Redox Active Oligomers Synthesized from Anthraquinone for Use in Rechargeable Lithium Batteries

Using redox active organic materials, that contain no scarce metal resources, as a positive-electrode active material instead of conventionally used rare-metal oxides can be a solution to the resource problem of the current rechargeable lithium batteries. Several types of organic positive-electrode materials have been proposed.  Among them, low-molecular-weight crystalline organic compounds have been drawing attentions recently, since these compounds tend to show high utilization ratios during the charge/discharge processes.  9,10-Anthraquinone (AQ) (Fig. 1a), one of those low-molecular-weight crystalline organic compounds, also shows a capacity close to its theoretical value [1,2]; however, the electrode using AQ tends to seriously degrade upon cycling because of the dissolution of the redox-related molecules into the electrolyte solution during cycling.

To suppress the dissolution and to improve their cycle stability, we focused our attention on the oligomerization technique. In this study, an anthraquinone dimer and a trimer connected by the -C≡C- bonds (Figs. 1b and 1c) were newly synthesized using the Sonogashira cross-coupling reaction; and their performances as positive-electrode active materials were compared with the AQ-monomer.  The positive electrodes using the dimer and trimer as active materials showed initial discharge capacities of about 200 mAh/g, with the average voltages of 2.2-2.3 V vs. Li⁺/Li.  These values are close to those of the AQ monomer, suggesting the two-electron redox reaction per AQ unit.  Fig. 2 compares the cycle-life performances of the electrode using the AQ-monomer, dimer, and trimer.  While the capacity of the AQ-monomer electrode rapidly decreased, the synthesized dimer and trimer showed better cycle stabilities. In particular, the trimer retained almost a constant capacity during 50 cycles without drastic decay.  The oligomerization of the AQ units effectively reduces the solubility. The present study revealed that oligomerization of the redox active molecular units was an effective way to extend the cycle-life without losing the discharge capacity.  

References

[1] Z. Song, H. Zhan, Y. Zhou, Chem. Commun., 4 (2009) 448.

[2] M. Yao, S. Yamazaki, H. Senoh, T. Sakai, T. Kiyobayashi, Mater. Sci. Eng. B, 177 (2012) 483.