Rechargaeble Organic Batteries Using Naphthazarin Derivatives: The Effect of Chloro-Substituents on the Battery Performance
Authors: Umetani Shinji , Yao Masaru , Ando Hisanori , Kiyobayashi Tetsu , Takeichi Nobuhiko , Kondo Ryota , Takeshita Hiroyuki T 
Published: February 2020

One of the recent requirements for the current rechargeable lithium battery systems is to reduce the amount of minor metal-based materials, especially from the positive-electrode. Our strategy to confront this issue is to replace the rare-metal oxides-based positive-electrode with redox active organic ones. So far, we have demonstrated that a series of low-molecular-weight quinone derivatives, which undergo multi-electron redox reactions, can be an alternative material category [1‒3]. To increase the discharge capacity of such organic compounds, we focused our attention on the naphthazarin (5,8-dihydroxy-1,4-naphthoquinone) skeleton as a new redox unit. This skeleton can exhibit a four-electron transfer redox reaction, which should lead to a large capacity of up to 550 mAh/g. This paper compares the battery performance of the electrodes incorporating the lithium salt of the naphthazarin skeleton (1)and some analogues carrying two or four peripheral chloro-substituents (1-Cl2
, 1-Cl4
) (Fig. 1a).
5,8-Dihydroxy-1,4-naphthoquinone and 2,3-dichloro-5,8-dihydroxy-1,4-naphthoquinone were purchased. 2,3,6,7-Tetrachloro-5,8-dihydroxy-1,4-naphthoquinone was synthesized by the chlorination of 2,3,6,7-tetrachloro-1,4,5,8-naphthodiquinone using sulfuryl chloride. The active material salts, 1, 1-Cl2
, and 1-Cl4
, were prepared by neutralizing the above-mentioned naphthazarin derivatives with lithium hydroxide. Their electrochemical properties as the positive-electrode active materials were evaluated in a coin-type half-cell using a metallic lithium sheet negative electrode and the electrolyte solution of tetraglyme/lithium bis(trifluoromethanesulfonyl)amide equimolar mixture.
Figure 1b shows the discharge curves of the prepared electrodes after the initial charge. In this figure, the capacities are expressed in the percentage of the theoretical values (1: 531; 1-Cl2
: 396; 1-Cl4
: 316 mAh/g) which assumes each four-electron transfer reactions. All discharge curves have complicated forms composed of some plateau potential regions, reflecting their multi-electron transfer reactions during the discharge processes. While the electrode of 1 was only about 50% of the theoretical value, the electrodes of 1-Cl2
and 1-Cl4
showed higher values of 88 and 87%, respectively. The actual specific discharge capacities of the electrodes of 1, 1-Cl2
, and 1-Cl4
were 259, 349, and 274 mAh/g, respectively. We consider that this difference in the initial utilization ratio results from the fact that the solubility of the charged (oxidized) state of 1 in the electrolyte solution might be higher than those of the chloro-substituted 1-Cl2
and 1-Cl4
. In the cycle test, the discharge capacity of the electrode using 1 quickly dropped upon cycling; it decreased to 13% of the theoretical value after 20 cycles as shown in Fig. 1c. On the other hand, the electrodes usingchloro-substituted ones exhibited better performance; the electrodes of 1-Cl2
and 1-Cl4
respectively maintained 28 and 57% of the theoretical value after 20 cycles. In addition to the battery tests, a potentiostatic polarization measurement combined with a UV-Vis spectroscopic apparatus was applied to solutions of 1, 1-Cl2
, and 1-Cl4
in a dimethyl sulfoxide/lithium bis(trifluoromethanesulfonyl)amide system. The measurement revealed that the solution state of the unsubstituted 1 is less stable than those of the chloro-substituted 1-Cl2
and 1-Cl4
during their redox reactions, and 1-Cl4
was most stable among them.
As is often the case, the discharge capacity of low-molecular-weight organic active materials tend to decrease upon cycling. One of the reasons of this capacity decay is suggested to be the loss of the active materials from the electrodes by the dissolution of the organic molecules in the electrolyte solutions. In this study, there seems a correlation between the electrochemical stability of the monomer state and the cycle-life performance, i.e., higher the electrochemical stability, the longer the cycle-life. The chloro substituent is considered to work as a protecting group to suppress the undesired side reactions during the redox reaction.
This study suggests that both enhancing the chemical durability and suppressing the solubility in the electrolyte solution are important to design a long cycle-life organic active material.

References
[1] M. Yao, et al., J. Power Sources, 195, 8336 (2010).
[2] M. Yao, et al., Mat. Sci. Eng. B, 177, 483 (2012).
[3] M. Yao, et al., Energy Procedia, 56, 228 (2014).

Figure 1