Comparative Study of the Influence of Anion Size on the Electrochemical Anion Intercalation into Graphitic Carbons
Authors: Meister Paul , Beltrop Kolja , Rothermel Sergej , Winter Martin , Placke Tobias 
Published: February 2020

The intercalation of anions into graphite forming so-called acceptor-type graphite intercalation compounds (GICs) [1] was presented for the first time by Rüdorff and Hofmann [2] in 1938. This storage mechanism was adopted and modified by McCullough et al. [3, 4] and Carlin et al. [5, 6] in their work about “dual-graphite” cells. Further studies on this topic were carried out by Seel and Dahn [7, 8], using a mixture of organic solvents and LiPF6. In this type of cells, a storage mechanism which is different from lithium-ion cells is shown, where upon charge the lithium ions intercalate into the graphitic anode, whereas the anions are simultaneously intercalated into the graphitic cathode. The reverse processes take place during discharge. Thus, the ions are released back into the electrolyte.
However, since the use of organic solvent-based electrolytes, which suffer from the highly oxidizing conditions, yields in electrolyte degradation, only an insufficient coulombic efficiency was achieved. To face this problem, Read et al. [9] investigated an electrolyte composed of LiPF6 and a fluorinated carbonate (monofluoroethylene carbonate, FEC) with an improved oxidation stability. Despite this improved anodic stability, only an insufficient efficiency of around 96% was achieved.
In recent publications [10, 11], we reported about an electrochemical energy storage system using an ionic liquid-based electrolyte as well as a lithium metal anode and a graphite cathode. To characterize the storage mechanism in this system we introduced the term “dual-ion” cell. Considering an identical storage mechanism presented in the references 3-9, one may conclude that the dual-graphite cell is just a special case of the dual-ion technology owing to an exchange of the anode material. Generally, ionic liquids possess different properties depending on the choice and combination of cation and anion. In particular, the ionic liquid N-butyl-N-methyl-pyrrolidinium bis(trifluoro-methanesulfonyl) imide (Pyr14TFSI) shows a high anodic stability, thus being an ideal candidate for dual-ion cells. Our studies have shown a high coulombic efficiency exceeding 99% for dual-ion cells utilizing a mixture of 1M LiTFSI and Pyr14TFSI. Due to the broad electrochemical stability window of Pyr14TFSI, the issue of graphite exfoliation arose by the application of this electrolyte in dual-graphite cells. Rothermel et al. [12] were able to solve this problem by the addition of the SEI-forming additive ethylene sulfite (ES). Based on the environmental, safety and cost benefits (e.g. no use of transition metals, but cheap graphite, non-flammability of the ionic liquid electrolyte), this technology might be a candidate for stationary energy storage.
One strategy to improve the discharge capacity as well as the specific energy of the system is the usage of smaller anions. Due to the decreased dimensions, the graphite host structure is capable to accommodate a higher amount of anions and therefore, a higher discharge capacity is obtained. In addition, the replacement of the anion leads also to a change of the stage number of the GIC. In this contribution, the electrochemical performance of dual-ion cells using ionic liquid-based electrolytes with different-sized anions will be evaluated. In particular, the influence of the upper cut-off potential on the discharge capacity as well as the coulombic efficiency will be shown. Furthermore, a correlation between discharge capacity and the anion size will be discussed. As example the anion fluorosulfonyl-(trifluoromethanesulfonyl) imide (FTFSI-, Figure 1) will be presented.

References

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Figure 1