Dilatometric Study of the Electrochemical Intercalation of Bis(trifluoromethanesulfonyl) imide and Hexafluorophosphate Anions into Carbon-Based Positive Electrodes

Abstract

Anion intercalation into graphitic carbons typically occurs at high cathode potentials, i.e., above 4.5 V vs. Li/Li+, and is accompanied by a gallery height expansion and volume increase. The volume increase by accommodation of guest ions such as hexafluorophosphate (PF6 -) or bis(trifluoromethanesulfonyl) imide (TFSI-) is in the range of 130% to 140% according to a gallery height increase from 3.35 Å for pristine graphite to 7.74 to 7.99 Å for the graphite intercalation compounds. The gallery height expansion upon intercalation/de-intercalation in turn also results in an expansion/shrinkage behavior of the thickness of the composite graphite cathode which thus influences the mechanical integrity of the electrode and therefore the overall cycling stability. Furthermore, the type of polymeric binder will have a strong influence on the mechanical stability of the graphite-based cathodes. In this work, the expansion and shrinkage behavior of graphite positive electrodes related to PF6 - and TFSI- anion intercalation/de-intercalation into/from graphite was studied with electrochemical dilatometry technique. In particular, the reversibility of the expansion behavior as well as the maximum electrode thickness for the different anions/electrolytes is studied.