Improvement in Cycle-Stability of Naphthoquinone-Based Rechargeable Organic Batteries By Polymerization

Using redox active organic materials, which 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. We have focused our attention on the redox active quinone-based materials which can lead to a high discharge capacity due to their multi-electron redox reactions and found that certain low-molecular-weight crystalline quinone derivatives exhibit high discharge capacities [1]. However, their discharge capacities tend to decrease upon cycling resulting from the loss of the active materials from the electrodes by the dissolution of the quinone molecules in the electrolyte solutions. To suppress the dissolution level of the quinone skeleton, we examined the polymerization of a quinone skeleton. In this paper, polymeric compounds were synthesized from 5,8-dihydroxy-1,4-naphthoquinone (DHNQ) (Fig. 1a) and their cycle-life performances were compared to that of the monomer.

Polymers including the DHNQ skeleton (PDNQs) were synthesized by the condensation reaction between DHNQ and formaldehyde in an acidic media condition at the temperatures of 90, 100, and 150 ^oC. The obtained materials were characterized by the ¹H-NMR, FT-IR spectroscopies, and the gel permeation chromatography (GPC). The positive-electrode was prepared by applying a composite sheet composed of the powder of the organic active material, acetylene black, and polytetrafluoroethylene in the weight ratio of 4:5:1 to a mesh-type aluminum current collector. The prepared positive-electrode was evaluated by making a coin-type half cell.

The solubility of the quinone-based materials in the ordinary organic solvents was successfully decreased by polymerization. Fig. 1b compares the cycle-life performance of the prepared materials (PDNQs) to that of monomer (DHNQ). The electrodes using DHNQ showed a discharge capacity of 110 mAh g⁻¹ in the first cycle. The obtained value is less than one fourth of the theoretical value based on the assumption of the four-electron redox reaction per molecule. The discharge capacity decreased rapidly upon cycling. On the other hand, the electrodes using PDNQs showed larger capacities than that of monomer at the first cycle. The obtained capacities are about 240-290 mAh g⁻¹, and the utilization ratio of these polymeric materials were improved to 45 to 55 %. Furthermore, the stabilities were better than that of the monomer.

There is a report about the charge/discharge behavior of a naphthoquinone polymer which was synthesized in a different way [2]. The previously reported polymer only showed a discharge capacity of about 60 mAh g⁻¹ during cycling although its cycle-life was fair. On the other hand, our samples in this study showed relatively large capacities. We also performed another polymerization on the DHNQ moiety with sulfur. While the cycle-stability of the sulfur-containing polymer was also good, the capacity was not large enough during initial cycles. These results indicate that polymerization can be effective to improve the cycle-life performance of naphthoquinone-based active materials; however the synthesis method has a profound effect on their discharge capacities. Further optimization in the synthesis condition is considered to improve the discharge property.

[1] M. Yao et al., J. Power Sources, 195 (2010) 8336.

[2] A. Kassam et al., Electrochem. Solid-State Lett., 14, (2011) A22.