New insights into the uptake/release of FTFSI- anions into graphite by means of in situ powder X-ray diffraction

The redox-amphoteric character of graphite enables its utilization as intercalation host for various types of cations and anions to form either donor-type or acceptor-type graphite intercalation compounds (GICs), respectively. While the donor-type GIC LiC6 is commonly used in the field of lithium ion batteries, acceptor-type GICs were suggested for application in dual-ion cells. In this contribution, the electrochemical intercalation/de-intercalation of fluorosulfonyl-(trifluoromethanesulfonyl) imide (FTFSI-) anions into graphite was studied for dual-ion cells during a cyclic voltammetry experiment using in situ powder X-ray diffraction. For the GICs, a series of most dominant stages could be assigned and the periodic repeat distance as well as the FTFSI-gallery height/gallery expansion were determined. The obtained dominant stage numbers of the formed GICs were correlated to cell voltage ranges. Upon charge, a transition of the different stages was observed, while upon discharge stage 1 was still preserved for a broad voltage range. These novel findings indicate different mechanisms for the uptake and release of the anions. (C) 2016 Elsevier B.V. All rights reserved.