A research team at the University of Tokyo has discovered a promising lithium-ion conducting liquid using water, which is expected to contribute to the improved safety and lower cost of Li-ion batteries. We here report on the highly safe new electrolyte for Li-ion batteries by introducing an article released on August 30, 2016, by Smart Japan, an online media services provider specializing in energy conservation, storage, and generation. This article is reproduced in an edited form with permission from Smart Japan.
The research team, led by Assistant Professor Yuki Yamada and Professor Atsuo Yamada at the Faculty of Engineering of the University of Tokyo, announced on Aug. 26, 2016 in Nature Energy, an American science journal, that they have discovered a new aqueous Li-ion electrolyte, a room-temperature hydrate melt, which will significantly contribute to realizing advanced, low-cost and safe storage batteries. It was a joint work with Keitaro Sodeyama, a Strategic Basic Research Programs researcher of the Japan Science and Technology Agency, and Yoshitaka Tateyama and his research group at the National Institute for Materials Science.
Development of new and advanced storage batteries is required in order to realize a society where power sources are dispersed. At present, the main line of storage batteries are Li-ion batteries, which use organic solvents as electrolyte. Because of the use of organic solvent, Li-ion batteries have a risk of high combustibility, and safety measures are necessary to prevent fire or explosion. Aqueous Li-ion batteries, using water instead of organic solvent, are attracting attention as a future target, but are associated with another issue. Water has lower voltage-resistance properties than organic solvent, and is easily decomposed into hydrogen and oxygen, even by low-voltage electric current.
Here, the team succeeded in finding a novel aqueous Li-ion conducting liquid, a hydrate melt, with sufficient stability for practical use. They verified this when two specific lithium salts of different kinds were mixed with water in a certain ratio, lithium salt dihydrate forms a stable liquid at room temperature, at which point lithium salt dihydrate is generally a solid.
While general aqueous solution is decomposed into oxygen and hydrogen with an electric current of about 1.2 volts, the newly discovered hydrate melt was not decomposed by an electric current of 3 volts or more. The analysis using the K computer of RIKEN, a comprehensive chemical research institute in Japan, showed that the high voltage-resistance properties of this hydrate melt are due to its unique liquid construction. According to this analysis, the hydrate melt also shows superior lithium ion transport properties; therefore, it can be used as an aqueous electrolyte for Li-ion batteries.
The team experimentally produced Li-ion batteries of 3.1 volts and 2.4 volts levels with the hydrate melt electrolyte, and these batteries succeeded in performing a reversible reaction, the first such example of an aqueous electrolyte performing at those levels. The capacity of aqueous Li-ion batteries has been limited to 2 volts or less; however, the success of the newly developed aqueous Li-ion batteries shows significant progress toward performance comparable to that of commercial non-aqueous batteries.
The hydrate melt batteries also showed good energy density, comparable to commercial 2.4-volt Li-ion batteries, and they require less than 6 minutes for charging, thus ensuring high-speed charge-discharge.
The group notes that use of an aqueous conducting liquid instead of an organic solvent, which is flammable, harmful and expensive, helps reduce the risk of combustion and fire accidents due to overcharging or accidental breakage. In the event of electrolyte leakage, damage to people or the environment is minimal because water is an atoxic solvent. The hydrate melt is therefore a highly safe battery storage system with high energy density.
The team also notes that the newly discovered hydrate melt will contribute to a cost reduction of batteries. The hydrate melt is produced from water, which is abundantly present in the nature. Moreover, while existing Li-ion batteries need rigorous dry rooms during the production process, the new hydrate melt batteries does not require such rooms; therefore, production equipment can be simplified.
The group now intends to investigate the roots of anomalous properties shown by the hydrate melt, and to explore other novel functions, aiming to establish of a new academic field. The group also plans to develop commercial storage battery devices that can use the hydrate melt electrolyte, identifying and solving problems while progressing toward their goals.
Source: Smart Japan (in Japanese)