Is it possible to exceed the lithium redox potential in electrochemical systems? One may think the answer to be “No” because among all the elements the redox potential of the elemental lithium is the lowest, which leads to the high voltage characteristics of the widely used lithium ion batteries. However, the answer should turn “Yes” in principle when we use a molecule-based ion which is not reduced even at the lithium potential. Here we examined a model battery system using a few molecular ion charge carriers such as PF₆^– and Me₄N⁺with organic active materials. Although the potential of the negative-electrode is not yet lower than that of lithium at present, this study reveals that molecular ions can work as a “rocking chair” type charge carrier in a battery [1]. There are so far a few studies [2-5] describing the use of a molecular ion as the charge carrier; however, its stoichiometery has rarely been analyzed.

     To construct a “rocking chair” type molecular ion battery using, for instance, an anion charge carrier, we used a PF₆^–-based electrolyte solution with two redox polymers (poly(N-vinylcarbazole): PVK, poly(1,1'-pentyl-4,4'-bipyridinium dihexafluorophosphate): PBPy) (Fig. 1a) as active materials. As for the molecular ion battery which uses a cation charge carrier, we chose a Me₄N⁺-based electrolyte with some low molecular weight quinone type organic active materials.

     As shown in Fig.1b, the prepared sealed full cell using a PF₆^–-based electrolyte system exhibited the potential difference of about 1.8 V with about 100 mAh/g_(PVK). The cell exhibited a fairly good cycle stability; the cell maintained about 65% of the initial capacity after 100 cycles.  The cell using the Me₄N⁺-based electrolyte system also showed a repeatable charge/discharge behavior.

     The concept of these molecular ion batteries was proved by energy dispersive X-ray (EDX) measurement: we evaluated the change in stoichiometry of PF₆^– in the electrode of the former cell ex situ by EDX and found that the charge carrier in this system is PF₆^– and the prepared system is certainly a PF₆^–-based “rocking chair type” battery.

     When succeeded in further developing the molecular ion battery, the benefits we would gain are: Breaking through the Li potential limitation, High ion conductivity in the electrolyte solution, Free from the dendrite risk, Minor metal free, and so on. To enjoy these benefits, the development in the active materials of both the positive- and negative-electrodes is indispensable. Searching for a negative electrode active material with a low redox potential is especially significant, which, we believe, will pave the way for the progress of this new battery system.

References:

[1] M. Yao, et al., Sci. Rep. 5, 10962 (2015).

[2] J.O. Besenhard, Carbon, 14, 111-115 (1976).

[3] P. Novák, et al., Chem. Rev. 97, 207-281 (1997).

[4] T. Boinowitz, et al., J. Power Sources, 56, 179-187(1995).

[5] H. Nakano et al., J. Mater. Chem. A, 2, 7588-7592 (2014).