Ethylene carbonate-free electrolytes for Li-ion battery: Study of the solid electrolyte interphases formed on graphite anodes

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We investigated the Solid Electrolyte Interphase (SEI) formed onto graphite using adiponitrile/dimethyl carbonate-based electrolytes with either lithium difluoro(oxalato)borate (LiDFOB) or lithium bis(fluorosulfonyl)imide (LiFSI), with or without fluoroethylene carbonate (FEC) as SEI-forming additive, by a combination of X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy and scanning electron microscopy. Without FEC, LiDFOB leads to a more protective SEI layer than LiFSI. FEC leads to improvements in both cases and, in combination with LiDFOB, allows forming an SEI richer in LiF with an overall impedance lower than without FEC. It also prevents the dissolution of the SEI upon cycling. For the graphite electrodes cycled with the LiFSI electrolytes, the interface is greatly influenced by the presence of FEC. Nevertheless, with or without FEC, the SEI layer thicknesses decreases upon cycling. In presence of FEC though, this effect is mitigated, but localized exfoliation of graphite was observed after 50 cycles.