Prof. Dr. Martin Winter and his colleagues at the University of Münster report promising results for the use of graphite as both the anode and cathode material in a so-called “dual-graphite” or “dual-carbon” cell (earlier post) based on anion intercalation. Dr. Winter and his colleagues have been working with anion intercalation in dual carbon cells for several years. (Because both cations and anions participate in the charge/discharge reaction and other anode materials than graphite are possible, the Münster team earlier proposed the name “dual-ion cells” for these systems.)
In such a cell, the electrolyte cations intercalate into the graphite anode and the electrolyte anions intercalate into the graphite cathode during charge. On discharge, cations and anions are released back into the electrolyte.
A major goal for such as system is to find suitable electrolyte mixtures which exhibit not only a high oxidative stability at the cathode but also form a stable solid electrolyte interphase (SEI) at the graphite anode, they note. Of particular interest are electrolytes which display a high stability vs. oxidation due to the very high operation potentials of the cathode, which may exceed 5 V vs. Li/Li+
In their latest report, published in the RSC journal Energy & Environmental Science they used as an electrolyte system the ionic liquid-based electrolyte mixture Pyr14TFSI-LiTFSI in combination with the SEI-forming additive ethylene sulfite (ES). This combination allows a stable and highly reversible Li+ ion and TFSI- anion intercalation/de-intercalation into/from the graphite anode and cathode, respectively.
With the addition of ES, the discharge capacity for the anion intercalation can be remarkably increased from 50 mAh g-1 to 97 mAh g-1.
Sergej Rothermel, Paul Meister, Guido Schmuelling, Olga Fromm, Hinrich-Wilhelm Meyer, Sascha Nowak, Martin Winter and Tobias Placke (2014) “Dual-Graphite Cells based on the Reversible Intercalation of Bis(trifluoromethanesulfonyl)imide Anions from an Ionic Liquid Electrolyte” Energy Environ. Sci. doi: 10.1039/C4EE01873G
Tobias Placke, Olga Fromm, Simon Franz Lux, Peter Bieker, Sergej Rothermel, Hinrich-Wilhelm Meyer, Stefano Passerini and Martin Winter (2012) “Reversible Intercalation of Bis(trifluoromethanesulfonyl)imide Anions from an Ionic Liquid Electrolyte into Graphite for High Performance Dual-Ion Cells” J. Electrochem. Soc. volume 159, issue 11, A1755-A1765 doi: 10.1149/2.011211jes