Differing Electrolyte Implication on Anion and Cation Intercalation into Graphite

Emerging dual-graphite batteries (DGBs) capture extensive interest for their high output voltage and exceptional cost-effectiveness. Yet, developing electrolytes compatible with both the cathode and anode stands to be a tremendous challenge, and how electrolyte impacts anion and cation intercalation into graphite remains inexplicit or controversial. Herein, we have evaluated the performance of graphite anode and cathode in typical ethyl methyl carbonate (EMC) based electrolytes and unveiled their electrode-electrolyte interphase using Cryogenic transmission electron microscopy (Cryo-TEM). The addition of fluoroethylene carbonate (FEC) brings substantial improvement in cycle stability and Coulombic efficiency for both the graphite cathode and anode, but its implication on cation and anion intercalation differs. FEC is involved in anodic side reactions to produce a LiF-embedded solid-electrolyte interphase layer. It is much thinner and more uniform than that formed in the electrolyte without FEC, which is correlated with less graphite exfoliation and enhanced stability. As for the graphite cathode, both basal and edge planes are largely bare, and only few scattered byproducts are found. In addition, we also reveal layer bending and local lattice disordering of the graphite cathode based on multiple Cryo-TEM images, which are speculated to be caused by high lattice strain induced by anion intercalation and local oxidation under high voltage. The absence of cathode-electrolyte interphase (CEI) layers overturns the paradigm of attributing cathodic performance to CEI features and is regarded as a fundamental reason for severe self-discharge of graphite cathode. FEC helps to alleviate graphite exfoliation issues and enhance cycle stability, and we ascribe it to weakened solvation, which means reduced probability of solvent co-intercalation during charging, rather than compositional changes of cathodic byproducts.