycling using a highly concentrated acetonitrile-LiTFSI-based electrolyte. The origin of the more facile cycling behavior is associated with a thicker SEI originating from a condensation reaction of N- and O-containing reduction products. Full cells constructed using the CEC treatment exhibit cycling behaviors equivalent to those seen using the FEC treatment. Replacing the LiTFSI electrolyte with LiFSI diminishes the salubrious effect of surface treatment by either FEC or CEC due to consequences arising from the more complete decomposition of the FSI^-anion upon Li metal exposure.