Scientists from University of Yamanashi and Waseda University in Japan created a new type of a solid-state air battery in which instead of a metal anode organic substance is used, an anthraquinone derivative. Such battery can accumulate energy owing to the reaction with the air oxygen, but at the same time is devoid of a number of problems inherent in traditional metal-air batteries.
Air batteries have long been considered one of the most promising technologies to accumulate energy. In such devices oxygen is taken directly from the ambient air. Owing to this it is possible to appreciably reduce the battery weight and to increase its theoretical energy capacity. For instance, lithium-air batteries are potentially capable of storing several times more energy than modern lithium-ion batteries. However, there are serious problems that hinder their wide applications: metal electrodes gradually deteriorate, form dendrites, can cause a short-circuit, while liquid electrolytes may get ignited or leak.
Japanese researchers decided to give up both the liquid electrolyte and the metal negative electrode. Instead they created a solid-state structure in which the role of the anode is played by the organic molecule of anthraquinone-2-carboxylic acid, while the charge transfer is provided by the solid-state membrane Nafion, one of the most common and effective proton-conductive polymers. When the battery operates oxygen is supplied from the positive electrode, while the organic compound at the negative electrode receives and gives off electrons, thus storing energy.
One of the key advantages of the new material is the low oxidation-reduction potential. Thanks to it the battery could develop a higher voltage, than the previous organic air batteries of a similar type. During the experiments voltage reached 1.16 V versus 0.88 of the earlier tested systems.
Scientists paid special attention to the thickness of the solid-state electrolyte. It turned out that a thicker membrane significantly improves the battery characteristics. The reason is that it lets oxygen pass inside the device worse. If oxygen penetrates towards the organic electrode too actively, it begins to destroy the active material by the side chemical reactions. A thick membrane allowed this effect to be reduced: battery capacity rose from 18 to 65 mA/h per gram, while the charge-discharge efficiency from 33% to 88%.
In order to reduce undesirable oxidation even more scientists replaced pure oxygen with the usual air which was also applied to the positive electrode. At the first glance, it might seem a step back, for the oxygen concentration in the air is less. However, in practice such approach gave an even better result: the discharge capacity rose to 80mA/h per gram, while the coulombic efficiency to 95%. This is attributable to the fact that at a lower concentration of oxygen less molecules of the latter penetrate through the membrane, thus reducing losses for the side chemical reactions.
Tests also showed good battery stability at intensive work. Even at very high charging and discharging speeds the system kept about 85% of efficiency. After 20 operating cycles the battery, when air was used, kept about 92% of the initial efficiency, which is a good indicator for experimental organic batteries.
Researchers believe that in the future such solid-state air batteries may become a perspective alternative to traditional lithium-ion batteries. Use of organic materials potentially reduces the production costs and diminishes dependence on deficit materials, while the solid-state electrolyte enhances the security of the device.
