Metals have traditionally been the go-to materials for the negative electrodes in batteries. However, there has been a recent shift towards using redox-active organic molecules, such as quinone- and amine-based compounds, as negative electrodes in rechargeable metal-air batteries. These batteries pair the organic molecules with oxygen-reducing positive electrodes, and the redox reactions involve protons and hydroxide ions. The use of organic molecules has led to high-performance batteries that approach the maximum theoretical capacity.
One of the key advantages of using redox-active organic molecules in rechargeable air batteries is that it circumvents issues associated with metals, such as the formation of dendrites. Dendrites can adversely affect battery performance and have negative environmental consequences. However, these batteries still rely on liquid electrolytes, similar to their metal-based counterparts. This reliance on liquid electrolytes brings about significant safety concerns, including high electrical resistance, leaching effects, and flammability.
To address these concerns, a recent study published in Angewandte Chemie International Edition describes the development of an all-solid-state rechargeable air battery (SSAB) by a group of Japanese researchers led by Professor Kenji Miyatake from Waseda University and the University of Yamanashi. The study, which was co-authored by Professor Kenichi Oyaizu from Waseda University, focuses on investigating the capacity and durability of the SSAB.
For the negative electrode’s active materials, the researchers selected a chemical called 2,5-dihydroxy-1,4-benzoquinone (DHBQ) and its polymer poly(2,5-dihydroxy-1,4-benzoquinone-3,6-methylene) (PDBM). These materials were chosen due to their stable and reversible redox reactions in acidic conditions. In addition, the researchers replaced conventional liquid electrolytes with a proton-conductive polymer called Nafion, serving as the solid electrolyte in the SSAB.
By adopting this solid-state design, the researchers have taken a significant step towards addressing the safety concerns associated with liquid electrolytes in batteries. The use of stable and reversible redox reactions in the organic molecule-based negative electrode, along with the proton-conductive solid electrolyte, offers promise for improved battery performance, enhanced safety, and reduced environmental impact.
According to Professor Miyatake, there have been no previous developments of air batteries using organic electrodes and solid polymer electrolytes, to the best of his knowledge. To address this gap, the researchers established an all-solid-state rechargeable air battery (SSAB) and conducted various assessments to evaluate its performance.
Unlike conventional air batteries with metallic negative electrodes and organic liquid electrolytes, the SSAB demonstrated remarkable stability in the presence of water and oxygen. This durability was a significant advantage of the SSAB, highlighting its potential as a viable alternative to existing battery technologies.
The researchers discovered that replacing the redox-active molecule DHBQ with its polymeric variant, PDBM, resulted in a superior negative electrode. At a constant current density of 1 mAcm-2, the SSAB-PDBM exhibited a discharge capacity of 176.1 mAh per gram, in contrast to the 29.7 mAh per gram of the SSAB-DHBQ.
Furthermore, the coulombic efficiency of the SSAB-PDBM was found to be 84% at a 4 C rate, gradually decreasing to 66% at a higher rate of 101°C. Although the discharge capacity of SSAB-PDBM decreased to 44% after 30 cycles, the researchers successfully improved it to 78% by increasing the content of the proton-conductive polymer in the negative electrode. Electron microscopic images confirmed the positive impact of Nafion addition on the performance and durability of the PDBM-based electrode.
Source: Waseda University